pcm_utils: use the custom PRNG for volume dithering

Don't use libc's rand() function, because it is slow.  Our own trivial
linear congruential generator is good enough for dithering.

src/Makefile.am

@@ -54,6 +54,7 @@ mpd_headers = \
 	pcm_channels.h \
 	pcm_resample.h \
 	pcm_dither.h \
+	pcm_prng.h \
 	permission.h \
 	player_thread.h \
 	player_control.h \

src/pcm_dither.c

@@ -17,11 +17,7 @@
  */
 
 #include "pcm_dither.h"
-
-static unsigned long prng(unsigned long state)
-{
-	return (state * 0x0019660dL + 0x3c6ef35fL) & 0xffffffffL;
-}
+#include "pcm_prng.h"
 
 static int16_t
 pcm_dither_sample_24_to_16(int32_t sample, struct pcm_dither_24 *dither)

src/pcm_prng.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2003-2009 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifndef PCM_PRNG_H
+#define PCM_PRNG_H
+
+/**
+ * A very simple linear congruential PRNG.  It's good enough for PCM
+ * dithering.
+ */
+static unsigned long prng(unsigned long state)
+{
+	return (state * 0x0019660dL + 0x3c6ef35fL) & 0xffffffffL;
+}
+
+#endif

src/pcm_utils.c

@@ -18,6 +18,7 @@
 
 #include "pcm_utils.h"
 #include "pcm_channels.h"
+#include "pcm_prng.h"
 #include "utils.h"
 #include "conf.h"
 #include "audio_format.h"
@@ -33,7 +34,12 @@
 static inline int
 pcm_dither(void)
 {
-	return (rand() & 511) - (rand() & 511);
+	static unsigned long state;
+	uint32_t r;
+
+	r = state = prng(state);
+
+	return (r & 511) - ((r >> 9) & 511);
 }
 
 /**