media: add AudioTrack::getMinFrameCount().

Change-Id: If15162583d1d16d89f59be0793106afe77417f35

include/media/AudioTrack.h

@@ -102,6 +102,17 @@ public:
 
     typedef void (*callback_t)(int event, void* user, void *info);
 
+    /* Returns the minimum frame count required for the successful creation of
+     * an AudioTrack object.
+     * Returned status (from utils/Errors.h) can be:
+     *  - NO_ERROR: successful operation
+     *  - NO_INIT: audio server or audio hardware not initialized
+     */
+
+     static status_t getMinFrameCount(int* frameCount,
+                                      int streamType      =-1,
+                                      uint32_t sampleRate = 0);
+
     /* Constructs an uninitialized AudioTrack. No connection with
      * AudioFlinger takes place.
      */

media/libmedia/AudioTrack.cpp

@@ -41,6 +41,35 @@
 #define UNLIKELY( exp )     (__builtin_expect( (exp) != 0, false ))
 
 namespace android {
+// ---------------------------------------------------------------------------
+
+// static
+status_t AudioTrack::getMinFrameCount(
+        int* frameCount,
+        int streamType,
+        uint32_t sampleRate)
+{
+    int afSampleRate;
+    if (AudioSystem::getOutputSamplingRate(&afSampleRate, streamType) != NO_ERROR) {
+        return NO_INIT;
+    }
+    int afFrameCount;
+    if (AudioSystem::getOutputFrameCount(&afFrameCount, streamType) != NO_ERROR) {
+        return NO_INIT;
+    }
+    uint32_t afLatency;
+    if (AudioSystem::getOutputLatency(&afLatency, streamType) != NO_ERROR) {
+        return NO_INIT;
+    }
+
+    // Ensure that buffer depth covers at least audio hardware latency
+    uint32_t minBufCount = afLatency / ((1000 * afFrameCount) / afSampleRate);
+    if (minBufCount < 2) minBufCount = 2;
+
+    *frameCount = (sampleRate == 0) ? afFrameCount * minBufCount :
+              afFrameCount * minBufCount * sampleRate / afSampleRate;
+    return NO_ERROR;
+}
 
 // ---------------------------------------------------------------------------