如何在 iOS 上将 flac 转换为 wav？

Question

我有一个使用 FLAC 编码的文件，我想将其转换为 WAV。

我已将this FFMpeg lib 添加到我的项目并导入它。

我看到this answer的一些代码，但我不清楚如何使用它：

#import "avformat.h"

// Some code goes here

/*
 *   avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options)
 */
int openInputValue = avformat_open_input(&pFormatCtx, utf8FilePath, inputFormat, nil);
NSLog(@"%s - %d # openInputValue = %d", __PRETTY_FUNCTION__, __LINE__, openInputValue);

我目前在一个函数中有这个代码，它需要NSData 保存 FLAC 文件。 如果avformat_open_input 是正确的调用，我该如何设置变量？如果不是正确的调用，那是什么？

This question 似乎是重复的，但实际上并没有很好的答案。

另外请注意，我不想要玩家。这个文件包含 MQA，所以我需要通过我自己的自定义解码器来运行它。

Answer 1

我能够使用this code 进行解码并使用this code 来实际编写WAV 标头/正文。

另外，this 在解码 NSData 而不是文件方面非常有帮助。

这是我完成的解码器，虽然我不希望它在任何情况下都能正常工作，除了我的。

//
//  FlacToWavConverter.m
//  SuperpoweredMQAExample
//
//  Created by Tony Lawrence on 5/18/17.
//  Copyright © 2017 imect. All rights reserved.
//

#import "FlacToWavConverter.h"
#import "avformat.h"
#import "avcodec.h"
#import "avutil.h"
#import "swresample.h"
#import "file.h"

@implementation FlacToWavConverter

+(NSURL*)convertFlacToWav:(NSData*)data {

    //const char* input_filename = [filePath UTF8String];
    int buffer_size = 16384;

    // This call is necessarily done once in your app to initialize
    // libavformat to register all the muxers, demuxers and protocols.
    av_register_all();

    // A media container
    AVFormatContext* container = avformat_alloc_context();

    //Make a custom IO context so that we can read from memory instead of a file...
    unsigned char* iobuffer = av_malloc(buffer_size);
    struct buffer_data bd = { 0 };
    bd.ptr = (uint8_t*)data.bytes;
    bd.size = data.length;
    AVIOContext* ioContext = avio_alloc_context(iobuffer, buffer_size, 0, &bd, &read_packet, NULL, NULL);

    container->pb = ioContext;

    if (avformat_open_input(&container, "arbitrary", NULL, NULL) < 0) {
        NSLog(@"Could not open file");
    }

    if (avformat_find_stream_info(container, NULL) < 0) {
        NSLog(@"Could not find file info");
    }

    int stream_id = -1;

    // To find the first audio stream. This process may not be necessary
    // if you can gurarantee that the container contains only the desired
    // audio stream
    int i;
    for (i = 0; i < container->nb_streams; i++) {
        if (container->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            stream_id = i;
            break;
        }
    }

    if (stream_id == -1) {
        NSLog(@"Could not find an audio stream");
    }

    // Extract some metadata
    AVDictionary* metadata = container->metadata;

    // Find the apropriate codec and open it
    AVCodecContext* codec_context = container->streams[stream_id]->codec;

    AVCodec* codec = avcodec_find_decoder(codec_context->codec_id);

    if (avcodec_open2(codec_context, codec, NULL) < 0) {
        NSLog(@"Could not find open the needed codec");
    }

    NSMutableData *pcmFile = [NSMutableData new];

    AVPacket packet;
    int8_t buffer[buffer_size];

    while (1) {

        //buffer_size = AVCODEC_MAX_AUDIO_FRAME_SIZE;

        // Read one packet into `packet`
        if (av_read_frame(container, &packet) < 0) {
            break;  // End of stream. Done decoding.
        }

        // Decodes from `packet` into the buffer
        if (avcodec_decode_audio3(codec_context, (int16_t*)buffer, &buffer_size, &packet) < 1) {
            break;  // Error in decoding
        }

        // Send the buffer contents to the audio device
        [pcmFile appendBytes:buffer length:buffer_size];

    }

    avformat_close_input(&container);

    //fprintf(stdout, "Done playing. Exiting...");

    NSURL *file = [FlacToWavConverter getAndCreatePlayableFileFromPcmData:pcmFile];

    NSLog(@"Got a playable file maybe? %@", [file absoluteString]);

    return file;
}

+(NSURL *) getAndCreatePlayableFileFromPcmData:(NSData *)data
{
    NSArray *dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *docsDir = [dirPaths objectAtIndex:0];
    NSString *wavFilePath = [docsDir stringByAppendingPathComponent:@"output.wav"];

    //NSLog(@"PCM data : %@",data);

    FILE *fout;

    short NumChannels = 2;
    short BitsPerSample = 16;
    int SamplingRate = 44100;
    int numOfSamples = [data length];

    int ByteRate = NumChannels*BitsPerSample*SamplingRate/8;
    short BlockAlign = NumChannels*BitsPerSample/8;
    int DataSize = NumChannels*numOfSamples*BitsPerSample/8;
    int chunkSize = 16;
    int totalSize = 46 + DataSize;
    short audioFormat = 1;

    if((fout = fopen([wavFilePath cStringUsingEncoding:1], "w")) == NULL)
    {
        printf("Error opening out file ");
    }

    fwrite("RIFF", sizeof(char), 4,fout);
    fwrite(&totalSize, sizeof(int), 1, fout);
    fwrite("WAVE", sizeof(char), 4, fout);
    fwrite("fmt ", sizeof(char), 4, fout);
    fwrite(&chunkSize, sizeof(int),1,fout);
    fwrite(&audioFormat, sizeof(short), 1, fout);
    fwrite(&NumChannels, sizeof(short),1,fout);
    fwrite(&SamplingRate, sizeof(int), 1, fout);
    fwrite(&ByteRate, sizeof(int), 1, fout);
    fwrite(&BlockAlign, sizeof(short), 1, fout);
    fwrite(&BitsPerSample, sizeof(short), 1, fout);
    fwrite("data", sizeof(char), 4, fout);
    fwrite(&DataSize, sizeof(int), 1, fout);

    fclose(fout);

    NSMutableData *pamdata = [NSMutableData dataWithData:data];
    NSFileHandle *handle;
    handle = [NSFileHandle fileHandleForUpdatingAtPath:wavFilePath];
    [handle seekToEndOfFile];
    [handle writeData:pamdata];
    [handle closeFile];

    NSLog(@"Saved wav: %@", wavFilePath);

    return [NSURL URLWithString:wavFilePath];
}

struct buffer_data {
    uint8_t *ptr;
    size_t size; ///< size left in the buffer
};


static int read_packet(void *opaque, uint8_t *buf, int buf_size)
{
    struct buffer_data *bd = (struct buffer_data *)opaque;
    buf_size = FFMIN(buf_size, bd->size);
    //printf("ptr:%p size:%zu\n", bd->ptr, bd->size);
    /* copy internal buffer data to buf */
    memcpy(buf, bd->ptr, buf_size);
    bd->ptr  += buf_size;
    bd->size -= buf_size;
    return buf_size;
}

@end