add some comments for the distributive operation and add another test

Signed-off-by: Robin Appelman <robin@icewind.nl>

lib/private/Files/Search/QueryOptimizer/MergeDistributiveOperations.php

@@ -10,26 +10,41 @@
 /**
  * Attempt to transform
  *
- * (A AND B) OR (A AND C) into A AND (B OR C)
+ * (A AND B) OR (A AND C) OR (A AND D AND E) into A AND (B OR C OR (D AND E))
+ *
+ * This is always valid because logical 'AND' and 'OR' are distributive[1].
+ *
+ * [1]: https://en.wikipedia.org/wiki/Distributive_property
  */
 class MergeDistributiveOperations extends ReplacingOptimizerStep {
 	public function processOperator(ISearchOperator &$operator): bool {
 		if (
 			$operator instanceof SearchBinaryOperator &&
 			$this->isAllSameBinaryOperation($operator->getArguments())
 		) {
+			// either 'AND' or 'OR'
 			$topLevelType = $operator->getType();
 
+			// split the arguments into groups that share a first argument
+			// (we already know that all arguments are binary operators with at least 1 child)
 			$groups = $this->groupBinaryOperatorsByChild($operator->getArguments(), 0);
 			$outerOperations = array_map(function (array $operators) use ($topLevelType) {
+				// no common operations, no need to change anything
 				if (count($operators) === 1) {
 					return $operators[0];
 				}
 				/** @var ISearchBinaryOperator $firstArgument */
 				$firstArgument = $operators[0];
-				$outerType = $firstArgument->getType();
+
+				// we already checked that all arguments have the same type, so this type applies for all, either 'AND' or 'OR'
+				$innerType = $firstArgument->getType();
+
+				// the common operation we move out ('A' from the example)
 				$extractedLeftHand = $firstArgument->getArguments()[0];
 
+				// for each argument we remove the extracted operation to get the leftovers ('B', 'C' and '(D AND E)' in the example)
+				// note that we leave them inside the "inner" binary operation for when the "inner" operation contained more than two parts
+				// in the (common) case where the "inner" operation only has 1 item left it will be cleaned up in a follow up step
 				$rightHandArguments = array_map(function (ISearchOperator $inner) {
 					/** @var ISearchBinaryOperator $inner */
 					$arguments = $inner->getArguments();
@@ -39,12 +54,17 @@ public function processOperator(ISearchOperator &$operator): bool {
 					}
 					return new SearchBinaryOperator($inner->getType(), $arguments);
 				}, $operators);
+
+				// combine the extracted operation ('A') with the remaining bit ('(B OR C OR (D AND E))')
+				// note that because of how distribution work, we use the "outer" type "inside" and the "inside" type "outside".
 				$extractedRightHand = new SearchBinaryOperator($topLevelType, $rightHandArguments);
 				return new SearchBinaryOperator(
-					$outerType,
+					$innerType,
 					[$extractedLeftHand, $extractedRightHand]
 				);
 			}, $groups);
+
+			// combine all groups again
 			$operator = new SearchBinaryOperator($topLevelType, $outerOperations);
 			parent::processOperator($operator);
 			return true;
@@ -90,7 +110,6 @@ private function groupBinaryOperatorsByChild(array $operators, int $index = 0):
 		foreach ($operators as $operator) {
 			/** @var SearchBinaryOperator|SearchComparison $child */
 			$child = $operator->getArguments()[$index];
-			;
 			$childKey = (string) $child;
 			$result[$childKey][] = $operator;
 		}

tests/lib/Files/Search/QueryOptimizer/MergeDistributiveOperationsTest.php

@@ -130,4 +130,31 @@ public function testOptimizeInside() {
 
 		$this->assertEquals('((storage eq 1 and (path eq "foo" or path eq "bar" or path eq "asd")) and mimetype eq "text")', $operator->__toString());
 	}
+
+	public function testMoveInnerOperations() {
+		$operator = new SearchBinaryOperator(
+			ISearchBinaryOperator::OPERATOR_OR,
+			[
+				new SearchBinaryOperator(ISearchBinaryOperator::OPERATOR_AND, [
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "storage", 1),
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "path", "foo"),
+				]),
+				new SearchBinaryOperator(ISearchBinaryOperator::OPERATOR_AND, [
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "storage", 1),
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "path", "bar"),
+				]),
+				new SearchBinaryOperator(ISearchBinaryOperator::OPERATOR_AND, [
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "storage", 1),
+					new SearchComparison(ISearchComparison::COMPARE_EQUAL, "path", "asd"),
+					new SearchComparison(ISearchComparison::COMPARE_GREATER_THAN, "size", "100"),
+				])
+			]
+		);
+		$this->assertEquals('((storage eq 1 and path eq "foo") or (storage eq 1 and path eq "bar") or (storage eq 1 and path eq "asd" and size gt "100"))', $operator->__toString());
+
+		$this->optimizer->processOperator($operator);
+		$this->simplifier->processOperator($operator);
+
+		$this->assertEquals('(storage eq 1 and (path eq "foo" or path eq "bar" or (path eq "asd" and size gt "100")))', $operator->__toString());
+	}
 }