feat: add kbucket prefix map

bucket.go

@@ -108,3 +108,5 @@ func (b *Bucket) Split(cpl int, target ID) *Bucket {
 	}
 	return newbuck
 }
+
+//go:generate go run ./generate

generate/main.go

+package main
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"os"
+	"strings"
+
+	mh "github.com/multiformats/go-multihash"
+)
+
+const bits = 16
+const target = 1 << bits
+const idLen = 32 + 2
+
+func main() {
+	pkg := os.Getenv("GOPACKAGE")
+	file := os.Getenv("GOFILE")
+	targetFile := strings.TrimSuffix(file, ".go") + "_prefixmap.go"
+
+	ids := new([target]uint64)
+	found := new([target]bool)
+	count := int32(0)
+
+	out := make([]byte, 32)
+	inp := [idLen]byte{mh.SHA2_256, 32}
+	hasher := sha256.New()
+
+	for i := uint64(0); count < target; i++ {
+		binary.BigEndian.PutUint64(inp[idLen-8:], i)
+		hasher.Write(inp[:])
+		out = hasher.Sum(out[:0])
+		hasher.Reset()
+
+		prefix := binary.BigEndian.Uint32(out) >> (32 - bits)
+		if !found[prefix] {
+			found[prefix] = true
+			ids[prefix] = i
+			count++
+		}
+	}
+
+	f, err := os.Create(targetFile)
+	if err != nil {
+		panic(err)
+	}
+
+	printf := func(s string, args ...interface{}) {
+		_, err := fmt.Fprintf(f, s, args...)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	printf("package %s\n\n", pkg)
+	printf("// Code generated by generate/generate_map.go DO NOT EDIT\n")
+	printf("var keyPrefixMap = [...]uint32{")
+	for i, j := range ids[:] {
+		if i%16 == 0 {
+			printf("\n\t")
+		} else {
+			printf(" ")
+		}
+		printf("%d,", j)
+	}
+	printf("\n}")
+	f.Close()
+}

go.mod

@@ -6,4 +6,5 @@ require (
 	github.com/libp2p/go-libp2p-core v0.0.1
 	github.com/libp2p/go-libp2p-peerstore v0.1.0
 	github.com/minio/sha256-simd v0.0.0-20190131020904-2d45a736cd16
+	github.com/multiformats/go-multihash v0.0.1
 )