<?php
/**
 * Minimal ZIP writer in plain PHP — no ZipArchive, no Composer.
 *
 * Only what a gallery archive needs: **stored** entries (method 0, no
 * compression) and ZIP64. Photos are JPEG/RAW and do not compress, so deflating
 * them would burn CPU for nothing; storing them means an entry's bytes are a
 * byte-for-byte copy of the S3 object, which is what lets the builder stream
 * each photo straight through (see app/archive.php).
 *
 * The functions here only produce byte strings — they never touch S3, and the
 * only file handle they see is the one zip_patch_local_header() writes back to.
 *
 * Layout produced:
 *
 *   [local header][file bytes] … [central directory][ZIP64 EOCD + locator][EOCD]
 *
 * Local headers are written **before** the file's bytes are known, so their CRC
 * and size fields start as placeholders and are patched afterwards
 * (zip_patch_local_header). That is why every local header carries a ZIP64 extra
 * field unconditionally: the layout has to be fixed before the size is known,
 * and a >4 GB entry must not require a different one. Central directory entries
 * are built at the end, when the real values are known, so those use ZIP64 only
 * where a field actually overflows.
 *
 * No data descriptors are used (the patch makes them unnecessary), which keeps
 * the output readable by the widest range of tools.
 */

declare(strict_types=1);

/** Fixed part of a local file header, before filename and extra field. */
const ZIP_LOCAL_FIXED = 30;
/** ZIP64 extra field in a local header: id + size + 8-byte sizes. */
const ZIP_ZIP64_EXTRA = 20;
/** Value meaning "look in the ZIP64 extra field for the real number". */
const ZIP_MAX32 = 0xFFFFFFFF;

/** Convert a Unix timestamp to the DOS date/time pair ZIP stores. */
function zip_dos_time(int $timestamp): array
{
    $t = getdate($timestamp);
    // DOS time cannot represent anything before 1980; clamp rather than wrap.
    if ($t['year'] < 1980) {
        return [0, 0x21]; // 1980-01-01 00:00:00
    }
    $time = ($t['hours'] << 11) | ($t['minutes'] << 5) | (int)($t['seconds'] / 2);
    $date = (($t['year'] - 1980) << 9) | ($t['mon'] << 5) | $t['mday'];
    return [$time, $date];
}

/**
 * Total length of the local header for $name, i.e. how far the file's bytes sit
 * from the start of the entry. Callers need this to track archive offsets.
 */
function zip_local_header_size(string $name): int
{
    return ZIP_LOCAL_FIXED + strlen($name) + ZIP_ZIP64_EXTRA;
}

/**
 * Local file header with placeholder CRC and sizes, to be patched once the
 * file's bytes have been written (zip_patch_local_header).
 *
 * Flag bit 11 marks the filename as UTF-8 so "Straße.jpg" survives; without it
 * readers fall back to CP437 and mangle anything non-ASCII.
 */
function zip_local_header(string $name, int $mtime): string
{
    [$time, $date] = zip_dos_time($mtime);

    return pack('V', 0x04034b50)   // local file header signature
        . pack('v', 45)            // version needed to extract: 4.5 (ZIP64)
        . pack('v', 0x0800)        // flags: UTF-8 filename
        . pack('v', 0)             // method: stored
        . pack('v', $time)
        . pack('v', $date)
        . pack('V', 0)             // CRC-32      — patched later
        . pack('V', ZIP_MAX32)     // compressed size   → ZIP64 extra
        . pack('V', ZIP_MAX32)     // uncompressed size → ZIP64 extra
        . pack('v', strlen($name))
        . pack('v', ZIP_ZIP64_EXTRA)
        . $name
        . pack('v', 0x0001)        // ZIP64 extended information extra field
        . pack('v', 16)            // ... holding two 8-byte sizes
        . pack('P', 0)             // uncompressed size — patched later
        . pack('P', 0);            // compressed size   — patched later
}

/**
 * Fill in the CRC and size a local header was written without.
 *
 * $entryOffset is the position of the header's signature within $fh. The handle
 * is left at end-of-file so the caller can carry on appending.
 *
 * @param resource $fh
 */
function zip_patch_local_header($fh, int $entryOffset, string $name, int $crc, int $size): void
{
    // CRC-32 sits 14 bytes into the fixed header; the two ZIP64 sizes sit 4
    // bytes into the extra field, which follows the filename.
    fseek($fh, $entryOffset + 14);
    fwrite($fh, pack('V', $crc));

    fseek($fh, $entryOffset + ZIP_LOCAL_FIXED + strlen($name) + 4);
    fwrite($fh, pack('P', $size) . pack('P', $size));

    fseek($fh, 0, SEEK_END);
}

/**
 * One central directory entry.
 *
 * $entry: ['name' => string, 'crc' => int, 'size' => int, 'offset' => int,
 *          'mtime' => int] — offset being the entry's local header position.
 *
 * ZIP64 fields appear only when a value genuinely overflows 32 bits, and the
 * spec requires them in a fixed order (uncompressed, compressed, offset), each
 * present only if its fixed-record counterpart was set to 0xFFFFFFFF.
 */
function zip_central_entry(array $entry): string
{
    [$time, $date] = zip_dos_time($entry['mtime']);
    $name = $entry['name'];
    $size = $entry['size'];
    $offset = $entry['offset'];

    $bigSize = $size >= ZIP_MAX32;
    $bigOffset = $offset >= ZIP_MAX32;

    $extra = '';
    if ($bigSize) {
        $extra .= pack('P', $size) . pack('P', $size);
    }
    if ($bigOffset) {
        $extra .= pack('P', $offset);
    }
    if ($extra !== '') {
        $extra = pack('v', 0x0001) . pack('v', strlen($extra)) . $extra;
    }

    return pack('V', 0x02014b50)   // central file header signature
        . pack('v', 45)            // version made by: 4.5, MS-DOS
        . pack('v', 45)            // version needed to extract
        . pack('v', 0x0800)        // flags: UTF-8 filename
        . pack('v', 0)             // method: stored
        . pack('v', $time)
        . pack('v', $date)
        . pack('V', $entry['crc'])
        . pack('V', $bigSize ? ZIP_MAX32 : $size)
        . pack('V', $bigSize ? ZIP_MAX32 : $size)
        . pack('v', strlen($name))
        . pack('v', strlen($extra))
        . pack('v', 0)             // file comment length
        . pack('v', 0)             // disk number start
        . pack('v', 0)             // internal file attributes
        . pack('V', 0)             // external file attributes
        . pack('V', $bigOffset ? ZIP_MAX32 : $offset)
        . $name
        . $extra;
}

/**
 * The archive trailer: ZIP64 end-of-central-directory record, its locator, and
 * the classic EOCD.
 *
 * The ZIP64 pair is always emitted. A reader that predates ZIP64 skips straight
 * to the classic EOCD at the end of the file and works as long as nothing
 * overflows; one that supports ZIP64 finds the locator immediately before it and
 * gets the wide values. Emitting both is what lets a 300 MB archive and a 40 GB
 * archive share a single code path.
 */
function zip_end_of_central_directory(int $count, int $cdSize, int $cdOffset): string
{
    $zip64Eocd = pack('V', 0x06064b50)      // ZIP64 EOCD signature
        . pack('P', 44)                     // size of the rest of this record
        . pack('v', 45)                     // version made by
        . pack('v', 45)                     // version needed
        . pack('V', 0)                      // this disk
        . pack('V', 0)                      // disk with start of CD
        . pack('P', $count)                 // entries on this disk
        . pack('P', $count)                 // entries total
        . pack('P', $cdSize)
        . pack('P', $cdOffset);

    $locator = pack('V', 0x07064b50)        // ZIP64 EOCD locator signature
        . pack('V', 0)                      // disk with the ZIP64 EOCD
        . pack('P', $cdOffset + $cdSize)    // its offset: right after the CD
        . pack('V', 1);                     // total number of disks

    $eocd = pack('V', 0x06054b50)           // EOCD signature
        . pack('v', 0)                      // this disk
        . pack('v', 0)                      // disk with start of CD
        . pack('v', min($count, 0xFFFF))
        . pack('v', min($count, 0xFFFF))
        . pack('V', min($cdSize, ZIP_MAX32))
        . pack('V', min($cdOffset, ZIP_MAX32))
        . pack('v', 0);                     // archive comment length

    return $zip64Eocd . $locator . $eocd;
}

/**
 * Make $name unique within the archive, remembering what has been used.
 *
 * Gallery images keep their original filename while their S3 key gets a random
 * token, so two cameras both producing DSC_0001.jpg is entirely normal — and a
 * ZIP with duplicate names silently loses files on extraction.
 */
function zip_dedupe_name(string $name, array &$seen): string
{
    $key = strtolower($name);
    if (!isset($seen[$key])) {
        $seen[$key] = 1;
        return $name;
    }

    $ext = pathinfo($name, PATHINFO_EXTENSION);
    $base = $ext !== '' ? substr($name, 0, -(strlen($ext) + 1)) : $name;
    do {
        $candidate = $base . ' (' . (++$seen[$key]) . ')' . ($ext !== '' ? '.' . $ext : '');
    } while (isset($seen[strtolower($candidate)]));

    $seen[strtolower($candidate)] = 1;
    return $candidate;
}