Apply thousands of fixes in time linear in their number

Patcher copied the accepted runs of a file for each group of diffs:
the runs looked up for the group still referenced them when array_splice()
inserted the group's runs, and array_splice() builds a new array even to
append one at the end, which is where the runs of diffs coming in the order
of their lines go.

Removing all 50,000 entries of a baseline with --fix spent 15.3 s in
Patcher::apply() and now 0.3 s, the whole --fix of the project took 41.9 s
and now 17.5 s.

Amp-Thread-ID: https://ampcode.com/threads/T-01a1a198-fa57-7583-9394-63372a7e2fa8

Author: ondrejmirtes

src/Fixable/Patcher.php

@@ -71,13 +71,20 @@ public function apply(array $contentsByFile, array $diffGroups): array
 
 				$insertionsByFile[$diff->file] = $insertions;
 			}
+			// a reference to the runs would make the insertions below copy them all, for each group
+			unset($fileRuns);
 
 			foreach ($insertionsByFile as $file => $insertions) {
 				$acceptedRuns[$file] ??= [];
 
 				// runs of a single diff are ordered and non-overlapping,
 				// inserting from the last one keeps the earlier indexes valid
 				for ($i = count($insertions) - 1; $i >= 0; $i--) {
+					if ($insertions[$i][0] === count($acceptedRuns[$file])) {
+						// the diffs usually come in the order of their lines, array_splice() would copy the runs to append one
+						$acceptedRuns[$file][] = $insertions[$i][1];
+						continue;
+					}
 					array_splice($acceptedRuns[$file], $insertions[$i][0], 0, [$insertions[$i][1]]);
 				}
 			}

tests/PHPStan/Fixable/PatcherTest.php

@@ -36,4 +36,25 @@ public function testGroupIsSkippedInAllFilesWhenItConflictsInOne(): void
 		$this->assertSame(2, $result['skippedCount']);
 	}
 
+	public function testRunsAreSplicedInTheOrderOfTheirLines(): void
+	{
+		$code = "1\n2\n3\n4\n5\n6\n";
+		$group = static fn (string $diff): array => [new FixedErrorDiff('a.php', hash('sha256', $code), $diff)];
+
+		$result = (new Patcher())->apply(['a.php' => $code], [
+			$group("@@ -4,1 +4,1 @@\n-4\n+D\n"),
+			// before the run of an earlier error
+			$group("@@ -2,1 +2,1 @@\n-2\n+B\n"),
+			// one run before the accepted ones and one after them
+			$group("@@ -1,1 +1,1 @@\n-1\n+A\n@@ -6,1 +6,1 @@\n-6\n+F\n"),
+			// between them
+			$group("@@ -5,1 +5,1 @@\n-5\n+E\n"),
+			// the same change as another error
+			$group("@@ -4,1 +4,1 @@\n-4\n+D\n"),
+		]);
+
+		$this->assertSame(['a.php' => "A\nB\n3\nD\nE\nF\n"], $result['contents']);
+		$this->assertSame(5, $result['appliedCount']);
+	}
+
 }