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std.math: Add O(log N) implementation of log2(x) for comptime_int

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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.
This commit is contained in:
Cody Tapscott 2022-06-07 10:58:49 -07:00 committed by Andrew Kelley
parent 6ff7b437ff
commit 70dc910086
1 changed files with 19 additions and 5 deletions
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24
lib/std/math/log2.zig
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@ -17,12 +17,20 @@ pub fn log2(x: anytype) @TypeOf(x) {
},
.Float => return @log2(x),
.ComptimeInt => comptime {
var result = 0;
var x_shifted = x;
while (b: {
x_shifted >>= 1;
break :b x_shifted != 0;
}) : (result += 1) {}
// First, calculate floorPowerOfTwo(x)
var shift_amt = 1;
while (x_shifted >> (shift_amt << 1) != 0) shift_amt <<= 1;
// Answer is in the range [shift_amt, 2 * shift_amt - 1]
// We can find it in O(log(N)) using binary search.
var result = 0;
while (shift_amt != 0) : (shift_amt >>= 1) {
if (x_shifted >> shift_amt != 0) {
x_shifted >>= shift_amt;
result += shift_amt;
}
}
return result;
},
.Int => |IntType| switch (IntType.signedness) {
@ -36,4 +44,10 @@ pub fn log2(x: anytype) @TypeOf(x) {
test "log2" {
try expect(log2(@as(f32, 0.2)) == @log2(0.2));
try expect(log2(@as(f64, 0.2)) == @log2(0.2));
comptime {
try expect(log2(1) == 0);
try expect(log2(15) == 3);
try expect(log2(16) == 4);
try expect(log2(1 << 4073) == 4073);
}
}