class TimeEfficientLongestCommonSubsequenceCalculator

final class TimeEfficientLongestCommonSubsequenceCalculator implements LongestCommonSubsequenceCalculator {
    
    /**
     * @inheritDoc
     */
    public function calculate(array $from, array $to) : array {
        $common = [];
        $fromLength = count($from);
        $toLength = count($to);
        $width = $fromLength + 1;
        $matrix = new SplFixedArray($width * ($toLength + 1));
        for ($i = 0; $i <= $fromLength; $i++) {
            $matrix[$i] = 0;
        }
        for ($j = 0; $j <= $toLength; $j++) {
            $matrix[$j * $width] = 0;
        }
        for ($i = 1; $i <= $fromLength; $i++) {
            for ($j = 1; $j <= $toLength; $j++) {
                $o = $j * $width + $i;
                // don't use max() to avoid function call overhead
                $firstOrLast = $from[$i - 1] === $to[$j - 1] ? $matrix[$o - $width - 1] + 1 : 0;
                if ($matrix[$o - 1] > $matrix[$o - $width]) {
                    if ($firstOrLast > $matrix[$o - 1]) {
                        $matrix[$o] = $firstOrLast;
                    }
                    else {
                        $matrix[$o] = $matrix[$o - 1];
                    }
                }
                else {
                    if ($firstOrLast > $matrix[$o - $width]) {
                        $matrix[$o] = $firstOrLast;
                    }
                    else {
                        $matrix[$o] = $matrix[$o - $width];
                    }
                }
            }
        }
        $i = $fromLength;
        $j = $toLength;
        while ($i > 0 && $j > 0) {
            if ($from[$i - 1] === $to[$j - 1]) {
                $common[] = $from[$i - 1];
                $i--;
                $j--;
            }
            else {
                $o = $j * $width + $i;
                if ($matrix[$o - $width] > $matrix[$o - 1]) {
                    $j--;
                }
                else {
                    $i--;
                }
            }
        }
        return array_reverse($common);
    }

}