feat: add ConsumeVarint

Signed-off-by: Christian Stewart <christian@aperture.us>

protobuf-go-lite.go

@@ -155,6 +155,110 @@ func AppendVarint(b []byte, v uint64) []byte {
 	return b
 }
 
+// ConsumeVarint parses b as a varint-encoded uint64, reporting its length.
+// This returns -1 upon any error, -1 for parse error and -2 for overflow.
+func ConsumeVarint(b []byte) (v uint64, n int) {
+	var y uint64
+	if len(b) <= 0 {
+		return 0, -1
+	}
+	v = uint64(b[0])
+	if v < 0x80 {
+		return v, 1
+	}
+	v -= 0x80
+
+	if len(b) <= 1 {
+		return 0, -1
+	}
+	y = uint64(b[1])
+	v += y << 7
+	if y < 0x80 {
+		return v, 2
+	}
+	v -= 0x80 << 7
+
+	if len(b) <= 2 {
+		return 0, -1
+	}
+	y = uint64(b[2])
+	v += y << 14
+	if y < 0x80 {
+		return v, 3
+	}
+	v -= 0x80 << 14
+
+	if len(b) <= 3 {
+		return 0, -1
+	}
+	y = uint64(b[3])
+	v += y << 21
+	if y < 0x80 {
+		return v, 4
+	}
+	v -= 0x80 << 21
+
+	if len(b) <= 4 {
+		return 0, -1
+	}
+	y = uint64(b[4])
+	v += y << 28
+	if y < 0x80 {
+		return v, 5
+	}
+	v -= 0x80 << 28
+
+	if len(b) <= 5 {
+		return 0, -1
+	}
+	y = uint64(b[5])
+	v += y << 35
+	if y < 0x80 {
+		return v, 6
+	}
+	v -= 0x80 << 35
+
+	if len(b) <= 6 {
+		return 0, -1
+	}
+	y = uint64(b[6])
+	v += y << 42
+	if y < 0x80 {
+		return v, 7
+	}
+	v -= 0x80 << 42
+
+	if len(b) <= 7 {
+		return 0, -1
+	}
+	y = uint64(b[7])
+	v += y << 49
+	if y < 0x80 {
+		return v, 8
+	}
+	v -= 0x80 << 49
+
+	if len(b) <= 8 {
+		return 0, -1
+	}
+	y = uint64(b[8])
+	v += y << 56
+	if y < 0x80 {
+		return v, 9
+	}
+	v -= 0x80 << 56
+
+	if len(b) <= 9 {
+		return 0, -1
+	}
+	y = uint64(b[9])
+	v += y << 63
+	if y < 2 {
+		return v, 10
+	}
+	return 0, -2
+}
+
 // SizeOfVarint returns the size of the varint-encoded value.
 func SizeOfVarint(x uint64) (n int) {
 	return (bits.Len64(x|1) + 6) / 7