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Joined 2026-09-21 Message
Incoming links: Demand-driven filtering iterator
A demand-driven filtering iterator needs one cached accepted value. Otherwise The representation invariant says that
hasNext would either lose an element or consume another accepted value every time it is queried.def filterIter[T](p: T => Boolean, it: Iter[T]): Iter[T] =
new Iter[T] {
private var buffered: Option[T] = None
private var finished = false
private def fill(): Unit = {
while (buffered.isEmpty && !finished) {
if (!it.hasNext) finished = true
else {
val x = it.next()
if (p(x)) buffered = Some(x)
}
}
}
def hasNext: Boolean = {
fill()
buffered.nonEmpty
}
def next(): T = {
fill()
require(buffered.nonEmpty)
val answer = buffered.get
buffered = None
answer
}
}buffered holds at most one accepted value. The represented sequence is the buffered value, if present, followed by the subsequence of the underlying remainder satisfying the predicate. Filling moves the next accepted value into the cache and discards only rejected values, so this abstract sequence is unchanged by hasNext. Clearing the cache in next removes its head. Thusnext works even when the client has not first called hasNext; exhaustion violates its stated precondition. As with ordinary iterator adapters, clients should give this wrapper exclusive consumption of it, and the predicate should give a stable answer for each element. One call may scan several rejected elements, but across a complete traversal each source element is consumed once. Cache storage is constant.Keep a buffered first match and a flag recording whether the initial prefix still needs to be skipped:
class DropUntil[T] private (
p: T => Boolean,
iter: Iter[T],
private var cached: Option[T],
private var seeking: Boolean) extends Iter[T] {
def this(p: T => Boolean, iter: Iter[T]) =
this(p, iter, None, true)
private def prepare(): Unit = {
if (seeking) {
while (cached.isEmpty && iter.hasNext) {
val x = iter.next()
if (p(x)) cached = Some(x)
}
seeking = false
}
}
def hasNext: Boolean = {
prepare()
cached.nonEmpty || iter.hasNext
}
def next(): T = {
prepare()
cached match {
case Some(x) => cached = None; x
case None =>
if (!iter.hasNext) throw new NoSuchElementException("DropUntil.next")
iter.next()
}
}
def copy(): Iter[T] = {
prepare()
new DropUntil[T](p, iter.copy(), cached, false)
}
}While seeking,
prepare drops exactly the rejected prefix and caches the first accepted value, including a possible null payload as Some(null). Afterwards the abstract remainder is the cache, if any, followed by the underlying remainder; later values are returned even when they fail p. Repeated hasNext calls do not lose the cached value.The iterator snapshot copies the underlying cursor and the cached value, then marks the initial search complete. It does not reconstruct a fresh two-argument
DropUntil, which could incorrectly discard later values. Resolving the initial search before copying also avoids having two snapshots independently rerun a privately stateful predicate. The wrapper owns consumption of the supplied source cursor. If that source is infinite and never produces an accepted value, seeking cannot finish. This is a prefix-dropping iterator, rather than a repeatedly demand-driven filtering iterator.Use one cached optional result. Only a request for availability or the next result fills that cache:This is a demand-driven filtering iterator. When the cache is empty, its abstract remaining sequence is the filtered remaining input. When it holds
class FilterIterator[T](test: T => Boolean, iter: Iterator[T])
extends Iterator[T] {
private var cached: Option[T] = None
private def prepare(): Unit = {
while (cached.isEmpty && iter.hasNext) {
val x = iter.next()
if (test(x)) cached = Some(x)
}
}
def hasNext: Boolean = {
prepare()
cached.nonEmpty
}
def next(): T = {
prepare()
cached match {
case Some(x) =>
cached = None
x
case None => throw new NoSuchElementException("Filtered iterator exhausted")
}
}
}Some(x), the abstract sequence is x followed by the filtered remaining input. The loop invariant for prepare says that all input elements consumed during the present search have failed test, except for the one cached passing value if one has been found. The first passing value stops the loop, so it cannot read past a result that the client has not yet consumed.Construction performs no underlying reads, and repeated
hasNext calls reuse a cached result. next also works without a preceding hasNext, clears only the result it returns, and does not prefetch its successor. Some(x), rather than Option(x), allows a passing null value to remain a genuine cached value. For finite remaining input, its length is a loop variant; for infinite input, availability can legitimately run forever if no further passing element exists. Deterministic terminating tests and exclusive ownership of the consuming input are assumed. One cache slot is extra storage. Prefix-dropping iterator 2026-10-31
Discard input values until the first one satisfying a predicate, then return that value and every remaining value without testing the predicate again. A demand-driven filtering iterator instead keeps testing and can discard later values too. A buffered first match must survive repeated availability tests and iterator snapshots. A flag distinguishes the initial seeking phase from the already-positioned remainder.