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+import { Observable } from '../Observable';
+import { identity } from '../util/identity';
+import { ObservableInput, SchedulerLike } from '../types';
+import { isScheduler } from '../util/isScheduler';
+import { defer } from './defer';
+import { scheduleIterable } from '../scheduled/scheduleIterable';
+
+type ConditionFunc<S> = (state: S) => boolean;
+type IterateFunc<S> = (state: S) => S;
+type ResultFunc<S, T> = (state: S) => T;
+
+export interface GenerateBaseOptions<S> {
+  /**
+   * Initial state.
+   */
+  initialState: S;
+  /**
+   * Condition function that accepts state and returns boolean.
+   * When it returns false, the generator stops.
+   * If not specified, a generator never stops.
+   */
+  condition?: ConditionFunc<S>;
+  /**
+   * Iterate function that accepts state and returns new state.
+   */
+  iterate: IterateFunc<S>;
+  /**
+   * SchedulerLike to use for generation process.
+   * By default, a generator starts immediately.
+   */
+  scheduler?: SchedulerLike;
+}
+
+export interface GenerateOptions<T, S> extends GenerateBaseOptions<S> {
+  /**
+   * Result selection function that accepts state and returns a value to emit.
+   */
+  resultSelector: ResultFunc<S, T>;
+}
+
+/**
+ * Generates an observable sequence by running a state-driven loop
+ * producing the sequence's elements, using the specified scheduler
+ * to send out observer messages.
+ *
+ * ![](generate.png)
+ *
+ * ## Examples
+ *
+ * Produces sequence of numbers
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate(0, x => x < 3, x => x + 1, x => x);
+ *
+ * result.subscribe(x => console.log(x));
+ *
+ * // Logs:
+ * // 0
+ * // 1
+ * // 2
+ * ```
+ *
+ * Use `asapScheduler`
+ *
+ * ```ts
+ * import { generate, asapScheduler } from 'rxjs';
+ *
+ * const result = generate(1, x => x < 5, x => x * 2, x => x + 1, asapScheduler);
+ *
+ * result.subscribe(x => console.log(x));
+ *
+ * // Logs:
+ * // 2
+ * // 3
+ * // 5
+ * ```
+ *
+ * @see {@link from}
+ * @see {@link Observable}
+ *
+ * @param initialState Initial state.
+ * @param condition Condition to terminate generation (upon returning false).
+ * @param iterate Iteration step function.
+ * @param resultSelector Selector function for results produced in the sequence.
+ * @param scheduler A {@link SchedulerLike} on which to run the generator loop.
+ * If not provided, defaults to emit immediately.
+ * @returns The generated sequence.
+ * @deprecated Instead of passing separate arguments, use the options argument.
+ * Signatures taking separate arguments will be removed in v8.
+ */
+export function generate<T, S>(
+  initialState: S,
+  condition: ConditionFunc<S>,
+  iterate: IterateFunc<S>,
+  resultSelector: ResultFunc<S, T>,
+  scheduler?: SchedulerLike
+): Observable<T>;
+
+/**
+ * Generates an Observable by running a state-driven loop
+ * that emits an element on each iteration.
+ *
+ * <span class="informal">Use it instead of nexting values in a for loop.</span>
+ *
+ * ![](generate.png)
+ *
+ * `generate` allows you to create a stream of values generated with a loop very similar to
+ * a traditional for loop. The first argument of `generate` is a beginning value. The second argument
+ * is a function that accepts this value and tests if some condition still holds. If it does,
+ * then the loop continues, if not, it stops. The third value is a function which takes the
+ * previously defined value and modifies it in some way on each iteration. Note how these three parameters
+ * are direct equivalents of three expressions in a traditional for loop: the first expression
+ * initializes some state (for example, a numeric index), the second tests if the loop can perform the next
+ * iteration (for example, if the index is lower than 10) and the third states how the defined value
+ * will be modified on every step (for example, the index will be incremented by one).
+ *
+ * Return value of a `generate` operator is an Observable that on each loop iteration
+ * emits a value. First of all, the condition function is ran. If it returns true, then the Observable
+ * emits the currently stored value (initial value at the first iteration) and finally updates
+ * that value with iterate function. If at some point the condition returns false, then the Observable
+ * completes at that moment.
+ *
+ * Optionally you can pass a fourth parameter to `generate` - a result selector function which allows you
+ * to immediately map the value that would normally be emitted by an Observable.
+ *
+ * If you find three anonymous functions in `generate` call hard to read, you can provide
+ * a single object to the operator instead where the object has the properties: `initialState`,
+ * `condition`, `iterate` and `resultSelector`, which should have respective values that you
+ * would normally pass to `generate`. `resultSelector` is still optional, but that form
+ * of calling `generate` allows you to omit `condition` as well. If you omit it, that means
+ * condition always holds, or in other words the resulting Observable will never complete.
+ *
+ * Both forms of `generate` can optionally accept a scheduler. In case of a multi-parameter call,
+ * scheduler simply comes as a last argument (no matter if there is a `resultSelector`
+ * function or not). In case of a single-parameter call, you can provide it as a
+ * `scheduler` property on the object passed to the operator. In both cases, a scheduler decides when
+ * the next iteration of the loop will happen and therefore when the next value will be emitted
+ * by the Observable. For example, to ensure that each value is pushed to the Observer
+ * on a separate task in the event loop, you could use the `async` scheduler. Note that
+ * by default (when no scheduler is passed) values are simply emitted synchronously.
+ *
+ *
+ * ## Examples
+ *
+ * Use with condition and iterate functions
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate(0, x => x < 3, x => x + 1);
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!')
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1
+ * // 2
+ * // 'Complete!'
+ * ```
+ *
+ * Use with condition, iterate and resultSelector functions
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate(0, x => x < 3, x => x + 1, x => x * 1000);
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!')
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1000
+ * // 2000
+ * // 'Complete!'
+ * ```
+ *
+ * Use with options object
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate({
+ *   initialState: 0,
+ *   condition(value) { return value < 3; },
+ *   iterate(value) { return value + 1; },
+ *   resultSelector(value) { return value * 1000; }
+ * });
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!')
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1000
+ * // 2000
+ * // 'Complete!'
+ * ```
+ *
+ * Use options object without condition function
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate({
+ *   initialState: 0,
+ *   iterate(value) { return value + 1; },
+ *   resultSelector(value) { return value * 1000; }
+ * });
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!') // This will never run
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1000
+ * // 2000
+ * // 3000
+ * // ...and never stops.
+ * ```
+ *
+ * @see {@link from}
+ *
+ * @param initialState Initial state.
+ * @param condition Condition to terminate generation (upon returning false).
+ * @param iterate Iteration step function.
+ * @param scheduler A {@link Scheduler} on which to run the generator loop. If not
+ * provided, defaults to emitting immediately.
+ * @return The generated sequence.
+ * @deprecated Instead of passing separate arguments, use the options argument.
+ * Signatures taking separate arguments will be removed in v8.
+ */
+export function generate<S>(
+  initialState: S,
+  condition: ConditionFunc<S>,
+  iterate: IterateFunc<S>,
+  scheduler?: SchedulerLike
+): Observable<S>;
+
+/**
+ * Generates an observable sequence by running a state-driven loop
+ * producing the sequence's elements, using the specified scheduler
+ * to send out observer messages.
+ * The overload accepts options object that might contain initial state, iterate,
+ * condition and scheduler.
+ *
+ * ![](generate.png)
+ *
+ * ## Examples
+ *
+ * Use options object with condition function
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate({
+ *   initialState: 0,
+ *   condition: x => x < 3,
+ *   iterate: x => x + 1
+ * });
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!')
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1
+ * // 2
+ * // 'Complete!'
+ * ```
+ *
+ * @see {@link from}
+ * @see {@link Observable}
+ *
+ * @param options Object that must contain initialState, iterate and might contain condition and scheduler.
+ * @returns The generated sequence.
+ */
+export function generate<S>(options: GenerateBaseOptions<S>): Observable<S>;
+
+/**
+ * Generates an observable sequence by running a state-driven loop
+ * producing the sequence's elements, using the specified scheduler
+ * to send out observer messages.
+ * The overload accepts options object that might contain initial state, iterate,
+ * condition, result selector and scheduler.
+ *
+ * ![](generate.png)
+ *
+ * ## Examples
+ *
+ * Use options object with condition and iterate function
+ *
+ * ```ts
+ * import { generate } from 'rxjs';
+ *
+ * const result = generate({
+ *   initialState: 0,
+ *   condition: x => x < 3,
+ *   iterate: x => x + 1,
+ *   resultSelector: x => x
+ * });
+ *
+ * result.subscribe({
+ *   next: value => console.log(value),
+ *   complete: () => console.log('Complete!')
+ * });
+ *
+ * // Logs:
+ * // 0
+ * // 1
+ * // 2
+ * // 'Complete!'
+ * ```
+ *
+ * @see {@link from}
+ * @see {@link Observable}
+ *
+ * @param options Object that must contain initialState, iterate, resultSelector and might contain condition and scheduler.
+ * @returns The generated sequence.
+ */
+export function generate<T, S>(options: GenerateOptions<T, S>): Observable<T>;
+
+export function generate<T, S>(
+  initialStateOrOptions: S | GenerateOptions<T, S>,
+  condition?: ConditionFunc<S>,
+  iterate?: IterateFunc<S>,
+  resultSelectorOrScheduler?: ResultFunc<S, T> | SchedulerLike,
+  scheduler?: SchedulerLike
+): Observable<T> {
+  let resultSelector: ResultFunc<S, T>;
+  let initialState: S;
+
+  // TODO: Remove this as we move away from deprecated signatures
+  // and move towards a configuration object argument.
+  if (arguments.length === 1) {
+    // If we only have one argument, we can assume it is a configuration object.
+    // Note that folks not using TypeScript may trip over this.
+    ({
+      initialState,
+      condition,
+      iterate,
+      resultSelector = identity as ResultFunc<S, T>,
+      scheduler,
+    } = initialStateOrOptions as GenerateOptions<T, S>);
+  } else {
+    // Deprecated arguments path. Figure out what the user
+    // passed and set it here.
+    initialState = initialStateOrOptions as S;
+    if (!resultSelectorOrScheduler || isScheduler(resultSelectorOrScheduler)) {
+      resultSelector = identity as ResultFunc<S, T>;
+      scheduler = resultSelectorOrScheduler as SchedulerLike;
+    } else {
+      resultSelector = resultSelectorOrScheduler as ResultFunc<S, T>;
+    }
+  }
+
+  // The actual generator used to "generate" values.
+  function* gen() {
+    for (let state = initialState; !condition || condition(state); state = iterate!(state)) {
+      yield resultSelector(state);
+    }
+  }
+
+  // We use `defer` because we want to defer the creation of the iterator from the iterable.
+  return defer(
+    (scheduler
+      ? // If a scheduler was provided, use `scheduleIterable` to ensure that iteration/generation
+        // happens on the scheduler.
+        () => scheduleIterable(gen(), scheduler!)
+      : // Otherwise, if there's no scheduler, we can just use the generator function directly in
+        // `defer` and executing it will return the generator (which is iterable).
+        gen) as () => ObservableInput<T>
+  );
+}