<?php

/**
 * Class for converting between different unit-lengths as specified by
 * CSS.
 */
class HTMLPurifier_UnitConverter {
	const ENGLISH = 1;
	const METRIC = 2;
	const DIGITAL = 3;
	
	/**
	 * Units information array.
	 * Units are grouped into measuring systems
	 * (English, Metric), and are assigned an integer representing
	 * the conversion factor between that unit and the smallest unit in
	 * the system. Numeric indexes are actually magical constants that
	 * encode conversion data from one system to the next, with a O(n^2)
	 * constraint on memory (this is generally not a problem, since
	 * the number of measuring systems is small.)
	 */
	protected static $units = array (
			self::ENGLISH => array (
					'px' => 3, // This is as per CSS 2.1 and Firefox. Your mileage may vary
					'pt' => 4,
					'pc' => 48,
					'in' => 288,
					self::METRIC => array (
							'pt',
							'0.352777778',
							'mm' 
					) 
			),
			self::METRIC => array (
					'mm' => 1,
					'cm' => 10,
					self::ENGLISH => array (
							'mm',
							'2.83464567',
							'pt' 
					) 
			) 
	);
	
	/**
	 * Minimum bcmath precision for output.
	 * @type int
	 */
	protected $outputPrecision;
	
	/**
	 * Bcmath precision for internal calculations.
	 * @type int
	 */
	protected $internalPrecision;
	
	/**
	 * Whether or not BCMath is available.
	 * @type bool
	 */
	private $bcmath;
	public function __construct($output_precision = 4, $internal_precision = 10, $force_no_bcmath = false) {
		$this->outputPrecision = $output_precision;
		$this->internalPrecision = $internal_precision;
		$this->bcmath = ! $force_no_bcmath && function_exists ( 'bcmul' );
	}
	
	/**
	 * Converts a length object of one unit into another unit.
	 * 
	 * @param HTMLPurifier_Length $length
	 *        	Instance of HTMLPurifier_Length to convert. You must validate()
	 *        	it before passing it here!
	 * @param string $to_unit
	 *        	Unit to convert to.
	 * @return HTMLPurifier_Length|bool @note
	 *         About precision: This conversion function pays very special
	 *         attention to the incoming precision of values and attempts
	 *         to maintain a number of significant figure. Results are
	 *         fairly accurate up to nine digits. Some caveats:
	 *         - If a number is zero-padded as a result of this significant
	 *         figure tracking, the zeroes will be eliminated.
	 *         - If a number contains less than four sigfigs ($outputPrecision)
	 *         and this causes some decimals to be excluded, those
	 *         decimals will be added on.
	 */
	public function convert($length, $to_unit) {
		if (! $length->isValid ()) {
			return false;
		}
		
		$n = $length->getN ();
		$unit = $length->getUnit ();
		
		if ($n === '0' || $unit === false) {
			return new HTMLPurifier_Length ( '0', false );
		}
		
		$state = $dest_state = false;
		foreach ( self::$units as $k => $x ) {
			if (isset ( $x [$unit] )) {
				$state = $k;
			}
			if (isset ( $x [$to_unit] )) {
				$dest_state = $k;
			}
		}
		if (! $state || ! $dest_state) {
			return false;
		}
		
		// Some calculations about the initial precision of the number;
		// this will be useful when we need to do final rounding.
		$sigfigs = $this->getSigFigs ( $n );
		if ($sigfigs < $this->outputPrecision) {
			$sigfigs = $this->outputPrecision;
		}
		
		// BCMath's internal precision deals only with decimals. Use
		// our default if the initial number has no decimals, or increase
		// it by how ever many decimals, thus, the number of guard digits
		// will always be greater than or equal to internalPrecision.
		$log = ( int ) floor ( log ( abs ( $n ), 10 ) );
		$cp = ($log < 0) ? $this->internalPrecision - $log : $this->internalPrecision; // internal precision
		
		for($i = 0; $i < 2; $i ++) {
			
			// Determine what unit IN THIS SYSTEM we need to convert to
			if ($dest_state === $state) {
				// Simple conversion
				$dest_unit = $to_unit;
			} else {
				// Convert to the smallest unit, pending a system shift
				$dest_unit = self::$units [$state] [$dest_state] [0];
			}
			
			// Do the conversion if necessary
			if ($dest_unit !== $unit) {
				$factor = $this->div ( self::$units [$state] [$unit], self::$units [$state] [$dest_unit], $cp );
				$n = $this->mul ( $n, $factor, $cp );
				$unit = $dest_unit;
			}
			
			// Output was zero, so bail out early. Shouldn't ever happen.
			if ($n === '') {
				$n = '0';
				$unit = $to_unit;
				break;
			}
			
			// It was a simple conversion, so bail out
			if ($dest_state === $state) {
				break;
			}
			
			if ($i !== 0) {
				// Conversion failed! Apparently, the system we forwarded
				// to didn't have this unit. This should never happen!
				return false;
			}
			
			// Pre-condition: $i == 0
			
			// Perform conversion to next system of units
			$n = $this->mul ( $n, self::$units [$state] [$dest_state] [1], $cp );
			$unit = self::$units [$state] [$dest_state] [2];
			$state = $dest_state;
			
			// One more loop around to convert the unit in the new system.
		}
		
		// Post-condition: $unit == $to_unit
		if ($unit !== $to_unit) {
			return false;
		}
		
		// Useful for debugging:
		// echo "<pre>n";
		// echo "$n\nsigfigs = $sigfigs\nnew_log = $new_log\nlog = $log\nrp = $rp\n</pre>\n";
		
		$n = $this->round ( $n, $sigfigs );
		if (strpos ( $n, '.' ) !== false) {
			$n = rtrim ( $n, '0' );
		}
		$n = rtrim ( $n, '.' );
		
		return new HTMLPurifier_Length ( $n, $unit );
	}
	
	/**
	 * Returns the number of significant figures in a string number.
	 * 
	 * @param string $n
	 *        	Decimal number
	 * @return int number of sigfigs
	 */
	public function getSigFigs($n) {
		$n = ltrim ( $n, '0+-' );
		$dp = strpos ( $n, '.' ); // decimal position
		if ($dp === false) {
			$sigfigs = strlen ( rtrim ( $n, '0' ) );
		} else {
			$sigfigs = strlen ( ltrim ( $n, '0.' ) ); // eliminate extra decimal character
			if ($dp !== 0) {
				$sigfigs --;
			}
		}
		return $sigfigs;
	}
	
	/**
	 * Adds two numbers, using arbitrary precision when available.
	 * 
	 * @param string $s1        	
	 * @param string $s2        	
	 * @param int $scale        	
	 * @return string
	 */
	private function add($s1, $s2, $scale) {
		if ($this->bcmath) {
			return bcadd ( $s1, $s2, $scale );
		} else {
			return $this->scale ( ( float ) $s1 + ( float ) $s2, $scale );
		}
	}
	
	/**
	 * Multiples two numbers, using arbitrary precision when available.
	 * 
	 * @param string $s1        	
	 * @param string $s2        	
	 * @param int $scale        	
	 * @return string
	 */
	private function mul($s1, $s2, $scale) {
		if ($this->bcmath) {
			return bcmul ( $s1, $s2, $scale );
		} else {
			return $this->scale ( ( float ) $s1 * ( float ) $s2, $scale );
		}
	}
	
	/**
	 * Divides two numbers, using arbitrary precision when available.
	 * 
	 * @param string $s1        	
	 * @param string $s2        	
	 * @param int $scale        	
	 * @return string
	 */
	private function div($s1, $s2, $scale) {
		if ($this->bcmath) {
			return bcdiv ( $s1, $s2, $scale );
		} else {
			return $this->scale ( ( float ) $s1 / ( float ) $s2, $scale );
		}
	}
	
	/**
	 * Rounds a number according to the number of sigfigs it should have,
	 * using arbitrary precision when available.
	 * 
	 * @param float $n        	
	 * @param int $sigfigs        	
	 * @return string
	 */
	private function round($n, $sigfigs) {
		$new_log = ( int ) floor ( log ( abs ( $n ), 10 ) ); // Number of digits left of decimal - 1
		$rp = $sigfigs - $new_log - 1; // Number of decimal places needed
		$neg = $n < 0 ? '-' : ''; // Negative sign
		if ($this->bcmath) {
			if ($rp >= 0) {
				$n = bcadd ( $n, $neg . '0.' . str_repeat ( '0', $rp ) . '5', $rp + 1 );
				$n = bcdiv ( $n, '1', $rp );
			} else {
				// This algorithm partially depends on the standardized
				// form of numbers that comes out of bcmath.
				$n = bcadd ( $n, $neg . '5' . str_repeat ( '0', $new_log - $sigfigs ), 0 );
				$n = substr ( $n, 0, $sigfigs + strlen ( $neg ) ) . str_repeat ( '0', $new_log - $sigfigs + 1 );
			}
			return $n;
		} else {
			return $this->scale ( round ( $n, $sigfigs - $new_log - 1 ), $rp + 1 );
		}
	}
	
	/**
	 * Scales a float to $scale digits right of decimal point, like BCMath.
	 * 
	 * @param float $r        	
	 * @param int $scale        	
	 * @return string
	 */
	private function scale($r, $scale) {
		if ($scale < 0) {
			// The f sprintf type doesn't support negative numbers, so we
			// need to cludge things manually. First get the string.
			$r = sprintf ( '%.0f', ( float ) $r );
			// Due to floating point precision loss, $r will more than likely
			// look something like 4652999999999.9234. We grab one more digit
			// than we need to precise from $r and then use that to round
			// appropriately.
			$precise = ( string ) round ( substr ( $r, 0, strlen ( $r ) + $scale ), - 1 );
			// Now we return it, truncating the zero that was rounded off.
			return substr ( $precise, 0, - 1 ) . str_repeat ( '0', - $scale + 1 );
		}
		return sprintf ( '%.' . $scale . 'f', ( float ) $r );
	}
}

// vim: et sw=4 sts=4