add xxhash partitioner

Author: vadiminshakov

xxhashpartitioner.go

@@ -0,0 +1,41 @@
+package partmap
+
+import (
+	"fmt"
+
+	"github.com/cespare/xxhash/v2"
+)
+
+type XXHashPartitioner struct {
+	partitionsNum uint
+	mask          uint64
+	useMask       bool
+}
+
+func NewXXHashPartitioner(partitionsNum uint) (*XXHashPartitioner, error) {
+	if partitionsNum == 0 {
+		return nil, fmt.Errorf("partmap: %w", ErrInvalidPartitions)
+	}
+
+	// power-of-two fast path
+	useMask := (partitionsNum & (partitionsNum - 1)) == 0
+	var mask uint64
+	if useMask {
+		mask = uint64(partitionsNum - 1)
+	}
+
+	return &XXHashPartitioner{
+		partitionsNum: partitionsNum,
+		mask:          mask,
+		useMask:       useMask,
+	}, nil
+}
+
+func (h *XXHashPartitioner) Find(key string) uint {
+	sum := xxhash.Sum64String(key)
+	if h.useMask {
+		return uint(sum & h.mask)
+	}
+	
+	return uint(sum % uint64(h.partitionsNum))
+}