Simple Audio Mixer

LFAudioMixer.m

//
//  LFAudioMixer.m
//  Garena
//
//  Created by Omer Iqbal on 8/3/17.
//  Copyright © 2017 Garena. All rights reserved.
//

#import "LFAudioMixer.h"
#import <AVFoundation/AVFoundation.h>

@interface LFAudioMixer ()

@property (nonatomic, strong) NSMutableArray<AVAudioPCMBuffer *> *micBuffers;
@property (nonatomic, strong) NSMutableArray<AVAudioPCMBuffer *> *appBuffers;

@end

@implementation LFAudioMixer

- (instancetype)init {
    self = [super init];
    if (self) {
        _micBuffers = [NSMutableArray array];
        _appBuffers = [NSMutableArray array];
    }
    return self;
}

static inline SInt16 TPMixSamples(SInt16 a, SInt16 b) {
    return  
    // If both samples are negative, mixed signal must have an amplitude between the lesser of A and B, and the minimum permissible negative amplitude
    a < 0 && b < 0 ?
    ((int)a + (int)b) - (((int)a * (int)b)/INT16_MIN) :
    
    // If both samples are positive, mixed signal must have an amplitude between the greater of A and B, and the maximum permissible positive amplitude
    ( a > 0 && b > 0 ?
     ((int)a + (int)b) - (((int)a * (int)b)/INT16_MAX)
     
     // If samples are on opposite sides of the 0-crossing, mixed signal should reflect that samples cancel each other out somewhat
     :
     a + b);
}

- (AVAudioPCMBuffer *)mixPCMBuffer:(AVAudioPCMBuffer *)b1 withBuffer:(AVAudioPCMBuffer *)b2 {
    AVAudioPCMBuffer *smaller = b1.frameLength < b2.frameLength ? b1: b2;
    AVAudioPCMBuffer *larger = b1.frameLength >= b2.frameLength ? b1: b2;
    
    NSLog(@"b1: %@, b2:%@", b1, b2);
    
    uint16_t mixedBuffer[larger.frameLength];
    memset(mixedBuffer, 0, larger.frameLength * sizeof(uint16_t));
    
    for (NSUInteger i = 0; i < smaller.frameLength; i++) {
        mixedBuffer[i] = TPMixSamples(*smaller.int16ChannelData[i], *larger.int16ChannelData[i]);
    }
    
    for (NSUInteger i = smaller.frameLength; i < larger.frameLength; i++) {
        mixedBuffer[i] = *larger.int16ChannelData[i];
    }
    
    AudioBuffer mixedAudioBuffer;
    mixedAudioBuffer.mData = mixedBuffer;
    mixedAudioBuffer.mDataByteSize = larger.frameLength * 2;
    mixedAudioBuffer.mNumberChannels = 1;
    
    AudioBufferList bufferList;
    bufferList.mBuffers[0] = mixedAudioBuffer;
    bufferList.mNumberBuffers = 1;
    
    AVAudioPCMBuffer *mixed = [[AVAudioPCMBuffer alloc] initWithPCMFormat:larger.format frameCapacity:larger.frameLength];
    mixed.mutableAudioBufferList->mBuffers[0] = mixedAudioBuffer;
    mixed.mutableAudioBufferList->mNumberBuffers = 1;
    
    return mixed;
}

- (void)queueMicAudioBuffer:(AVAudioPCMBuffer *)buffer {
    [self.micBuffers addObject:buffer];
    //[self processQueue];
}

- (void)queueAppAudioBuffer:(AVAudioPCMBuffer *)buffer {
    [self.appBuffers addObject:buffer];
    //[self processQueue];
}

- (void)processQueue {
    if (self.micBuffers.count == 0 || self.appBuffers.count == 0) {
        return;
    }
    
    AVAudioPCMBuffer *firstMic = [self.micBuffers firstObject];
    AVAudioPCMBuffer *firstApp = [self.appBuffers firstObject];
    
    AVAudioPCMBuffer *mixed = [self mixPCMBuffer:firstMic withBuffer:firstApp];
    [self.delegate mixer:self didMixPCMBuffer:mixed];
    
    [self.micBuffers removeObjectAtIndex:0];
    [self.appBuffers removeObjectAtIndex:0];
    
    [self processQueue];
}

@end