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chore: make sw pext generic over T

This commit is contained in:
Rekai Nyangadzayi Musuka 2023-12-02 16:44:29 -06:00
parent 611549c8e9
commit 64e1daf31f
1 changed files with 23 additions and 49 deletions
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72
src/lib.zig
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@ -326,62 +326,36 @@ const pext = struct {
};
}
fn sw(comptime T: type, value: T, mask: T) T {
// TODO: rewrite in idiomatic zig / understand the algorithm and write an original implementation
// code source: https://stackoverflow.com/questions/41720249/detecting-matching-bits-in-c
// note: will be replaced in the future by https://github.com/ziglang/zig/issues/14995 (hopefully?)
inline fn sw(comptime T: type, value: T, mask: T) T {
// FIXME: will be replaced in the future by https://github.com/ziglang/zig/issues/14995 (hopefully?)
return switch (T) {
u32 => {
var _value: T = value;
var _mask: T = mask;
u32, u64 => {
// code source: https://stackoverflow.com/questions/41720249/detecting-matching-bits-in-c
// TODO: rewrite more in generic/idiomatic zig
const log2_bits = @typeInfo(Log2Int(T)).Int.bits;
_value &= _mask;
var mk: T = ~_mask << 1;
var mp: T = undefined;
var mv: T = undefined;
var t: T = undefined;
var val: T = value & mask; // immediately clear irrelevant bits
var msk: T = mask;
var mk: T = ~msk << 1; // count 0s to the right
inline for (0..log2_bits) |i| {
var mp: T = mk ^ (mk << 1);
inline for (1..log2_bits) |j| {
mp = mp ^ (mp << (1 << j)); // parallel suffix
}
const mv = (mp & msk); // bits to move
msk = ((msk ^ mv) | (mv >> (1 << i))); // compress mask
const t = (val & mv);
val = ((val ^ t) | (t >> (1 << i))); // compress val
inline for (0..@typeInfo(Log2Int(T)).Int.bits) |i| {
mp = mk ^ (mk << 1); // parallel suffix
mp = mp ^ (mp << 2);
mp = mp ^ (mp << 4);
mp = mp ^ (mp << 8);
mp = mp ^ (mp << 16);
mv = (mp & _mask); // bits to move
_mask = ((_mask ^ mv) | (mv >> (1 << i))); // compress _mask
t = (_value & mv);
_value = ((_value ^ t) | (t >> (1 << i))); // compress _value
mk &= ~mp;
}
return _value;
},
u64 => {
var _value: T = value;
var _mask: T = mask;
_value &= _mask;
var mk: T = ~_mask << 1;
var mp: T = undefined;
var mv: T = undefined;
var t: T = undefined;
inline for (0..@typeInfo(Log2Int(T)).Int.bits) |i| {
mp = mk ^ (mk << 1); // parallel suffix
mp = mp ^ (mp << 2);
mp = mp ^ (mp << 4);
mp = mp ^ (mp << 8);
mp = mp ^ (mp << 16);
mp = mp ^ (mp << 32);
mv = (mp & _mask); // bits to move
_mask = ((_mask ^ mv) | (mv >> (1 << i))); // compress _mask
t = (_value & mv);
_value = ((_value ^ t) | (t >> (1 << i))); // compress _value
mk &= ~mp;
}
return _value;
return val;
},
else => @compileError("pext is sunsupported for " ++ @typeName(T) ++ "."),
};