Fix Alphabet enumerator producing non-letters for large lists#715
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winklemad wants to merge 1 commit into
Open
Fix Alphabet enumerator producing non-letters for large lists#715winklemad wants to merge 1 commit into
winklemad wants to merge 1 commit into
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list.Alphabet used a naive per-digit div/mod formula that ignored the
"no zero digit" borrow of bijective base-26 (spreadsheet-style A..Z,
AA..ZZ, AAA..). Past index 1352 the middle digit underflowed to '@'
('A'-1) and the leading digit overflowed past 'Z' to '[', and the 4+
letter range was not handled at all, so large lists rendered enumerators
like "B@A." instead of "AZA.".
Replace the hard-coded branches with a correct bijective base-26
conversion (decrement-before-divide borrow, digits built least-
significant first then reversed). Behavior-preserving for the
previously-correct range and correct for all indices thereafter.
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Description
list.Alphabetimplements spreadsheet-style bijective base-26 labels (A..Z, AA..ZZ, AAA..), but used a naive per-digitdiv/modformula that ignores the "no zero digit" borrow. Past index 1352 the middle digit underflows to'@'('A'-1) when(i/26)%26 == 0, and the leading digit overflows past'Z'to'['; the 4+ letter range isn't handled at all. So a long list renders non-letter enumerators:First corruption is at index 1352 (the 1353rd item); a broad check finds 156 wrong outputs in 0..4999. This is reachable through the public API (
list.New(...).Enumerator(list.Alphabet)).Fix
Replace the hard-coded 1/2/3-letter branches with a correct bijective base-26 conversion (decrement-before-divide borrow, digits built least-significant-first then reversed). It's behavior-preserving for the previously-correct range (0..1351, which the existing tests cover) and correct for all indices beyond.
Test
Added regression rows to the existing
TestBullettable (1352→AZA,1377→AZZ,1378→BAA,18277→ZZZ,18278→AAAA), which fail on the old code and pass with the fix.go test ./...is green;gofmt/go vetclean.