/* gunzip.c
 * PUBLIC DOMAIN - Jon Mayo - August 30, 2006
 * dumps the Deflate stream from a GZIP stream (RFC 1951) */

/* TODO: 
 * accumulate crc16 for the header (FHCRC)
 */
#include <stdio.h>
#include <stdlib.h>

#define FTEXT 1
#define FHCRC 2
#define FEXTRA 4
#define FNAME 8
#define FCOMMENT 16

int main() {
	unsigned char header[10];
	unsigned char buf[333];
	int buflen;

	if(fread(header, sizeof header, 1, stdin)!=1) {
		perror("header");
		return EXIT_FAILURE;
	}

	if(header[0]!=31 && header[1]!=139) {
		fprintf(stderr, "Not a GZIP\n");
		return EXIT_FAILURE;
	}
	
	if(header[3] & FEXTRA) {
		unsigned char len_data[2];
		unsigned char *data;
		unsigned fextra_len;
		if(fread(len_data, sizeof len_data, 1, stdin)!=1) {
			perror("fextra");
			return EXIT_FAILURE;
		}
		fextra_len=len_data[0] | len_data[1]<<8;
		data=malloc(fextra_len);
		if(fread(data, fextra_len, 1, stdin)!=1) {
			perror("fextra");
			return EXIT_FAILURE;
		}
		free(data);
		fprintf(stderr, "FEXTRA %d\n", fextra_len);
	}

	if(header[3] & FNAME) {
		unsigned char name[64];
		int i;
		int ch;
		int fname_len=0;
		for(i=0;i<sizeof name;i++) {
			ch=fgetc(stdin);
			if(ch==EOF) {
				perror("fname");
				return EXIT_FAILURE;
			}
			fname_len++;
			if(ch==0) break;
			name[i]=ch;
		}
		name[i]=0;
		/* if the name was truncated, consume the rest of the fname */
		while(!ch) {
			ch=fgetc(stdin);
			if(ch==EOF) {
				perror("fname");
				return EXIT_FAILURE;
			}
			fname_len++;
		}
		fprintf(stderr, "FNAME %d\n", fname_len);
	}
	
	/* swallow the comment and throw it away */
	if(header[3] & FCOMMENT) {
		int ch;
		unsigned fcomment_len=0;
		while((ch=fgetc(stdin))) {
			if(ch==EOF) {
				perror("fcomment");
				return EXIT_FAILURE;
			}
			fcomment_len++;
		}
		fprintf(stderr, "FCOMMENT %d\n", fcomment_len);
	}
	
	if(header[3] & FHCRC) {
		unsigned char hcrc_data[2];
		if(fread(hcrc_data, sizeof hcrc_data, 1, stdin)!=1) {
			perror("fhcrc");
			return EXIT_FAILURE;
		}
		fprintf(stderr, "FHCRC %02X%02X\n", hcrc_data[0], hcrc_data[1]);
	}

	fprintf(stderr, "Method = %d\n", header[2]);
	/* dump inflate data, leaves CRC32 and ISIZE fields on the end */
	while((buflen=fread(buf, 1, sizeof buf, stdin))>0) {
		fwrite(buf, 1, buflen, stdout);
	}
	if(buflen<0) {
		perror("fread()");
		return EXIT_FAILURE;
	}
	return 0;
}