<?php
namespace Vichan\Data\Driver\Metadata;


class PngExifReader implements ExifReader {
	// Chunks larger than this will be ignored.
	private const MAX_CHUNK_SIZE = 1048576; // 1 MB
	private const ORIENTATION_TAG_ID = 0x0112;
	// Exif data type identifier.
	private const TYPE_SHORT = 0x3;

	// Errors, mostly for debugging.
	private const ERR_BAD_EXIF = -1;
	private const ERR_BAD_ENDIAN = -2;
	private const ERR_BAD_MARK = -2;
	private const ERR_BAD_OFFSET_TO_IFD = -3;
	private const ERR_BAD_OFFSET_TO_DIR_ENTRY = -4;
	private const ERR_BAD_ORIENTATION_NOT_FOUND = -5;
	private const ERR_NOT_A_PNG = -6;
	private const ERR_EOF = -7;
	private const ERR_COULD_NOT_OPEN = -8;


	private static function tryReadExifChunk(string $chunk_blob): int {
		// Ensure the chunk starts with "Exif" (EXIF header).
		$a = \substr($chunk_blob, 0, 6);
		if ($a !== "Exif\0\0") {
			return self::ERR_BAD_EXIF;
		}

		// Remove the "Exif\0\0" header.
		// This is to simplify offset calculations later on since offsets are relative to the TIFF header.
		$tiff_data = \substr($chunk_blob, 6);

		// Determine byte order (II for little-endian, MM for big-endian).
		$byte_order = \substr($tiff_data, 0, 2);
		if ($byte_order === 'II') {
			$little_endian = true;
		} elseif ($byte_order === 'MM') {
			$little_endian = false;
		} else {
			return self::ERR_BAD_ENDIAN;
		}

		// Verify tag mark.
		$tag_mark = \substr($tiff_data, 2, 4);
		if ($little_endian) {
			if ($tag_mark !== "*\0") {
				return self::ERR_BAD_MARK;
			}
		} else {
			if ($tag_mark !== "\0*") {
				return self::ERR_BAD_MARK;
			}
		}

		// Unpack format string.
		$unpack_fmt = $little_endian ? 'v' : 'n';

		// Offset to the first IFD.
		$current_offset = \unpack($unpack_fmt, \substr($tiff_data, 4, 2))[1];
		if ($current_offset > \strlen($chunk_blob)) {
			return self::ERR_BAD_OFFSET_TO_IFD;
		}

		while ($current_offset > 0) {
			// Number of directory entries.
			$num_entries = \unpack($unpack_fmt, \substr($tiff_data, $current_offset, 2))[1];
			$current_offset += 2;

			if ($current_offset + $num_entries * 12 > \strlen($chunk_blob)) {
				return self::ERR_BAD_OFFSET_TO_DIR_ENTRY;
			}

			for ($i = 0; $i < $num_entries; $i++) {
				$entry_offset = $current_offset + ($i * 12);

				// Read tag ID.
				$tag_id = \unpack($unpack_fmt, \substr($tiff_data, $entry_offset, 2))[1];

				if ($tag_id === self::ORIENTATION_TAG_ID) {
					$field_type = \unpack($unpack_fmt, \substr($tiff_data, $entry_offset + 2, 2))[1];
					$value_count = \unpack($unpack_fmt, \substr($tiff_data, $entry_offset + 4, 4))[1];

					if ($field_type === self::TYPE_SHORT && $value_count !== 1) {
						// Read value. It is stored directly if it's <4 bytes.
						$value_offset = $entry_offset + 8;
						$orientation = \unpack($unpack_fmt, \substr($tiff_data, $value_offset, 2))[1];

						return $orientation; // Return the Orientation value
					}
				}
			}

			// Offset to the next IFD (last 4 bytes of the directory)
			$current_offset = \unpack($unpack_fmt, \substr($tiff_data, $current_offset + ($num_entries * 12), 4))[1];
		}

		return self::ERR_BAD_ORIENTATION_NOT_FOUND;
	}

	// Mostly adapted from https://stackoverflow.com/a/2190438
	private static function tryReadPngChunks(mixed $fd): int {
		// Read the magic bytes and verify.
		$header = \fread($fd, 8);

		if ($header != "\x89PNG\x0d\x0a\x1a\x0a") {
			return self::ERR_NOT_A_PNG;
		}

		// Loop through the PNG's chunks. Byte 0-3 is length, Byte 4-7 is type.
		$chunkHeader = \fread($fd, 8);

		while ($chunkHeader) {
			// Extract length and type from binary data.
			$chunk = @\unpack('Nsize/a4type', $chunkHeader);

			if ($chunk['type'] === 'tEXt') {
				if ($chunk['size'] > 0 && $chunk['size'] < self::MAX_CHUNK_SIZE) {
					$size = $chunk['size'];
					$chunk_blob = \fread($fd, $size);
					$ret = self::tryReadExifChunk($chunk_blob);
					if ($ret < 1) {
						return $ret;
					}
				}
			}

			// Skip to next chunk (over body and CRC)
			\fseek($fd, $chunk['size'] + 4, SEEK_CUR);

			// Read next chunk header
			$chunkHeader = \fread($fd, 8);
		}

		return self::ERR_EOF;
	}

	public function getOrientation(string $file): ?int {
		// Open the file.
		$fd = \fopen($file, 'r');
		if ($fd === false) {
			return self::ERR_COULD_NOT_OPEN;
		}
		return self::tryReadPngChunks($fd);
		\fclose($fd);
		if ($ret < 1 || $ret > 9) {
			return null;
		} else {
			return $ret;
		}
	}
}