rust/library/core/tests/num/int_log.rs

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//! This tests the `Integer::{log,log2,log10}` methods. These tests are in a
//! separate file because there's both a large number of them, and not all tests
//! can be run on Android. This is because in Android `log2` uses an imprecise
//! approximation:https://github.com/rust-lang/rust/blob/4825e12fc9c79954aa0fe18f5521efa6c19c7539/src/libstd/sys/unix/android.rs#L27-L53
#[test]
fn checked_log() {
assert_eq!(999u32.checked_log(10), Some(2));
assert_eq!(1000u32.checked_log(10), Some(3));
assert_eq!(555u32.checked_log(13), Some(2));
assert_eq!(63u32.checked_log(4), Some(2));
assert_eq!(64u32.checked_log(4), Some(3));
assert_eq!(10460353203u64.checked_log(3), Some(21));
assert_eq!(10460353202u64.checked_log(3), Some(20));
assert_eq!(147808829414345923316083210206383297601u128.checked_log(3), Some(80));
assert_eq!(147808829414345923316083210206383297600u128.checked_log(3), Some(79));
assert_eq!(22528399544939174411840147874772641u128.checked_log(19683), Some(8));
assert_eq!(22528399544939174411840147874772631i128.checked_log(19683), Some(7));
assert_eq!(0u8.checked_log(4), None);
assert_eq!(0u16.checked_log(4), None);
assert_eq!(0i8.checked_log(4), None);
assert_eq!(0i16.checked_log(4), None);
for i in i16::MIN..=0 {
assert_eq!(i.checked_log(4), None);
}
for i in 1..=i16::MAX {
assert_eq!(i.checked_log(13), Some((i as f32).log(13.0) as u32));
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}
for i in 1..=u16::MAX {
assert_eq!(i.checked_log(13), Some((i as f32).log(13.0) as u32));
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}
}
#[test]
fn checked_log2() {
assert_eq!(5u32.checked_log2(), Some(2));
assert_eq!(0u64.checked_log2(), None);
assert_eq!(128i32.checked_log2(), Some(7));
assert_eq!((-55i16).checked_log2(), None);
assert_eq!(0u8.checked_log2(), None);
assert_eq!(0u16.checked_log2(), None);
assert_eq!(0i8.checked_log2(), None);
assert_eq!(0i16.checked_log2(), None);
for i in 1..=u8::MAX {
assert_eq!(i.checked_log2(), Some((i as f32).log2() as u32));
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}
for i in 1..=u16::MAX {
// Guard against Android's imprecise f32::log2 implementation.
if i != 8192 && i != 32768 {
assert_eq!(i.checked_log2(), Some((i as f32).log2() as u32));
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}
}
for i in i8::MIN..=0 {
assert_eq!(i.checked_log2(), None);
}
for i in 1..=i8::MAX {
assert_eq!(i.checked_log2(), Some((i as f32).log2() as u32));
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}
for i in i16::MIN..=0 {
assert_eq!(i.checked_log2(), None);
}
for i in 1..=i16::MAX {
// Guard against Android's imprecise f32::log2 implementation.
if i != 8192 {
assert_eq!(i.checked_log2(), Some((i as f32).log2() as u32));
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}
}
}
// Validate cases that fail on Android's imprecise float log2 implementation.
#[test]
#[cfg(not(target_os = "android"))]
fn checked_log2_not_android() {
assert_eq!(8192u16.checked_log2(), Some((8192f32).log2() as u32));
assert_eq!(32768u16.checked_log2(), Some((32768f32).log2() as u32));
assert_eq!(8192i16.checked_log2(), Some((8192f32).log2() as u32));
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}
#[test]
fn checked_log10() {
assert_eq!(0u8.checked_log10(), None);
assert_eq!(0u16.checked_log10(), None);
assert_eq!(0i8.checked_log10(), None);
assert_eq!(0i16.checked_log10(), None);
for i in i16::MIN..=0 {
assert_eq!(i.checked_log10(), None);
}
for i in 1..=i16::MAX {
assert_eq!(i.checked_log10(), Some((i as f32).log10() as u32));
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}
for i in 1..=u16::MAX {
assert_eq!(i.checked_log10(), Some((i as f32).log10() as u32));
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}
Speed up integer log10. This is achieved with a branchless bit-twiddling implementation of the case x < 100_000, and using this as building block. Benchmark on an Intel i7-8700K (Coffee Lake): name old ns/iter new ns/iter diff ns/iter diff % speedup num::int_log::u8_log10_predictable 165 169 4 2.42% x 0.98 num::int_log::u8_log10_random 438 423 -15 -3.42% x 1.04 num::int_log::u8_log10_random_small 438 423 -15 -3.42% x 1.04 num::int_log::u16_log10_predictable 633 417 -216 -34.12% x 1.52 num::int_log::u16_log10_random 908 471 -437 -48.13% x 1.93 num::int_log::u16_log10_random_small 945 471 -474 -50.16% x 2.01 num::int_log::u32_log10_predictable 1,496 1,340 -156 -10.43% x 1.12 num::int_log::u32_log10_random 1,076 873 -203 -18.87% x 1.23 num::int_log::u32_log10_random_small 1,145 874 -271 -23.67% x 1.31 num::int_log::u64_log10_predictable 4,005 3,171 -834 -20.82% x 1.26 num::int_log::u64_log10_random 1,247 1,021 -226 -18.12% x 1.22 num::int_log::u64_log10_random_small 1,265 921 -344 -27.19% x 1.37 num::int_log::u128_log10_predictable 39,667 39,579 -88 -0.22% x 1.00 num::int_log::u128_log10_random 6,456 6,696 240 3.72% x 0.96 num::int_log::u128_log10_random_small 4,108 3,903 -205 -4.99% x 1.05 Benchmark on an M1 Mac Mini: name old ns/iter new ns/iter diff ns/iter diff % speedup num::int_log::u8_log10_predictable 143 130 -13 -9.09% x 1.10 num::int_log::u8_log10_random 375 325 -50 -13.33% x 1.15 num::int_log::u8_log10_random_small 376 325 -51 -13.56% x 1.16 num::int_log::u16_log10_predictable 500 322 -178 -35.60% x 1.55 num::int_log::u16_log10_random 794 405 -389 -48.99% x 1.96 num::int_log::u16_log10_random_small 1,035 405 -630 -60.87% x 2.56 num::int_log::u32_log10_predictable 1,144 894 -250 -21.85% x 1.28 num::int_log::u32_log10_random 832 786 -46 -5.53% x 1.06 num::int_log::u32_log10_random_small 832 787 -45 -5.41% x 1.06 num::int_log::u64_log10_predictable 2,681 2,057 -624 -23.27% x 1.30 num::int_log::u64_log10_random 1,015 806 -209 -20.59% x 1.26 num::int_log::u64_log10_random_small 1,004 795 -209 -20.82% x 1.26 num::int_log::u128_log10_predictable 56,825 56,526 -299 -0.53% x 1.01 num::int_log::u128_log10_random 9,056 8,861 -195 -2.15% x 1.02 num::int_log::u128_log10_random_small 1,528 1,527 -1 -0.07% x 1.00 The 128 bit case remains ridiculously slow because llvm fails to optimize division by a constant 128-bit value to multiplications. This could be worked around but it seems preferable to fix this in llvm. From u32 up, table lookup (like suggested here https://github.com/rust-lang/rust/issues/70887#issuecomment-881099813) is still faster, but requires a hardware leading_zero to be viable, and might clog up the cache.
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for i in 1..=100_000u32 {
assert_eq!(i.checked_log10(), Some((i as f32).log10() as u32));
}
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}
macro_rules! log10_loop {
($T:ty, $log10_max:expr) => {
assert_eq!(<$T>::MAX.log10(), $log10_max);
for i in 0..=$log10_max {
let p = (10 as $T).pow(i as u32);
if p >= 10 {
assert_eq!((p - 9).log10(), i - 1);
assert_eq!((p - 1).log10(), i - 1);
}
assert_eq!(p.log10(), i);
assert_eq!((p + 1).log10(), i);
if p >= 10 {
assert_eq!((p + 9).log10(), i);
}
// also check `x.log(10)`
if p >= 10 {
assert_eq!((p - 9).log(10), i - 1);
assert_eq!((p - 1).log(10), i - 1);
}
assert_eq!(p.log(10), i);
assert_eq!((p + 1).log(10), i);
if p >= 10 {
assert_eq!((p + 9).log(10), i);
}
}
};
}
#[test]
fn log10_u8() {
log10_loop! { u8, 2 }
}
#[test]
fn log10_u16() {
log10_loop! { u16, 4 }
}
#[test]
fn log10_u32() {
log10_loop! { u32, 9 }
}
#[test]
fn log10_u64() {
log10_loop! { u64, 19 }
}
#[test]
fn log10_u128() {
log10_loop! { u128, 38 }
}