app/Beat/Decoder.php

   1 <?php
   2
   3 namespace App\Beat;
   4
   5 class Decoder {
   6
   7         public function __construct($source) {
   8                 $this->source = $source;
   9         }
  10
  11         public function applyPatch($patch) {
  12                 $this->patch = $patch;
  13                 $this->patchCursor = 0;
  14                 $this->outputOffset = 0;
  15                 $this->sourceRelativeOffset = 0;
  16                 $this->targetRelativeOffset = 0;
  17
  18                 $this->readHeader();
  19                 while ($this->hasAction()) {
  20                         $this->handleAction();
  21                 }
  22
  23                 $sourceCrc = $this->read32();
  24                 $targetCrc = $this->read32();
  25                 $patchCrc = $this->read32();
  26
  27                 if (crc32($this->source) != $sourceCrc) {
  28                         throw new Exception('source checksum mismatch');
  29                 }
  30                 if (crc32($this->target) != $targetCrc) {
  31                         throw new Exception('target checksum mismatch');
  32                 }
  33                 if (crc32(substr($this->patch, 0, -4)) != $patchCrc) {
  34                         throw new Exception('patch checksum mismatch');
  35                 }
  36
  37                 return $this->target;
  38         }
  39
  40
  41         private function readHeader() {
  42                 $header = $this->readString(4);
  43                 if ($header != 'BPS1') {
  44                         throw new Exception('invalid header: '.$header);
  45                 }
  46
  47                 $sourceSize = $this->readNumber();
  48                 $targetSize = $this->readNumber();
  49                 $metadataSize = $this->readNumber();
  50                 $this->metadata = $this->readString($metadataSize);
  51                 if ($sourceSize != strlen($this->source)) {
  52                         throw new Exception('source size mismatch. source: '.strlen($this->source).', patch: '.$sourceSize);
  53                 }
  54                 $this->target = str_repeat("\0", $targetSize);
  55         }
  56
  57         private function hasAction() {
  58                 return $this->patchCursor < strlen($this->patch) - 12;
  59         }
  60
  61         private function handleAction() {
  62                 $data = $this->readNumber();
  63                 $command = $data & 3;
  64                 $length = ($data >> 2) + 1;
  65                 switch ($command) {
  66                         case 0:
  67                                 $this->handleSourceRead($length);
  68                                 break;
  69                         case 1:
  70                                 $this->handleTargetRead($length);
  71                                 break;
  72                         case 2:
  73                                 $this->handleSourceCopy($length);
  74                                 break;
  75                         case 3:
  76                                 $this->handleTargetCopy($length);
  77                                 break;
  78                 }
  79         }
  80
  81         private function handleSourceRead($length) {
  82                 while ($length--) {
  83                         $this->target[$this->outputOffset] = $this->source[$this->outputOffset];
  84                         ++$this->outputOffset;
  85                 }
  86         }
  87
  88         private function handleTargetRead($length) {
  89                 while ($length--) {
  90                         $this->target[$this->outputOffset++] = $this->readByte();
  91                 }
  92         }
  93
  94         private function handleSourceCopy($length) {
  95                 $this->sourceRelativeOffset += $this->readSignedNumber();
  96                 while ($length--) {
  97                         $this->target[$this->outputOffset++] = $this->source[$this->sourceRelativeOffset++];
  98                 }
  99         }
 100
 101         private function handleTargetCopy($length) {
 102                 $this->targetRelativeOffset += $this->readSignedNumber();
 103                 while ($length--) {
 104                         $this->target[$this->outputOffset++] = $this->target[$this->targetRelativeOffset++];
 105                 }
 106         }
 107
 108         private function read32() {
 109                 $a = ord($this->readByte());
 110                 $b = ord($this->readByte());
 111                 $c = ord($this->readByte());
 112                 $d = ord($this->readByte());
 113                 return $a | ($b << 8) | ($c << 16) | ($d << 24);
 114         }
 115
 116         private function readByte() {
 117                 return $this->patch[$this->patchCursor++];
 118         }
 119
 120         private function readNumber() {
 121                 $data = 0;
 122                 $shift = 1;
 123                 for ($i = 0; $i < 16; ++$i) {
 124                         $x = ord($this->readByte());
 125                         $data += ($x & 0x7f) * $shift;
 126                         if ($x & 0x80) break;
 127                         $shift <<= 7;
 128                         $data += $shift;
 129                 }
 130                 return $data;
 131         }
 132
 133         private function readSignedNumber() {
 134                 $data = $this->readNumber();
 135                 return ($data & 1 ? -1 : 1) * ($data >> 1);
 136         }
 137
 138         private function readString($length) {
 139                 $string = substr($this->patch, $this->patchCursor, $length);
 140                 $this->patchCursor += $length;
 141                 return $string;
 142         }
 143
 144
 145         private $source;
 146         private $patch = '';
 147         private $target = '';
 148         private $metadata = '';
 149
 150         private $patchCursor = 0;
 151         private $outputOffset = 0;
 152         private $sourceRelativeOffset = 0;
 153         private $targetRelativeOffset = 0;
 154
 155 }