neilconway opened a new pull request, #10998:
URL: https://github.com/apache/arrow-rs/pull/10998
# Which issue does this PR close?
- N/A
# Rationale for this change
`parse_decimal` validated the parsed value against the precision with
`is_valid_decimal_precision`, an out-of-line call that loads the bounds for the
precision and compares the value against both.
The parser already knows how many digits it kept. We can modify the mantissa
scan to return the digit count and then use this count to skip the precision
check for the common case that the number of parsed digits implies the parsed
value is well within the allowed precision.
`parse_decimal` microbenchmark, Apple M4 Max, µs per 1024 inputs:
```
case before after change
123.123 7.93 7.85 -1.0%
123.1234 11.39 10.96 -3.7%
123.1 11.22 10.27 -8.5%
123 10.49 9.47 -9.7%
-123.123 8.16 8.07 -1.0%
-123.1234 11.47 11.09 -3.3%
-123.1 11.45 10.53 -8.0%
-123 10.71 9.84 -8.1%
0.0000123 8.37 7.89 -5.7%
12. 10.21 9.54 -6.5%
-12. 10.44 9.76 -6.5%
00.1 10.69 9.90 -7.4%
-00.1 10.93 10.00 -8.5%
12345678912345678.1234 24.49 24.53 +0.1%
-12345678912345678.1234 24.67 24.77 +0.4%
99999999999999999.999 20.31 19.90 -2.0%
-99999999999999999.999 20.25 20.40 +0.7%
.123 6.34 6.18 -2.6%
-.123 6.59 6.41 -2.7%
123. 10.67 10.12 -5.1%
-123. 10.94 10.30 -5.8%
string decimal128 short 8.20 7.85 -4.2%
string decimal128 integer 11.19 10.89 -2.7%
string decimal128 exact scale 11.43 11.05 -3.3%
string decimal128 padded scale 10.50 10.42 -0.8%
string decimal128 rounded scale 15.23 14.63 -3.9%
string decimal128 signed 15.28 14.78 -3.3%
string decimal128 38 digits 24.19 23.65 -2.2%
string decimal128 exponent 20.28 20.18 -0.5%
string decimal128 negative exponent 19.52 18.79 -3.7%
string decimal128 negative scale 18.32 17.97 -1.9%
string decimal128 long fraction 22.39 22.14 -1.1%
string decimal256 76 digits 59.03 57.76 -2.1%
string decimal256 rounded scale 55.72 54.71 -1.8%
string decimal32 short 8.51 7.74 -9.1%
string decimal32 9 digits 10.45 9.76 -6.6%
string decimal64 short 7.29 6.84 -6.2%
string decimal64 18 digits 12.04 11.66 -3.2%
```
# What changes are included in this PR?
See above.
# Are these changes tested?
Yes; new test added, existing tests pass.
# Are there any user-facing changes?
No.
# AI usage
Developed with Claude Code Fable 5.1; reviewed with Codex Astra 6. I
reviewed, revised, and understand the resulting code.
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