MemoizedSequence.kt
/*
* Copyright 2022 The Android Open Source Project
*
* 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 androidx.room.compiler.processing.util
/**
* A [Sequence] implementation that caches values so that another collector can avoid
* re-computing the source sequence.
*
* Note that collecting on these sequence is not thread safe.
*/
@Suppress("SyntheticAccessor")
internal class MemoizedSequence<T>(
private val buildSequence: () -> Sequence<T>
) : Sequence<T> {
/**
* Shared cache between iterators.
*/
private val cache = mutableListOf<T>()
private val delegateIterator by lazy {
buildSequence().iterator()
}
private inner class CachedIterator : Iterator<T> {
private var yieldedCount = 0
override fun hasNext(): Boolean {
return yieldedCount < cache.size || delegateIterator.hasNext()
}
override fun next(): T {
if (cache.size == yieldedCount) {
// no point in checking hasNext, let it throw if we get a next() call without a
// previous hasNext call.
cache.add(delegateIterator.next())
}
return cache[yieldedCount].also {
yieldedCount++
}
}
}
override fun iterator(): Iterator<T> {
return CachedIterator()
}
}