new(slideThrough, slideThroughCyclic): add a new pair of operators

The newly added operators `slideThrough` and `slideThroughCyclic` are a generalization of `pairwise` and `pairwiseCyclic`.
They will both yield tuples with `windowSize` elements by sliding a window through the source iterable.

Closes #17

src/operators/index.ts

@@ -27,3 +27,4 @@ export { join } from './join'
 export { endWith } from './end-with'
 export { startWith } from './start-with'
 export { scan } from './scan'
+export { slideThrough, slideThroughCyclic } from './slide-through'

src/operators/pairwise.ts

@@ -16,6 +16,8 @@ import { Operator } from '../core/types'
  * Empty iterator and an iterator with a single value will both result in an
  * empty iterator.
  *
+ * This is a specialization of {@link slideThrough}.
+ *
  * @returns
  * Operator<T, [T, T]>
  *
@@ -68,6 +70,8 @@ export function pairwise<T> (): Operator<T, [T, T]> {
  * yielded pair will consist of the last and the first value of the source
  * iterator.
  *
+ * This is a specialization of {@link slideThroughCyclic}.
+ *
  * @returns
  * Operator<T, [T, T]>
  *

src/operators/slide-through.ts

@@ -0,0 +1,212 @@
+import { Operator } from '../core/types'
+
+
+/**
+ * @short
+ * Like `pairwise`, but you *slide through* a custom window size.
+ *
+ * @categories
+ * operator
+ *
+ * @description
+ * Processes an iterable by yielding successive overlapping tuples of a specified
+ * window size. Each tuple contains `windowSize` consecutive elements from the source
+ * iterable. As iteration progresses, the window slides forward by one position,
+ * excluding the first element of the previous window and including the next element
+ * from the iterable. This operation continues until the source iterable is fully
+ * consumed.
+ *
+ * It follows that the size of the resulting iterable will be the same as the window
+ * size, unless the size of the source iterable is less than the window size -- in
+ * that case, the resulting iterable is empty.
+ *
+ * This is a generalization of {@link pairwise}.
+ *
+ * @since
+ * 0.0.4
+ *
+ * @parameter
+ * windowSize
+ * number
+ * The size of each yielded tuple.
+ *
+ * @returns
+ * Operator<T, Array<T>>
+ *
+ * @example
+ * j.pipe(
+ *   [1, 2, 3, 4, 5],
+ *   j.slideThrough(3),
+ * )
+ * // => [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
+ *
+ * @example
+ * j.pipe(
+ *   [1, 2, 3],
+ *   j.slideThrough(4),
+ * )
+ * // => []
+ */
+export function slideThrough<T> (windowSize: 1): Operator<T, [T]>
+export function slideThrough<T> (windowSize: 2): Operator<T, [T, T]>
+export function slideThrough<T> (windowSize: 3): Operator<T, [T, T, T]>
+export function slideThrough<T> (windowSize: 4): Operator<T, [T, T, T, T]>
+export function slideThrough<T> (windowSize: 5): Operator<T, [T, T, T, T, T]>
+export function slideThrough<T> (windowSize: 6): Operator<T, [T, T, T, T, T, T]>
+export function slideThrough<T> (windowSize: 7): Operator<T, [T, T, T, T, T, T, T]>
+export function slideThrough<T> (windowSize: 8): Operator<T, [T, T, T, T, T, T, T, T]>
+export function slideThrough<T> (windowSize: 9): Operator<T, [T, T, T, T, T, T, T, T, T]>
+export function slideThrough<T> (windowSize: 10): Operator<T, [T, T, T, T, T, T, T, T, T, T]>
+export function slideThrough<T> (windowSize: number): Operator<T, Array<T>>
+export function slideThrough<T> (windowSize: number): Operator<T, Array<T>> {
+
+  if (!Number.isInteger(windowSize) || windowSize < 1) {
+    throw new RangeError(`Window size must be an integer not less than 1; an attempt was made to define the window size as ${windowSize}.`)
+  }
+
+  return function* (iterable: Iterable<T>): IterableIterator<Array<T>> {
+    const window: Array<T> = []
+    for (const value of iterable) {
+      window.push(value)
+      if (window.length == windowSize) {
+        yield [...window]
+        window.shift()
+      }
+    }
+  }
+
+}
+
+/**
+ * @short
+ * Like `slideThrough`, but *cyclic*.
+ *
+ * @categories
+ * operator
+ *
+ * @description
+ * Processes an iterable by yielding successive overlapping sub-arrays of a specified
+ * window size, similar to `slideThrough`. However, upon fully consuming the source
+ * iterable, this operator wraps around to the beginning, continuing the sliding
+ * window operation in a cyclic manner. The final sub-arrays combine elements from
+ * the end of the iterable with those from the beginning, forming a continuous loop.
+ *
+ * It follows that the size of the resulting iterable will always be either the size
+ * of the source iterable or the window size, whichever is smaller.
+ *
+ * This is a generalization of {@link pairwiseCyclic}.
+ *
+ * @since
+ * 0.0.4
+ *
+ * @parameter
+ * windowSize
+ * number
+ * The size of each yielded tuple.
+ *
+ * @returns
+ * Operator<T, Array<T>>
+ *
+ * @example
+ * j.pipe(
+ *   [1, 2, 3, 4, 5],
+ *   j.slideThrough(3),
+ * )
+ * // => [
+ * //   [1, 2, 3],
+ * //   [2, 3, 4],
+ * //   [3, 4, 5],
+ * //   [4, 5, 1],
+ * //   [5, 1, 2],
+ * // ]
+ *
+ * @example
+ * j.pipe(
+ *   [1, 2, 3],
+ *   j.slideThrough(4),
+ * )
+ * // => [
+ * //   [1, 2, 3, 1],
+ * //   [2, 3, 1, 2],
+ * //   [3, 1, 2, 3],
+ * // ]
+ */
+export function slideThroughCyclic<T> (windowSize: 1): Operator<T, [T]>
+export function slideThroughCyclic<T> (windowSize: 2): Operator<T, [T, T]>
+export function slideThroughCyclic<T> (windowSize: 3): Operator<T, [T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 4): Operator<T, [T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 5): Operator<T, [T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 6): Operator<T, [T, T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 7): Operator<T, [T, T, T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 8): Operator<T, [T, T, T, T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 9): Operator<T, [T, T, T, T, T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: 10): Operator<T, [T, T, T, T, T, T, T, T, T, T]>
+export function slideThroughCyclic<T> (windowSize: number): Operator<T, Array<T>>
+export function slideThroughCyclic<T> (windowSize: number): Operator<T, Array<T>> {
+
+  if (!Number.isInteger(windowSize) || windowSize < 1) {
+    throw new RangeError(`Window size must be an integer not less than 1; an attempt was made to define the window size as ${windowSize}.`)
+  }
+
+  return function* (iterable: Iterable<T>): IterableIterator<Array<T>> {
+
+    // This will slide across the source iterable.
+    // We'll be mutating this array, but we'll yield a shallow copy.
+    const window: Array<T> = []
+
+    // We need the head later due to the cyclic nature of the operator.
+    const head: Array<T> = []
+
+    for (const value of iterable) {
+      if (head.length < windowSize) {
+        head.push(value)
+      }
+      window.push(value)
+      if (window.length == windowSize) {
+        yield [...window]
+        window.shift()
+      }
+    }
+
+    if (head.length === windowSize) {
+
+      // The source iterable is at least as long as the window size, so everything plays out regularly.
+      // We just need to yield `windowSize - 1` more tuples, and wrap the window to the beginning.
+      for (let i = 0; i < windowSize - 1; i++) {
+        window.push(head[i])
+        yield [...window]
+        window.shift()
+      }
+
+    } else /* head.length < windowSize */ {
+
+      // The source iterable was consumed before the window was fully filled in.
+      // This means that we didn't yield anything yet, and that both head and window are populated with all
+      // values yielded from the source observable.
+
+      // Before proceeding, we must handle a special edge case here: the source iterable can be empty.
+      // In that case we're already done, as the correct result is an empty iterable.
+      if (head.length == 0) return
+
+      // We firstly populate the window by iterating over the head (which is in this case the whole source iterable)
+      // as many times as it's needed to fully fill it in.
+      let index = head.length
+      while (window.length < windowSize) {
+        window.push(head[index++ % head.length])
+      }
+
+      // We're ready for the first tuple immediately.
+      yield [...window]
+
+      // Now we slide for the other tuples.
+      for (let i = 0; i < head.length - 1; i++) {
+        window.shift()
+        window.push(head[index++ % head.length])
+        yield [...window]
+      }
+
+    }
+
+  }
+
+}