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package main
import (
"encoding/binary"
"io"
"github.com/chmduquesne/rollinghash/rabinkarp64"
)
// SketchChunk produces a sketch for a chunk based on wSize: the window size,
// sfCount: the number of super-features, and fCount: the number of feature
// per super-feature
func SketchChunk(chunk Chunk, wSize int, sfCount int, fCount int) ([]uint64, error) {
var fSize = chunkSize / (sfCount * fCount)
superfeatures := make([]uint64, 0, sfCount)
features := make([]uint64, 0, fCount)
buff := make([]byte, 8*fCount)
r, err := chunk.Reader()
if err != nil {
return nil, err
}
hasher := rabinkarp64.New()
for sf := 0; sf < sfCount; sf++ {
features = features[:0]
for f := 0; f < fCount; f++ {
hasher.Reset()
io.CopyN(hasher, r, int64(wSize))
max := hasher.Sum64()
for w := 0; w < fSize-wSize; w++ {
b, _ := r.ReadByte()
hasher.Roll(b)
h := hasher.Sum64()
if h > max {
max = h
}
}
features = append(features, max)
}
for i, f := range features {
binary.LittleEndian.PutUint64(buff[i*8:i*8+8], f)
}
hasher.Reset()
hasher.Write(buff)
superfeatures = append(superfeatures, hasher.Sum64())
}
return superfeatures, nil
}
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