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Handle NaNs in a reasonable way in rmath's ulp float functions.

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This commit is contained in:
Nathan Vegdahl 2023-08-03 23:55:26 +02:00
parent f9acc8096c
commit 911542c534
1 changed files with 17 additions and 9 deletions
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26
sub_crates/rmath/src/utils.rs
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@ -1,5 +1,3 @@
const TOP_BIT: u32 = 1 << 31;
/// Compute how different two floats are in ulps.
///
/// Notes:
@ -7,9 +5,12 @@ const TOP_BIT: u32 = 1 << 31;
/// implications of that to the rest of the numbers. E.g. the numbers
/// just before and after 0.0/-0.0 are only two ulps apart, not three.
/// - Infinity is one ulp past float max, and converse for -infinity.
/// - This doesn't handle NaNs in any really useful way.
/// - If either number is NaN, returns `u32::MAX`.
#[inline(always)]
pub fn ulp_diff(a: f32, b: f32) -> u32 {
const TOP_BIT: u32 = 1 << 31;
const NAN_THRESHOLD: u32 = 0x7f800000;
let a = a.to_bits();
let b = b.to_bits();
@ -18,7 +19,10 @@ pub fn ulp_diff(a: f32, b: f32) -> u32 {
let a_abs = a & !TOP_BIT;
let b_abs = b & !TOP_BIT;
if a_sign == b_sign {
if a_abs > NAN_THRESHOLD || b_abs > NAN_THRESHOLD {
// NaNs always return maximum ulps apart.
u32::MAX
} else if a_sign == b_sign {
a_abs.max(b_abs) - a_abs.min(b_abs)
} else {
a_abs + b_abs
@ -28,7 +32,8 @@ pub fn ulp_diff(a: f32, b: f32) -> u32 {
/// Checks if two floats are approximately equal, within `max_ulps`.
#[inline(always)]
pub fn ulps_eq(a: f32, b: f32, max_ulps: u32) -> bool {
!a.is_nan() && !b.is_nan() && (ulp_diff(a, b) <= max_ulps)
// The minimum ensures that NaNs never return true.
ulp_diff(a, b) <= max_ulps.min(u32::MAX - 1)
}
#[cfg(test)]
@ -49,10 +54,12 @@ mod tests {
assert_eq!(ulp_diff(-0.0, 1.0), 0x3f800000);
assert_eq!(ulp_diff(0.0, -1.0), 0x3f800000);
assert_eq!(ulp_diff(-0.0, -1.0), 0x3f800000);
assert_eq!(
ulp_diff(std::f32::INFINITY, -std::f32::INFINITY),
0xff000000
);
assert_eq!(ulp_diff(f32::INFINITY, -f32::INFINITY), 0xff000000);
assert_eq!(ulp_diff(f32::NAN, f32::NAN), 0xffffffff);
assert_eq!(ulp_diff(f32::NAN, 1.0), 0xffffffff);
assert_eq!(ulp_diff(1.0, f32::NAN), 0xffffffff);
assert_eq!(ulp_diff(-f32::NAN, 1.0), 0xffffffff);
assert_eq!(ulp_diff(1.0, -f32::NAN), 0xffffffff);
assert_eq!(ulp_diff(0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
assert_eq!(ulp_diff(-0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
}
@ -82,6 +89,7 @@ mod tests {
assert!(!ulps_eq(std::f32::NAN, std::f32::NAN, 0));
assert!(!ulps_eq(-std::f32::NAN, -std::f32::NAN, 0));
assert!(!ulps_eq(std::f32::NAN, std::f32::NAN, u32::MAX));
assert!(!ulps_eq(std::f32::NAN, std::f32::INFINITY, 1 << 31));
assert!(!ulps_eq(std::f32::INFINITY, std::f32::NAN, 1 << 31));
}