mp3: moved dropSamplesAtEnd check out of the loop

Simplifying loops for performance: why check dropSamplesAtEnd in every iteration, when we could modify the loop boundary? The (writable) variable samplesLeft can be eliminated; add a write-once variable pcm_length instead, which is used for the loop condition.

mp3: moved dropSamplesAtEnd check out of the loop
e99536e8 · Max Kellermann · e4c6c019 · e99536e8
Commit e99536e8 authored Aug 26, 2008 by Max Kellermann
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Showing with 18 additions and 21 deletions

mp3_plugin.c src/inputPlugins/mp3_plugin.c +18 -21

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--- a/src/inputPlugins/mp3_plugin.c
+++ b/src/inputPlugins/mp3_plugin.c
@@ -839,7 +839,7 @@ static int openMp3FromInputStream(InputStream * inStream, mp3DecodeData * data,
 static int mp3Read(mp3DecodeData * data, struct decoder *decoder,
 		   ReplayGainInfo ** replayGainInfo)
 {
-	unsigned int samplesLeft;
+	unsigned int pcm_length;
 	unsigned int i;
 	int ret;
 	int skip;
@@ -917,32 +917,23 @@ static int mp3Read(mp3DecodeData * data, struct decoder *decoder,
 			freeMpdTag(tag);
 		}

-		samplesLeft = (data->synth).pcm.length;
-
 		if (!data->decodedFirstFrame) {
-			if (data->dropSamplesAtStart >= samplesLeft) {
-				i = samplesLeft;
-				samplesLeft = 0;
-			} else {
-				i = data->dropSamplesAtStart;
-				samplesLeft -= data->dropSamplesAtStart;
-			}
+			i = data->dropSamplesAtStart;
 			data->decodedFirstFrame = 1;
 		} else
 			i = 0;

-		for (; i < (data->synth).pcm.length; i++) {
-			unsigned int num_samples;
-
-			samplesLeft--;
+		pcm_length = data->synth.pcm.length;
+		if (data->dropSamplesAtEnd &&
+		    (data->currentFrame == data->maxFrames - data->dropFramesAtEnd)) {
+			if (data->dropSamplesAtEnd >= pcm_length)
+				pcm_length = 0;
+			else
+				pcm_length -= data->dropSamplesAtEnd;
+		}

-			if (data->dropSamplesAtEnd &&
-			           (data->currentFrame == (data->maxFrames - data->dropFramesAtEnd)) &&
-				   (samplesLeft < data->dropSamplesAtEnd)) {
-				/* stop decoding, effectively dropping
-				 * all remaining samples */
-				return DECODE_BREAK;
-			}
+		for (; i < pcm_length; i++) {
+			unsigned int num_samples;

 			num_samples = dither_buffer((mpd_sint16 *) data->outputPtr,
 						    &data->synth, &data->dither,
@@ -968,6 +959,12 @@ static int mp3Read(mp3DecodeData * data, struct decoder *decoder,
 			}
 		}

+		if (data->dropSamplesAtEnd &&
+		    (data->currentFrame == data->maxFrames - data->dropFramesAtEnd))
+			/* stop decoding, effectively dropping
+			 * all remaining samples */
+			return DECODE_BREAK;
+
 		if (decoder_get_command(decoder) == DECODE_COMMAND_SEEK &&
 		    data->inStream->seekable) {
 			unsigned long j = 0;