#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <zlib.h>

uint32_t endianswap( uint32_t n )
{
	// if we're in a big endian system, we don't need this
	uint16_t testme = 0x1234;
	if ( *(uint8_t*)(&testme) == 0x12 ) return n;
	uint32_t on;
	for ( int i=0; i<4; i++ )
		*(((uint8_t*)(&on))+i) = *(((uint8_t*)(&n))+(3-i));
	return on;
}

typedef struct
{
	uint8_t highbitcheck;
	char sig[3];
	char crlf[2];
	char dosstop;
	char lf;
} pnghead_t;

typedef struct
{
	uint32_t length;
	char type[4];
	uint8_t *data;
	uint32_t crc;
} pngchunk_t;

typedef struct
{
	uint32_t width, height;
	uint8_t depth, type, compression, filter, interlace;
} ihdr_t;

typedef struct
{
	uint32_t xppu, yppu;
	uint8_t unit;
} phys_t;

int exitval = 0;
FILE *f = 0;
int nchunk = 0;
pnghead_t hdr = {0,0,0,0,0};
pngchunk_t chk = {0,0,0,0};	// make sure this is null at the very start
uint32_t curcrc = 0;
int validcrc = 0;
uint8_t *tdat = 0;	// accumulated data for all IDAT chunks
uint32_t tdatsiz = 0;	// size of it

void readIHDR( ihdr_t* ihdr )
{
	printf(" width: %u\n height: %u\n depth: %u\n type: %u\n compression: %u\n filter: %u\n interlace: %u\n",
		endianswap(ihdr->width),endianswap(ihdr->height),ihdr->depth,ihdr->type,ihdr->compression,ihdr->filter,ihdr->interlace);
}

// read up a stream chunk
void readIDAT( uint8_t* dat, uint32_t len )
{
	if ( tdat ) tdat = realloc(tdat,tdatsiz+len);
	else tdat = malloc(len);
	memcpy(tdat+tdatsiz,dat,len);
	tdatsiz += len;
}

// process the whole thing
void processIDAT( void )
{
}

void readpHYs( phys_t* phys )
{
	printf(" xppu: %u\n yppu: %u\n unit: %u\n",endianswap(phys->xppu),endianswap(phys->yppu),phys->unit);
}

void readtEXt( uint8_t* data, uint32_t len )
{
	uint8_t *key = data;
	uint8_t *val = data+strlen(data)+1;
	uint32_t vallen = len-(val-key);
	printf(" '%s' :: '%.*s'\n",key,vallen,val);
}

void readchunkdata( pngchunk_t* chk )
{
	if ( !strncmp(chk->type,"IHDR",4) )
		readIHDR((ihdr_t*)chk->data);
	else if ( !strncmp(chk->type,"pHYs",4) )
		readpHYs((phys_t*)chk->data);
	else if ( !strncmp(chk->type,"tEXt",4) )
		readtEXt(chk->data,chk->length);
}


int main( int argc, char **argv )
{
	if ( argc < 2 )
	{
		printf("No file supplied.\n");
		exitval = 0;
		goto endmii;
	}
	f = fopen(argv[1],"rb");
	if ( !f )
	{
		printf("Failed to open file.\n");
		exitval = 1;
		goto endmii;
	}
	fread(&hdr,1,sizeof(hdr),f);
	if ( hdr.highbitcheck != 0x89 )
	{
		printf("High bit check failed.\n");
		exitval = 2;
		goto endmii;
	}
	if ( strncmp(hdr.sig,"PNG",3) )
	{
		printf("PNG signature check failed.\n");
		exitval = 4;
		goto endmii;
	}
	if ( strncmp(hdr.crlf,"\r\n",2) )
	{
		printf("CRLF check failed.\n");
		exitval = 8;
		goto endmii;
	}
	if ( hdr.dosstop != '\032' )
	{
		printf("EOF check failed.\n");
		exitval = 16;
		goto endmii;
	}
	if ( hdr.lf != '\n' )
	{
		printf("LF check failed.\n");
		exitval = 32;
		goto endmii;
	}
readchunk:
	fread(&chk,1,8,f);	// read size and type first
	chk.length = endianswap(chk.length);	// swap from BE
	if ( (nchunk == 0) && strncmp(chk.type,"IHDR",4) )
	{
		printf("First chunk is not IHDR.\n");
		exitval = 64;
		goto endmii;
	}
	// (re)allocate data
	if ( chk.data ) chk.data = realloc(chk.data,chk.length);
	else chk.data = malloc(chk.length);
	fread(chk.data,1,chk.length,f);	// read data
	fread(&(chk.crc),1,4,f);	// read CRC
	chk.crc = endianswap(chk.crc);
	curcrc = crc32(0,chk.type,4);
	if ( chk.length ) curcrc = crc32(curcrc,chk.data,chk.length);
	validcrc = (curcrc == chk.crc);
	printf("%.4s chunk of length %u (CRC32: %08X, %s).\n",chk.type,chk.length,chk.crc,validcrc?"OK":"FAILED");
	if ( !validcrc ) printf("  CRC32 ACTUAL: %08X\n",curcrc);
	else readchunkdata(&chk);
	nchunk++;
	if ( strncmp(chk.type,"IEND",4) ) goto readchunk;
	long cpos = ftell(f);
	fseek(f,0,SEEK_END);
	long epos = ftell(f);
	if ( epos > cpos ) printf("%lu bytes of extra data after IEND.\n",epos-cpos);
endmii:
	if ( f ) fclose(f);
	if ( chk.data ) free(chk.data);
	return exitval;
}