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Use core.bitop.popcnt in std.bitmanip.countBitsSet

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Nathan Sashihara 2019-05-24 01:45:34 -04:00
parent 7972cca21f
commit 3be1b14f96
1 changed files with 7 additions and 12 deletions
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19
std/bitmanip.d
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@ -4492,24 +4492,17 @@ For signed integers, the sign bit is included in the count.
private uint countBitsSet(T)(T value) @nogc pure nothrow private uint countBitsSet(T)(T value) @nogc pure nothrow
if (isIntegral!T) if (isIntegral!T)
{ {
// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
static if (T.sizeof == 8) static if (T.sizeof == 8)
{ {
T c = value - ((value >> 1) & 0x55555555_55555555); import core.bitop : popcnt;
c = ((c >> 2) & 0x33333333_33333333) + (c & 0x33333333_33333333); const c = popcnt(cast(ulong) value);
c = ((c >> 4) + c) & 0x0F0F0F0F_0F0F0F0F;
c = ((c >> 8) + c) & 0x00FF00FF_00FF00FF;
c = ((c >> 16) + c) & 0x0000FFFF_0000FFFF;
c = ((c >> 32) + c) & 0x00000000_FFFFFFFF;
} }
else static if (T.sizeof == 4) else static if (T.sizeof == 4)
{ {
T c = value - ((value >> 1) & 0x55555555); import core.bitop : popcnt;
c = ((c >> 2) & 0x33333333) + (c & 0x33333333); const c = popcnt(cast(uint) value);
c = ((c >> 4) + c) & 0x0F0F0F0F;
c = ((c >> 8) + c) & 0x00FF00FF;
c = ((c >> 16) + c) & 0x0000FFFF;
} }
// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
else static if (T.sizeof == 2) else static if (T.sizeof == 2)
{ {
uint c = value - ((value >> 1) & 0x5555); uint c = value - ((value >> 1) & 0x5555);
@ -4559,6 +4552,8 @@ if (isIntegral!T)
{ {
assert(countBitsSet(T.max) == 8 * T.sizeof); assert(countBitsSet(T.max) == 8 * T.sizeof);
} }
// Check CTFE compiles.
static assert(countBitsSet(cast(T) 1) == 1);
} }
assert(countBitsSet(1_000_000) == 7); assert(countBitsSet(1_000_000) == 7);
foreach (i; 0 .. 63) foreach (i; 0 .. 63)