1const std = @import("../std.zig");
 2const builtin = @import("builtin");
 3const math = std.math;
 4const expect = std.testing.expect;
 5
 6/// Returns the base-2 logarithm of x.
 7///
 8/// Special Cases:
 9///  - log2(+inf)  = +inf
10///  - log2(0)     = -inf
11///  - log2(x)     = nan if x < 0
12///  - log2(nan)   = nan
13pub fn log2(x: anytype) @TypeOf(x) {
14    const T = @TypeOf(x);
15    return switch (@typeInfo(T)) {
16        .comptime_float, .float => @log2(x),
17        .comptime_int => comptime {
18            std.debug.assert(x > 0);
19            var x_shifted = x;
20            // First, calculate floorPowerOfTwo(x)
21            var shift_amt = 1;
22            while (x_shifted >> (shift_amt << 1) != 0) shift_amt <<= 1;
23
24            // Answer is in the range [shift_amt, 2 * shift_amt - 1]
25            // We can find it in O(log(N)) using binary search.
26            var result = 0;
27            while (shift_amt != 0) : (shift_amt >>= 1) {
28                if (x_shifted >> shift_amt != 0) {
29                    x_shifted >>= shift_amt;
30                    result += shift_amt;
31                }
32            }
33            return result;
34        },
35        .int => |int_info| math.log2_int(switch (int_info.signedness) {
36            .signed => @Int(.unsigned, int_info.bits -| 1),
37            .unsigned => T,
38        }, @intCast(x)),
39        else => @compileError("log2 not implemented for " ++ @typeName(T)),
40    };
41}
42
43test log2 {
44    // https://github.com/ziglang/zig/issues/13703
45    if (builtin.cpu.arch == .aarch64 and builtin.os.tag == .windows) return error.SkipZigTest;
46
47    try expect(log2(@as(f32, 0.2)) == @log2(0.2));
48    try expect(log2(@as(f64, 0.2)) == @log2(0.2));
49    comptime {
50        try expect(log2(1) == 0);
51        try expect(log2(15) == 3);
52        try expect(log2(16) == 4);
53        try expect(log2(1 << 4073) == 4073);
54    }
55}