05092163bb
Float formatting uses a multiprecision fallback path where Grisu3 algorithm fails. This has a bug during the rounding phase: the difference between the decimal value and the upper bound is examined byte-by-byte and doesn't properly handle the case where the first divergence has a difference of 1. For instance (using an example from #29491), for the number 498484681984085570, roundShortest examines the three decimal values: lower: 498484681984085536 d: 498484681984085568 upper: 498484681984085600 After examining the 16th digit, we know that rounding d up will fall within the bounds unless all remaining digits of d are 9 and all remaining digits of upper are 0: d: ...855xx upper: ...856xx However, the loop forgets that d and upper have already diverged and then on the next iteration sees that the 17th digit of d is actually lower than the 17th digit of upper and decides that we still can't round up: d: ...8556x upper: ...8560x Thus the original value is incorrectly rounded down to 498484681984085560 instead of the closer (and equally short) 498484681984085570. Thanks to Brian Kessler for diagnosing this bug. Fix it by remembering when we've seen divergence in previous digits. This CL also fixes another bug in the same loop: for some inputs, the decimal value d or the lower bound may have fewer digits than the upper bound, yet the iteration through the digits starts at i=0 for each of them. For instance, given the float64 value 1e23, we have d: 99999999999999991611392 upper: 100000000000000000000000 but the loop starts by comparing '9' to '1' rather than '0' to '1'. I haven't found any cases where this second bug causes incorrect output because when the digit comparison fails on the first loop iteration the upper bound always has more nonzero digits (i.e., the expression 'i+1 < upper.nd' is always true). Fixes #29491 Change-Id: I58856a7a2e47935ec2f233d9f717ef15c78bb2d0 Reviewed-on: https://go-review.googlesource.com/c/go/+/157697 Run-TryBot: Caleb Spare <cespare@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rémy Oudompheng <remyoudompheng@gmail.com> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> |
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