RecorderThread: Send input timestamps to MediaCodec encoders

This was previously disabled due to incorrect troubleshooting of an
issue where the `c2.android.flac.encoder` component would crash during
recording. It turns out the crash was caused by incorrect math:

    inputTimestamp += frames * 1000000 / audioRecord.sampleRate

`frames` and `1000000` are both 32-bit integers, so the multiplication
result overflowed and caused `inputTimestamp` (a 64-bit integer) to
accumulate negative values, which would crash the encoder. This bug was
inadvertently fixed during refactorying in commit
3d3eb82.

This commit reenables the submission of timestamps to MediaCodec. To
avoid accumulated error due to integer division when the microseconds
per sample count is not perfectly divisible, the recording loop will
keep track of the total frame count and only calculate the timestamp
during buffer submission.

This should fix issues where the encoded output files had overlapping
audio or other weird audible artifacts. These were caused by
MediaCodec's multithreaded encoding, where the lack of timestamps would
cause encoded samples to be produced out of order.

Fixes: #39 #54
Signed-off-by: Andrew Gunnerson <[email protected]>

app/src/main/java/com/chiller3/bcr/RecorderThread.kt

@@ -341,7 +341,7 @@ class RecorderThread(
         mediaFormat: MediaFormat,
         container: Container,
     ) {
-        var inputTimestamp = 0L
+        var numFrames = 0L
         val bufferInfo = MediaCodec.BufferInfo()
         val frameSize = audioRecord.format.frameSizeInBytesCompat
 
@@ -364,25 +364,24 @@ class RecorderThread(
                 logE( "Unexpected EOF from AudioRecord")
                 isCancelled = true
             } else {
-                val frames = n / frameSize
-                inputTimestamp += frames * 1_000_000L / audioRecord.sampleRate
-
                 bufferInfo.offset = 0
                 bufferInfo.size = buffer.limit()
-                bufferInfo.presentationTimeUs = 0
+                bufferInfo.presentationTimeUs = numFrames * 1_000_000L / audioRecord.sampleRate
                 bufferInfo.flags = if (isCancelled) {
                     MediaCodec.BUFFER_FLAG_END_OF_STREAM
                 } else {
                     0
                 }
 
+                numFrames += n / frameSize
+
                 container.writeSamples(trackIndex, buffer, bufferInfo)
                 buffer.clear()
             }
 
             if (isCancelled) {
-                val duration = "%.1f".format(inputTimestamp / 1_000_000.0)
-                logD("Input complete after ${duration}s")
+                val durationSecs = numFrames.toDouble() / audioRecord.sampleRate
+                logD("Input complete after ${"%.1f".format(durationSecs)}s")
                 break
             }
         }
@@ -407,7 +406,7 @@ class RecorderThread(
      * @throws MediaCodec.CodecException if the codec encounters an error
      */
     private fun encodeLoop(audioRecord: AudioRecord, mediaCodec: MediaCodec, container: Container) {
-        var inputTimestamp = 0L
+        var numFrames = 0L
         var inputComplete = false
         val bufferInfo = MediaCodec.BufferInfo()
         val frameSize = audioRecord.format.frameSizeInBytesCompat
@@ -436,25 +435,23 @@ class RecorderThread(
                         logE( "MediaCodec's ByteBuffer was not a direct buffer")
                         isCancelled = true
                     } else {
-                        val frames = n / frameSize
-                        inputTimestamp += frames * 1_000_000L / audioRecord.sampleRate
+                        val flags = if (isCancelled) {
+                            MediaCodec.BUFFER_FLAG_END_OF_STREAM
+                        } else {
+                            0
+                        }
+
+                        val timestampUs = numFrames * 1_000_000L / audioRecord.sampleRate
+                        mediaCodec.queueInputBuffer(inputBufferId, 0, n, timestampUs, flags)
+
+                        numFrames += n / frameSize
                     }
 
                     if (isCancelled) {
-                        val duration = "%.1f".format(inputTimestamp / 1_000_000.0)
-                        logD("Input complete after ${duration}s")
+                        val durationSecs = numFrames.toDouble() / audioRecord.sampleRate
+                        logD("Input complete after ${"%.1f".format(durationSecs)}s")
                         inputComplete = true
                     }
-
-                    val flags = if (inputComplete) {
-                        MediaCodec.BUFFER_FLAG_END_OF_STREAM
-                    } else {
-                        0
-                    }
-
-                    // Setting the presentation timestamp will cause `c2.android.flac.encoder`
-                    // software encoder to crash with SIGABRT
-                    mediaCodec.queueInputBuffer(inputBufferId, 0, n, 0, flags)
                 } else if (inputBufferId != MediaCodec.INFO_TRY_AGAIN_LATER) {
                     logW("Unexpected input buffer dequeue error: $inputBufferId")
                 }