From e4fa1e69e7ebfb627c7198fd1a9881e9327ec4d4 Mon Sep 17 00:00:00 2001
From: Pinapelz <yukais@pinapelz.com>
Date: Sat, 28 Jun 2025 17:26:46 -0700
Subject: initial commit: scaffolding

---
 node_modules/rxjs/src/internal/operators/scan.ts | 95 ++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 node_modules/rxjs/src/internal/operators/scan.ts

(limited to 'node_modules/rxjs/src/internal/operators/scan.ts')

diff --git a/node_modules/rxjs/src/internal/operators/scan.ts b/node_modules/rxjs/src/internal/operators/scan.ts
new file mode 100644
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+++ b/node_modules/rxjs/src/internal/operators/scan.ts
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+import { OperatorFunction } from '../types';
+import { operate } from '../util/lift';
+import { scanInternals } from './scanInternals';
+
+export function scan<V, A = V>(accumulator: (acc: A | V, value: V, index: number) => A): OperatorFunction<V, V | A>;
+export function scan<V, A>(accumulator: (acc: A, value: V, index: number) => A, seed: A): OperatorFunction<V, A>;
+export function scan<V, A, S>(accumulator: (acc: A | S, value: V, index: number) => A, seed: S): OperatorFunction<V, A>;
+
+// TODO: link to a "redux pattern" section in the guide (location TBD)
+
+/**
+ * Useful for encapsulating and managing state. Applies an accumulator (or "reducer function")
+ * to each value from the source after an initial state is established -- either via
+ * a `seed` value (second argument), or from the first value from the source.
+ *
+ * <span class="informal">It's like {@link reduce}, but emits the current
+ * accumulation state after each update</span>
+ *
+ * ![](scan.png)
+ *
+ * This operator maintains an internal state and emits it after processing each value as follows:
+ *
+ * 1. First value arrives
+ *   - If a `seed` value was supplied (as the second argument to `scan`), let `state = seed` and `value = firstValue`.
+ *   - If NO `seed` value was supplied (no second argument), let `state = firstValue` and go to 3.
+ * 2. Let `state = accumulator(state, value)`.
+ *   - If an error is thrown by `accumulator`, notify the consumer of an error. The process ends.
+ * 3. Emit `state`.
+ * 4. Next value arrives, let `value = nextValue`, go to 2.
+ *
+ * ## Examples
+ *
+ * An average of previous numbers. This example shows how
+ * not providing a `seed` can prime the stream with the
+ * first value from the source.
+ *
+ * ```ts
+ * import { of, scan, map } from 'rxjs';
+ *
+ * const numbers$ = of(1, 2, 3);
+ *
+ * numbers$
+ *   .pipe(
+ *     // Get the sum of the numbers coming in.
+ *     scan((total, n) => total + n),
+ *     // Get the average by dividing the sum by the total number
+ *     // received so far (which is 1 more than the zero-based index).
+ *     map((sum, index) => sum / (index + 1))
+ *   )
+ *   .subscribe(console.log);
+ * ```
+ *
+ * The Fibonacci sequence. This example shows how you can use
+ * a seed to prime accumulation process. Also... you know... Fibonacci.
+ * So important to like, computers and stuff that its whiteboarded
+ * in job interviews. Now you can show them the Rx version! (Please don't, haha)
+ *
+ * ```ts
+ * import { interval, scan, map, startWith } from 'rxjs';
+ *
+ * const firstTwoFibs = [0, 1];
+ * // An endless stream of Fibonacci numbers.
+ * const fibonacci$ = interval(1000).pipe(
+ *   // Scan to get the fibonacci numbers (after 0, 1)
+ *   scan(([a, b]) => [b, a + b], firstTwoFibs),
+ *   // Get the second number in the tuple, it's the one you calculated
+ *   map(([, n]) => n),
+ *   // Start with our first two digits :)
+ *   startWith(...firstTwoFibs)
+ * );
+ *
+ * fibonacci$.subscribe(console.log);
+ * ```
+ *
+ * @see {@link expand}
+ * @see {@link mergeScan}
+ * @see {@link reduce}
+ * @see {@link switchScan}
+ *
+ * @param accumulator A "reducer function". This will be called for each value after an initial state is
+ * acquired.
+ * @param seed The initial state. If this is not provided, the first value from the source will
+ * be used as the initial state, and emitted without going through the accumulator. All subsequent values
+ * will be processed by the accumulator function. If this is provided, all values will go through
+ * the accumulator function.
+ * @return A function that returns an Observable of the accumulated values.
+ */
+export function scan<V, A, S>(accumulator: (acc: V | A | S, value: V, index: number) => A, seed?: S): OperatorFunction<V, V | A> {
+  // providing a seed of `undefined` *should* be valid and trigger
+  // hasSeed! so don't use `seed !== undefined` checks!
+  // For this reason, we have to check it here at the original call site
+  // otherwise inside Operator/Subscriber we won't know if `undefined`
+  // means they didn't provide anything or if they literally provided `undefined`
+  return operate(scanInternals(accumulator, seed as S, arguments.length >= 2, true));
+}
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