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bits.go
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bits.go
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// (c) 2019-2020, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.
package ids
import (
"bytes"
"math/bits"
)
// NumBits is the number of bits this patricia tree manages
const NumBits = 256
// BitsPerByte is the number of bits per byte
const BitsPerByte = 8
// EqualSubset takes in two indices and two ids and returns if the ids are
// equal from bit start to bit end (non-inclusive). Bit indices are defined as:
// [7 6 5 4 3 2 1 0] [15 14 13 12 11 10 9 8] ... [255 254 253 252 251 250 249 248]
// Where index 7 is the MSB of byte 0.
func EqualSubset(start, stop int, id1, id2 ID) bool {
stop--
if start > stop || stop < 0 {
return true
}
if stop >= NumBits {
return false
}
startIndex := start / BitsPerByte
stopIndex := stop / BitsPerByte
// If there is a series of bytes between the first byte and the last byte, they must be equal
if startIndex+1 < stopIndex && !bytes.Equal(id1[startIndex+1:stopIndex], id2[startIndex+1:stopIndex]) {
return false
}
startBit := uint(start % BitsPerByte) // Index in the byte that the first bit is at
stopBit := uint(stop % BitsPerByte) // Index in the byte that the last bit is at
startMask := -1 << startBit // 111...0... The number of 0s is equal to startBit
stopMask := (1 << (stopBit + 1)) - 1 // 000...1... The number of 1s is equal to stopBit+1
if startIndex == stopIndex {
// If we are looking at the same byte, both masks need to be applied
mask := startMask & stopMask
// The index here could be startIndex or stopIndex, as they are equal
b1 := mask & int(id1[startIndex])
b2 := mask & int(id2[startIndex])
return b1 == b2
}
start1 := startMask & int(id1[startIndex])
start2 := startMask & int(id2[startIndex])
stop1 := stopMask & int(id1[stopIndex])
stop2 := stopMask & int(id2[stopIndex])
return start1 == start2 && stop1 == stop2
}
// FirstDifferenceSubset takes in two indices and two ids and returns the index
// of the first difference between the ids inside bit start to bit end
// (non-inclusive). Bit indices are defined above
func FirstDifferenceSubset(start, stop int, id1, id2 ID) (int, bool) {
stop--
if start > stop || stop < 0 || stop >= NumBits {
return 0, false
}
startIndex := start / BitsPerByte
stopIndex := stop / BitsPerByte
startBit := uint(start % BitsPerByte) // Index in the byte that the first bit is at
stopBit := uint(stop % BitsPerByte) // Index in the byte that the last bit is at
startMask := -1 << startBit // 111...0... The number of 0s is equal to startBit
stopMask := (1 << (stopBit + 1)) - 1 // 000...1... The number of 1s is equal to stopBit+1
if startIndex == stopIndex {
// If we are looking at the same byte, both masks need to be applied
mask := startMask & stopMask
// The index here could be startIndex or stopIndex, as they are equal
b1 := mask & int(id1[startIndex])
b2 := mask & int(id2[startIndex])
if b1 == b2 {
return 0, false
}
bd := b1 ^ b2
return bits.TrailingZeros8(uint8(bd)) + startIndex*BitsPerByte, true
}
// Check the first byte, may have some bits masked
start1 := startMask & int(id1[startIndex])
start2 := startMask & int(id2[startIndex])
if start1 != start2 {
bd := start1 ^ start2
return bits.TrailingZeros8(uint8(bd)) + startIndex*BitsPerByte, true
}
// Check all the interior bits
for i := startIndex + 1; i < stopIndex; i++ {
b1 := int(id1[i])
b2 := int(id2[i])
if b1 != b2 {
bd := b1 ^ b2
return bits.TrailingZeros8(uint8(bd)) + i*BitsPerByte, true
}
}
// Check the last byte, may have some bits masked
stop1 := stopMask & int(id1[stopIndex])
stop2 := stopMask & int(id2[stopIndex])
if stop1 != stop2 {
bd := stop1 ^ stop2
return bits.TrailingZeros8(uint8(bd)) + stopIndex*BitsPerByte, true
}
// No difference was found
return 0, false
}