Merge range-based set reconciliation implementation

backend/syncing/rbsr/fingerprint.go

@@ -0,0 +1,56 @@
+package rbsr
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"unsafe"
+)
+
+type Fingerprint [fingerprintSize]byte
+
+type accumulator struct {
+	len int
+	sum [32]byte
+}
+
+func (acc *accumulator) Add(other [32]byte) {
+	var currCarry, nextCarry uint64
+
+	// Treating [32]byte as [4]uint64 when adding.
+	p := (*[4]uint64)(unsafe.Pointer(&acc.sum[0]))
+	po := (*[4]uint64)(unsafe.Pointer(&other[0]))
+
+	for i := 0; i < 4; i++ {
+		orig := p[i]
+		otherV := po[i]
+
+		next := orig
+
+		next += currCarry
+		if next < orig {
+			nextCarry = 1
+		}
+
+		next += otherV
+		if next < otherV {
+			nextCarry = 1
+		}
+
+		p[i] = next
+
+		currCarry = nextCarry
+		nextCarry = 0
+	}
+}
+
+func (acc *accumulator) Fingerprint() Fingerprint {
+	buf := make([]byte, 0, len(acc.sum)+8) // sum + len will be hashed.
+	buf = append(buf, acc.sum[:]...)
+	buf = binary.LittleEndian.AppendUint64(buf, uint64(acc.len))
+
+	hash := sha256.Sum256(buf)
+
+	var fingerprint Fingerprint
+	copy(fingerprint[:], hash[:fingerprintSize])
+	return fingerprint
+}