mirror of
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d29871977f
We already have a LICENSE file that covers the Zig Standard Library. We no longer need to remind everyone that the license is MIT in every single file. Previously this was introduced to clarify the situation for a fork of Zig that made Zig's LICENSE file harder to find, and replaced it with their own license that required annual payments to their company. However that fork now appears to be dead. So there is no need to reinforce the copyright notice in every single file.
324 lines
7.9 KiB
Zig
324 lines
7.9 KiB
Zig
// Ported from musl, which is licensed under the MIT license:
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// https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
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//
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// https://git.musl-libc.org/cgit/musl/tree/src/math/expmf.c
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// https://git.musl-libc.org/cgit/musl/tree/src/math/expm.c
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// TODO: Updated recently.
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const std = @import("../std.zig");
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const math = std.math;
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const expect = std.testing.expect;
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/// Returns e raised to the power of x, minus 1 (e^x - 1). This is more accurate than exp(e, x) - 1
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/// when x is near 0.
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///
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/// Special Cases:
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/// - expm1(+inf) = +inf
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/// - expm1(-inf) = -1
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/// - expm1(nan) = nan
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pub fn expm1(x: anytype) @TypeOf(x) {
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const T = @TypeOf(x);
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return switch (T) {
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f32 => expm1_32(x),
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f64 => expm1_64(x),
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else => @compileError("exp1m not implemented for " ++ @typeName(T)),
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};
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}
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fn expm1_32(x_: f32) f32 {
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if (math.isNan(x_))
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return math.nan(f32);
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const o_threshold: f32 = 8.8721679688e+01;
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const ln2_hi: f32 = 6.9313812256e-01;
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const ln2_lo: f32 = 9.0580006145e-06;
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const invln2: f32 = 1.4426950216e+00;
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const Q1: f32 = -3.3333212137e-2;
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const Q2: f32 = 1.5807170421e-3;
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var x = x_;
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const ux = @bitCast(u32, x);
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const hx = ux & 0x7FFFFFFF;
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const sign = hx >> 31;
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// TODO: Shouldn't need this check explicitly.
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if (math.isNegativeInf(x)) {
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return -1.0;
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}
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// |x| >= 27 * ln2
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if (hx >= 0x4195B844) {
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// nan
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if (hx > 0x7F800000) {
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return x;
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}
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if (sign != 0) {
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return -1;
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}
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if (x > o_threshold) {
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x *= 0x1.0p127;
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return x;
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}
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}
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var hi: f32 = undefined;
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var lo: f32 = undefined;
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var c: f32 = undefined;
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var k: i32 = undefined;
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// |x| > 0.5 * ln2
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if (hx > 0x3EB17218) {
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// |x| < 1.5 * ln2
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if (hx < 0x3F851592) {
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if (sign == 0) {
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hi = x - ln2_hi;
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lo = ln2_lo;
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k = 1;
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} else {
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hi = x + ln2_hi;
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lo = -ln2_lo;
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k = -1;
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}
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} else {
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var kf = invln2 * x;
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if (sign != 0) {
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kf -= 0.5;
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} else {
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kf += 0.5;
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}
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k = @floatToInt(i32, kf);
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const t = @intToFloat(f32, k);
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hi = x - t * ln2_hi;
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lo = t * ln2_lo;
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}
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x = hi - lo;
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c = (hi - x) - lo;
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}
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// |x| < 2^(-25)
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else if (hx < 0x33000000) {
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if (hx < 0x00800000) {
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math.doNotOptimizeAway(x * x);
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}
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return x;
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} else {
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k = 0;
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}
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const hfx = 0.5 * x;
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const hxs = x * hfx;
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const r1 = 1.0 + hxs * (Q1 + hxs * Q2);
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const t = 3.0 - r1 * hfx;
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var e = hxs * ((r1 - t) / (6.0 - x * t));
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// c is 0
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if (k == 0) {
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return x - (x * e - hxs);
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}
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e = x * (e - c) - c;
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e -= hxs;
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// exp(x) ~ 2^k (x_reduced - e + 1)
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if (k == -1) {
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return 0.5 * (x - e) - 0.5;
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}
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if (k == 1) {
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if (x < -0.25) {
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return -2.0 * (e - (x + 0.5));
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} else {
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return 1.0 + 2.0 * (x - e);
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}
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}
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const twopk = @bitCast(f32, @intCast(u32, (0x7F +% k) << 23));
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if (k < 0 or k > 56) {
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var y = x - e + 1.0;
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if (k == 128) {
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y = y * 2.0 * 0x1.0p127;
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} else {
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y = y * twopk;
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}
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return y - 1.0;
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}
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const uf = @bitCast(f32, @intCast(u32, 0x7F -% k) << 23);
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if (k < 23) {
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return (x - e + (1 - uf)) * twopk;
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} else {
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return (x - (e + uf) + 1) * twopk;
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}
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}
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fn expm1_64(x_: f64) f64 {
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if (math.isNan(x_))
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return math.nan(f64);
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const o_threshold: f64 = 7.09782712893383973096e+02;
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const ln2_hi: f64 = 6.93147180369123816490e-01;
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const ln2_lo: f64 = 1.90821492927058770002e-10;
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const invln2: f64 = 1.44269504088896338700e+00;
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const Q1: f64 = -3.33333333333331316428e-02;
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const Q2: f64 = 1.58730158725481460165e-03;
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const Q3: f64 = -7.93650757867487942473e-05;
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const Q4: f64 = 4.00821782732936239552e-06;
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const Q5: f64 = -2.01099218183624371326e-07;
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var x = x_;
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const ux = @bitCast(u64, x);
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const hx = @intCast(u32, ux >> 32) & 0x7FFFFFFF;
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const sign = ux >> 63;
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if (math.isNegativeInf(x)) {
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return -1.0;
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}
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// |x| >= 56 * ln2
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if (hx >= 0x4043687A) {
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// exp1md(nan) = nan
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if (hx > 0x7FF00000) {
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return x;
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}
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// exp1md(-ve) = -1
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if (sign != 0) {
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return -1;
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}
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if (x > o_threshold) {
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math.raiseOverflow();
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return math.inf(f64);
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}
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}
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var hi: f64 = undefined;
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var lo: f64 = undefined;
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var c: f64 = undefined;
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var k: i32 = undefined;
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// |x| > 0.5 * ln2
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if (hx > 0x3FD62E42) {
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// |x| < 1.5 * ln2
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if (hx < 0x3FF0A2B2) {
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if (sign == 0) {
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hi = x - ln2_hi;
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lo = ln2_lo;
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k = 1;
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} else {
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hi = x + ln2_hi;
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lo = -ln2_lo;
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k = -1;
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}
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} else {
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var kf = invln2 * x;
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if (sign != 0) {
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kf -= 0.5;
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} else {
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kf += 0.5;
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}
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k = @floatToInt(i32, kf);
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const t = @intToFloat(f64, k);
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hi = x - t * ln2_hi;
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lo = t * ln2_lo;
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}
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x = hi - lo;
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c = (hi - x) - lo;
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}
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// |x| < 2^(-54)
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else if (hx < 0x3C900000) {
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if (hx < 0x00100000) {
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math.doNotOptimizeAway(@floatCast(f32, x));
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}
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return x;
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} else {
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k = 0;
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}
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const hfx = 0.5 * x;
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const hxs = x * hfx;
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const r1 = 1.0 + hxs * (Q1 + hxs * (Q2 + hxs * (Q3 + hxs * (Q4 + hxs * Q5))));
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const t = 3.0 - r1 * hfx;
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var e = hxs * ((r1 - t) / (6.0 - x * t));
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// c is 0
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if (k == 0) {
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return x - (x * e - hxs);
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}
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e = x * (e - c) - c;
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e -= hxs;
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// exp(x) ~ 2^k (x_reduced - e + 1)
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if (k == -1) {
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return 0.5 * (x - e) - 0.5;
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}
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if (k == 1) {
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if (x < -0.25) {
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return -2.0 * (e - (x + 0.5));
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} else {
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return 1.0 + 2.0 * (x - e);
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}
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}
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const twopk = @bitCast(f64, @intCast(u64, 0x3FF +% k) << 52);
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if (k < 0 or k > 56) {
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var y = x - e + 1.0;
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if (k == 1024) {
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y = y * 2.0 * 0x1.0p1023;
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} else {
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y = y * twopk;
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}
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return y - 1.0;
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}
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const uf = @bitCast(f64, @intCast(u64, 0x3FF -% k) << 52);
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if (k < 20) {
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return (x - e + (1 - uf)) * twopk;
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} else {
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return (x - (e + uf) + 1) * twopk;
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}
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}
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test "math.exp1m" {
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try expect(expm1(@as(f32, 0.0)) == expm1_32(0.0));
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try expect(expm1(@as(f64, 0.0)) == expm1_64(0.0));
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}
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test "math.expm1_32" {
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const epsilon = 0.000001;
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try expect(expm1_32(0.0) == 0.0);
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try expect(math.approxEqAbs(f32, expm1_32(0.0), 0.0, epsilon));
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try expect(math.approxEqAbs(f32, expm1_32(0.2), 0.221403, epsilon));
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try expect(math.approxEqAbs(f32, expm1_32(0.8923), 1.440737, epsilon));
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try expect(math.approxEqAbs(f32, expm1_32(1.5), 3.481689, epsilon));
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}
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test "math.expm1_64" {
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const epsilon = 0.000001;
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try expect(expm1_64(0.0) == 0.0);
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try expect(math.approxEqAbs(f64, expm1_64(0.0), 0.0, epsilon));
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try expect(math.approxEqAbs(f64, expm1_64(0.2), 0.221403, epsilon));
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try expect(math.approxEqAbs(f64, expm1_64(0.8923), 1.440737, epsilon));
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try expect(math.approxEqAbs(f64, expm1_64(1.5), 3.481689, epsilon));
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}
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test "math.expm1_32.special" {
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try expect(math.isPositiveInf(expm1_32(math.inf(f32))));
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try expect(expm1_32(-math.inf(f32)) == -1.0);
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try expect(math.isNan(expm1_32(math.nan(f32))));
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}
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test "math.expm1_64.special" {
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try expect(math.isPositiveInf(expm1_64(math.inf(f64))));
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try expect(expm1_64(-math.inf(f64)) == -1.0);
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try expect(math.isNan(expm1_64(math.nan(f64))));
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}
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