Mix partial stream PCM without copying stale buffer tails

tools/chatgui-android/app/src/main/java/com/utun/chat/data/StreamAudioMix.kt

@@ -30,14 +30,14 @@ internal class StreamMixBuffer(val frames: Int = 480) {
     private var gain = 1f
     fun clear() = sum.fill(0f)
 
-    fun add(pcm: ShortArray, fadeFrames: Int = 480): Int {
-        require(pcm.size <= sum.size && pcm.size % 2 == 0)
-        for (i in 0 until pcm.size / 2) {
+    fun add(pcm: ShortArray, fadeFrames: Int = 480, count: Int = pcm.size / 2): Int {
+        require(pcm.size % 2 == 0 && count in 0..min(frames, pcm.size / 2))
+        for (i in 0 until count) {
             val volume = min(1f, (fadeFrames + i + 1) / 480f) / 32768f
             sum[2 * i] += pcm[2 * i] * volume
             sum[2 * i + 1] += pcm[2 * i + 1] * volume
         }
-        return min(480, fadeFrames + pcm.size / 2)
+        return min(480, fadeFrames + count)
     }
 
     fun finish(): ShortArray {

tools/chatgui-android/app/src/test/java/com/utun/chat/data/StreamAudioMixTest.kt

@@ -7,6 +7,16 @@ import kotlin.math.abs
 import kotlin.math.sin
 
 class StreamAudioMixTest {
+    @Test fun reusedPcmOnlyMixesTheNewlyPreparedPrefix() {
+        val mix = StreamMixBuffer()
+        val pcm = ShortArray(960) { 12000 }
+        mix.clear(); assertEquals(480, mix.add(pcm, 480, 120))
+        val output = mix.finish()
+        for (i in output.indices) assertEquals(if (i < 240) 12000 else 0, output[i].toInt())
+        mix.clear(); mix.add(pcm, 480, 0)
+        assertTrue(mix.finish().all { it == 0.toShort() })
+    }
+
     @Test fun joiningAndResetUseTheirOwnOutputOrigin() {
         val first = StreamMixTimeline()
         first.append(0, 480); first.append(480, 480)