1 files changed, 126 insertions, 0 deletions
diff --git a/lib/std/math/round.zig b/lib/std/math/round.zig
new file mode 100644
index 0000000000..0b80a46ce5
--- /dev/null
+++ b/lib/std/math/round.zig
@@ -0,0 +1,126 @@
+// 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/roundf.c
+// https://git.musl-libc.org/cgit/musl/tree/src/math/round.c
+
+const builtin = @import("builtin");
+const expect = std.testing.expect;
+const std = @import("../std.zig");
+const math = std.math;
+
+/// Returns x rounded to the nearest integer, rounding half away from zero.
+///
+/// Special Cases:
+///  - round(+-0)   = +-0
+///  - round(+-inf) = +-inf
+///  - round(nan)   = nan
+pub fn round(x: var) @typeOf(x) {
+    const T = @typeOf(x);
+    return switch (T) {
+        f32 => round32(x),
+        f64 => round64(x),
+        else => @compileError("round not implemented for " ++ @typeName(T)),
+    };
+}
+
+fn round32(x_: f32) f32 {
+    var x = x_;
+    const u = @bitCast(u32, x);
+    const e = (u >> 23) & 0xFF;
+    var y: f32 = undefined;
+
+    if (e >= 0x7F + 23) {
+        return x;
+    }
+    if (u >> 31 != 0) {
+        x = -x;
+    }
+    if (e < 0x7F - 1) {
+        math.forceEval(x + math.f32_toint);
+        return 0 * @bitCast(f32, u);
+    }
+
+    y = x + math.f32_toint - math.f32_toint - x;
+    if (y > 0.5) {
+        y = y + x - 1;
+    } else if (y <= -0.5) {
+        y = y + x + 1;
+    } else {
+        y = y + x;
+    }
+
+    if (u >> 31 != 0) {
+        return -y;
+    } else {
+        return y;
+    }
+}
+
+fn round64(x_: f64) f64 {
+    var x = x_;
+    const u = @bitCast(u64, x);
+    const e = (u >> 52) & 0x7FF;
+    var y: f64 = undefined;
+
+    if (e >= 0x3FF + 52) {
+        return x;
+    }
+    if (u >> 63 != 0) {
+        x = -x;
+    }
+    if (e < 0x3ff - 1) {
+        math.forceEval(x + math.f64_toint);
+        return 0 * @bitCast(f64, u);
+    }
+
+    y = x + math.f64_toint - math.f64_toint - x;
+    if (y > 0.5) {
+        y = y + x - 1;
+    } else if (y <= -0.5) {
+        y = y + x + 1;
+    } else {
+        y = y + x;
+    }
+
+    if (u >> 63 != 0) {
+        return -y;
+    } else {
+        return y;
+    }
+}
+
+test "math.round" {
+    expect(round(f32(1.3)) == round32(1.3));
+    expect(round(f64(1.3)) == round64(1.3));
+}
+
+test "math.round32" {
+    expect(round32(1.3) == 1.0);
+    expect(round32(-1.3) == -1.0);
+    expect(round32(0.2) == 0.0);
+    expect(round32(1.8) == 2.0);
+}
+
+test "math.round64" {
+    expect(round64(1.3) == 1.0);
+    expect(round64(-1.3) == -1.0);
+    expect(round64(0.2) == 0.0);
+    expect(round64(1.8) == 2.0);
+}
+
+test "math.round32.special" {
+    expect(round32(0.0) == 0.0);
+    expect(round32(-0.0) == -0.0);
+    expect(math.isPositiveInf(round32(math.inf(f32))));
+    expect(math.isNegativeInf(round32(-math.inf(f32))));
+    expect(math.isNan(round32(math.nan(f32))));
+}
+
+test "math.round64.special" {
+    expect(round64(0.0) == 0.0);
+    expect(round64(-0.0) == -0.0);
+    expect(math.isPositiveInf(round64(math.inf(f64))));
+    expect(math.isNegativeInf(round64(-math.inf(f64))));
+    expect(math.isNan(round64(math.nan(f64))));
+}