Extensions for Kotlin sequences
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package moe.kageru.sekwences
/**
* Sequence that holds a [CoalescingIterator].
*/
internal class CoalescingSequence<T>(private val source: Sequence<T>, private inline val merger: (T, T) -> T?) : Sequence<T> {
override fun iterator(): Iterator<T> {
return CoalescingIterator(source.iterator(), merger)
}
/**
* Iterator that will attempt to merge adjacent elements as described in the KDoc for [coalesce].
*/
internal class CoalescingIterator<T>(private val source: Iterator<T>, private inline val merger: (T, T) -> T?) : Iterator<T> {
private var previous: T = if (source.hasNext()) source.next() else error("Please don’t pass empty iterators in here")
// The reason we need this marker is that our iterator can still hold one value, even if the source has been drained.
private var hasNext = true
override fun hasNext() = hasNext
override tailrec fun next(): T {
if (!source.hasNext()) {
hasNext = false
return previous
}
val current = source.next()
val merged = merger(previous, current)
// If the elements can’t be merged, return the first, then save the second for next time.
return if (merged == null) {
val ret = previous
previous = current
ret
} else {
previous = merged
next()
}
}
}
}