Adding instrument metadata to a caf file

add_caf_metadata.m

/*
 * Adding instrument metadata (node value) to a given caf file.
 * 
 * Original author: https://gist.github.com/2640513
 * Skovhus encountered a bug using gcc 4.2.1. This is the fixed version.
 *
 * 2012-10-21
 */

#include <stdio.h>

#include <AudioToolbox/AudioToolbox.h>
#include <Foundation/NSData.h>
#include <Foundation/NSAutoreleasePool.h>

// generic error handler - if result is nonzero, prints error message and exits program.
static void CheckResult(OSStatus result, const char *operation)
{
	if (result == noErr) return;
	
	char errorString[20];
	// see if it appears to be a 4-char-code
	*(UInt32 *)(errorString + 1) = CFSwapInt32HostToBig(result);
	if (isprint(errorString[1]) && isprint(errorString[2]) && isprint(errorString[3]) && isprint(errorString[4])) {
		errorString[0] = errorString[5] = '\'';
		errorString[6] = '\0';
	} else
		// no, format it as an integer
		sprintf(errorString, "%d", (int)result);
	
	fprintf(stderr, "Error: %s (%s)\n", operation, errorString);
	
	exit(1);
}

int	main(int argc, const char *argv[])
{
	UInt8 note = 0;
	const char* input_filename;
	
	if(argc < 3)
	{
		printf("Not enough arguments provided - need an input file and a note for the instrument\n");
		return 0;
	}
	else
	{
		input_filename = argv[1];
		note = atoi(argv[2]);
	}
	
	CFStringRef inputFileLocation = CFStringCreateWithCString(kCFAllocatorDefault, input_filename, kCFStringEncodingASCII);
	
	CFURLRef inputFileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, inputFileLocation, kCFURLPOSIXPathStyle, false);
	
	// create output file
	char output_filename[strlen(input_filename) + 6]; // bug fixed
	
	//copy minus extension
	strncpy(output_filename, input_filename, strlen(input_filename) - 4);
	sprintf(&output_filename[strlen(input_filename) - 4], "_%d.caf", note);
	
	CFStringRef outputFileLocation = CFStringCreateWithCString(kCFAllocatorDefault, output_filename, kCFStringEncodingASCII);
	CFURLRef outputFileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, outputFileLocation, kCFURLPOSIXPathStyle, false);
	
	
	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
	
	NSData *data = [NSData dataWithContentsOfURL:(NSURL*)inputFileURL];
	
	UInt32 curr_byte_index = 0;
	
	UInt8* p_bytes = (UInt8*)[data bytes];
	UInt32* p_word = (UInt32*)(&p_bytes[curr_byte_index]);
	
	UInt32 fourcc = CFSwapInt32BigToHost(*p_word);
	
	if(fourcc == 'caff')
	{
		printf("Searching caff file for free chunk...\n");
		
		//step over fourcc and version number
		curr_byte_index += 8;
		
		//walk through the chunks in the file looking for the 'free' chunk.
		while(1)
		{
			fourcc = CFSwapInt32BigToHost(*((UInt32*)(&p_bytes[curr_byte_index])));
			UInt64 num_bytes = CFSwapInt64BigToHost(*((UInt64*)(&p_bytes[curr_byte_index + 4])));
			printf("Found \'%c%c%c%c\' chunk - %qi bytes\n", p_bytes[curr_byte_index], p_bytes[curr_byte_index+1], p_bytes[curr_byte_index+2], p_bytes[curr_byte_index+3], num_bytes);
			
			if(fourcc == 'free')
			{
				UInt64 num_bytes_in_free_chunk = num_bytes;
				num_bytes_in_free_chunk -= sizeof(CAFChunkHeader);
				num_bytes_in_free_chunk -= sizeof(CAFInstrumentChunk);
				
				//write instrument chunk header
				CAFChunkHeader instrument_header;
				instrument_header.mChunkType = CFSwapInt32HostToBig('inst');
				instrument_header.mChunkSize = CFSwapInt64HostToBig(sizeof(CAFInstrumentChunk));
				
				*(CAFChunkHeader*)(&p_bytes[curr_byte_index]) = instrument_header;
				curr_byte_index += sizeof(CAFChunkHeader);
				
				//write instrument chunk
				CAFInstrumentChunk instrument_chunk = {0};
				
				instrument_chunk.mMIDILowVelocity = 0;
				instrument_chunk.mMIDIHighVelocity = 127;
				instrument_chunk.mMIDILowNote = note;
				instrument_chunk.mMIDIHighNote = note;
				
				*(CAFInstrumentChunk*)(&p_bytes[curr_byte_index]) = instrument_chunk;
				
				//byte swapping floats is a pain in the arse...
				CFSwappedFloat32 base_note = CFConvertFloatHostToSwapped((float)note);
				*(CFSwappedFloat32*)(&p_bytes[curr_byte_index]) = base_note;
				
				curr_byte_index += sizeof(CAFInstrumentChunk);
				
				num_bytes = sizeof(CAFInstrumentChunk) + sizeof(CAFChunkHeader);
				printf("Inserting %qi bytes of instrument data\n", num_bytes);
				
				//Update the free header
				CAFChunkHeader free_header;
				free_header.mChunkType = CFSwapInt32HostToBig('free');
				free_header.mChunkSize = CFSwapInt64HostToBig(num_bytes_in_free_chunk);
				
				*(CAFChunkHeader*)(&p_bytes[curr_byte_index]) = free_header;
				
				break;
			}
			else
			{
				curr_byte_index += num_bytes + sizeof(CAFChunkHeader);
			}
		}
	}
	else
	{
		printf("Not a caff file");
	}
	
	[data writeToURL:(NSURL*)outputFileURL atomically:TRUE];
	
	[pool drain];
	
	return 0;
}