// Copyright 2018-2026 the Deno authors. MIT license.
// This module is browser compatible.

/**
 * Returns the first element that is the smallest value of the given function or
 * undefined if there are no elements.
 *
 * @typeParam T The type of the elements in the array.
 *
 * @param array The array to find the minimum element in.
 * @param selector The function to get the value to compare from each element.
 * The function receives the element and its index.
 *
 * @returns The first element that is the smallest value of the given function
 * or undefined if there are no elements.
 *
 * @example Basic usage
 * ```ts
 * import { minBy } from "min_by.ts";
 * import { assertEquals } from "../assert/mod.ts";
 *
 * const people = [
 *   { name: "Anna", age: 34 },
 *   { name: "Kim", age: 42 },
 *   { name: "John", age: 23 },
 * ];
 *
 * const personWithMinAge = minBy(people, (i) => i.age);
 *
 * assertEquals(personWithMinAge, { name: "John", age: 23 });
 * ```
 *
 * @example Using the index parameter
 * ```ts
 * import { minBy } from "min_by.ts";
 * import { assertEquals } from "../assert/mod.ts";
 *
 * const array = [4, 3, 2, 1];
 * const result = minBy(array, (_, index) => index);
 *
 * assertEquals(result, 4);
 * ```
 */
export function minBy<T>(
  array: Iterable<T>,
  selector: (el: T, index: number) => number,
): T | undefined;
/**
 * Returns the first element that is the smallest value of the given function or
 * undefined if there are no elements.
 *
 * @typeParam T The type of the elements in the array.
 *
 * @param array The array to find the minimum element in.
 * @param selector The function to get the value to compare from each element.
 * The function receives the element and its index.
 *
 * @returns The first element that is the smallest value of the given function
 * or undefined if there are no elements.
 *
 * @example Basic usage
 * ```ts
 * import { minBy } from "min_by.ts";
 * import { assertEquals } from "../assert/mod.ts";
 *
 * const people = [
 *   { name: "Anna" },
 *   { name: "Kim" },
 *   { name: "John" },
 * ];
 *
 * const personWithMinName = minBy(people, (person) => person.name);
 *
 * assertEquals(personWithMinName, { name: "Anna" });
 * ```
 */
export function minBy<T>(
  array: Iterable<T>,
  selector: (el: T, index: number) => string,
): T | undefined;
/**
 * Returns the first element that is the smallest value of the given function or
 * undefined if there are no elements.
 *
 * @typeParam T The type of the elements in the array.
 *
 * @param array The array to find the minimum element in.
 * @param selector The function to get the value to compare from each element.
 * The function receives the element and its index.
 *
 * @returns The first element that is the smallest value of the given function
 * or undefined if there are no elements.
 *
 * @example Basic usage
 * ```ts
 * import { minBy } from "min_by.ts";
 * import { assertEquals } from "../assert/mod.ts";
 *
 * const people = [
 *   { name: "Anna", age: 34n },
 *   { name: "Kim", age: 42n },
 *   { name: "John", age: 23n },
 * ];
 *
 * const personWithMinAge = minBy(people, (i) => i.age);
 *
 * assertEquals(personWithMinAge, { name: "John", age: 23n });
 * ```
 */
export function minBy<T>(
  array: Iterable<T>,
  selector: (el: T, index: number) => bigint,
): T | undefined;
/**
 * Returns the first element that is the smallest value of the given function or
 * undefined if there are no elements.
 *
 * @typeParam T The type of the elements in the array.
 *
 * @param array The array to find the minimum element in.
 * @param selector The function to get the value to compare from each element.
 * The function receives the element and its index.
 *
 * @returns The first element that is the smallest value of the given function
 * or undefined if there are no elements.
 *
 * @example Basic usage
 * ```ts
 * import { minBy } from "min_by.ts";
 * import { assertEquals } from "../assert/mod.ts";
 *
 * const people = [
 *   { name: "Anna", startedAt: new Date("2020-01-01") },
 *   { name: "Kim", startedAt: new Date("2020-03-01") },
 *   { name: "John", startedAt: new Date("2019-01-01") },
 * ];
 *
 * const personWithMinStartedAt = minBy(people, (person) => person.startedAt);
 * ```
 */
export function minBy<T>(
  array: Iterable<T>,
  selector: (el: T, index: number) => Date,
): T | undefined;
export function minBy<T>(
  array: Iterable<T>,
  selector:
    | ((el: T, index: number) => number)
    | ((el: T, index: number) => string)
    | ((el: T, index: number) => bigint)
    | ((el: T, index: number) => Date),
): T | undefined {
  if (Array.isArray(array)) {
    const length = array.length;
    if (length === 0) return undefined;

    let min: T = array[0]!;
    let minValue = selector(min, 0);

    for (let i = 1; i < length; i++) {
      const current = array[i]!;
      const currentValue = selector(current, i);
      if (currentValue < minValue) {
        min = current;
        minValue = currentValue;
      }
    }

    return min;
  }

  let index = 0;
  const iter = array[Symbol.iterator]();
  const first = iter.next();

  if (first.done) return undefined;

  let min: T = first.value;
  let minValue = selector(min, index++);

  let next = iter.next();
  while (!next.done) {
    const currentValue = selector(next.value, index++);
    if (currentValue < minValue) {
      min = next.value;
      minValue = currentValue;
    }
    next = iter.next();
  }

  return min;
}