The previous code computed `frac_part * pow10_negative[n]`, where
pow10_negative[n] is already a rounded approximation of 10^-n (since
10^-n is not exactly representable in binary). The multiplication then
adds another rounding step, leaving up to ~1 ulp of error.
Dividing `frac_part` by `pow10_positive[n]` is exact for n <= 22 (the
range where 10^n fits exactly in a double), so the division is the
only rounding and the result is correctly rounded. For example,
"0.3".to_f now matches the 0.3 literal's bit pattern (and libc strtod).
Replace expensive pow() calls with pre-computed lookup tables for powers of 10.
Use integer arithmetic during parsing to avoid floating-point precision loss.
Add overflow detection for large numbers while maintaining compatibility.
Co-authored-by: Claude <noreply@anthropic.com>
This commit adds a descriptive comment at the beginning of the `mrb_read_float` function in `src/readfloat.c`.
The comment explains the function's purpose, its parameters (`str`, `endp`, `fp`), and its return value (`TRUE` or `FALSE`). This improves code readability and understanding.
The new version gives more accurate values for decimal number
representation that are not divisible in binary representations, for
example `0.3`.
The function uses `long double` for precision. Please report if `long
double` causes problems on any platform (especially microcontrollers).
Ref #6182
Yasuhiro Matsumoto