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   1  <?php
   2  
   3  /**
   4   * Class for converting between different unit-lengths as specified by
   5   * CSS.
   6   */
   7  class HTMLPurifier_UnitConverter
   8  {
   9  
  10      const ENGLISH = 1;
  11      const METRIC = 2;
  12      const DIGITAL = 3;
  13  
  14      /**
  15       * Units information array. Units are grouped into measuring systems
  16       * (English, Metric), and are assigned an integer representing
  17       * the conversion factor between that unit and the smallest unit in
  18       * the system. Numeric indexes are actually magical constants that
  19       * encode conversion data from one system to the next, with a O(n^2)
  20       * constraint on memory (this is generally not a problem, since
  21       * the number of measuring systems is small.)
  22       */
  23      protected static $units = array(
  24          self::ENGLISH => array(
  25              'px' => 3, // This is as per CSS 2.1 and Firefox. Your mileage may vary
  26              'pt' => 4,
  27              'pc' => 48,
  28              'in' => 288,
  29              self::METRIC => array('pt', '0.352777778', 'mm'),
  30          ),
  31          self::METRIC => array(
  32              'mm' => 1,
  33              'cm' => 10,
  34              self::ENGLISH => array('mm', '2.83464567', 'pt'),
  35          ),
  36      );
  37  
  38      /**
  39       * Minimum bcmath precision for output.
  40       */
  41      protected $outputPrecision;
  42  
  43      /**
  44       * Bcmath precision for internal calculations.
  45       */
  46      protected $internalPrecision;
  47  
  48      /**
  49       * Whether or not BCMath is available
  50       */
  51      private $bcmath;
  52  
  53      public function __construct($output_precision = 4, $internal_precision = 10, $force_no_bcmath = false) {
  54          $this->outputPrecision = $output_precision;
  55          $this->internalPrecision = $internal_precision;
  56          $this->bcmath = !$force_no_bcmath && function_exists('bcmul');
  57      }
  58  
  59      /**
  60       * Converts a length object of one unit into another unit.
  61       * @param HTMLPurifier_Length $length
  62       *      Instance of HTMLPurifier_Length to convert. You must validate()
  63       *      it before passing it here!
  64       * @param string $to_unit
  65       *      Unit to convert to.
  66       * @note
  67       *      About precision: This conversion function pays very special
  68       *      attention to the incoming precision of values and attempts
  69       *      to maintain a number of significant figure. Results are
  70       *      fairly accurate up to nine digits. Some caveats:
  71       *          - If a number is zero-padded as a result of this significant
  72       *            figure tracking, the zeroes will be eliminated.
  73       *          - If a number contains less than four sigfigs ($outputPrecision)
  74       *            and this causes some decimals to be excluded, those
  75       *            decimals will be added on.
  76       */
  77      public function convert($length, $to_unit) {
  78  
  79          if (!$length->isValid()) return false;
  80  
  81          $n    = $length->getN();
  82          $unit = $length->getUnit();
  83  
  84          if ($n === '0' || $unit === false) {
  85              return new HTMLPurifier_Length('0', false);
  86          }
  87  
  88          $state = $dest_state = false;
  89          foreach (self::$units as $k => $x) {
  90              if (isset($x[$unit])) $state = $k;
  91              if (isset($x[$to_unit])) $dest_state = $k;
  92          }
  93          if (!$state || !$dest_state) return false;
  94  
  95          // Some calculations about the initial precision of the number;
  96          // this will be useful when we need to do final rounding.
  97          $sigfigs = $this->getSigFigs($n);
  98          if ($sigfigs < $this->outputPrecision) $sigfigs = $this->outputPrecision;
  99  
 100          // BCMath's internal precision deals only with decimals. Use
 101          // our default if the initial number has no decimals, or increase
 102          // it by how ever many decimals, thus, the number of guard digits
 103          // will always be greater than or equal to internalPrecision.
 104          $log = (int) floor(log(abs($n), 10));
 105          $cp = ($log < 0) ? $this->internalPrecision - $log : $this->internalPrecision; // internal precision
 106  
 107          for ($i = 0; $i < 2; $i++) {
 108  
 109              // Determine what unit IN THIS SYSTEM we need to convert to
 110              if ($dest_state === $state) {
 111                  // Simple conversion
 112                  $dest_unit = $to_unit;
 113              } else {
 114                  // Convert to the smallest unit, pending a system shift
 115                  $dest_unit = self::$units[$state][$dest_state][0];
 116              }
 117  
 118              // Do the conversion if necessary
 119              if ($dest_unit !== $unit) {
 120                  $factor = $this->div(self::$units[$state][$unit], self::$units[$state][$dest_unit], $cp);
 121                  $n = $this->mul($n, $factor, $cp);
 122                  $unit = $dest_unit;
 123              }
 124  
 125              // Output was zero, so bail out early. Shouldn't ever happen.
 126              if ($n === '') {
 127                  $n = '0';
 128                  $unit = $to_unit;
 129                  break;
 130              }
 131  
 132              // It was a simple conversion, so bail out
 133              if ($dest_state === $state) {
 134                  break;
 135              }
 136  
 137              if ($i !== 0) {
 138                  // Conversion failed! Apparently, the system we forwarded
 139                  // to didn't have this unit. This should never happen!
 140                  return false;
 141              }
 142  
 143              // Pre-condition: $i == 0
 144  
 145              // Perform conversion to next system of units
 146              $n = $this->mul($n, self::$units[$state][$dest_state][1], $cp);
 147              $unit = self::$units[$state][$dest_state][2];
 148              $state = $dest_state;
 149  
 150              // One more loop around to convert the unit in the new system.
 151  
 152          }
 153  
 154          // Post-condition: $unit == $to_unit
 155          if ($unit !== $to_unit) return false;
 156  
 157          // Useful for debugging:
 158          //echo "<pre>n";
 159          //echo "$n\nsigfigs = $sigfigs\nnew_log = $new_log\nlog = $log\nrp = $rp\n</pre>\n";
 160  
 161          $n = $this->round($n, $sigfigs);
 162          if (strpos($n, '.') !== false) $n = rtrim($n, '0');
 163          $n = rtrim($n, '.');
 164  
 165          return new HTMLPurifier_Length($n, $unit);
 166      }
 167  
 168      /**
 169       * Returns the number of significant figures in a string number.
 170       * @param string $n Decimal number
 171       * @return int number of sigfigs
 172       */
 173      public function getSigFigs($n) {
 174          $n = ltrim($n, '0+-');
 175          $dp = strpos($n, '.'); // decimal position
 176          if ($dp === false) {
 177              $sigfigs = strlen(rtrim($n, '0'));
 178          } else {
 179              $sigfigs = strlen(ltrim($n, '0.')); // eliminate extra decimal character
 180              if ($dp !== 0) $sigfigs--;
 181          }
 182          return $sigfigs;
 183      }
 184  
 185      /**
 186       * Adds two numbers, using arbitrary precision when available.
 187       */
 188      private function add($s1, $s2, $scale) {
 189          if ($this->bcmath) return bcadd($s1, $s2, $scale);
 190          else return $this->scale($s1 + $s2, $scale);
 191      }
 192  
 193      /**
 194       * Multiples two numbers, using arbitrary precision when available.
 195       */
 196      private function mul($s1, $s2, $scale) {
 197          if ($this->bcmath) return bcmul($s1, $s2, $scale);
 198          else return $this->scale($s1 * $s2, $scale);
 199      }
 200  
 201      /**
 202       * Divides two numbers, using arbitrary precision when available.
 203       */
 204      private function div($s1, $s2, $scale) {
 205          if ($this->bcmath) return bcdiv($s1, $s2, $scale);
 206          else return $this->scale($s1 / $s2, $scale);
 207      }
 208  
 209      /**
 210       * Rounds a number according to the number of sigfigs it should have,
 211       * using arbitrary precision when available.
 212       */
 213      private function round($n, $sigfigs) {
 214          $new_log = (int) floor(log(abs($n), 10)); // Number of digits left of decimal - 1
 215          $rp = $sigfigs - $new_log - 1; // Number of decimal places needed
 216          $neg = $n < 0 ? '-' : ''; // Negative sign
 217          if ($this->bcmath) {
 218              if ($rp >= 0) {
 219                  $n = bcadd($n, $neg . '0.' .  str_repeat('0', $rp) . '5', $rp + 1);
 220                  $n = bcdiv($n, '1', $rp);
 221              } else {
 222                  // This algorithm partially depends on the standardized
 223                  // form of numbers that comes out of bcmath.
 224                  $n = bcadd($n, $neg . '5' . str_repeat('0', $new_log - $sigfigs), 0);
 225                  $n = substr($n, 0, $sigfigs + strlen($neg)) . str_repeat('0', $new_log - $sigfigs + 1);
 226              }
 227              return $n;
 228          } else {
 229              return $this->scale(round($n, $sigfigs - $new_log - 1), $rp + 1);
 230          }
 231      }
 232  
 233      /**
 234       * Scales a float to $scale digits right of decimal point, like BCMath.
 235       */
 236      private function scale($r, $scale) {
 237          if ($scale < 0) {
 238              // The f sprintf type doesn't support negative numbers, so we
 239              // need to cludge things manually. First get the string.
 240              $r = sprintf('%.0f', (float) $r);
 241              // Due to floating point precision loss, $r will more than likely
 242              // look something like 4652999999999.9234. We grab one more digit
 243              // than we need to precise from $r and then use that to round
 244              // appropriately.
 245              $precise = (string) round(substr($r, 0, strlen($r) + $scale), -1);
 246              // Now we return it, truncating the zero that was rounded off.
 247              return substr($precise, 0, -1) . str_repeat('0', -$scale + 1);
 248          }
 249          return sprintf('%.' . $scale . 'f', (float) $r);
 250      }
 251  
 252  }
 253  
 254  // vim: et sw=4 sts=4