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Microptimization.

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Fix range obscurity and compile error.

Revert change for converting comptime int.
This commit is contained in:
Said Kadrioski 2025-09-27 21:01:52 +02:00
parent 53e615b920
commit ed88533fcf
1 changed files with 18 additions and 14 deletions
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32
lib/std/math/gcd.zig
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@ -9,12 +9,14 @@ pub fn gcd(a: anytype, b: anytype) @TypeOf(a, b) {
comptime_int => std.math.IntFittingRange(@min(a, b), @max(a, b)),
else => |T| T,
};
if (@typeInfo(N) != .int or @typeInfo(N).int.signedness != .unsigned) {
@compileError("`a` and `b` must be usigned integers");
@compileError("`a` and `b` must be unsigned integers");
}
// using an optimised form of Stein's algorithm:
// https://en.wikipedia.org/wiki/Binary_GCD_algorithm
std.debug.assert(a != 0 or b != 0);
if (a == 0) return b;
@ -26,25 +28,27 @@ pub fn gcd(a: anytype, b: anytype) @TypeOf(a, b) {
const xz = @ctz(x);
const yz = @ctz(y);
const shift = @min(xz, yz);
x >>= @intCast(xz);
y >>= @intCast(yz);
x = @shrExact(x, @intCast(xz));
y = @shrExact(y, @intCast(yz));
var diff = y -% x;
while (diff != 0) : (diff = y -% x) {
// ctz is invariant under negation, we
// put it here to ease data dependencies,
// makes the CPU happy.
const zeros = @ctz(diff);
if (x > y) diff = -%diff;
y = @min(x, y);
x = diff >> @intCast(zeros);
var y_minus_x = y -% x;
while (y_minus_x != 0) : (y_minus_x = y -% x) {
const copy_x = x;
const zeros = @ctz(y_minus_x);
const carry = x < y;
x -%= y;
if (carry) {
x = y_minus_x;
y = copy_x;
}
x = @shrExact(x, @intCast(zeros));
}
return y << @intCast(shift);
return @shlExact(y, @intCast(shift));
}
test gcd {
const expectEqual = std.testing.expectEqual;
try expectEqual(gcd(0, 5), 5);
try expectEqual(gcd(5, 0), 5);
try expectEqual(gcd(8, 12), 4);