cosmopolitan/libc/str/getzipcdir.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/zip.h"

/**
 * Locates End Of Central Directory record in ZIP file.
 *
 * The ZIP spec says this header can be anywhere in the last 64kb.
 * We search it backwards for the ZIP-64 "PK♠♠" magic number. If that's
 * not found, then we search again for the original "PK♣♠" magnum. The
 * caller needs to check the first four bytes of the returned value to
 * determine whether to use ZIP_CDIR_xxx() or ZIP_CDIR64_xxx() macros.
 *
 * @param p points to file memory
 * @param n is byte size of file
 * @return pointer to EOCD64 or EOCD, or NULL if not found
 */
uint8_t *GetZipCdir(const uint8_t *p, size_t n) {
  size_t i, j;
  if (n >= kZipCdirHdrMinSize) {
    i = n - kZipCdirHdrMinSize;
    do {
      if (READ32LE(p + i) == kZipCdir64HdrMagic && IsZipCdir64(p, n, i)) {
        return (/*unconst*/ uint8_t *)(p + i);
      } else if (READ32LE(p + i) == kZipCdirHdrMagic && IsZipCdir32(p, n, i)) {
        j = i;
        do {
          if (READ32LE(p + j) == kZipCdir64HdrMagic && IsZipCdir64(p, n, j)) {
            return (/*unconst*/ uint8_t *)(p + j);
          }
        } while (j-- && i - j < 64 * 1024);
        return (/*unconst*/ uint8_t *)(p + i);
      }
    } while (i--);
  }
  return NULL;
}