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70dc910086
Since Zig provides @clz and not @ffs (find-first-set), log2 for comptime integers needs to be computed algorithmically. To avoid hitting the backward branch quota, this updates log2(x) to use a simple O(log N) algorithm.
53 lines
1.7 KiB
Zig
53 lines
1.7 KiB
Zig
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 the base-2 logarithm of x.
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///
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/// Special Cases:
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/// - log2(+inf) = +inf
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/// - log2(0) = -inf
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/// - log2(x) = nan if x < 0
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/// - log2(nan) = nan
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pub fn log2(x: anytype) @TypeOf(x) {
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const T = @TypeOf(x);
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switch (@typeInfo(T)) {
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.ComptimeFloat => {
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return @as(comptime_float, @log2(x));
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},
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.Float => return @log2(x),
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.ComptimeInt => comptime {
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var x_shifted = x;
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// First, calculate floorPowerOfTwo(x)
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var shift_amt = 1;
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while (x_shifted >> (shift_amt << 1) != 0) shift_amt <<= 1;
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// Answer is in the range [shift_amt, 2 * shift_amt - 1]
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// We can find it in O(log(N)) using binary search.
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var result = 0;
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while (shift_amt != 0) : (shift_amt >>= 1) {
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if (x_shifted >> shift_amt != 0) {
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x_shifted >>= shift_amt;
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result += shift_amt;
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}
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}
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return result;
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},
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.Int => |IntType| switch (IntType.signedness) {
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.signed => @compileError("log2 not implemented for signed integers"),
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.unsigned => return math.log2_int(T, x),
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},
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else => @compileError("log2 not implemented for " ++ @typeName(T)),
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}
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}
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test "log2" {
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try expect(log2(@as(f32, 0.2)) == @log2(0.2));
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try expect(log2(@as(f64, 0.2)) == @log2(0.2));
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comptime {
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try expect(log2(1) == 0);
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try expect(log2(15) == 3);
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try expect(log2(16) == 4);
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try expect(log2(1 << 4073) == 4073);
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}
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}
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