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Refuse a branch to a line the program did not number

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GOTO 100 in a script written without line numbers finds the hundredth line
and branches there. Silent, plausible and wrong: the test for it loops
forever printing the second line when the check is removed.

akbasic_runtime_check_targets() is a fourth prescan beside the label, DATA
and TYPE ones, run on every entry into MODE_RUN -- the earliest the check
can be made and the only place all four ways a program arrives pass
through. A target naming an empty line is still allowed, for the same
reason RENUMBER leaves one alone, and a fully numbered program is
unaffected, which is every program that existed before this.

It shares RENUMBER's walk rather than repeating it. renumber.c grows an
akbasic_TargetWalk -- a self pointer and a visit function -- and
rewrite_line() takes one. RENUMBER's visitor substitutes the number a line
moved to; the check's substitutes the number unchanged and raises. One
walk, so the two cannot disagree about what a branch target is.

The check points environment->lineno at the line being walked so the "? N :"
prefix names the offending line. The other three prescans do not and report
whichever line the loader stopped on; TODO.md section 5 item 64 records it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Andrew Kesterson
2026-08-01 16:38:54 -04:00
parent 8bc253ebbf
commit 6273be580a
7 changed files with 341 additions and 20 deletions
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23
include/akbasic/runtime.h
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@@ -388,6 +388,29 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_settime(akbasic_Runtime *obj,
*/
akerr_ErrorContext AKERR_NOIGNORE *akbasic_renumber(akbasic_Runtime *obj, int64_t newstart, int64_t increment, int64_t oldstart);
/**
* @brief Refuse a program that branches by number to a line it did not number.
*
* The fourth prescan, run beside the label, `DATA` and `TYPE` ones on every entry
* into AKBASIC_MODE_RUN. Since a loaded line may be given its number rather than
* carry one, `GOTO 100` in a script written without numbers finds the hundredth
* line and branches there -- plausible, silent and wrong. This says so before any
* line executes, which is the earliest it can be said and the only place that
* covers all four ways a program arrives.
*
* It walks the same `GOTO`, `GOSUB`, `RUN`, `RESTORE`, `TRAP`, `ON ... GOTO` and
* `COLLISION` targets akbasic_renumber() rewrites, sharing that walk rather than
* repeating it. A target naming an *empty* line is allowed through, for the same
* reason RENUMBER leaves one alone. A `GOTO` to a label never reaches here.
*
* @param obj Object to initialize, inspect, or modify.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER When `obj` is NULL.
* @throws AKBASIC_ERR_SYNTAX When a numeric target names a line the program did not number.
* @throws AKBASIC_ERR_BOUNDS When a line does not fit the walk's buffer.
*/
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_check_targets(akbasic_Runtime *obj);
/**
* @brief File every `LABEL` in the stored program before any of it runs.
*