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// Copyright 2014 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.collect;
import com.google.common.base.Joiner;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;
import com.google.devtools.build.lib.collect.nestedset.NestedSet;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
* An immutable chain of immutable Iterables.
*
* <p>This class is defined for the sole purpose of being able to check for immutability (using
* instanceof). Otherwise, we could use a plain Iterable (as returned by {@code
* Iterables.concat()}).
*
* @see CollectionUtils#checkImmutable(Iterable)
*/
public final class IterablesChain<T> implements Iterable<T> {
public static <T> Iterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b) {
return IterablesChain.<T>builder().add(a).add(b).build();
}
private final Iterable<T> chain;
IterablesChain(Iterable<T> chain) {
this.chain = chain;
}
@Override
public Iterator<T> iterator() {
return chain.iterator();
}
public static <T> Builder<T> builder() {
return new Builder<>();
}
@Override
public String toString() {
return "[" + Joiner.on(", ").join(this) + "]";
}
/** Builder for IterablesChain. */
public static class Builder<T> {
private List<Iterable<? extends T>> iterables = new ArrayList<>();
private Builder() {}
/**
* Adds an immutable iterable to the end of the chain.
*
* <p>If the iterable can not be confirmed to be immutable, a runtime error is thrown.
*/
public Builder<T> add(Iterable<? extends T> iterable) {
CollectionUtils.checkImmutable(iterable);
// Avoid unnecessarily expanding a NestedSet.
boolean isEmpty =
iterable instanceof NestedSet
? ((NestedSet<?>) iterable).isEmpty()
: Iterables.isEmpty(iterable);
if (!isEmpty) {
iterables.add(iterable);
}
return this;
}
/** Adds a single element to the chain. */
public Builder<T> addElement(T element) {
iterables.add(ImmutableList.of(element));
return this;
}
/** Returns true if the chain is empty. */
public boolean isEmpty() {
return iterables.isEmpty();
}
/** Builds an iterable that iterates through all elements in this chain. */
@SuppressWarnings("unchecked")
public Iterable<T> build() {
int size = iterables.size();
if (size == 0) {
return ImmutableList.of();
} else if (size == 1) {
// cast is type safe since Iterable<T> doesn't allow mutation
return (Iterable<T>) iterables.get(0);
}
return new IterablesChain<>(Iterables.concat(ImmutableList.copyOf(iterables)));
}
}
}
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