akbasic/TODO.md

# TODO

Implementation plan for the Go → C port of `deps/basicinterpret`.

This file is written to be executed by AI agents, not read by humans for inspiration. Every
item names the files it touches, the exact deliverable, and the command that proves it done.
Work the phases in order. Inside a phase, items with no `Depends:` line may be done in
parallel.

---

## 0. Agent protocol

**Read these before touching anything**, in this order:

1. `CLAUDE.md` in this repository — project goals, the `libakerror` convention, the error-code
   range map, and the dependency versions.
2. `deps/libakerror/AGENTS.md` — the `ATTEMPT`/`CLEANUP`/`PROCESS`/`HANDLE`/`FINISH` protocol.
3. `deps/libakerror/UPGRADING.md` — 1.0.0's status registry. Required before writing an error
   code; the mechanism it replaced is gone.
4. `deps/libakstdlib/TODO.md` §2.1 — six **confirmed** defects in the library this port calls
   into. §1.9 below says which calls are cleared for use; that section is not optional
   reading, it bans a family of functions the port would otherwise reach for by reflex.
5. `deps/libakgl/AGENTS.md` — the no-`malloc` rule and the commit co-author requirement.
6. `deps/basicinterpret/README.md` — the language reference and the unimplemented list.

**Rules for working this file:**

- **Do not** mark an item done until its acceptance command passes on a clean out-of-tree
  build. "It compiles" is not acceptance.
- **Do** update this file in the same commit as the work: strike the item, or replace it with
  the defect it uncovered. This file holds outstanding items only.
- **Do** add the agent program name, model name and version as a commit co-author. That rule
  comes from `libakgl` and applies here.
- **Never** hand-edit generated output. `build/` trees and the generated `akerror.h` are
  off-limits; change the generator.
- **Never** reformat a file you are not otherwise changing.
- When a step is blocked because `libakgl` cannot supply a capability, **do not work around
  it here.** File it in `deps/libakgl/TODO.md` in the numbered prose style of that file's
  "Carried over" section and note the block in §7 below.

**Style**, restated so nobody has to go look: C99, 4-space indent, tabs at width 8
(`stroustrup`), function-body braces in column 0 on their own line, control braces on the same
line, **always brace**, spaces inside control-flow parens — `if ( x == y ) {`. Pointer star
binds to the identifier: `char *name`. Prefix is `akbasic_` for functions,
`akbasic_TypeName` for types, `AKBASIC_UPPER_SNAKE` for macros. `static` helpers drop the
prefix. Parameter names must match between header and source.

---

## 1. Design decisions already made

These are settled. Do not relitigate them mid-port; if evidence says one is wrong, say so in
a commit that changes it deliberately, and update this section.

### 1.1 Reflection becomes one aligned dispatch table

The Go runtime resolves verbs with `reflect.MethodByName("Command" + NAME)`
(`basicruntime.go:401`), functions with `"Function" + NAME`, and special parse paths with
`"ParseCommand" + NAME` (`basicparser.go:103`). C has no reflection and we are not adding
any. Replace all three with **one** static table in `src/verbs.c`, sorted by name, searched
with `bsearch(3)`:

```c
/* name        token type              parse handler          exec handler        */
{ "AUTO",      AKBASIC_TOK_CMDIMM,     NULL,                  cmd_auto      },
{ "DATA",      AKBASIC_TOK_COMMAND,    parse_data,            cmd_data      },
{ "DEF",       AKBASIC_TOK_COMMAND,    parse_def,             cmd_def       },
```

A `NULL` parse handler means "parse the rval as a plain expression", which is exactly what
`commandByReflection` returning `(nil, nil)` means today. Adding a verb is adding one row plus
two functions. **Keep the table column-aligned and one row per verb** — it is a table, so it
gets laid out as one.

This also kills the Go scanner's three separate maps (`reservedwords`, `commands`,
`functions`, `basicscanner.go:64-67`): the token type lives in the same row.

### 1.2 Strings are fixed-size and live inline

`libakstdlib` has no string type. `libakgl` has `akgl_String` but it is `PATH_MAX` bytes,
refcounted, and pool-allocated — wrong shape for a value that gets copied on every
assignment, and it would drag a `libakgl` dependency into the core interpreter.

Define in `include/akbasic/types.h`:

```c
#define AKBASIC_MAX_STRING_LENGTH    256    /* matches AKBASIC_MAX_LINE_LENGTH */
```

and give `akbasic_Value` a `char stringval[AKBASIC_MAX_STRING_LENGTH]` **inline**. `clone()`
becomes a struct assignment. No allocator, no refcount, no lifetime question.

**Tradeoff, stated:** every `akbasic_Value` is ~300 bytes, so one environment's
`values[AKBASIC_MAX_VALUES]` pool is ~19KB, and 32 environments is ~610KB of BSS. That is
fine on a PC and is the price of never calling `malloc`. If it ever isn't fine, the knob is
`AKBASIC_MAX_STRING_LENGTH`, not the allocator.

Truncation is an **error**, not a silent clamp: `FAIL_RETURN(e, AKBASIC_ERR_VALUE, ...)`.

### 1.3 Maps become fixed-capacity open-addressed tables

Five Go maps need replacing:

| Go site | Purpose | C replacement |
|---|---|---|
| `BasicScanner.reservedwords/commands/functions` | keyword → token type | the §1.1 static table + `bsearch` |
| `BasicEnvironment.variables` | name → `*BasicVariable` | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_VARIABLES` |
| `BasicEnvironment.functions` | name → `*BasicFunctionDef` | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_FUNCTIONS` |
| `BasicEnvironment.labels` | name → line number | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_LABELS` |

One implementation, `src/symtab.c`, keyed by `aksl_strhash_djb2()` (already in
`libakstdlib`) with linear probing and a fixed slot array. **Use the existing hash; do not
write another one.** Table full is an error, not a resize.

Caveat, from `deps/libakstdlib/TODO.md` §1.6/§2.2.6: the wrapper sign-extends `char`, so a
high-bit byte hashes wrong — `"\xff\xfe"` returns 5859874 where the `unsigned char` answer is
5868578. BASIC identifiers are 7-bit ASCII (the scanner only accepts `IsLetter`/`IsDigit` plus
a type suffix), so this cannot bite the symbol tables. It **would** bite if anyone later keys
a table on a string literal or a filename. Do not work around it here; it is already filed
upstream.

### 1.4 Environments come from a pool and are released

Go calls `new(BasicEnvironment)` at `basicruntime.go:121` and `basicparser_commands.go:124`
and never frees one. A long-running `GOSUB` or `FOR` in Go leaks; the GC eventually catches
some of it, and nothing in the tests notices.

C gets `HEAP_ENVIRONMENT[AKBASIC_MAX_ENVIRONMENTS]` with
`akbasic_env_acquire()` / `akbasic_env_release()`, in the shape of `akgl_heap_next_*`.
`akbasic_runtime_prev_environment()` **must** release the environment it pops. Pool
exhaustion is `AKBASIC_ERR_ENVIRONMENT`, reported with the current line number.

Watch the one place this is not a clean stack: `userFunction` (`basicruntime.go:348`) stores
a `BasicEnvironment` **by value** inside `BasicFunctionDef` and re-`init()`s it on every call.
In C the funcdef holds an `akbasic_Environment *` acquired at `DEF` time and reset per call —
it is owned by the funcdef, not the pool's free list, until the funcdef dies.

### 1.5 Output goes through a text sink backend

`Write()` and `Println()` (`basicruntime_graphics.go:140,148`) mirror every line to stdout
*and* to an SDL surface. That mirror is the only reason the golden-file suite works. Do not
reproduce it as a hardcoded pair of calls.

Define a record of function pointers, populated by an initializer — the house pattern:

```c
typedef struct akbasic_TextSink
{
    void *self;
    akerr_ErrorContext AKERR_NOIGNORE *(*write)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*writeln)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*readline)(struct akbasic_TextSink *self, char *dest, size_t len);
    akerr_ErrorContext AKERR_NOIGNORE *(*clear)(struct akbasic_TextSink *self);
} akbasic_TextSink;
```

`akbasic_sink_init_stdio()` is in the library and `akbasic_sink_init_akgl()` is in the
akgl-backed module. The driver is supposed to pick: a default build selects stdio, while an
`AKBASIC_WITH_AKGL` build selects the SDL text path and mirrors its output to stdout. It does
not do that yet; §3 records the missing standalone frontend. Cursor arithmetic, wrapping and
scrolling belong to the akgl sink, not to the interpreter.

### 1.6 The interpreter steps; it does not run

Go's `run()` (`basicruntime.go:682`) is a `for {}` that owns the process until `MODE_QUIT`.
Goal 3 forbids that: a host game must be able to bound execution.

The library exposes:

```c
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_step(akbasic_Runtime *obj);
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_run(akbasic_Runtime *obj, int maxsteps);
```

`_step()` does exactly what one iteration of Go's `for {}` body does and returns.
`_run(obj, maxsteps)` loops `_step()` until the mode is `AKBASIC_MODE_QUIT` or `maxsteps`
steps have elapsed; `maxsteps <= 0` means unbounded, which is what the standalone driver
passes. **This is a deliberate restructure, and it must not change a single byte of golden
output.**

`QUIT` sets `AKBASIC_MODE_QUIT` and returns. **Nothing in the library calls `exit()`,
`abort()`, or `FINISH_NORETURN`.** `FINISH_NORETURN` appears exactly once in the tree, in
`src/main.c`.

### 1.7 Error codes are absolute, reserved from the 1.0.0 registry

`deps/libakerror` is at **1.0.0**. Read `deps/libakerror/UPGRADING.md` before writing an error
code; `CLAUDE.md`'s error-code section is the condensed version. Three things this changes
from what you may have seen in an older draft or in `libakgl`:

- `AKERR_MAX_ERR_VALUE` **no longer exists**. The name registry is sparse and takes any `int`.
  There is nothing for a consumer to size, and no compile definition to set. Anywhere you find
  one, delete it.
- `__AKERR_ERROR_NAMES` **no longer exists**. The table is private.
- Codes are **absolute integer constants at 256 or above**, never `AKERR_LAST_ERRNO_VALUE + N`.
  The offset scheme is what produced the live `libakgl` collision documented in `CLAUDE.md`.

`akbasic` owns **512–767** per the range map in `CLAUDE.md`. Declare it as an `enum`, so the
values stay compile-time integer constants (`HANDLE` expands to `case` labels, which require
that) and adding a code does not mean renumbering an offset:

```c
#define AKBASIC_OWNER "akbasic"

enum {
    AKBASIC_ERR_BASE        = 512,   /** Start of akbasic's reserved status range */
    AKBASIC_ERR_SYNTAX      = AKBASIC_ERR_BASE,
                                     /** Parse-time grammar violation */
    AKBASIC_ERR_TYPE,                /** Incompatible types in an operation */
    AKBASIC_ERR_UNDEFINED,           /** Reference to an undefined verb, function or label */
    AKBASIC_ERR_BOUNDS,              /** Array subscript or pool index out of range */
    AKBASIC_ERR_ENVIRONMENT,         /** Environment pool exhausted or orphaned environment */
    AKBASIC_ERR_VALUE,               /** A value was malformed, truncated or unconvertible */
    AKBASIC_ERR_STATE,               /** A verb ran outside the block structure it requires */
    AKBASIC_ERR_LIMIT       = AKBASIC_ERR_BASE + 256
};
```

`akbasic_init()` reserves the **whole** 256 in one call and then names each code. Both
registry calls return `akerr_ErrorContext *` and are `AKERR_NOIGNORE`, so a collision is an
ordinary error — `PASS` it and let it propagate out of init:

```c
akerr_ErrorContext AKERR_NOIGNORE *akbasic_init(void)
{
    PREPARE_ERROR(errctx);

    PASS(errctx, akerr_reserve_status_range(AKBASIC_ERR_BASE,
					    AKBASIC_ERR_LIMIT - AKBASIC_ERR_BASE,
					    AKBASIC_OWNER));
    PASS(errctx, akerr_register_status_name(AKBASIC_OWNER, AKBASIC_ERR_SYNTAX,
					    "Syntax Error"));
    /* ... one per code ... */
    SUCCEED_RETURN(errctx);
}
```

Reserve the whole range in **one** call — a subset or superset of your own range raises
`AKERR_STATUS_RANGE_OVERLAP`, not a no-op. Use `akerr_register_status_name()`, never the
two-argument `akerr_name_for_status(status, name)` set path: the owned form is the one that
catches a component writing into a range that is not its own, and the two-argument form is
what let `libakgl` silently clobber `libakerror`'s names.

There is no startup ceiling to assert any more — the BSS-overflow hazard the old draft
defended against was deleted along with `AKERR_MAX_ERR_VALUE`. What replaces it is the
reservation itself: if `akbasic_init()` returns an error, something else owns part of 512–767
and the process must not continue as though it does not.

Do not call `akerr_init()` first. Every registry entry point calls it, and since 1.0.0 it no
longer clears reservations made before it ran.

### 1.8 Error message text is part of the acceptance contract

`tests/language/array_outofbounds.txt` is, verbatim:

```
? 20 : RUNTIME ERROR Variable index access out of bounds at dimension 0: 4 (max 2)\n\n
```

The trailing double newline is real: `basicError` builds a string ending in `\n` and hands it
to `Println`, which adds another. **Reproduce the message strings and the newline behaviour
byte for byte** or the golden suite fails for reasons that have nothing to do with the
interpreter. Where a Go message reads awkwardly, keep it awkward and note it here.

Numeric formatting must match too: integers via `%" PRId64 "`, floats via `%f` (Go's `%f` and
C's `%f` both give six decimals — `tests/language/arithmetic/float.txt` confirms).

### 1.9 Which `libakstdlib` calls are cleared for use — **the bans are lifted**

`deps/libakstdlib` is at **0.2.0**, and that release *fixed all six* of the confirmed defects
this section was built around, plus seventeen contract gaps. The table of bans that used to
live here is gone because it is no longer true, and leaving it would send an agent around a
workaround for a function that now works.

What changed, in the four that mattered to this port:

| Was banned | Now |
|---|---|
| `aksl_atoi` / `_atol` / `_atoll` / `_atof` | Every one takes a `dest` pointer and raises `AKERR_VALUE` on no digits or trailing junk, `ERANGE` on overflow — exactly the contract this section demanded |
| `aksl_list_append` / `_iterate` | Append no longer truncates to two nodes; iterate no longer skips the first half |
| `aksl_tree_iterate` | `AKERR_ITERATOR_BREAK` stops the traversal |
| `aksl_realpath` | Rewritten; no longer reads uninitialised memory on its error path |

Two signature changes reach this repository. `aksl_fread` and `aksl_fwrite` now take a
required `size_t *nmemb_out` and report a short transfer as `AKERR_IO` instead of a silent
success — so a `DSAVE` onto a full disk is now an error a program sees, where before it
reported nothing. `src/runtime_commands.c` passes the count and discards it, which is correct:
the library does the noticing now.

**`src/convert.c` is gone.** It existed only because `aksl_ato*` could not report a conversion
failure, and its own note said what to do when that changed: *"When it grows it, delete
`src/convert.c` and switch the call sites over."* That is done. Six call sites now go straight
to the library — `newLiteralInt` and `newLiteralFloat` in `src/grammar.c`, the line number in
`src/scanner.c`, `FunctionVAL` in `src/runtime_functions.c`, `INPUT` in
`src/runtime_commands.c`, and `GSHAPE`'s handle in `src/runtime_graphics.c`.

Three things that fell out of it, recorded because each was checked rather than assumed:

- **`aksl_strtoll(str, NULL, base, &dest)` is the exact replacement**, not `aksl_atoll`, wherever
  a base is involved. The `ato*` forms are base 10; `src/grammar.c` picks base 8 or 16 from the
  lexeme's prefix. A `NULL` endptr is what makes trailing junk an error rather than a stopping
  point, and that is the whole contract this port needs.
- **The raised status changed** from `AKBASIC_ERR_VALUE` to `AKERR_VALUE`, and the message text
  with it — `VAL("garbage")` now says `no digits in "garbage"`. §1.8 makes message text part of
  the acceptance contract, so this was checked against the corpus first: **no golden file
  contained a conversion message**, which is also why §1.9 used to ask for one. Two now exist,
  `tests/language/numeric/val_refuses_garbage` and `octal_literal`, so the next change to that
  text has to move a golden file.
- **`tests/convert.c` became `tests/numeric_contract.c`.** The assertions did not become
  worthless when the wrapper went away — they became assertions about a contract this port
  depends on and no longer owns, which is worth pinning exactly because a regression in it would
  make `VAL("garbage")` return 0 silently. Same reasoning `libakstdlib`'s own
  `tests/test_status_registry.c` gives for pinning that it reserves no status range.

One caveat survives the upgrade unchanged: `aksl_strhash_djb2` still sign-extends `char`, so a
high-bit byte hashes differently from the `unsigned char` answer. BASIC identifiers are 7-bit
ASCII so the symbol tables cannot reach it — see §1.3, which is still accurate.

---

## 2. What exists — **the core port is complete and green**

Phases 0 through 6 of the original plan are done. The interpreter builds clean under
`-Wall -Wextra`, reproduces the reference byte for byte, and passes under ASan and UBSan.

```sh
cmake -S . -B build && cmake --build build --parallel
ctest --test-dir build --output-on-failure          # 59/59
cmake -S . -B build-asan -DAKBASIC_SANITIZE=ON      # 59/59
cmake -S . -B build-cov  -DAKBASIC_COVERAGE=ON      # 92.3% line, 96.9% function
```

| Module | Source | Reference |
|---|---|---|
| Symbol table | `src/symtab.c` | the five Go maps |
| Value | `src/value.c` | `basicvalue.go` |
| Variable | `src/variable.c` | `basicvariable.go` |
| Tokens and AST leaves | `src/grammar.c` | `basicgrammar.go` + `basicparser.go` |
| Dispatch table | `src/verbs.c` | the three reflection lookups |
| Scanner | `src/scanner.c` | `basicscanner.go` |
| Parser | `src/parser.c`, `src/parser_commands.c` | `basicparser*.go` |
| Environment | `src/environment.c` | `basicenvironment.go` |
| Runtime | `src/runtime.c` | `basicruntime.go` minus SDL |
| Verbs and functions | `src/runtime_commands.c`, `src/runtime_functions.c` | `basicruntime_{commands,functions}.go` |
| Text sink | `src/sink_stdio.c` | `basicruntime_graphics.go`'s stdout mirror |
| Stdio driver | `src/main.c` | `main.go` minus its SDL frontend |
| Embedding examples | `examples/embed.c`, `examples/hostvars.c` | *(new)* |

`akbasic_runtime_load(rt, source)` was added while writing the README's embedding
section: a host usually holds its script as a string and wants the sink reserved for output,
and the only path that existed — `AKBASIC_MODE_RUNSTREAM` reading through the sink's
`readline` — forces a game to point its output device at its source text. `examples/embed.c`
is the code the README quotes, built by every build and registered as a CTest case so a
signature change breaks the build rather than rotting the document.

**The acceptance suite is the reference's own corpus, driven in place.** All 41 `.bas` files
under `deps/basicinterpret/tests/` are registered as individual CTest cases and byte-compared
against their `.txt` — including the trailing double newline on an error line (§1.8). Nothing
is copied, so the corpus cannot drift from its upstream.

Eighteen unit tests cover the modules the corpus cannot reach on its own.
`tests/known_reference_defects.c` is registered in `AKBASIC_KNOWN_FAILING_TESTS` and asserts
the **correct** contract for six of the defects in §6; when one is fixed CTest reports
"unexpectedly passed", which is the cue to split that assertion out and strike the item.

Two budget numbers moved during implementation and are worth knowing:
`AKBASIC_MAX_ARRAY_ELEMENTS` (1024) caps one array and `AKBASIC_MAX_ARRAY_VALUES` (4096) caps
all of them together, drawn from an `akbasic_ValuePool` the runtime owns. The reference had no
such ceiling because it called `make()`.

---


## 3. The akgl-backed sink, devices and standalone frontend — **done**

`-DAKBASIC_WITH_AKGL=ON` builds and its suite passes, and the build option now changes what
the executable *does*: an AKGL build of `basic` opens a window, draws BASIC output into it,
pumps SDL events, lets you type at it, and still puts every byte on stdout. Four adaptors in
the `akbasic_akgl` target, which is the only thing here that links SDL, are complete and
tested:

| File | Backs |
|---|---|
| `src/sink_akgl.c` | the §1.5 text sink, over `akgl_text_measure` and `akgl_text_rendertextat` |
| `src/graphics_akgl.c` | `akbasic_GraphicsBackend`, over `akgl_draw_*`, and the SSHAPE surface pool |
| `src/audio_akgl.c` | `akbasic_AudioBackend`, over `akgl_audio_*` |
| `src/input_akgl.c` | `akbasic_InputBackend`, over `akgl_controller_poll_key` |

The character grid comes from `akgl_text_measure(font, "A", &w, &h)` — the direct equivalent of
the `font.SizeUTF8("A")` at `basicruntime.go:96`, and the call this was blocked on until
42b60f7. Wrapping is done on the character grid rather than by handing SDL_ttf a `wraplength`,
because the cursor has to land somewhere definite: a program that `PRINT`s a long string and
then `PRINT`s again expects the second to start on the row after the first ended, and only the
code that placed the characters knows which row that is.

**The interpreter owns no window, renderer or event loop.** Every initializer takes something
the host already made. Each calls `akgl_error_init()` first: `akgl_game_init()` would have,
but we drive subsystems directly and never call it, and a code raised before that registration
carries no name into its stack trace. It is idempotent.

*Acceptance:* `tests/akgl_backends.c`, a 128x128 software renderer under the dummy video
driver read back with `SDL_RenderReadPixels` — the pattern `deps/libakgl/tests/draw.c`
established, which needs no display and no offscreen harness. Registered as the `akgl_backends`
CTest case, and only when `AKBASIC_WITH_AKGL` is on.

### The standalone frontend

The standalone executable is itself a host, and that half of the Go frontend is now ported —
`src/frontend_akgl.c`, in its own `akbasic_frontend` target. **The separation is in the build
graph and not only in a comment**: `akbasic` is the interpreter, `akbasic_akgl` is the four
adaptors that draw through somebody else's renderer, and `akbasic_frontend` is the one thing in
the repository that creates a window. A game embedding the interpreter links the first two and
not the third.

| Piece | Where |
|---|---|
| Window, renderer, font, the four adaptors | `akbasic_frontend_akgl_init()` |
| Sink and devices onto a runtime | `akbasic_frontend_akgl_attach()` |
| One frame: pump events, draw the text layer, present | `akbasic_frontend_akgl_pump()` |
| The bounded frame loop | `akbasic_frontend_akgl_drive()` |
| stdout mirror | `src/sink_tee.c`, in the *core* library |

Four things are worth knowing about how it came out.

1. **The stdout mirror is a sink, not a second write.** §1.5 made the sink boundary precisely
   so the interpreter would never carry the reference's hardcoded pair of calls, and §3 item 5
   said it again. `akbasic_sink_init_tee()` composes two sinks into one and takes `readline`
   from whichever of the two was named as the reader — which is the whole difference between
   the two modes: a program read from a file comes in through the stdio half, and typed lines
   come in through the akgl half's line editor. It lives in the core library and is tested
   there (`tests/sink_tee.c`), because composing two function-pointer records needs no SDL.

2. **The line editor borrows the host's loop a frame at a time.** `readline` has to wait for a
   typed line, and §1.6 forbids the library blocking — and a sink that sat on the keyboard with
   no way to pump events would deadlock the process on its first `INPUT`. So the host installs
   an `akbasic_AkglPump` with `akbasic_sink_akgl_set_pump()`, and the editor calls it between
   keystrokes. The loop is still the host's. `akbasic_frontend_akgl_pump()` has that exact
   signature and is installed as-is. Without a pump the sink reports EOF, which is what it did
   before and what a host that wants no editor still gets.

3. **The frame loop is bounded, and that is what makes the close button work.** 256 steps per
   frame (`AKBASIC_FRONTEND_STEPS_PER_FRAME`), then pump and present. `10 GOTO 10` under an
   unbounded `run()` would own the process forever; `tests/akgl_frontend.c` runs exactly that
   and asserts the window close ends it.

4. **The frame is not cleared.** The graphics verbs draw straight to the renderer rather than
   into a display list, so clearing would wipe every `DRAW` at the end of the frame it was
   issued in. The text layer is drawn over whatever is there, which is also how a C128 stacks
   its text plane on its bitmap plane.

*Acceptance:* two things, and the second one is the stronger.

- `tests/akgl_frontend.c`, registered as the `akgl_frontend` CTest case: a program run through
  the frontend under the dummy driver, asserting the mirrored stdout **byte for byte** and
  reading the renderer back to prove the same characters were drawn in the Commodore font;
  synthetic key events pushed into SDL's own queue and read back through the frontend's input
  backend, so every link the host owns is in the path; the line editor's fold to upper case,
  its backspace and its escape; a whole REPL session typed at the window and ended with a typed
  `QUIT`; and the window-close path.
- **The entire golden corpus is driven through the AKGL binary.** A `-DAKBASIC_WITH_AKGL=ON`
  build registers the same 41 upstream cases against the same executable, which now opens a
  window to run them, and all 41 still match byte for byte. That is a much broader statement
  than any hand-written frontend test: the SDL path changes no observable output anywhere in
  the corpus.

Three `no_device.bas` local cases are **deliberately not registered** in an AKGL build. Their
premise is a driver that was given no devices, which is true of the stdio driver and
deliberately false of this one. The refusal path they cover is asserted directly against the
backend records in `tests/devices.c`, which both configurations build.

**Manual check against a real display, 2026-07-31.** The dummy driver cannot prove presentation,
so `build-akgl/basic` was run against X display `:0` on a program that prints a line, draws a
`BOX` and then loops. `xwininfo` found a real 800x600 window titled `BASIC`; the window was
captured with `import` and measured: the first text row carries glyph pixels (mean 65/255), the
box's left edge at x=100 is a solid white line (mean 255), the box interior is black — it
outlines rather than fills — and an empty corner is black. stdout carried `HELLO WORLD` at the
same time. `QUIT` exits cleanly with status 0.

**What that check does not cover, and could not here:** typing at a real focused window. No key
synthesis tool (`xdotool`, `xte`, python-xlib) is installed on this machine, so the keyboard
path is proved only through SDL's own event queue in `tests/akgl_frontend.c` — which is the
same queue a real keyboard delivers into, but not the same as a window manager giving the
window focus. Somebody should type at it once.

### Five things that had to be worked around to get there

All five are `libakgl`'s and all five are filed in `deps/libakgl/TODO.md`. Each workaround is
commented at its site with the words "filed upstream" so it can be found and deleted later.

1. **An embedded `libakgl` demands its dependencies be *installed*.** It builds its vendored
   SDL, SDL_image, SDL_mixer, SDL_ttf and jansson only when it is top-level; embedded, it goes
   down a `find_package` path and fails on a machine that has none of them — while the
   submodules sit right there in `deps/libakgl/deps`. Every lookup is guarded with
   `if(NOT TARGET ...)`, so our `CMakeLists.txt` adds those five subdirectories *before*
   `add_subdirectory(deps/libakgl)` and the vendored copies get used after all. Same trick and
   same ordering requirement `akerror::akerror` and `akstdlib::akstdlib` already need.

2. **`akgl/controller.h` does not compile on its own.** It declares two handler function
   pointers taking an `akgl_Actor *` and includes nothing that declares the type.
   `src/input_akgl.c` includes `akgl/actor.h` ahead of it.

3. **There is no way to attach a 2D backend to a renderer you already have.**
   `akgl_render_init2d()` installs the six vtable pointers, but it also creates its own window
   from the game properties and writes to the `camera` global, so it belongs to the
   `akgl_game_init()` path — which is exactly the path an embedding host is not on.
   `tests/akgl_backends.c` assigns the six pointers by hand. What is wanted upstream is the
   vtable half of `init2d` on its own.

4. **`akgl_text_rendertextat()` segfaults on a backend whose vtable is empty.** It reaches
   through `renderer->draw_texture` without checking it, so the first `PRINT` through the sink
   dereferences NULL. That is the same class of defect 42b60f7's own commit added a draw test
   for — "a backend that exists but was never given an `SDL_Renderer`" — and the text path
   still has it.

### Still missing from the sink

Nothing that blocks a verb. The line editor exists (see above), so `INPUT` and the REPL work in
the window. Two limits are worth stating because a program can reach both:

- **No shifted character can be typed.** The keystroke ring carries a keycode and no modifier
  state, so `"`, `!`, `(`, `)`, `:` and `;` are unreachable and a string literal cannot be typed
  at the window at all. Letters are folded to upper case, which is a C128 doing what a C128 does
  rather than a workaround, but punctuation has no such excuse. Filed upstream as `libakgl`
  item 10; until it lands, the way to run a program with a string literal in it is to pass the
  file on the command line, which is unaffected.
- **No cursor movement within a line.** Backspace and escape only. The arrow keys stay in the
  ring for a script's own `GET` loop, which is the more valuable use of them.

`WINDOW` and `KEY` in group E want a text-window rectangle and a function-key table on top of
this, which is ordinary work here rather than anything blocked.

---

## 4. Remaining work: language completion (goal 2)

The work queue is the "What Isn't Implemented" list in `deps/basicinterpret/README.md`.
Each verb is: table row (§1.1) → parse handler if it needs one → exec handler → unit test →
a new `.bas`/`.txt` pair. **Both** kinds of test; they answer different questions.

Order by what unblocks the most, and by what does not need `libakgl` to grow first:

| Group | Verbs | Blocked on |
|---|---|---|
| A. Structure | `DO`, `LOOP`, `WHILE`, `UNTIL`, `ON`, `BEGIN`, `BEND`, `END` | nothing — reuses `waitingForCommand` |
| B. Housekeeping | `NEW`, `CLR`, `CONT`, `RESTORE`, `RENUMBER`, `SWAP`, `TRON`, `TROFF`, `HELP` | nothing |
| C. Errors | `TRAP`, `RESUME`, `ER`, `ERR` | needs a BASIC-visible error object |
| D. Strings/format | `USING`, `PUDEF`, `WIDTH`, `CHAR` | nothing |
| E. Console | `WINDOW`, `KEY`, `SLEEP`, `WAIT`, `TI` | the sink, or the clock `akbasic_runtime_settime()` now carries |
| F. Disk | `DOPEN`, `DCLOSE`, `APPEND`, `RECORD`, `HEADER`, `COLLECT`, `BACKUP`, `COPY`, `CONCAT`, `RENAME`, `SCRATCH`, `DIRECTORY`, `CATALOG`, `DCLEAR`, `DVERIFY`, `SAVE`, `LOAD`, `VERIFY`, `BLOAD`, `BSAVE`, `BOOT` | `aksl_f*` only; no new akgl |
| ~~G. Graphics~~ | ~~`GRAPHIC`, `DRAW`, `BOX`, `CIRCLE`, `PAINT`, `COLOR`, `SCALE`, `SSHAPE`, `GSHAPE`, `LOCATE`~~ | **done** — `src/runtime_graphics.c`, `tests/graphics_verbs.c` |
| H. Sprites | `SPRITE`, `MOVSPR`, `SPRCOLOR`, `SPRDEF`, `SPRSAV`, `COLLISION` | `libakgl` sprite/actor API — check before filing |
| ~~I. Audio~~ | ~~`PLAY`, `SOUND`, `ENVELOPE`, `VOL`, `TEMPO`~~ | **done** — `src/runtime_audio.c`, `src/play.c`, `tests/audio_verbs.c` |
| I′. Audio, still gapped | `FILTER`, `SOUND`'s sweep arguments | `libakgl` has neither; both refused with `AKBASIC_ERR_DEVICE`, both filed in §7 |
| ~~E′. Console input~~ | ~~`GET`, `GETKEY`, `SCNCLR`~~ | **done** — `src/runtime_input.c`, `tests/input_verbs.c` |
| J. Machine | `SYS`, `FETCH`, `STASH`, `POKE`/`PEEK` variants | nothing |

`LOCATE` used to appear in both E and G. It belongs to G alone: in BASIC 7.0 `LOCATE` moves the
*graphics* pixel cursor that `DRAW` starts from, and the text cursor verb is `CHAR`, which is
already in group D.

**Out of scope, and staying that way** — the reference marks these as incompatible with a
modern PC and that reasoning stands: `BANK` (no bank switching), `FAST` (irrelevant CPU speed
control), `MONITOR` (no machine-language monitor).

Also on the queue, from the reference's own defect list:

- **Multiple statements per line** (`10 PRINT A$ : REM ...`). The `COLON` token exists
  (`basicscanner.go:40`) and nothing consumes it. This changes the parser's statement loop and
  should be done before group A, because `DO`/`LOOP` bodies read badly without it.
- **Array references in parameter lists** (`READ A$(0), B#`) currently fail to parse. Fix in
  `argumentList`.

---

## 5. Deliberate deviations from the reference

Keep this list current. Each of these changes structure without changing observable output,
and each one has to be defensible against the golden suite.

1. Reflection → static dispatch table (§1.1).
2. Go maps → fixed open-addressed tables (§1.3).
3. Unbounded `new(BasicEnvironment)` → pool with release (§1.4).
4. Hardcoded stdout+SDL mirror → text sink backend (§1.5).
5. `run()` owns the process → `step()` / bounded `run()` (§1.6).
6. `panic()` → `FAIL_RETURN`; no library call terminates the process (§1.6).
7. `debug.PrintStack()` on parse error → the `akerr` stack trace only (§3.1 of the original plan).
8. `BasicEnvironment.eval_clone_identifiers` deleted as dead state (§4.2 of the original plan).
9. `BasicValue.name` and `BasicEnvironment.update()` deleted as dead: nothing ever writes the
   field a non-empty string, and `update()` — its only reader — has no callers.
10. The `DEF`-statement bootstrap is gone. The reference declares every builtin by *running a
    BASIC program of `DEF` lines through the interpreter* at startup and then nulling out the
    expressions (`basicruntime_functions.go:14`); here a builtin's name, arity and handler are
    a row in the dispatch table, and `MOD`, `SPC` and `STR` are ordinary native handlers. This
    removes the need to run the interpreter before the interpreter is ready.
11. Undefined behaviour the reference reaches by a defined route is refused rather than
    inherited: a shift count outside 0..63, a negative string multiplier, and integer division
    by zero all raise. Go defines all three (or panics); C does not. No golden case exercises
    any of them, so observable behaviour is unchanged.
12. `CommandEXIT` clears the pending `NEXT` wait before popping. The reference does not, which
    leaves the parent waiting for a `NEXT` that never arrives (§6 item 8) — in C that is a hang
    rather than a misbehaviour, and no golden case depends on it.

### Deviations in the verbs the reference never implemented

Items 1–12 above are deviations *from ported code*. These are deviations from **Commodore
BASIC 7.0**, in verbs the reference lists as unimplemented and which therefore had no Go
behaviour to port. They matter to somebody typing in a listing out of a C128 manual.

13. **Hardware verbs reach a device through a backend record, and refuse when there is none.**
    `akbasic_GraphicsBackend`, `_AudioBackend` and `_InputBackend` are records of function
    pointers on the runtime; all three may be `NULL`, which is what the current stdio-only
    standalone driver gives them. That is correct for a default build and unfinished for an
    `AKBASIC_WITH_AKGL` build; see §3. A verb that needs a device it was not given raises
    `AKBASIC_ERR_DEVICE` naming itself. `COLOR`, `LOCATE`, `SCALE`, `ENVELOPE`, `TEMPO` and
    `VOL` deliberately do *not* require one — they change interpreter state, so a program can
    configure itself before the host lends it a renderer.

14. **`CIRCLE` is a polygon, always.** BASIC 7.0's `CIRCLE` takes two radii, a start and end
    angle, a rotation and a degree increment, which makes it an inc-degree polygon by
    definition. `akgl_draw_circle` exists and is deliberately **not** used: it draws one
    radius, full sweep, unrotated, so it could serve only the case where every optional
    argument is defaulted — and a shape that changed character depending on whether the two
    radii happened to be equal is worse than one that is uniformly a polygon.

15. **`SSHAPE` stores a handle in the string, not the pixels.** A real C128 packs the
    region's bitmap into the string variable. Here a value's string is a fixed 256 bytes
    (§1.2) and a saved region is a device surface, so the variable gets `SHAPE:<n>` and the
    surface stays in a fixed pool on the backend. `GSHAPE` accepts only a string carrying that
    prefix, so a hand-written one is refused rather than pasting an unrelated slot. **What
    this costs:** a shape cannot be written to disk, cannot be concatenated, and `LEN` of it
    is not its size. Nothing in BASIC does any of those to a shape string except a program
    deliberately poking at it.

16. **`BOX` fills on a negative angle rather than on a seventh argument.** 7.0's last
    argument selects outline or fill and sits *after* the rotation, which would make a filled
    box a seven-argument call. That is filed rather than implemented; today a rotation of zero
    outlines through the renderer's rectangle and any other rotation draws four lines.

17. **`GRAPHIC` records its mode but honours only one consequence of it.** 7.0's five modes
    differ in bitmap resolution and in whether the bottom of the screen stays text. Neither
    means anything against a host's renderer, whose size the interpreter does not know and
    does not own. The mode is stored, an out-of-range one is refused because that is a typo
    worth catching, and the one observable rule is that mode 0 is text.

18. **Coordinates reaching a backend are BASIC's 320x200 space, and scaling to the window is
    the host's job.** The interpreter cannot ask the renderer how big it is without owning it.
    `SCALE` maps user coordinates onto that space; with `SCALE` off they pass through.

19. **`PLAY` does not block.** On a C128 `PLAY` holds the program until the last note ends.
    §1.6 forbids that outright, so `PLAY` parses the string into a fixed queue and returns;
    `akbasic_runtime_step()` releases one note at a time against whatever time the host last
    passed to `akbasic_runtime_settime()`. **What this changes for a program:** the statement
    after a `PLAY` runs immediately rather than a bar later, so a listing that relied on `PLAY`
    for timing will race ahead. A program that wants to wait should test the queue rather than
    assume the verb waits for it.

20. **The host owns the clock, and an unset clock rushes the music.** The library reads no
    clock — it owns no loop and must not block. A host calls `akbasic_runtime_settime()` once a
    frame; the standalone driver calls it every step off `CLOCK_MONOTONIC`. Left unset it reads
    zero forever, so every duration has already expired and the queue drains as fast as
    `step()` is called. Audible, deterministic, and never a hang, which is the right way for
    that to fail.

21. **`PLAY`'s `M` (measure) is a no-op and `SOUND`'s frequency sweep is refused.** `M`
    synchronises the three voices on a C128; with one sequential queue there is nothing to
    synchronise, and a listing full of them should still play, so it is accepted and ignored.
    The sweep is the opposite call: `SOUND`'s `dir`/`min`/`step` arguments have no
    `akgl_audio_*` equivalent, and the only way to fake one is to re-issue tones from
    `step()` — which would tie audible pitch to how often the host happens to call us, a tune
    that changes key with the frame rate. Filed upstream as `akgl_audio_sweep`; see §7.

22. **`TEMPO`'s constant is a calibration choice, not a transcription.** BASIC 7.0 documents
    `TEMPO` as a relative duration from 1 to 255 defaulting to 8, and does not publish what a
    whole note actually lasts. `src/audio_tables.c` uses 16000 ms at `TEMPO` 1, which puts a
    default-tempo quarter note at 500 ms — 120 beats per minute. If the real constant turns
    up, that is the one line to change.

23. **`GETKEY` holds the step loop rather than blocking.** A C128 sits on the keyboard inside
    `GETKEY` until somebody types. §1.6 forbids the library blocking, so the verb sets a flag
    and `akbasic_runtime_step()` declines to advance the program until a key arrives. Every
    step still returns and a bounded `akbasic_runtime_run()` still comes back, so a host keeps
    its frame rate; the program simply does not move past the `GETKEY`, which is the part a
    program author cares about. The `PLAY` queue is serviced *before* this check on purpose —
    music should keep playing while a program waits for a keypress. Withdrawing the input
    device while a `GETKEY` is holding releases it rather than wedging the script forever.

24. **`GET` and `GETKEY` into a numeric variable yield the key code.** BASIC 7.0's `GET` takes
    a string variable. Accepting an integer one as well, and giving it the raw code, is what a
    program testing for cursor or function keys needs and costs nothing. No key is code zero,
    matching what a C128 reports. A float variable is refused: it is neither a character nor a
    code.

### Deviations in the standalone frontend

Items 1–12 are deviations from ported interpreter code and 13–24 from BASIC 7.0. These four are
deviations from the reference's *program*: `main.go` and the SDL half of
`basicruntime_graphics.go`.

25. **The frame loop is bounded and the reference's is not.** Go's `run()` is a `for {}` that
    owns the process until `MODE_QUIT` (`basicruntime.go:682`), which is why the reference's
    window never answers its close button. Here the driver runs
    `AKBASIC_FRONTEND_STEPS_PER_FRAME` steps, pumps, presents, and goes round again. **What
    this changes for a program:** nothing observable — a bounded run reproduces an unbounded one
    exactly, and the whole golden corpus is driven through this loop to prove it. What it
    changes for a *user* is that the window closes when asked.

26. **The frame is not cleared between frames.** The reference blits a text surface onto the
    window surface and calls `UpdateSurface`. Here the graphics verbs draw straight to the
    renderer rather than into a display list, so a `SDL_RenderClear` at the top of each frame
    would erase every `DRAW` at the end of the frame it was issued in. The text layer is drawn
    over whatever is already there. **What this costs:** text that has scrolled away leaves its
    pixels behind wherever a graphics verb has drawn, because nothing repaints that region. A
    `SCNCLR` does not repaint it either — it clears the text grid, not the bitmap. `GRAPHIC 0`
    is the verb that wipes the drawing surface.

27. **The window is closed by the host, and that is not `QUIT`.** Closing the window stops the
    frontend and leaves `obj->mode` alone; it does not set `AKBASIC_MODE_QUIT`. The distinction
    is invisible to the standalone program, which exits either way, and load-bearing for an
    embedding host: a game that closes its own window has not decided that the script is
    finished, and a script that ran `QUIT` has. `tests/akgl_frontend.c` asserts both halves.

28. **Piped input still works in an AKGL build.** With a terminal on stdin the window is the
    console and lines come from its editor; piped or redirected, they come from the pipe. This
    is not in the reference, which always reads `os.Stdin` in REPL mode. It exists so
    `basic < program.bas` behaves the same in both builds — and so the golden corpus can be
    driven through the SDL binary, which is where most of the frontend's real coverage comes
    from.

---

## 6. Reference defects — ported faithfully, fix them deliberately

These are real bugs in `deps/basicinterpret`. **Port the behaviour first** so the golden suite
passes and the port is provably faithful, then fix each one in its own commit with a test that
asserts the correct contract. Until fixed, the correct-contract test lives in
`AKBASIC_KNOWN_FAILING_TESTS`.

1. **`basicvariable.go:176`** — `toString()` tests `len(self.values) == 0` and then indexes
   `self.values[0]`. Inverted condition; on an empty variable it panics, and on a non-empty one
   it returns the "not implemented for arrays" string. *Consequence:* `PRINT` of a scalar
   through this path is wrong. Fix: `if ( count == 1 )`.
2. **`basicvariable.go:108`** — `setBoolean` builds a value with `valuetype: TYPE_STRING`.
   *Consequence:* a boolean stored in a variable reads back as a string with an empty
   `stringval`. Fix: `TYPE_BOOLEAN`.
3. **`basicenvironment.go:157`** — `stopWaiting(command)` ignores `command` and clears the
   wait unconditionally. *Consequence:* an inner block can clear an outer block's wait. Fix:
   compare before clearing, and error on a mismatch.
4. **`basicvalue.go:181`** — `mathPlus` mutates `self` in place when `self.mutable` is true,
   while every other operator always clones. *Consequence:* `A# + 1` can modify `A#` depending
   on where the value came from. This one is high blast radius; it interacts with
   `eval_clone_identifiers` and with `CommandNEXT`'s increment
   (`basicruntime_commands.go:663`), which **relies** on the mutation. Do not fix it before
   `FOR`/`NEXT` has full test coverage.
5. **`basicvalue.go:191` and siblings** — every binary operator adds both of the right-hand
   operand's numeric fields: `rval.intval + int64(rval.floatval)`. Works only because the
   unused field is always zero. *Consequence:* none today; it is a landmine for any future
   value that carries both.
6. **`basicvalue.go`, all comparisons** — `dest` is cloned from `self`, so the resulting
   boolean inherits `self.name`. *Consequence:* a comparison result can be written back to the
   wrong variable through `environment.update()`.
7. **`basicruntime_commands.go:484`** — `CommandIF` walks `expr.right` to the end of the chain
   looking for a `THEN` command, then dereferences `expr.right` after the loop guaranteed it
   is `nil`. *Consequence:* the "Malformed IF statement" check is unreachable and nested
   `IF ... THEN ... ELSE` is unverified.
8. **`basicruntime_commands.go:669`** — `CommandEXIT` pops the environment without
   `stopWaiting`, leaving the parent waiting on a `NEXT` that will never come.
9. **`basicruntime.go:149`** — `newVariable()` and `BasicRuntime.variables[MAX_VARIABLES]` are
   dead; variables live in the environment's map. Do not port them.
10. ~~**`basicgrammar.go:224`** — `newLiteralInt` selects base 8 whenever the lexeme starts
    with `0`.~~ **Fixed.** Base 10 unless prefixed `0x`; a leading zero in a listing is padding,
    not a radix. `PRINT 010` prints 10 and `PRINT 08` parses. Nothing in the upstream corpus
    had a leading-zero literal, which is why nothing there noticed. Regression tests in
    `tests/grammar_leaves.c` and `tests/language/numeric/octal_literal.bas`, which was rewritten
    from pinning the defect to pinning the fix.
11. **`basicruntime.go:121` / `basicparser_commands.go:124`** — environments are never freed.
    Addressed structurally by §1.4; no separate fix needed, but the C pool **will** exhaust
    where Go merely leaked, so `tests/environment_scope.c`'s exhaustion assertion matters.

### Found during the port

Five more, none of which the reference's own corpus catches. **Four are now fixed**, each with
a regression test in the suite that pinned it; the fifth is described below and is not a coding
question.

12. ~~**`basicparser.go:329` — `subtraction()` returns after one operator where `addition()`
    loops.**~~ **Fixed.** `1 - 2 - 3` computed `1 - 2` and abandoned the rest of the line; the
    remaining `- 3` was left in the token stream, picked up by the statement loop as a second
    statement, evaluated and discarded. A wrong answer, silently, which made this the
    highest-impact item on the list. `exponent()` has the identical shape — a `for` loop with a
    `return` inside it, in both the reference and the port — so `2 ^ 3 ^ 2` had it too. Both
    now fold left, which is what a C128 does with a run of same-precedence operators.
    Regression tests in `tests/parser_expressions.c`.

    Worth recording for whoever reads the reference next: `exponent()` there is
    `for self.match(CARAT) { ... return expr, nil }` (`basicparser.go:519-539`), so the loop is
    a loop in shape only. `subtraction()` is the same at `:357-377`. Neither is a
    transcription slip in the port.

13. ~~**`basicparser.go:565` — a unary-minus argument inflates a function's arity count.**~~
    **Fixed**, and the fix is structural rather than a smarter counter. The counter walked
    `.right`, which is where a unary leaf keeps its operand *and* where an argument list chains
    its arguments; the two meanings were indistinguishable, so `ABS(-9)` counted as two
    arguments and was refused with "function ABS takes 1 arguments, received 2". No builtin
    could be called with a negative literal at all. The corpus hid it — `sgn.bas` assigns `-1`
    to a variable first.

    **A unary leaf's operand now hangs off `.left`.** A unary leaf had no other use for that
    field, so the two meanings separate for the cost of one nobody was using — and counting
    then needs no special case at all. That also fixes the half of the defect the original
    entry did not mention: chaining the *next* argument through `.right` overwrote the operand,
    so `MOD(-7, 3)` destroyed its own first argument. Three readers moved (`src/runtime.c`,
    `src/grammar.c`'s renderer, `src/runtime_commands.c`'s `LIST` range parser) and that is the
    whole change. Regression tests in `tests/runtime_evaluate.c`, both halves.

    **The same collision remains for array subscripts**, which is why §4 still lists
    "array references in parameter lists" as outstanding: an identifier keeps its subscript
    list on `.right` too. The same move — onto `.left` — is very likely the fix, and it is not
    made here because it wants its own commit and its own tests.

14. ~~**`basicscanner.go:272` — `matchNextChar` returns without setting a token type when it
    cannot peek past the end of the line.**~~ **Fixed.** A comparison operator in the final
    column was silently dropped: `A# =` produced one token, not two, and the parse error that
    followed pointed somewhere else entirely. `falsetype` is now set before returning on the
    peek failure. Regression tests in `tests/scanner_tokens.c`, for `=`, `<` and `>`.

15. ~~**`basicscanner.go:318` — hex literals do not survive the scanner.**~~ **Fixed.**
    `matchNumber` let `x` through but not the hex digits after it, so `0xff` lexed as `0x`
    followed by an identifier `ff`, the base-16 branch in `newLiteralInt` was unreachable, and
    the README's hex support did not exist. Once the `x` is seen the run continues over hex
    digits. Accepted anywhere in a number run rather than only after a leading `0`, which is
    the reference's rule and is worth keeping: it is what makes `1x2` one malformed token that
    the line-number conversion diagnoses by name. Regression tests in `tests/scanner_tokens.c`
    and `tests/language/numeric/octal_literal.bas`.

16. **`basicscanner.go:349` — the "Reserved word in variable name" check never fires.** The
    lexeme still carries its type suffix when the keyword tables are searched, so `PRINT$`
    misses every table and becomes an ordinary string variable. *Consequence:* the diagnostic
    is dead code and `PRINT$ = 1` is accepted.

    **Deliberately not fixed, and this is the interesting one.** The fix is four lines — strip
    the suffix before the lookup — and it works. It also breaks a golden case: the reference's
    own `deps/basicinterpret/tests/examples/strreverse.bas` names a variable `INPUT$`, and with
    the check working that line is refused.

    A real C128 refuses it too, so the reference *program* is the wrong one here, not the
    diagnostic. But the corpus is the acceptance contract (§1.8), it lives in a submodule this
    repository may not edit, and reproducing it byte for byte is goal 1's headline claim. The
    two cannot both hold. Fixing this means deciding that the diagnostic is worth one
    permanently-excluded upstream case, which is a call for the author rather than for whoever
    is next through this file. Until then it stays pinned in
    `tests/known_reference_defects.c`, which is now the only thing left in there.

### Ours, not the reference's

17. **A host cannot reliably create a script variable while a script is suspended.** This one
    is not inherited — it falls out of §1.4's environment pool meeting §1.6's bounded `run()`,
    a combination the reference never had because its `run()` never returned.

    Exchanging variables with an embedding host works and is documented in `README.md`: the
    host calls `akbasic_environment_get()` to find or create a variable and
    `akbasic_variable_set_*` / `_get_subscript` to move values across. Seeding before
    `akbasic_runtime_start()` and reading after the script stops is safe, and so is *updating*
    an existing global at any time, because the parent chain is searched. **Creating** one
    mid-run is not, in two ways, and both are silent:

    - A script suspended part-way through a bounded `akbasic_runtime_run()` is usually inside a
      `FOR` or `GOSUB` scope, so `obj->environment` is that scope. A variable created through
      it lands in the loop's scope and dies when the environment pops — the script reads the
      value correctly inside the loop and gets `0` immediately after it.
    - Reaching for the root explicitly does not help. `akbasic_environment_get` only
      auto-creates when `obj->runtime->environment == obj` (`src/environment.c:242`), so with a
      child active it returns `NULL` through `dest` **without raising**, and an unchecked host
      dereferences it.

    `examples/hostvars.c` demonstrates both rather than describing them, and prints what it
    observes, so the day this is fixed that output changes and the example needs revisiting.

    Fix: add `akbasic_runtime_global(akbasic_Runtime *obj, const char *name, akbasic_Variable **dest)`
    that walks `obj->environment` to the root and creates there unconditionally, and point the
    README at it instead of at `akbasic_environment_get`. Roughly fifteen lines. The one design
    question worth settling first is what it should do when the script is suspended inside a
    *user function's* scope — that environment is owned by the funcdef rather than the pool
    (§1.4), and "root" is still the right answer there, but it is worth being deliberate about
    it rather than discovering it. Tests: create a global while suspended in a `FOR` body and
    assert the script still sees it after `NEXT`; the same from inside a `GOSUB`; and the
    NULL-without-error path, which should become an impossible state.

---

## 7. Filing gaps against `libakgl`

When phase 7 or phase 8 needs something `libakgl` does not have, **stop and file it** in
`deps/libakgl/TODO.md` in that file's numbered prose style: what the BASIC verb requires, what
the `akgl_*` entry point should look like, and what tests would cover it. Growing `libakgl` to
serve the interpreter is a wanted outcome, not a detour.

**All four gaps filed so far are now closed upstream**, at `libakgl` 42b60f7. Nothing here is
blocked on `libakgl` any more; what remains is akbasic-side work.

| Was blocking | Closed by |
|---|---|
| Text measurement (§3, the akgl sink) | `akgl_text_measure`, `akgl_text_measure_wrapped` |
| Immediate-mode drawing (§4 group G) | `akgl_draw_point`/`_line`/`_rect`/`_filled_rect`/`_circle`/`_flood_fill`/`_copy_region`/`_paste_region` |
| Audio (§4 group I) | `akgl_audio_init`/`_tone`/`_envelope`/`_waveform`/`_volume`/`_stop`/`_voice_active`/`_mix` |
| Console input (§4 group E) | `akgl_controller_poll_key`, `akgl_controller_flush_keys` |

Two notes for whoever picks up §3 next. The draw calls take an `akgl_RenderBackend *` as their
first argument rather than reaching for a global renderer, which fits the rule that the
interpreter draws through whatever renderer the host already initialized. And the audio API is
a synthesised-voice one — `akgl_audio_tone(voice, hz, ms)` with a separate ADSR envelope —
which is the shape `PLAY` and `ENVELOPE` need, rather than the sample playback SDL3_mixer
would have given.

Building against any of it needs `-DAKBASIC_WITH_AKGL=ON`, which pulls in `libakgl`'s own
submodules (SDL and friends). Those are not initialized in a default clone; `git submodule
update --init --recursive` gets them.

**Seven items are filed upstream and outstanding**, at `deps/libakgl/TODO.md` items 5-10 of "API
gaps blocking akbasic" and item 14 of "Defects". Four are workarounds this repository carries
today, each commented at its site with the words "filed upstream" so it can be found and
deleted: the embedded-dependency CMake trick in our `CMakeLists.txt`, the `akgl/actor.h`
include in `src/input_akgl.c` and `src/sink_akgl.c`, and the hand-populated vtable and its
NULL-deref hazard in `tests/akgl_backends.c` and `src/frontend_akgl.c`. §3 describes all four.
The fifth is `SOUND`'s frequency sweep, the sixth is the missing version bump recorded in §8,
and the seventh is new:

- **The keystroke ring carries a keycode and no modifier state**, so the line editor cannot see
  Shift. Every shifted character — `"`, `!`, `(`, `)`, `:`, `;` — is unreachable, which means a
  string literal cannot be typed at the window at all, and lower case is unreachable too.
  Letters are folded to upper case, which is what a C128 does and is therefore the right answer
  for letters rather than merely the available one; punctuation has no such excuse. It is not
  fixable from here: by the time a caller could ask `SDL_GetModState()` the key is long
  released, and decoupling reads from events is the entire point of a ring. The wanted API is
  an `akgl_controller_poll_keystroke()` returning keycode, modifiers and the composed UTF-8
  text SDL already computes from `SDL_EVENT_TEXT_INPUT` — which is also the only way a keyboard
  layout, a compose key or a dead key can ever work. Filed as item 10, alongside the existing
  `akgl_controller_poll_key()`, which stays exactly as it is: a game asking "was the up arrow
  pressed" wants precisely what it already gets.

**One further gap is outstanding, and upstream named it itself.** The commit that added the audio API
says plainly what it does not cover: *"Still missing for a complete BASIC sound vocabulary:
`FILTER` (SDL3 has no filter primitive; this would need writing), `TEMPO` and the `PLAY`
note-string parser, both of which belong in the interpreter rather than here."*

- **`TEMPO` and the `PLAY` parser are ours**, not a gap. They are group I work in this
  repository and nothing upstream is waiting on.
- **`FILTER` is a real gap.** BASIC 7.0's `FILTER freq, lp, bp, hp, res` sets the SID's
  filter cutoff, the three band-pass switches and the resonance. `akgl_audio_*` synthesises
  raw waveforms and mixes them; there is no filter stage to configure and SDL3 supplies no
  primitive to build one from. Until an `akgl_audio_filter()` exists, `FILTER` is accepted by
  the parser and refused at execution with `AKBASIC_ERR_DEVICE` rather than silently ignored —
  a program that asks for a low-pass and gets an unfiltered square wave has been lied to.

A second gap turned up while implementing group I, and is filed alongside it:

- **`SOUND` has no frequency sweep.** BASIC 7.0's `SOUND voice, freq, dur, dir, min, step`
  ramps the pitch from `freq` toward `min` in `step` increments per tick, in the direction
  `dir` selects. `akgl_audio_tone(voice, hz, ms)` holds one pitch for one duration and there
  is no entry point that changes pitch over the life of a note. The wanted API is roughly
  `akgl_audio_sweep(int voice, float32_t from_hz, float32_t to_hz, float32_t step_hz, uint32_t ms)`,
  advanced on the mixer's own frame counter — the same place the phase is already derived
  from, and the reason it belongs there rather than here. Faking it in the interpreter means
  re-issuing tones from `akbasic_runtime_step()`, which ties audible pitch to how often a host
  calls us: a tune that changes key with the frame rate. Refused with `AKBASIC_ERR_DEVICE`
  until it lands. Tests would drive `akgl_audio_mix` by hand and assert the frame at which the
  pitch has moved, exactly as `tests/audio.c` already does for the envelope.

When the next gap turns up, file it there rather than working around it here.

---

## 8. Status

**The port is done.** The C interpreter reproduces the Go reference byte for byte across its
entire test corpus and passes clean under ASan and UBSan.

| Gate | Result |
|---|---|
| `ctest` | 72/72 — 41 upstream golden cases, 5 local ones, 22 unit tests, 2 embedding examples, 1 known-failing, 1 version check |
| `ctest` with `-DAKBASIC_WITH_AKGL=ON` | 72/72 — the same, minus the three `no_device` cases the SDL driver contradicts, plus `akgl_backends` and `akgl_frontend`; the `akgl_build` CI job |
| Golden corpus | 41/41 byte-exact, driven in place from the submodule — **and 41/41 again through the SDL binary**, which is most of what proves the frontend changes no output |
| ASan + UBSan | 72/72 |
| Line coverage | 93.6% (3618/3867) — above the 90% gate |
| Function coverage | 97.8% (267/273) |
| Warnings | none under `-Wall -Wextra` |
| `doxygen Doxyfile` | clean |
| Mutation (`src/symtab.c`) | 74.1%, against a gate of 65 — see `.gitea/workflows/ci.yaml` |

Coverage of the verb groups added for goal 2, since they are the new surface: `src/audio_tables.c`
and `src/graphics_tables.c` 100%, `src/runtime_input.c` 100%, `src/runtime_graphics.c` and
`src/runtime_audio.c` 98%, `src/play.c` 87%. The `akbasic_akgl` target is not in the coverage
figure — it is not built in a default configuration.

**A note on reading that number, because it was misread once while producing it.** A stale
`build-cov/` left in the source directory from an earlier session was silently folded into the
report by `gcovr --root .`, which produced the *previous* run's 92.3% for a tree that had grown
by 800 lines. That is `libakgl` defect #13 happening here rather than there, and `build*/` being
gitignored is exactly what makes it invisible. **Keep build trees out of the source directory**,
and if a coverage number looks suspiciously unchanged, `find . -name '*.gcda'` before believing
it.

Branch coverage reads 18.0% and is not a target, for the reason `libakgl/TODO.md` and
`libakstdlib/TODO.md` both give: the akerror control-flow macros expand into large branch trees
at every call site, most of them unreachable in normal operation. Track line and function
coverage.

Dependency baseline:

| Submodule | Version | Notes |
|---|---|---|
| `deps/libakerror` | 1.0.0 | Private ownership-enforced status registry. akbasic reserves 512–767 in `akbasic_error_register()`. Does not namespace its `coverage` target when embedded — worked around in our `CMakeLists.txt`. |
| `deps/libakstdlib` | 0.2.0 | soname `libakstdlib.so.0.2`. `AKSL_VERSION_CHECK()` asserted in `tests/version_check.c`. This release fixed all six confirmed defects the port was working around — see §1.9, where the bans are now lifted. |
| `deps/libakgl` | 0.2.0 | soname `libakgl.so.0.2`. Owns status codes 256–260. Linked and tested under `-DAKBASIC_WITH_AKGL=ON`, which still defaults OFF so the core library and its whole suite build on a machine with no SDL. It requires `libakstdlib` 0.2, which is why the two moved together. |

**The `libakgl` requirement used to be pinned by commit, and no longer is.** 42b60f7 added 22
public symbols across four headers and left `project(akgl VERSION 0.1.0)` and the
`libakgl.so.0.1` soname unchanged, so nothing here could feature-test for the new API and the
submodule pointer was the only thing expressing the requirement. That was filed upstream as
`libakgl` defect #14 and fixed by its commit 1066ac7 — the two crossed rather than one
following the other. `libakgl` is 0.2.0 with an `libakgl.so.0.2` soname, so
`AKGL_VERSION_AT_LEAST(0, 2, 0)` now means something and an ABI mismatch is refused at load
time. Kept here because the *reason* is the reusable part: an additive release under an
unchanged soname costs its consumers the ability to detect it at all.

CI is split in two. `.gitea/workflows/ci.yaml` runs on push -- the suite, ASan+UBSan, coverage
and a two-file mutation run -- and `.gitea/workflows/release.yaml` is manual
(`workflow_dispatch`), carrying the doxygen gate and the whole-tree mutation run. The split is
about cost: the full mutation set is 3675 mutants and hours of runner time, which is a release
gate rather than a per-commit one, and the docs are wanted at release time. Three notes worth
keeping:

- The checkout is `submodules: true`, **not** `recursive`. The build needs `libakerror`,
  `libakstdlib` and `basicinterpret`; it does not need `libakgl`, which is guarded behind
  `AKBASIC_WITH_AKGL` and defaults OFF — and recursing into it would clone SDL, SDL_image,
  SDL_mixer, SDL_ttf and jansson for a target that is never configured. The `docs` job needs
  no submodules at all.
- The push-path `mutation_test` job is bounded to `src/symtab.c`, scoring 74.1% against a gate
  of 65. It was two files until `src/convert.c` was deleted. `src/audio_tables.c` was measured
  as a replacement and scored 64.7%, below the gate — a real gap rather than a reason to skip
  the file: almost every survivor is in `akbasic_audio_state_init`, where nothing asserts that
  a freshly initialised audio state is actually zeroed and defaulted. That is the same gap this
  job's history records closing for `src/symtab.c`, and closing it is what earns the file its
  place back in CI.

  Worth knowing before reading a score: the harness's `ICR` operator only rewrites the
  constants `0` and `1`, so a lookup table of other values produces no mutants at all and a
  high score over one says nothing about whether its entries are right. `tests/audio_verbs.c`
  asserts all 32 ADSR entries anyway — that catches a hand-edit typo, which is the real failure
  mode, but it did not and could not move the mutation number. `src/value.c` is the file most
  worth mutating and is deliberately not on that path: 368 mutants at roughly 11 seconds each
  is about 70 minutes, because almost everything links against it. It is covered by
  `release.yaml`, or locally with `cmake --build build --target mutation`.
- `release.yaml`'s threshold defaults to 65, the same as the push path, rather than something
  stricter. Only two files have ever been measured; a tighter whole-tree number would be a
  guess until a full run establishes a baseline. Raise it through the workflow input once one
  has.

What remains, in priority order:

1. ~~**The standalone AKGL frontend in §3.**~~ **Done** — `src/frontend_akgl.c` in its own
   `akbasic_frontend` target, `src/sink_tee.c` for the stdout mirror, and
   `tests/akgl_frontend.c`. The build option now changes what the executable does. The one
   thing not machine-verified is typing at a real focused window; §3 records why and what was
   verified instead.
2. ~~**A line editor for the akgl sink.**~~ **Done** — `readline` in `src/sink_akgl.c`, over
   the keystroke ring, borrowing frames from the host through an `akbasic_AkglPump`. It cannot
   type a shifted character, which is `libakgl` item 10 rather than work outstanding here.
3. **§4 — the language completion work queue.** Groups G and I and the `GET`/`GETKEY`/`SCNCLR`
   part of E are done. Of what is left, **multiple statements per line** should come first:
   the `COLON` token exists and nothing consumes it, and `DO`/`LOOP` in group A reads badly
   without it. Then groups A, B, D, F and J, none of which need anything from `libakgl`.
4. ~~**CI does not cover `-DAKBASIC_WITH_AKGL=ON`.**~~ **Done** — the `akgl_build` job in
   `.gitea/workflows/ci.yaml`. It turned out much cheaper than the deferral assumed, and the
   two reasons are worth keeping because both were guesses that measurement corrected:

   - **`submodules: recursive` is not needed and costs 461 MB.** It descends into
     `SDL_image/external`, `SDL_mixer/external` and `SDL_ttf/external` — aom, dav1d, libjxl,
     libtiff, mpg123, opus, flac and a dozen more — and *not one of them is used*. Every one
     configures as "Could NOT find". The job initialises exactly six submodules
     non-recursively instead, which takes about 25 seconds.
   - **The build is about a minute of CPU, not "a long build".** 330 object files, because
     SDL3 compiles out almost every backend that is not wanted here.

   The one real system dependency is `libfreetype-dev` and `libharfbuzz-dev`: SDL_ttf prefers
   the system copies and would otherwise reach for `external/freetype`, which the checkout
   deliberately does not clone. Two apt packages against two more submodules is an easy trade.

   Every step was verified from a clean clone before being written down, not inferred.
5. **§6 items 16 and 17.** Items 10 and 12–15 are fixed, each with a regression test in the
   suite that pinned it. What is left is not more of the same:
   - **Item 16** is a *decision*, not a fix. Making the "Reserved word in variable name" check
     fire costs an upstream golden case, because the reference's own `strreverse.bas` names a
     variable `INPUT$`. §6 states the trade; somebody has to make it.
   - **Item 17** is ours: a host cannot reliably create a script variable while a script is
     suspended. The fix is `akbasic_runtime_global()` and roughly fifteen lines. §6 describes
     it and names the one design question to settle first.
6. **Mutation survivors in `src/value.c`.** The harness is in place (`scripts/mutation_test.py`,
   the `mutation` CMake target, and a CI job on `src/symtab.c`), and a partial run over
   `src/value.c` turned up test gaps worth closing. These are *not* equivalent mutants; each
   is a real bug of that shape the suite would not notice:

   - `AKBASIC_MAX_STRING_LENGTH - 1` → `+ 1` and → `- 0` survive at `src/value.c:47-48`, in
     `set_string`'s `strncpy` and its NUL terminator. Nothing in the suite writes a
     *maximum-length* string, so the off-by-one that would truncate or overrun goes unseen.
     One test that round-trips a 255-character string kills all five.
   - `memset(obj, 0, ...)` → `memset(obj, 1, ...)` and its deletion survive in
     `akbasic_valuepool_init`, as does `obj->next = 0` → `= 1`. Nothing asserts a freshly
     initialised pool is actually empty and zeroed.

   Two survivors there are genuinely equivalent and cannot be killed: `rval_as_int` and
   `rval_as_float` (`src/value.c:30,35`) tolerate `+` → `-` because the reference's habit of
   adding *both* of the right operand's numeric fields only works when the unused one is zero
   — §6 item 5. Fixing that defect would make these two mutants killable, which is a small
   argument for doing it.

   `src/value.c` takes about 70 minutes to mutate in full (368 mutants, ~11s each, because
   almost everything links against it), which is why CI runs `src/symtab.c` instead and the
   whole-tree run is a local `cmake --build build --target mutation`.