aureooms/js-mergesort

export function _recursive ( merge ) {

    /**
     * if n = 2^k then
     *    input is in a if k is odd
     *    input is in b if k is even
     * otherwise input is in both a and b
     * output is in b
     */

    let sort = function ( compare , a , ai , aj , b , bi , bj ) {

        if ( aj - ai < 2 ) return ;

        let p = Math.floor( ( ai + aj ) / 2 ) ;

        sort( compare , b , bi , bi + p - ai , a , ai , p ) ;
        sort( compare , b , bi + p - ai , bj , a , p , aj ) ;

        merge( compare , a , ai , p , a , p , aj , b , bi ) ;

    } ;

    return sort ;

}