/*
 * Copyright (c) 2017 Kotlin Algorithm Club
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

package io.uuddlrlrba.ktalgs.sorts

/**
 * Invented in 1945 by John von Neumann, merge sort is an efficient algorithm using the divide and conquer approach
 * which is to divide a big problem into smaller problems and solve them. Conceptually, a merge sort works as follows:
 * 1) Divide the unsorted list into n sublists, each containing 1 element (a list of 1 element is considered sorted).
 * 2) Repeatedly merge sublists to produce new sorted sublists until there is only 1 sublist remaining.
 */
@ComparisonSort
@StableSort
class MergeSort: AbstractSortStrategy() {
    override fun <T : Comparable<T>> perform(arr: Array<T>) {
        val aux = arr.clone()
        sort(arr, aux, 0, arr.size - 1)
    }

    private fun <T : Comparable<T>> sort(arr: Array<T>, aux: Array<T>, lo: Int, hi: Int) {
        if (hi <= lo) return
        val mid = (lo + hi) / 2
        sort(arr, aux, lo, mid)
        sort(arr, aux, mid + 1, hi)
        merge(arr, aux, lo, mid, hi)
    }

    private fun <T : Comparable<T>> merge(arr: Array<T>, aux: Array<T>, lo: Int, mid: Int, hi: Int) {
        System.arraycopy(arr, lo, aux, lo, hi - lo + 1)

        var i = lo
        var j = mid + 1
        for (k in lo..hi) {
            when {
                i > mid -> arr[k] = aux[j++]
                j > hi -> arr[k] = aux[i++]
                aux[j] < aux[i] -> arr[k] = aux[j++]
                else -> arr[k] = aux[i++]
            }
        }
    }
}