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Clear most significant bit
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package main | |
import ( | |
"math/bits" | |
"math/rand" | |
"testing" | |
) | |
// DeBruijn64 from `math/bits` | |
var Input uint64 = 0x03f79d71b4ca8b09 | |
func bShift(x uint8) uint8 { | |
l := bits.Len8(x) | |
if l > 0 { | |
l -= 1 | |
} else { | |
return 0 | |
} | |
return x & ^(1 << l) | |
} | |
func BenchmarkShift(b *testing.B) { | |
for i := 0; i < b.N; i++ { | |
_ = bShift(uint8(Input) >> (uint(i) % 8)) | |
} | |
} | |
func bSub(x uint8) uint8 { | |
l := bits.Len8(x) | |
if l > 0 { | |
l -= 1 | |
} else { | |
return 0 | |
} | |
return x - (1 << l) | |
} | |
func BenchmarkSub(b *testing.B) { | |
for i := 0; i < b.N; i++ { | |
_ = bSub(uint8(Input) >> (uint(i) % 8)) | |
} | |
} | |
func bSwitch(x uint8) uint8 { | |
switch { | |
case x >= 128: | |
return x & 0b1111111 | |
case x >= 64: | |
return x & 0b111111 | |
case x >= 32: | |
return x & 0b11111 | |
case x >= 16: | |
return x & 0b1111 | |
case x >= 8: | |
return x & 0b111 | |
case x >= 4: | |
return x & 0b11 | |
case x >= 2: | |
return x & 0b1 | |
default: | |
return 0 | |
} | |
} | |
func BenchmarkSwitch(b *testing.B) { | |
for i := 0; i < b.N; i++ { | |
_ = bSwitch(uint8(Input) >> (uint(i) % 8)) | |
} | |
} | |
var tab = [256]uint8{ | |
0x00, 0x00, 0x00, 0x01, 0x00, 0x01, 0x02, 0x03, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, | |
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, | |
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, | |
0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, | |
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, | |
0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, | |
0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, | |
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, | |
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, | |
0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, | |
0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, | |
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, | |
0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, | |
0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f, | |
0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f, | |
0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f, | |
} | |
func bTab(x uint8) uint8 { | |
return tab[x] | |
} | |
func BenchmarkTab(b *testing.B) { | |
for i := 0; i < b.N; i++ { | |
_ = bTab(uint8(Input) >> (uint(i) % 8)) | |
} | |
} |
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