pcm_utils: added pcm_range()

Make the code more readable by moving the range checks to pcm_range().
gcc does quite a good job at optimizing it: the resulting binary is
exactly the same, although it contains a parametrized shift instead of
hard-coded boundaries.

src/pcm_utils.c

@@ -31,6 +31,20 @@ pcm_dither(void)
 	return (rand() & 511) - (rand() & 511);
 }
 
+/**
+ * Check if the value is within the range of the provided bit size,
+ * and caps it if necessary.
+ */
+static mpd_sint32
+pcm_range(mpd_sint32 sample, unsigned bits)
+{
+	if (mpd_unlikely(sample < (-1 << (bits - 1))))
+		return -1 << (bits - 1);
+	if (mpd_unlikely(sample >= (1 << (bits - 1))))
+		return (1 << (bits - 1)) - 1;
+	return sample;
+}
+
 void pcm_volumeChange(char *buffer, int bufferSize,
                       const struct audio_format *format,
                       int volume)
@@ -55,8 +69,7 @@ void pcm_volumeChange(char *buffer, int bufferSize,
 			temp32 += pcm_dither();
 			temp32 += 500;
 			temp32 /= 1000;
-			*buffer16 = temp32 > 32767 ? 32767 :
+			*buffer16 = pcm_range(temp32, 16);
-			    (temp32 < -32768 ? -32768 : temp32);
 			buffer16++;
 			bufferSize -= 2;
 		}
@@ -68,8 +81,7 @@ void pcm_volumeChange(char *buffer, int bufferSize,
 			temp32 += pcm_dither();
 			temp32 += 500;
 			temp32 /= 1000;
-			*buffer8 = temp32 > 127 ? 127 :
+			*buffer8 = pcm_range(temp32, 8);
-			    (temp32 < -128 ? -128 : temp32);
 			buffer8++;
 			bufferSize--;
 		}
@@ -99,9 +111,7 @@ static void pcm_add(char *buffer1, const char *buffer2, size_t bufferSize1,
 			temp32 += pcm_dither();
 			temp32 += 500;
 			temp32 /= 1000;
-			*buffer16_1 =
+			*buffer16_1 = pcm_range(temp32, 16);
-			    temp32 > 32767 ? 32767 : (temp32 <
-						      -32768 ? -32768 : temp32);
 			buffer16_1++;
 			buffer16_2++;
 			bufferSize1 -= 2;
@@ -117,9 +127,7 @@ static void pcm_add(char *buffer1, const char *buffer2, size_t bufferSize1,
 			temp32 += pcm_dither();
 			temp32 += 500;
 			temp32 /= 1000;
-			*buffer8_1 =
+			*buffer8_1 = pcm_range(temp32, 8);
-			    temp32 > 127 ? 127 : (temp32 <
-						  -128 ? -128 : temp32);
 			buffer8_1++;
 			buffer8_2++;
 			bufferSize1--;