Implement Checksum()

litestream.go

@@ -1,9 +1,16 @@
 package litestream
 
 import (
+	"encoding/binary"
 	"io"
 )
 
+// Magic number specified at the beginning of WAL files.
+const (
+	MagicLittleEndian = 0x377f0682
+	MagicBigEndian    = 0x377f0683
+)
+
 const (
 	WriteVersionOffset = 18
 	ReadVersionOffset  = 19
@@ -17,3 +24,19 @@ func ReadVersion(b []byte) (writeVersion, readVersion uint8, err error) {
 	}
 	return b[WriteVersionOffset], b[ReadVersionOffset], nil
 }
+
+// Checksum computes a running checksum over a byte slice.
+func Checksum(bo binary.ByteOrder, s uint64, b []byte) (_ uint64, err error) {
+	// Ensure byte slice length is divisible by 8.
+	if len(b)%8 != 0 {
+		return 0, ErrChecksumMisaligned
+	}
+
+	// Iterate over 8-byte units and compute checksum.
+	s0, s1 := uint32(s>>32), uint32(s&0xFFFFFFFF)
+	for i := 0; i < len(b); i += 8 {
+		s0 += bo.Uint32(b[i:]) + s1
+		s1 += bo.Uint32(b[i+4:]) + s0
+	}
+	return uint64(s0)<<32 | uint64(s1), nil
+}