diff options
Diffstat (limited to 'library/core/src/num/flt2dec')
| -rw-r--r-- | library/core/src/num/flt2dec/strategy/grisu.rs | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/library/core/src/num/flt2dec/strategy/grisu.rs b/library/core/src/num/flt2dec/strategy/grisu.rs index ed3e0edaf..b9f0d114c 100644 --- a/library/core/src/num/flt2dec/strategy/grisu.rs +++ b/library/core/src/num/flt2dec/strategy/grisu.rs @@ -487,6 +487,22 @@ pub fn format_exact_opt<'a>( let vint = (v.f >> e) as u32; let vfrac = v.f & ((1 << e) - 1); + let requested_digits = buf.len(); + + const POW10_UP_TO_9: [u32; 10] = + [1, 10, 100, 1000, 10_000, 100_000, 1_000_000, 10_000_000, 100_000_000, 1_000_000_000]; + + // We deviate from the original algorithm here and do some early checks to determine if we can satisfy requested_digits. + // If we determine that we can't, we exit early and avoid most of the heavy lifting that the algorithm otherwise does. + // + // When vfrac is zero, we can easily determine if vint can satisfy requested digits: + // If requested_digits >= 11, vint is not able to exhaust the count by itself since 10^(11 -1) > u32 max value >= vint. + // If vint < 10^(requested_digits - 1), vint cannot exhaust the count. + // Otherwise, vint might be able to exhaust the count and we need to execute the rest of the code. + if (vfrac == 0) && ((requested_digits >= 11) || (vint < POW10_UP_TO_9[requested_digits - 1])) { + return None; + } + // both old `v` and new `v` (scaled by `10^-k`) has an error of < 1 ulp (Theorem 5.1). // as we don't know the error is positive or negative, we use two approximations // spaced equally and have the maximal error of 2 ulps (same to the shortest case). |