121 lines
3.3 KiB
C
121 lines
3.3 KiB
C
/* id3v2.c - functions to handle ID3v2 tags prepended to shn files
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* Copyright (C) 2000-2004 Jason Jordan <shnutils@freeshell.org>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*/
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/*
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* $Id: id3v2.c 19 2005-06-07 04:16:15Z vspader $
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*/
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#include <stdio.h>
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#include "shorten.h"
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#ifdef HAVE_CONFIG_H
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#include "config.h"
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#endif
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#define ID3V2_MAGIC "ID3"
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typedef struct {
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char magic[3];
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unsigned char version[2];
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unsigned char flags[1];
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unsigned char size[4];
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} _id3v2_header;
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int tagcmp(char *got,char *expected)
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/* compare got against expected, up to the length of expected */
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{
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int i;
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for (i=0;*(expected+i);i++) {
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if (*(got+i) != *(expected+i))
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return i+1;
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}
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return 0;
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}
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unsigned long synchsafe_int_to_ulong(unsigned char *buf)
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/* converts 4 bytes stored in synchsafe integer format to an unsigned long */
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{
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return (unsigned long)(((buf[0] & 0x7f) << 21) | ((buf[1] & 0x7f) << 14) | ((buf[2] & 0x7f) << 7) | (buf[3] & 0x7f));
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}
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static unsigned long check_for_id3v2_tag(FILE *f)
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{
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_id3v2_header id3v2_header;
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unsigned long tag_size;
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/* read an ID3v2 header's size worth of data */
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if (sizeof(_id3v2_header) != fread(&id3v2_header,1,sizeof(_id3v2_header),f)) {
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return 0;
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}
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/* verify this is an ID3v2 header */
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if (tagcmp(id3v2_header.magic,ID3V2_MAGIC) ||
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0xff == id3v2_header.version[0] || 0xff == id3v2_header.version[1] ||
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0x80 <= id3v2_header.size[0] || 0x80 <= id3v2_header.size[1] ||
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0x80 <= id3v2_header.size[2] || 0x80 <= id3v2_header.size[3])
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{
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return 0;
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}
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/* calculate and return ID3v2 tag size */
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tag_size = synchsafe_int_to_ulong(id3v2_header.size);
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return tag_size;
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}
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FILE *shn_open_and_discard_id3v2_tag(char *filename,int *file_has_id3v2_tag,long *id3v2_tag_size)
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/* opens a file, and if it contains an ID3v2 tag, skips past it */
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{
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FILE *f;
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unsigned long tag_size;
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if (NULL == (f = fopen(filename,"rb"))) {
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return NULL;
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}
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if (file_has_id3v2_tag)
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*file_has_id3v2_tag = 0;
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if (id3v2_tag_size)
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*id3v2_tag_size = 0;
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/* check for ID3v2 tag on input */
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if (0 == (tag_size = check_for_id3v2_tag(f))) {
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fclose(f);
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return fopen(filename,"rb");
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}
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if (file_has_id3v2_tag)
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*file_has_id3v2_tag = 2;
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if (id3v2_tag_size)
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*id3v2_tag_size = (long)(tag_size + sizeof(_id3v2_header));
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fprintf(stderr, "Discarding %lu-byte ID3v2 tag at beginning of file '%s'.",tag_size+sizeof(_id3v2_header),filename);
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if (0 != fseek(f,(long)tag_size,SEEK_CUR)) {
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fprintf(stderr, "Error while discarding ID3v2 tag in file '%s'.",filename);
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fclose(f);
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return fopen(filename,"rb");
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}
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return f;
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}
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