package moe.kageru.sekwences

/**
 * Returns a sequences that optionally merges adjacent elements.
 * Heavily inspired by Itertools.coalesce() from the Rust crate.
 *
 * The merge function will be passed two adjacent elements in the sequence.
 * If the two can be merged, the result of the merging process should be returned.
 * If null is returned, the first of the two elements is returned,
 * and the second will be used as the first element in the next merge operation.
 *
 * https://docs.rs/itertools/0.8.0/itertools/trait.Itertools.html#method.coalesce
 */
fun <T> Sequence<T>.coalesce(merger: (T, T) -> T?): Sequence<T> {
    return if (this.iterator().hasNext()) {
        CoalescingSequence(this, merger)
    } else {
        emptySequence()
    }
}

/**
 * Modifies the current [Sequence] by adding a peek() method
 * that returns the next element or null without advancing the sequence.
 * Calling peek() multiple times without next() will return the same value.
 */
fun <T> Sequence<T>.peekable(): PeekableSequence<T> {
    return PeekableSequence(this)
}