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+import { OperatorFunction } from '../types';
+import { operate } from '../util/lift';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+
+/**
+ * Groups pairs of consecutive emissions together and emits them as an array of
+ * two values.
+ *
+ * <span class="informal">Puts the current value and previous value together as
+ * an array, and emits that.</span>
+ *
+ * ![](pairwise.png)
+ *
+ * The Nth emission from the source Observable will cause the output Observable
+ * to emit an array [(N-1)th, Nth] of the previous and the current value, as a
+ * pair. For this reason, `pairwise` emits on the second and subsequent
+ * emissions from the source Observable, but not on the first emission, because
+ * there is no previous value in that case.
+ *
+ * ## Example
+ *
+ * On every click (starting from the second), emit the relative distance to the previous click
+ *
+ * ```ts
+ * import { fromEvent, pairwise, map } from 'rxjs';
+ *
+ * const clicks = fromEvent<PointerEvent>(document, 'click');
+ * const pairs = clicks.pipe(pairwise());
+ * const distance = pairs.pipe(
+ *   map(([first, second]) => {
+ *     const x0 = first.clientX;
+ *     const y0 = first.clientY;
+ *     const x1 = second.clientX;
+ *     const y1 = second.clientY;
+ *     return Math.sqrt(Math.pow(x0 - x1, 2) + Math.pow(y0 - y1, 2));
+ *   })
+ * );
+ *
+ * distance.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link buffer}
+ * @see {@link bufferCount}
+ *
+ * @return A function that returns an Observable of pairs (as arrays) of
+ * consecutive values from the source Observable.
+ */
+export function pairwise<T>(): OperatorFunction<T, [T, T]> {
+  return operate((source, subscriber) => {
+    let prev: T;
+    let hasPrev = false;
+    source.subscribe(
+      createOperatorSubscriber(subscriber, (value) => {
+        const p = prev;
+        prev = value;
+        hasPrev && subscriber.next([p, value]);
+        hasPrev = true;
+      })
+    );
+  });
+}