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alsa: support SND_PCM_FORMAT_S24_3LE format

SND_PCM_FORMAT_S24_3LE (signed 24 bits, little-endian, packed into
three bytes) is the native encoding of some USB audio interfaces.
Added to the ALSA driver.

--- a/src/alsa.c

+++ b/src/alsa.c

@@ -39,6 +39,7 @@

     { 16, SND_PCM_FORMAT_U16, 2, SOX_ENCODING_UNSIGNED },

     { 24, SND_PCM_FORMAT_S24, 4, SOX_ENCODING_SIGN2 },

     { 24, SND_PCM_FORMAT_U24, 4, SOX_ENCODING_UNSIGNED },

+    { 24, SND_PCM_FORMAT_S24_3LE, 3, SOX_ENCODING_SIGN2 },

     { 32, SND_PCM_FORMAT_S32, 4, SOX_ENCODING_SIGN2 },

     { 32, SND_PCM_FORMAT_U32, 4, SOX_ENCODING_UNSIGNED },

     {  0, 0, 0, SOX_ENCODING_UNKNOWN } /* end of list */

@@ -225,6 +226,17 @@

         while (i--) *buf++ = SOX_SIGNED_24BIT_TO_SAMPLE(*buf1++,);

         break;

       }

+      case SND_PCM_FORMAT_S24_3LE: {

+        unsigned char *buf1 = (unsigned char *)p->buf;

+        while (i--) {

+          uint32_t temp;

+          temp  = *buf1++;

+          temp |= *buf1++ << 8;

+          temp |= *buf1++ << 16;

+          *buf++ = SOX_SIGNED_24BIT_TO_SAMPLE((sox_int24_t)temp,);

+        }

+        break;

+      }

       case SND_PCM_FORMAT_U24: {

         sox_uint24_t * buf1 = (sox_uint24_t *)p->buf;

         while (i--) *buf++ = SOX_UNSIGNED_24BIT_TO_SAMPLE(*buf1++,);

@@ -286,6 +298,16 @@

       case SND_PCM_FORMAT_S24: {

         sox_int24_t * buf1 = (sox_int24_t *)p->buf;

         while (i--) *buf1++ = SOX_SAMPLE_TO_SIGNED_24BIT(*buf++, ft->clips);

+        break;

+      }

+      case SND_PCM_FORMAT_S24_3LE: {

+        unsigned char *buf1 = (unsigned char *)p->buf;

+        while (i--) {

+          uint32_t temp = (uint32_t)SOX_SAMPLE_TO_SIGNED_24BIT(*buf++, ft->clips);

+          *buf1++ = (temp & 0x000000FF);

+          *buf1++ = (temp & 0x0000FF00) >> 8;

+          *buf1++ = (temp & 0x00FF0000) >> 16;

+        }

         break;

       }

       case SND_PCM_FORMAT_U24: {