tapir/primitives/bloom.go

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2019-06-05 18:44:26 +00:00
package primitives
import (
"crypto/sha256"
)
// BloomFilter implements a bloom filter
type BloomFilter struct {
B []bool
}
// Init constructs a bloom filter of size m
func (bf * BloomFilter) Init(m int) {
bf.B = make([]bool, m)
}
// Hash transforms a message to a set of bit flips
// Supports up to m == 65535
func (bf BloomFilter) Hash(msg []byte) []int {
hash := sha256.Sum256(msg)
pos1a := (int(hash[0])+ int(hash[1]) + int(hash[2]) + int(hash[3])) % 0xFF
pos1b := (int(hash[4])+ int(hash[5]) + int(hash[6]) + int(hash[7])) % 0xFF
pos1 := ((pos1a <<8) + pos1b) & (0xFFFF % len(bf.B))
pos2a := (int(hash[8])+ int(hash[9]) + int(hash[10]) + int(hash[11])) % 0xFF
pos2b := (int(hash[12])+ int(hash[13]) + int(hash[14]) + int(hash[15])) % 0xFF
pos2 := ((pos2a <<8) + pos2b) & (0xFFFF % len(bf.B))
pos3a := (int(hash[16])+ int(hash[17]) + int(hash[18]) + int(hash[19])) % 0xFF
pos3b := (int(hash[20])+ int(hash[21]) + int(hash[22]) + int(hash[23])) % 0xFF
pos3:= ((pos3a <<8) + pos3b) & (0xFFFF % len(bf.B))
pos4a := (int(hash[24])+ int(hash[25]) + int(hash[26]) + int(hash[27])) % 0xFF
pos4b := (int(hash[28])+ int(hash[29]) + int(hash[30]) + int(hash[31])) % 0xFF
pos4:= ((pos4a <<8) + pos4b) & (0xFFFF % len(bf.B))
return []int{pos1, pos2, pos3, pos4}
}
// Insert updates the BloomFilter
func (bf * BloomFilter) Insert(msg []byte) {
pos := bf.Hash(msg)
bf.B[pos[0]] = true
bf.B[pos[1]] = true
bf.B[pos[2]] = true
bf.B[pos[3]] = true
}
// Check returns true if the messages might be in the BloomFilter
// (No false positives, possible false negatives due to the probabilistic nature of the filter)
func (bf BloomFilter) Check(msg []byte) bool {
pos := bf.Hash(msg)
if bf.B[pos[0]] && bf.B[pos[1]] && bf.B[pos[2]] && bf.B[pos[3]]{
return true
}
return false
}