<?php

namespace App\Beat;

class Decoder {

	public function __construct($source) {
		$this->source = $source;
	}

	public function applyPatch($patch) {
		$this->patch = $patch;
		$this->patchCursor = 0;
		$this->outputOffset = 0;
		$this->sourceRelativeOffset = 0;
		$this->targetRelativeOffset = 0;

		$this->readHeader();
		while ($this->hasAction()) {
			$this->handleAction();
		}

		$sourceCrc = $this->read32();
		$targetCrc = $this->read32();
		$patchCrc = $this->read32();

		if (crc32($this->source) != $sourceCrc) {
			throw new Exception('source checksum mismatch');
		}
		if (crc32($this->target) != $targetCrc) {
			throw new Exception('target checksum mismatch');
		}
		if (crc32(substr($this->patch, 0, -4)) != $patchCrc) {
			throw new Exception('patch checksum mismatch');
		}

		return $this->target;
	}


	private function readHeader() {
		$header = $this->readString(4);
		if ($header != 'BPS1') {
			throw new Exception('invalid header: '.$header);
		}

		$sourceSize = $this->readNumber();
		$targetSize = $this->readNumber();
		$metadataSize = $this->readNumber();
		$this->metadata = $this->readString($metadataSize);
		if ($sourceSize != strlen($this->source)) {
			throw new Exception('source size mismatch. source: '.strlen($this->source).', patch: '.$sourceSize);
		}
		$this->target = str_repeat("\0", $targetSize);
	}

	private function hasAction() {
		return $this->patchCursor < strlen($this->patch) - 12;
	}

	private function handleAction() {
		$data = $this->readNumber();
		$command = $data & 3;
		$length = ($data >> 2) + 1;
		switch ($command) {
			case 0:
				$this->handleSourceRead($length);
				break;
			case 1:
				$this->handleTargetRead($length);
				break;
			case 2:
				$this->handleSourceCopy($length);
				break;
			case 3:
				$this->handleTargetCopy($length);
				break;
		}
	}

	private function handleSourceRead($length) {
		while ($length--) {
			$this->target[$this->outputOffset] = $this->source[$this->outputOffset];
			++$this->outputOffset;
		}
	}

	private function handleTargetRead($length) {
		while ($length--) {
			$this->target[$this->outputOffset++] = $this->readByte();
		}
	}

	private function handleSourceCopy($length) {
		$this->sourceRelativeOffset += $this->readSignedNumber();
		while ($length--) {
			$this->target[$this->outputOffset++] = $this->source[$this->sourceRelativeOffset++];
		}
	}

	private function handleTargetCopy($length) {
		$this->targetRelativeOffset += $this->readSignedNumber();
		while ($length--) {
			$this->target[$this->outputOffset++] = $this->target[$this->targetRelativeOffset++];
		}
	}

	private function read32() {
		$a = ord($this->readByte());
		$b = ord($this->readByte());
		$c = ord($this->readByte());
		$d = ord($this->readByte());
		return $a | ($b << 8) | ($c << 16) | ($d << 24);
	}

	private function readByte() {
		return $this->patch[$this->patchCursor++];
	}

	private function readNumber() {
		$data = 0;
		$shift = 1;
		for ($i = 0; $i < 16; ++$i) {
			$x = ord($this->readByte());
			$data += ($x & 0x7f) * $shift;
			if ($x & 0x80) break;
			$shift <<= 7;
			$data += $shift;
		}
		return $data;
	}

	private function readSignedNumber() {
		$data = $this->readNumber();
		return ($data & 1 ? -1 : 1) * ($data >> 1);
	}

	private function readString($length) {
		$string = substr($this->patch, $this->patchCursor, $length);
		$this->patchCursor += $length;
		return $string;
	}


	private $source;
	private $patch = '';
	private $target = '';
	private $metadata = '';

	private $patchCursor = 0;
	private $outputOffset = 0;
	private $sourceRelativeOffset = 0;
	private $targetRelativeOffset = 0;

}