// Copyright Fauna, Inc.
// SPDX-License-Identifier: MIT-0

import faunadb, { CreateFunction } from 'faunadb';

const q = faunadb.query
const {
  Drop,
  Abort,
  If,
  IsArray,
  Lambda,
  Query,
  Var
} = q;

/**
 * @param {Array<any>} array - The array to take the tail from.
 * @returns {Array<any>} - The tail of the array.
 * 
 * Tail() accepts a Fauna Array and returns a new array consisting of every
 * element except the first. If the array is empty, Tail() returns an empty array.
 *
 * ### Consumes per call
 * * 0 Read Ops
 * * 0 Write Ops
 * * 1 ComputeOp (8 FQL verbs)
 * 
 * ### Dependencies:
 * none
 *
 * ### Usage in Fauna Schema Migrate:
 * ```js
 * import { Tail } from '@fauna-labs/fql-utilities/collections';
 * export default Tail();
 * ```
 *
 * ### Usage in FQL:
 * ```fql
 * Let(
 *   {
 *     array: [0, 1, 2],
 *     shortArray: Call("Tail", Var("array"))
 *   },
 *   Equals(Count(Var("shortArray")), 2) // true
 * )
 *```
 */

const body = Query(Lambda(
  ["array"],
  If(
    IsArray(Var("array")),
    Drop(1, Var("array")),
    Abort("Tail() requires one argument that is an array.")
  )
));

export function Tail(replace: boolean = false): faunadb.Expr {
  return CreateFunction({
    name: "Tail",
    body: body 
  });
}