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File: Differ.php
<?php declare(strict_types = 1);

namespace PHPStan\PhpDocParser\Printer;

use Exception;
use function array_reverse;
use function count;

/**
 * Inspired by https://github.com/nikic/PHP-Parser/tree/36a6dcd04e7b0285e8f0868f44bd4927802f7df1
 *
 * Copyright (c) 2011, Nikita Popov
 * All rights reserved.
 *
 * Implements the Myers diff algorithm.
 *
 * Myers, Eugene W. "An O (ND) difference algorithm and its variations."
 * Algorithmica 1.1 (1986): 251-266.
 *
 * @template T
 * @internal
 */
class Differ
{

	/** @var callable(T, T): bool */
	private $isEqual;

	/**
	 * Create differ over the given equality relation.
	 *
	 * @param callable(T, T): bool $isEqual Equality relation
	 */
	public function __construct(callable $isEqual)
	{
		$this->isEqual = $isEqual;
	}

	/**
	 * Calculate diff (edit script) from $old to $new.
	 *
	 * @param T[] $old Original array
	 * @param T[] $new New array
	 *
	 * @return DiffElem[] Diff (edit script)
	 */
	public function diff(array $old, array $new): array
	{
		[$trace, $x, $y] = $this->calculateTrace($old, $new);
		return $this->extractDiff($trace, $x, $y, $old, $new);
	}

	/**
	 * Calculate diff, including "replace" operations.
	 *
	 * If a sequence of remove operations is followed by the same number of add operations, these
	 * will be coalesced into replace operations.
	 *
	 * @param T[] $old Original array
	 * @param T[] $new New array
	 *
	 * @return DiffElem[] Diff (edit script), including replace operations
	 */
	public function diffWithReplacements(array $old, array $new): array
	{
		return $this->coalesceReplacements($this->diff($old, $new));
	}

	/**
	 * @param T[] $old
	 * @param T[] $new
	 * @return array{array<int, array<int, int>>, int, int}
	 */
	private function calculateTrace(array $old, array $new): array
	{
		$n = count($old);
		$m = count($new);
		$max = $n + $m;
		$v = [1 => 0];
		$trace = [];
		for ($d = 0; $d <= $max; $d++) {
			$trace[] = $v;
			for ($k = -$d; $k <= $d; $k += 2) {
				if ($k === -$d || ($k !== $d && $v[$k - 1] < $v[$k + 1])) {
					$x = $v[$k + 1];
				} else {
					$x = $v[$k - 1] + 1;
				}

				$y = $x - $k;
				while ($x < $n && $y < $m && ($this->isEqual)($old[$x], $new[$y])) {
					$x++;
					$y++;
				}

				$v[$k] = $x;
				if ($x >= $n && $y >= $m) {
					return [$trace, $x, $y];
				}
			}
		}
		throw new Exception('Should not happen');
	}

	/**
	 * @param array<int, array<int, int>> $trace
	 * @param T[] $old
	 * @param T[] $new
	 * @return DiffElem[]
	 */
	private function extractDiff(array $trace, int $x, int $y, array $old, array $new): array
	{
		$result = [];
		for ($d = count($trace) - 1; $d >= 0; $d--) {
			$v = $trace[$d];
			$k = $x - $y;

			if ($k === -$d || ($k !== $d && $v[$k - 1] < $v[$k + 1])) {
				$prevK = $k + 1;
			} else {
				$prevK = $k - 1;
			}

			$prevX = $v[$prevK];
			$prevY = $prevX - $prevK;

			while ($x > $prevX && $y > $prevY) {
				$result[] = new DiffElem(DiffElem::TYPE_KEEP, $old[$x - 1], $new[$y - 1]);
				$x--;
				$y--;
			}

			if ($d === 0) {
				break;
			}

			while ($x > $prevX) {
				$result[] = new DiffElem(DiffElem::TYPE_REMOVE, $old[$x - 1], null);
				$x--;
			}

			while ($y > $prevY) {
				$result[] = new DiffElem(DiffElem::TYPE_ADD, null, $new[$y - 1]);
				$y--;
			}
		}
		return array_reverse($result);
	}

	/**
	 * Coalesce equal-length sequences of remove+add into a replace operation.
	 *
	 * @param DiffElem[] $diff
	 * @return DiffElem[]
	 */
	private function coalesceReplacements(array $diff): array
	{
		$newDiff = [];
		$c = count($diff);
		for ($i = 0; $i < $c; $i++) {
			$diffType = $diff[$i]->type;
			if ($diffType !== DiffElem::TYPE_REMOVE) {
				$newDiff[] = $diff[$i];
				continue;
			}

			$j = $i;
			while ($j < $c && $diff[$j]->type === DiffElem::TYPE_REMOVE) {
				$j++;
			}

			$k = $j;
			while ($k < $c && $diff[$k]->type === DiffElem::TYPE_ADD) {
				$k++;
			}

			if ($j - $i === $k - $j) {
				$len = $j - $i;
				for ($n = 0; $n < $len; $n++) {
					$newDiff[] = new DiffElem(
						DiffElem::TYPE_REPLACE,
						$diff[$i + $n]->old,
						$diff[$j + $n]->new,
					);
				}
			} else {
				for (; $i < $k; $i++) {
					$newDiff[] = $diff[$i];
				}
			}
			$i = $k - 1;
		}
		return $newDiff;
	}

}