Allow customizing PostMachine blank and mark symbols#56
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Adds an optional second constructor argument
`{ blankSymbol?, markSymbol? }`. When either is supplied, PostMachine
builds a per-instance Alphabet/TapeBlock and threads the chosen symbols
into CommandContext so mark/erase/check use the per-instance pair.
Defaults are unchanged (' ' / '*'); module-level constants remain
exported as the defaults.
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Closes #55.
Summary
new PostMachine(instructions, { blankSymbol?, markSymbol? }).Alphabet+TapeBlock. The default path still clonesoriginalTapeBlockto keep existing behavior identical.CommandContextgainsblankSymbolandmarkSymbol;mark/erase/checknow read these from the context instead of the module-level constants.left/right/noopare unaffected (no symbol writes).validateSymbolPairrejects multi-char or equal symbols up front with a clear message;null/undefinedfall back to the defaults via??.Public API
PostMachineconstructor signature widened — the second arg is optional, so existing callers compile and run unchanged.PostMachineOptions.blankSymbol/markSymbol/alphabetmodule exports unchanged — they remain the defaults.Tests
packages/machine/test/custom-alphabet.spec.tscovering: default path regression, validation errors, partial overrides, full override (mark/erase/checksemantics, subroutines, groups,summarizePostMachineinvariants, non-ASCII glyphs␣/•).Custom symbolsREADME example with matching test inexamples.spec.ts(per repo doc-parity rule).Known limitation (out of scope)
equivalentPostMachinesclones the originating tapeBlock per case. Comparing a custom-alphabet machine against a default-alphabet machine via the wrapper will hit symbol-interning mismatches in upstreamequivalentOn. Cross-alphabet equivalence is an upstream concern; the bareequivalentOnre-export is the existing escape hatch. Worth a follow-up if it bites in practice.Test plan
npm test— 119 passing (was 96, +22 new + 1 README example).npm run lint— clean.npm run build— builds; pre-existing Rollupthis-at-module-top warnings are unchanged.