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ISSUES.md

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BasiCoCo Issues

Not Yet Implemented (from Extended Color BASIC)

Low priority — rarely needed or hard to emulate meaningfully:

  • PEEK/POKE — memory access (would need a simulated memory map)
  • FIELD — file field definition (random-access file I/O)
  • GET/PUT (file) — file record I/O (random-access file I/O)
  • VARPTR — variable pointer (rarely used outside machine language interfacing)
  • EXEC — execute machine language (no-op or educational stub)
  • USR — user-defined machine language function (no-op or educational stub)

Testing

  • interactive_session() test harness helper — create helper in conftest.py for pexpect-based program audit tests to reduce boilerplate in test_program_audit.py.

Robustness Exploration

Areas that work but haven't been stress-tested with adversarial or exotic inputs:

  • File I/O edge cases — partial reads, large files, error recovery mid-stream, multiple files open simultaneously, EOF boundary conditions
  • ON ERROR GOTO / RESUME edge cases — error handler inside nested GOSUB with LOCAL, RESUME NEXT across subline boundaries, re-raising errors from within handlers
  • Graphics edge cases — PAINT on complex shapes with narrow corridors, LINE/DRAW at screen boundaries, rapid PMODE switching
  • Exotic control flow combinations — ON ERROR inside nested GOSUB with LOCAL variables, CHAIN with ALL preserving complex state (arrays, open files, error handlers), GOTO/GOSUB from inside deeply nested IF/FOR/WHILE blocks
  • String expression edge cases — very long strings, empty strings in concatenation/MID$/INSTR, string comparisons with trailing spaces

Known Bugs

  • Rubik's cube z-sorting artifacts — DrawCube in lib_rubiks_engine.bas uses a painter's algorithm with per-subcube depth sorting. This produces visible artifacts:
    • Static render: at boundaries between the three visible faces (front, right, top), stickers from back subcubes bleed through — e.g., yellow on the front face, blue on the top face in the "CUBE SOLVED!" view.
    • Animation: during face rotation, rotating subcubes' corners get occluded by static faces, and stickers appear to change colors mid-turn.
    • Root cause: per-subcube average depth doesn't correctly resolve overlap ordering between faces from adjacent subcubes that extend in different directions. The cross-product cull alone isn't sufficient. Fixed iteration order (back→front) was tried but made animation worse.
    • Possible approaches: (1) Only paint the three camera-visible face orientations (front/right/top) for non-rotating subcubes, with special handling during rotation when back/left/bottom faces can rotate into view. (2) Render static and rotating layers separately with different strategies. (3) Rethink depth metric — e.g., per-face depth for the rotating layer only. The internal 4D CL array is verified correct by sticker-stability tests against pycuber.

Known Behavioral Limitations

  • GOTO out of multi-line IF leaves a stale if_stack entry (cleared on next RUN). This matches real CoCo behavior where GOTO from structured blocks is undefined. Note: RETURN out of IF blocks inside GOSUB is handled correctly — GOSUB saves if_stack/for_stack depth and RETURN restores it.

Language Extension Ideas

(No pending language extension ideas at this time.)

Refactoring Opportunities

(No pending refactoring opportunities at this time.)