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use super::ln;
use core::f32;
pub(super) fn log2_ln_approx(x: f32) -> f32 {
let ln2_recip: f32 = f32::consts::LOG2_E;
let fract_base_ln = ln2_recip;
let value_ln = ln::ln_1to2_series_approximation(x);
value_ln * fract_base_ln
}
#[cfg(test)]
mod tests {
use super::super::abs;
use super::log2_ln_approx;
pub(crate) const MAX_ERROR: f32 = 0.001;
pub(crate) const TEST_VECTORS: &[(f32, f32)] = &[
(1e-20, -66.43856),
(1e-19, -63.116634),
(1e-18, -59.794704),
(1e-17, -56.47278),
(1e-16, -53.15085),
(1e-15, -49.828922),
(1e-14, -46.506992),
(1e-13, -43.185066),
(1e-12, -39.863136),
(1e-11, -36.54121),
(1e-10, -33.21928),
(1e-09, -29.897352),
(1e-08, -26.575424),
(1e-07, -23.253496),
(1e-06, -19.931568),
(1e-05, -16.60964),
(1e-04, -13.287712),
(0.001, -9.965784),
(0.01, -6.643856),
(0.1, -3.321928),
(10.0, 3.321928),
(100.0, 6.643856),
(1000.0, 9.965784),
(10000.0, 13.287712),
(100000.0, 16.60964),
(1000000.0, 19.931568),
(10000000.0, 23.253496),
(100000000.0, 26.575424),
(1000000000.0, 29.897352),
(10000000000.0, 33.21928),
(100000000000.0, 36.54121),
(1000000000000.0, 39.863136),
(10000000000000.0, 43.185066),
(100000000000000.0, 46.506992),
(1000000000000000.0, 49.828922),
(1e+16, 53.15085),
(1e+17, 56.47278),
(1e+18, 59.794704),
(1e+19, 63.116634),
];
#[test]
fn sanity_check() {
assert_eq!(log2_ln_approx(1_f32), 0_f32);
for (x, expected) in TEST_VECTORS {
let ln_x = log2_ln_approx(*x);
let relative_error = abs::abs(ln_x - *expected) / *expected;
assert!(
relative_error <= MAX_ERROR,
"relative_error {} too large: {} vs {}",
relative_error,
ln_x,
expected
);
}
}
}
