mapNotNullToIterable

mapNotNullToIterable.kt

// variations of filterNotNull that returns Iterable.
// it proxies iteration to avoid cost of create/update a list.
fun <T> Iterable<T?>.filterNotNullToIterable() =
	object : Iterable<T> {
		override fun iterator() = object : Iterator<T> {
			val source = this@filterNotNullToIterable.iterator()
			var item: T? = null
			private fun read() {
				while (source.hasNext()) {
					item = source.next() ?: continue
					return
				}
				item = null
			}

			init {
				read()
			}

			override fun hasNext(): Boolean = item != null
			override fun next(): T = item!!.also { read() }
		}
	}

// variations of map() that returns Iterable.
// it proxies iteration to avoid cost of create/update a list.
fun <I : Any?, T : Any?> Iterable<I>.mapToIterable(transform: (I) -> T) =
	object : Iterable<T> {
		override fun iterator() = object : Iterator<T> {
			val source = this@mapToIterable.iterator()
			override fun hasNext(): Boolean = source.hasNext()
			override fun next(): T = transform(source.next())
		}
	}

// variations of mapNotNull() that returns Iterable.
// it proxies iteration to avoid cost of create/update a list.
fun <I : Any?, T> Iterable<I>.mapNotNullToIterable(transform: (I) -> T?) =
	this.mapToIterable(transform).filterNotNullToIterable()