iterable-iterator/group

import {iter} from '@iterable-iterator/iter';

/**
 * Yields elements of the input iterable by grouping them into tuples
 * consecutive elements from the same equivalence class.
 *
 * @example
 * // A B C D A B
 * list( map( ( [ k , g ] ) => k , group( x => x , 'AAAABBBCCDAABBB' ) ) )
 *
 * @example
 * // AAAA BBB CC D
 * list( map( ( [ k , g ] ) => list( g ) , group( x => x , 'AAAABBBCCD' ) ) )
 *
 * @param {Function} key - The function used to determine the equivalence class
 * of an element.
 * @param {Iterable} iterable - The input iterable.
 * @returns {Iterator}
 */
export default function* group(key, iterable) {
    const iterator = iter(iterable);

    const first = iterator.next();

    if (first.done) {
        return;
    }

    let currval = first.value;
    let currkey = key(currval);

    const grouper = function* (tgtkey) {
        while (true) {
            yield currval;

            const event = iterator.next();
            if (event.done) {
                return;
            }

            currval = event.value;
            currkey = key(currval);

            if (currkey !== tgtkey) {
                return;
            }
        }
    };

    while (true) {
        const tgtkey = currkey;

        const g = grouper(tgtkey);

        yield [tgtkey, g];

        while (currkey === tgtkey) {
            const event = iterator.next();
            if (event.done) {
                return;
            }

            currval = event.value;
            currkey = key(currval);
        }
    }
}