com.google.devtools.build.lib.query2.engine.VariableContext.java Source code

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Here is the source code for com.google.devtools.build.lib.query2.engine.VariableContext.java

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// Copyright 2016 The Bazel Authors. 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.collect.ImmutableMap;
import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;

import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/** An immutable context of variable bindings for variables introduced by {@link LetExpression}s. */
@Immutable
@ThreadSafe
public class VariableContext<T> {
    private final ImmutableMap<String, Set<T>> context;

    private VariableContext(ImmutableMap<String, Set<T>> context) {
        this.context = context;
    }

    /**
     * Returns the value bound to the specified variable given by {@code name}, or {@code null} if
     * there is no such binding.
     */
    @Nullable
    Set<T> get(String name) {
        return context.get(name);
    }

    /** Returns a {@link VariableContext} with no variables defined. */
    public static <T> VariableContext<T> empty() {
        return new VariableContext<>(ImmutableMap.<String, Set<T>>of());
    }

    /**
     * Returns a {@link VariableContext} that has all the same bindings as the given
     * {@code variableContext} and also the binding of {@code name} to {@code value}.
     */
    static <T> VariableContext<T> with(VariableContext<T> variableContext, String name, Set<T> value) {
        ImmutableMap.Builder<String, Set<T>> newContextBuilder = ImmutableMap.builder();
        for (Map.Entry<String, Set<T>> entry : variableContext.context.entrySet()) {
            if (!entry.getKey().equals(name)) {
                // The binding of 'name' to 'value' should override any existing binding of name in
                // 'variableContext'. These are the semantics we want in order for nested let-expressions
                // to have the semantics we want.
                newContextBuilder.put(entry);
            }
        }
        newContextBuilder.put(name, value);
        return new VariableContext<>(newContextBuilder.build());
    }
}