# The reference interpreter's own test corpus

These 41 `.bas` files and their 41 `.txt` expectations are **not this project's tests.** They
are a byte-for-byte copy of the acceptance corpus belonging to the Go implementation this
interpreter was ported from:

| | |
|---|---|
| Source | `https://source.starfort.tech/andrew/basicinterpreter`, `tests/` |
| Copied at | commit `d76162cb37eeaccddae2cf6cab7af7bc8175d6d2` ("Update README.md") |
| Copied on | 2026-07-31 |
| Verified | every file `cmp`-identical to the submodule at the time of the copy |

They are registered as the `golden_*` CTest cases and run against `basic` on every build, in
both the default and the `-DAKBASIC_WITH_AKGL=ON` configurations.

## Why they are a copy, when they deliberately were not

Until 2026-07-31 these were driven *in place* out of `deps/basicinterpret`, and the reasoning
was written down in `CMakeLists.txt`: the corpus is a submodule, and copying a submodule's
corpus guarantees drift.

That reasoning was sound and it stopped applying. The Go project is deprecated and will not be
updated, so there is nothing left to drift *from* — and a build that cannot run its own
acceptance suite without cloning the implementation it replaced is not finished.

## What these are for now

**A regression suite, not a specification.** That distinction is the whole of how to treat
them, and it changed on 2026-07-31 (`TODO.md` §0.1): matching the Go implementation is no
longer a goal, so a difference between it and this interpreter is not automatically a bug in
this one.

What they are still good for is unchanged and is worth a lot: forty-one real BASIC programs
with known-good output, covering arithmetic, arrays, flow control, every builtin function,
`READ`/`DATA`, labels and three worked examples. That catches regressions whatever its
provenance.

## The rule for editing these

**Diverge deliberately, never by accident.**

A failure here means this interpreter's behaviour changed. Nine times in ten that is a bug you
just introduced and the answer is to fix the code. The tenth time it is an improvement on the
reference, and then the answer is:

1. Change the expectation in the same commit as the code.
2. Say why in the commit message.
3. Add a line to `TODO.md` §5, which is the list of deliberate deviations.

What is not acceptable is editing a `.txt` to make a red suite green without deciding which of
the two you are doing.

New cases for this project's own behaviour go in `tests/language/`, registered under `local_*`.
The prefix says which corpus a failure came from, and a diff touching `tests/reference/` stands
out as the thing it is.

## Divergences so far

| Case | Change | Why |
|---|---|---|
| `language/arithmetic/integer.txt` | Expectation grew from `4 4 2` to `4 4 2 2` | The program has **four** `PRINT` statements and the expectation had three values. The file ends in a blank line, and both interpreters filed a blank line under the loader's cursor — which for a line with no number of its own is the number of the line before it. So `40 PRINT 4 - 2` was erased before the program ran. The reference lost it the same way, which is why the expectation was written short. Fixed by skipping blank lines in RUNSTREAM, as `akbasic_runtime_load()` and `DLOAD` already did. `TODO.md` §5. |
| `examples/strreverse.bas` | Variable `INPUT$` renamed to `SOURCE$` | `INPUT$` is a reserved word with a type suffix, which a real C128 refuses and which this interpreter now refuses too (`TODO.md` §6 item 16). The reference accepted it only because its own reserved-word check never fired. The expectation is byte-for-byte unchanged — the program still prints `REVERSED: OLLEH` — so the case keeps all of its value. |