60 lines
2.1 KiB
Kotlin
60 lines
2.1 KiB
Kotlin
package moe.kageru.sekwences
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/**
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* Returns a sequences that optionally merges adjacent elements.
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* Heavily inspired by Itertools.coalesce() from the Rust crate.
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*
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* The merge function will be passed two adjacent elements in the sequence.
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* If the two can be merged, the result of the merging process should be returned.
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* If null is returned, the first of the two elements is returned,
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* and the second will be used as the first element in the next merge operation.
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*
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* https://docs.rs/itertools/0.8.0/itertools/trait.Itertools.html#method.coalesce
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*/
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fun <T> Sequence<T>.coalesce(merger: (T, T) -> T?): Sequence<T> {
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return if (this.iterator().hasNext()) {
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CoalescingSequence(this, merger)
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} else {
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emptySequence()
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}
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}
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/**
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* Sequence that holds a [CoalescingIterator].
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*/
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internal class CoalescingSequence<T>(private val source: Sequence<T>, private inline val merger: (T, T) -> T?) : Sequence<T> {
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override fun iterator(): Iterator<T> {
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return CoalescingIterator(source.iterator(), merger)
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}
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}
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/**
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* Iterator that will attempt to merge adjacent elements as described in the KDoc for [coalesce].
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*/
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internal class CoalescingIterator<T>(private val source: Iterator<T>, private inline val merger: (T, T) -> T?) : Iterator<T> {
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private var previous: T = if (source.hasNext()) source.next() else error("Please don’t pass empty iterators in here")
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// The reason we need this marker is that our iterator can still hold one value, even if the source has been drained.
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private var hasNext = true
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override fun hasNext() = hasNext
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override tailrec fun next(): T {
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if (!source.hasNext()) {
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hasNext = false
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return previous
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}
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val current = source.next()
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val merged = merger(previous, current)
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// If the elements can’t be merged, return the first, then save the second for next time.
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return if (merged == null) {
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val ret = previous
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previous = current
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ret
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} else {
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previous = merged
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next()
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}
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}
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}
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