1 files changed, 74 insertions, 0 deletions
diff --git a/lib/std/math/copysign.zig b/lib/std/math/copysign.zig
new file mode 100644
index 0000000000..e4d90c395e
--- /dev/null
+++ b/lib/std/math/copysign.zig
@@ -0,0 +1,74 @@
+// Ported from musl, which is licensed under the MIT license:
+// https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+//
+// https://git.musl-libc.org/cgit/musl/tree/src/math/copysignf.c
+// https://git.musl-libc.org/cgit/musl/tree/src/math/copysign.c
+
+const std = @import("../std.zig");
+const math = std.math;
+const expect = std.testing.expect;
+const maxInt = std.math.maxInt;
+
+/// Returns a value with the magnitude of x and the sign of y.
+pub fn copysign(comptime T: type, x: T, y: T) T {
+    return switch (T) {
+        f16 => copysign16(x, y),
+        f32 => copysign32(x, y),
+        f64 => copysign64(x, y),
+        else => @compileError("copysign not implemented for " ++ @typeName(T)),
+    };
+}
+
+fn copysign16(x: f16, y: f16) f16 {
+    const ux = @bitCast(u16, x);
+    const uy = @bitCast(u16, y);
+
+    const h1 = ux & (maxInt(u16) / 2);
+    const h2 = uy & (u16(1) << 15);
+    return @bitCast(f16, h1 | h2);
+}
+
+fn copysign32(x: f32, y: f32) f32 {
+    const ux = @bitCast(u32, x);
+    const uy = @bitCast(u32, y);
+
+    const h1 = ux & (maxInt(u32) / 2);
+    const h2 = uy & (u32(1) << 31);
+    return @bitCast(f32, h1 | h2);
+}
+
+fn copysign64(x: f64, y: f64) f64 {
+    const ux = @bitCast(u64, x);
+    const uy = @bitCast(u64, y);
+
+    const h1 = ux & (maxInt(u64) / 2);
+    const h2 = uy & (u64(1) << 63);
+    return @bitCast(f64, h1 | h2);
+}
+
+test "math.copysign" {
+    expect(copysign(f16, 1.0, 1.0) == copysign16(1.0, 1.0));
+    expect(copysign(f32, 1.0, 1.0) == copysign32(1.0, 1.0));
+    expect(copysign(f64, 1.0, 1.0) == copysign64(1.0, 1.0));
+}
+
+test "math.copysign16" {
+    expect(copysign16(5.0, 1.0) == 5.0);
+    expect(copysign16(5.0, -1.0) == -5.0);
+    expect(copysign16(-5.0, -1.0) == -5.0);
+    expect(copysign16(-5.0, 1.0) == 5.0);
+}
+
+test "math.copysign32" {
+    expect(copysign32(5.0, 1.0) == 5.0);
+    expect(copysign32(5.0, -1.0) == -5.0);
+    expect(copysign32(-5.0, -1.0) == -5.0);
+    expect(copysign32(-5.0, 1.0) == 5.0);
+}
+
+test "math.copysign64" {
+    expect(copysign64(5.0, 1.0) == 5.0);
+    expect(copysign64(5.0, -1.0) == -5.0);
+    expect(copysign64(-5.0, -1.0) == -5.0);
+    expect(copysign64(-5.0, 1.0) == 5.0);
+}