skipDuplicates.ts

import { Event, isValue, Observer, Scope } from "./abstractions"
import { applyScopeMaybe } from "./applyscope"
import {
  transform,
  Transformer,
  UnaryTransformOp,
  UnaryTransformOpScoped,
} from "./transform"

export type Predicate2<A> = (prev: A, next: A) => boolean

export function skipDuplicates<A>(fn: Predicate2<A>): UnaryTransformOp<A>
export function skipDuplicates<A>(
  fn: Predicate2<A>,
  scope: Scope
): UnaryTransformOpScoped<A>
export function skipDuplicates<A>(fn: Predicate2<A>, scope?: Scope): any {
  return transform(
    ["skipDuplicates", [fn]],
    skipDuplicatesT(fn),
    scope as Scope
  )
}

function skipDuplicatesT<A>(fn: Predicate2<A>): Transformer<A, A> {
  let current: A | typeof uninitialized = uninitialized
  return {
    changes: (subscribe) => (onValue, onEnd) =>
      subscribe((value) => {
        if (current === uninitialized || !fn(current, value)) {
          current = value
          onValue(value)
        }
      }, onEnd),
    init: (value: A) => {
      current = value
      return value
    },
  }
}

const uninitialized: unique symbol = Symbol()