fix: NumDigits underreports for some exact powers of ten#425
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c-tonneslan wants to merge 1 commit into
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fix: NumDigits underreports for some exact powers of ten#425c-tonneslan wants to merge 1 commit into
c-tonneslan wants to merge 1 commit into
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math.Log10(1eN) rounds down to N-1 for several N (15, 24, 28, ...) because the exact decimal can't survive a float64 round-trip. The fast path returned int(math.Log10(...)) + 1, so a coefficient like 1000000000000000 (16 digits) came back as 15. Count digits via int64 division for the IsInt64 branch. The slow big.Int path is unchanged. Regression tests cover 1e15 and -1e15 (both via plain literal and scientific-notation input) and 1<<53 (the old fast-path boundary). Fixes shopspring#420 Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>
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From the reporter in #420:
The fast path returns
int(math.Log10(math.Abs(float64(i64)))) + 1, andmath.Log10of an exact power of ten doesn't always round to the exact integer. On my machine:So 1e15 takes the wrong branch and
int(...) + 1returns 15 instead of 16. Same shape at 1e24, 1e28, and other powers where the float64 representation drifts down.Count digits via plain int64 division instead. No floats, no estimate. The
big.Intslow path is unchanged.Regression cases added to
TestDecimal_NumDigitscover1e15directly, with sign and scientific-notation variants, plus1<<53(the boundary of the old fast path). Fails on master, passes here.go test ./...is clean.Fixes #420.