zig/std/math/asinh.zig
2018-10-26 15:01:51 -04:00

129 lines
3.7 KiB
Zig

// Special Cases:
//
// - asinh(+-0) = +-0
// - asinh(+-inf) = +-inf
// - asinh(nan) = nan
const std = @import("../index.zig");
const math = std.math;
const assert = std.debug.assert;
const maxInt = std.math.maxInt;
pub fn asinh(x: var) @typeOf(x) {
const T = @typeOf(x);
return switch (T) {
f32 => asinh32(x),
f64 => asinh64(x),
else => @compileError("asinh not implemented for " ++ @typeName(T)),
};
}
// asinh(x) = sign(x) * log(|x| + sqrt(x * x + 1)) ~= x - x^3/6 + o(x^5)
fn asinh32(x: f32) f32 {
const u = @bitCast(u32, x);
const i = u & 0x7FFFFFFF;
const s = i >> 31;
var rx = @bitCast(f32, i); // |x|
// TODO: Shouldn't need this explicit check.
if (math.isNegativeInf(x)) {
return x;
}
// |x| >= 0x1p12 or inf or nan
if (i >= 0x3F800000 + (12 << 23)) {
rx = math.ln(rx) + 0.69314718055994530941723212145817656;
}
// |x| >= 2
else if (i >= 0x3F800000 + (1 << 23)) {
rx = math.ln(2 * x + 1 / (math.sqrt(x * x + 1) + x));
}
// |x| >= 0x1p-12, up to 1.6ulp error
else if (i >= 0x3F800000 - (12 << 23)) {
rx = math.log1p(x + x * x / (math.sqrt(x * x + 1) + 1));
}
// |x| < 0x1p-12, inexact if x != 0
else {
math.forceEval(x + 0x1.0p120);
}
return if (s != 0) -rx else rx;
}
fn asinh64(x: f64) f64 {
const u = @bitCast(u64, x);
const e = (u >> 52) & 0x7FF;
const s = u >> 63;
var rx = @bitCast(f64, u & (maxInt(u64) >> 1)); // |x|
if (math.isNegativeInf(x)) {
return x;
}
// |x| >= 0x1p26 or inf or nan
if (e >= 0x3FF + 26) {
rx = math.ln(rx) + 0.693147180559945309417232121458176568;
}
// |x| >= 2
else if (e >= 0x3FF + 1) {
rx = math.ln(2 * x + 1 / (math.sqrt(x * x + 1) + x));
}
// |x| >= 0x1p-12, up to 1.6ulp error
else if (e >= 0x3FF - 26) {
rx = math.log1p(x + x * x / (math.sqrt(x * x + 1) + 1));
}
// |x| < 0x1p-12, inexact if x != 0
else {
math.forceEval(x + 0x1.0p120);
}
return if (s != 0) -rx else rx;
}
test "math.asinh" {
assert(asinh(f32(0.0)) == asinh32(0.0));
assert(asinh(f64(0.0)) == asinh64(0.0));
}
test "math.asinh32" {
const epsilon = 0.000001;
assert(math.approxEq(f32, asinh32(0.0), 0.0, epsilon));
assert(math.approxEq(f32, asinh32(0.2), 0.198690, epsilon));
assert(math.approxEq(f32, asinh32(0.8923), 0.803133, epsilon));
assert(math.approxEq(f32, asinh32(1.5), 1.194763, epsilon));
assert(math.approxEq(f32, asinh32(37.45), 4.316332, epsilon));
assert(math.approxEq(f32, asinh32(89.123), 5.183196, epsilon));
assert(math.approxEq(f32, asinh32(123123.234375), 12.414088, epsilon));
}
test "math.asinh64" {
const epsilon = 0.000001;
assert(math.approxEq(f64, asinh64(0.0), 0.0, epsilon));
assert(math.approxEq(f64, asinh64(0.2), 0.198690, epsilon));
assert(math.approxEq(f64, asinh64(0.8923), 0.803133, epsilon));
assert(math.approxEq(f64, asinh64(1.5), 1.194763, epsilon));
assert(math.approxEq(f64, asinh64(37.45), 4.316332, epsilon));
assert(math.approxEq(f64, asinh64(89.123), 5.183196, epsilon));
assert(math.approxEq(f64, asinh64(123123.234375), 12.414088, epsilon));
}
test "math.asinh32.special" {
assert(asinh32(0.0) == 0.0);
assert(asinh32(-0.0) == -0.0);
assert(math.isPositiveInf(asinh32(math.inf(f32))));
assert(math.isNegativeInf(asinh32(-math.inf(f32))));
assert(math.isNan(asinh32(math.nan(f32))));
}
test "math.asinh64.special" {
assert(asinh64(0.0) == 0.0);
assert(asinh64(-0.0) == -0.0);
assert(math.isPositiveInf(asinh64(math.inf(f64))));
assert(math.isNegativeInf(asinh64(-math.inf(f64))));
assert(math.isNan(asinh64(math.nan(f64))));
}