import {InternMap} from "internmap";

import identity from "./identity.js";

export default function group(values, ...keys) {

return nest(values, identity, identity, keys);

}

export function groups(values, ...keys) {

return nest(values, Array.from, identity, keys);

}

function flatten(groups, keys) {

for (let i = 1, n = keys.length; i < n; ++i) {

groups = groups.flatMap(g => g.pop().map(([key, value]) => [...g, key, value]));

}

return groups;

}

export function flatGroup(values, ...keys) {

return flatten(groups(values, ...keys), keys);

}

export function flatRollup(values, reduce, ...keys) {

return flatten(rollups(values, reduce, ...keys), keys);

}

export function rollup(values, reduce, ...keys) {

return nest(values, identity, reduce, keys);

}

export function rollups(values, reduce, ...keys) {

return nest(values, Array.from, reduce, keys);

}

export function index(values, ...keys) {

return nest(values, identity, unique, keys);

}

export function indexes(values, ...keys) {

return nest(values, Array.from, unique, keys);

}

function unique(values) {

if (values.length !== 1) throw new Error("duplicate key");

return values[0];

}

function nest(values, map, reduce, keys) {

return (function regroup(values, i) {

if (i >= keys.length) return reduce(values);

const groups = new InternMap();

const keyof = keys[i++];

let index = -1;

for (const value of values) {

const key = keyof(value, ++index, values);

const group = groups.get(key);

if (group) group.push(value);

else groups.set(key, [value]);

}

for (const [key, values] of groups) {

groups.set(key, regroup(values, i));

}

return map(groups);

})(values, 0);

}