ADPCM: added helper function for adding int16s without overflowing

stream/adpcm/adpcm.go

@@ -97,9 +97,9 @@ func encodeSample(sample int16) byte {
 
 	// adjust predicted sample based on calculated difference
 	if nibble&8 != 0 {
-		encPred -= diff
+		encPred = capAdd16(encPred, -diff)
 	} else {
-		encPred += diff
+		encPred = capAdd16(encPred, diff)
 	}
 
 	// check for underflow and overflow
@@ -142,20 +142,8 @@ func decodeSample(nibble byte) int16 {
 		diff = -diff
 	}
 
-	// adjust predicted sample based on calculated difference, check for overflow
-	if diff > 0 {
-		if decPred > math.MaxInt16-diff {
-			decPred = math.MaxInt16
-		} else {
-			decPred += diff
-		}
-	} else {
-		if decPred < math.MaxInt16-diff {
-			decPred = math.MaxInt16
-		} else {
-			decPred += diff
-		}
-	}
+	// adjust predicted sample based on calculated difference
+	decPred = capAdd16(decPred, diff)
 
 	// adjust index into step size lookup table using nibble
 	decIndex += indexTable[nibble]
@@ -173,6 +161,19 @@ func decodeSample(nibble byte) int16 {
 	return decPred
 }
 
+// capAdd16 adds two int16s together and caps at max/min int16 instead of overflowing
+func capAdd16(a, b int16) int16 {
+	c := int32(a) + int32(b)
+	switch {
+	case c < math.MinInt16:
+		return math.MinInt16
+	case c > math.MaxInt16:
+		return math.MaxInt16
+	default:
+		return int16(c)
+	}
+}
+
 func calcHead(sample []byte) ([]byte, error) {
 	// check that we are given 1 16-bit sample (2 bytes)
 	sampSize := 2