audio: Resampler now special-cases stereo and mono processing.

Turns out that iterating from 0 to channels-1 was a serious performance hit!

These cases now tend to match or beat the original audio resampler's speed!

src/audio/SDL_audiocvt.c

@@ -249,16 +249,52 @@ SDL_ResampleAudioSimple(const int chans, const double rate_incr,
     SDL_assert((dest_samples * framelen) <= outbuflen);
     SDL_assert((inbuflen % framelen) == 0);
 
-    if (rate_incr > 1.0) {
+    if (rate_incr > 1.0) {  /* upsample */
         float *target = (outbuf + chans);
-        const float *earlier_sample = &inbuf[finalpos];
-        float final_sample[8];
         dst = outbuf + (dest_samples * chans);
         idx = (double) total;
 
-        /* save this off so we can correctly maintain state between runs. */
+        if (chans == 1) {
+            const float final_sample = inbuf[finalpos];
+            float earlier_sample = inbuf[finalpos];
+            while (dst > target) {
+                const int pos = ((int) idx) * chans;
+                const float *src = &inbuf[pos];
+                const float val = *(--src);
+                SDL_assert(pos >= 0.0);
+                *(--dst) = (val + earlier_sample) * 0.5f;
+                earlier_sample = val;
+                idx -= src_incr;
+            }
+            /* do last sample, interpolated against previous run's state. */
+            *(--dst) = (inbuf[0] + last_sample[0]) * 0.5f;
+            *last_sample = final_sample;
+        } else if (chans == 2) {
+            const float final_sample2 = inbuf[finalpos+1];
+            const float final_sample1 = inbuf[finalpos];
+            float earlier_sample2 = inbuf[finalpos];
+            float earlier_sample1 = inbuf[finalpos-1];
+            while (dst > target) {
+                const int pos = ((int) idx) * chans;
+                const float *src = &inbuf[pos];
+                const float val2 = *(--src);
+                const float val1 = *(--src);
+                SDL_assert(pos >= 0.0);
+                *(--dst) = (val2 + earlier_sample2) * 0.5f;
+                *(--dst) = (val1 + earlier_sample1) * 0.5f;
+                earlier_sample2 = val2;
+                earlier_sample1 = val1;
+                idx -= src_incr;
+            }
+            /* do last sample, interpolated against previous run's state. */
+            *(--dst) = (inbuf[1] + last_sample[1]) * 0.5f;
+            *(--dst) = (inbuf[0] + last_sample[0]) * 0.5f;
+            last_sample[1] = final_sample2;
+            last_sample[0] = final_sample1;
+        } else {
+            const float *earlier_sample = &inbuf[finalpos];
+            float final_sample[8];
             SDL_memcpy(final_sample, &inbuf[finalpos], framelen);
-
             while (dst > target) {
                 const int pos = ((int) idx) * chans;
                 const float *src = &inbuf[pos];
@@ -270,18 +306,47 @@ SDL_ResampleAudioSimple(const int chans, const double rate_incr,
                 earlier_sample = src;
                 idx -= src_incr;
             }
-
             /* do last sample, interpolated against previous run's state. */
             for (i = chans - 1; i >= 0; i--) {
                 const float val = inbuf[i];
                 *(--dst) = (val + last_sample[i]) * 0.5f;
             }
             SDL_memcpy(last_sample, final_sample, framelen);
+        }
+
         dst = (outbuf + (dest_samples * chans)) - 1;
-    } else {
+    } else {  /* downsample */
         float *target = (outbuf + (dest_samples * chans));
         dst = outbuf;
         idx = 0.0;
+        if (chans == 1) {
+            float last = *last_sample;
+            while (dst < target) {
+                const int pos = ((int) idx) * chans;
+                const float val = inbuf[pos];
+                SDL_assert(pos <= finalpos);
+                *(dst++) = (val + last) * 0.5f;
+                last = val;
+                idx += src_incr;
+            }
+            *last_sample = last;
+        } else if (chans == 2) {
+            float last1 = last_sample[0];
+            float last2 = last_sample[1];
+            while (dst < target) {
+                const int pos = ((int) idx) * chans;
+                const float val1 = inbuf[pos];
+                const float val2 = inbuf[pos+1];
+                SDL_assert(pos <= finalpos);
+                *(dst++) = (val1 + last1) * 0.5f;
+                *(dst++) = (val2 + last2) * 0.5f;
+                last1 = val1;
+                last2 = val2;
+                idx += src_incr;
+            }
+            last_sample[0] = last1;
+            last_sample[1] = last2;
+        } else {
             while (dst < target) {
                 const int pos = ((int) idx) * chans;
                 const float *src = &inbuf[pos];
@@ -294,6 +359,7 @@ SDL_ResampleAudioSimple(const int chans, const double rate_incr,
                 idx += src_incr;
             }
         }
+    }
 
     return (int) ((dst - outbuf) * ((int) sizeof (float)));
 }