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Use math/bits to find leading and trailing zeros #35

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Use math/bits to find leading and trailing zeros #35

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39 changes: 13 additions & 26 deletions s2/cellid.go
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@ import (
"fmt"
"io"
"math"
"math/bits"
"sort"
"strconv"
"strings"
Expand Down Expand Up @@ -737,37 +738,23 @@ func (ci CellID) CommonAncestorLevel(other CellID) (level int, ok bool) {
}

// findMSBSetNonZero64 returns the index (between 0 and 63) of the most
// significant set bit. Passing zero to this function has undefined behavior.
func findMSBSetNonZero64(bits uint64) int {
val := []uint64{0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000, 0xFFFFFFFF00000000}
shift := []uint64{1, 2, 4, 8, 16, 32}
var msbPos uint64
for i := 5; i >= 0; i-- {
if bits&val[i] != 0 {
bits >>= shift[i]
msbPos |= shift[i]
}
// significant set bit.
func findMSBSetNonZero64(value uint64) int {
if value == 0 {
return 0
}
return int(msbPos)
}

const deBruijn64 = 0x03f79d71b4ca8b09
const digitMask = uint64(1<<64 - 1)

var deBruijn64Lookup = []byte{
0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4,
62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5,
63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11,
54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6,
return 63 - bits.LeadingZeros64(value)
}

// findLSBSetNonZero64 returns the index (between 0 and 63) of the least
// significant set bit. Passing zero to this function has undefined behavior.
//
// This code comes from trailingZeroBits in https://golang.org/src/math/big/nat.go
// which references (Knuth, volume 4, section 7.3.1).
func findLSBSetNonZero64(bits uint64) int {
return int(deBruijn64Lookup[((bits&-bits)*(deBruijn64&digitMask))>>58])
// significant set bit.
func findLSBSetNonZero64(value uint64) int {
if value == 0 {
return 0
}

return bits.TrailingZeros64(value)
}

// Advance advances or retreats the indicated number of steps along the
Expand Down