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diff --git a/src/main/java/com/google/devtools/build/lib/query2/engine/BinaryOperatorExpression.java b/src/main/java/com/google/devtools/build/lib/query2/engine/BinaryOperatorExpression.java
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+// Copyright 2014 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.query2.engine;
+
+import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+
+import java.util.Collection;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Set;
+
+/**
+ * A binary algebraic set operation.
+ *
+ * <pre>
+ * expr ::= expr (INTERSECT expr)+
+ * | expr ('^' expr)+
+ * | expr (UNION expr)+
+ * | expr ('+' expr)+
+ * | expr (EXCEPT expr)+
+ * | expr ('-' expr)+
+ * </pre>
+ */
+class BinaryOperatorExpression extends QueryExpression {
+
+ private final Lexer.TokenKind operator; // ::= INTERSECT/CARET | UNION/PLUS | EXCEPT/MINUS
+ private final ImmutableList<QueryExpression> operands;
+
+ BinaryOperatorExpression(Lexer.TokenKind operator,
+ List<QueryExpression> operands) {
+ Preconditions.checkState(operands.size() > 1);
+ this.operator = operator;
+ this.operands = ImmutableList.copyOf(operands);
+ }
+
+ @Override
+ public <T> Set<T> eval(QueryEnvironment<T> env) throws QueryException {
+ Set<T> lhsValue = new LinkedHashSet<>(operands.get(0).eval(env));
+
+ for (int i = 1; i < operands.size(); i++) {
+ Set<T> rhsValue = operands.get(i).eval(env);
+ switch (operator) {
+ case INTERSECT:
+ case CARET:
+ lhsValue.retainAll(rhsValue);
+ break;
+ case UNION:
+ case PLUS:
+ lhsValue.addAll(rhsValue);
+ break;
+ case EXCEPT:
+ case MINUS:
+ lhsValue.removeAll(rhsValue);
+ break;
+ default:
+ throw new IllegalStateException("operator=" + operator);
+ }
+ }
+ return lhsValue;
+ }
+
+ @Override
+ public void collectTargetPatterns(Collection<String> literals) {
+ for (QueryExpression subExpression : operands) {
+ subExpression.collectTargetPatterns(literals);
+ }
+ }
+
+ @Override
+ public String toString() {
+ StringBuilder result = new StringBuilder();
+ for (int i = 1; i < operands.size(); i++) {
+ result.append("(");
+ }
+ result.append(operands.get(0));
+ for (int i = 1; i < operands.size(); i++) {
+ result.append(" " + operator.getPrettyName() + " " + operands.get(i) + ")");
+ }
+ return result.toString();
+ }
+}