Task: Sorting algorithms_Merge sort
C#: namespace RosettaCode {
  using System;

  public class MergeSort<T> where T : IComparable {
    #region Constants
    public const UInt32 INSERTION_LIMIT_DEFAULT = 12;
    public const Int32 MERGES_DEFAULT = 6;
    #endregion

    #region Properties
    public UInt32 InsertionLimit { get; }
    protected UInt32[] Positions { get; set; }

    private Int32 merges;
    public Int32 Merges {
      get { return merges; }
      set {
        
        if (value > 1)
          merges = value;
        else
          throw new ArgumentOutOfRangeException($"value = {value} must be greater than one", nameof(Merges));

        if (Positions == null || Positions.Length != merges)
          Positions = new UInt32[merges];
      }
    }
    #endregion

    #region Constructors
    public MergeSort(UInt32 insertionLimit, Int32 merges) {
      InsertionLimit = insertionLimit;
      Merges = merges;
    }

    public MergeSort()
      : this(INSERTION_LIMIT_DEFAULT, MERGES_DEFAULT) {
    }
    #endregion

    #region Sort Methods
    public void Sort(T[] entries) {
      
      var entries2 = new T[entries.Length];
      Sort(entries, entries2, 0, entries.Length - 1);
    }

    
    public void Sort(T[] entries1, T[] entries2, Int32 first, Int32 last) {
      var length = last + 1 - first;
      if (length < 2) return;      
      if (length < Merges || length < InsertionLimit) {
        InsertionSort<T>.Sort(entries1, first, last);
        return;
      }

      var left = first;
      var size = ceiling(length, Merges);
      for (var remaining = length; remaining > 0; remaining -= size, left += size) {
        var right = left + Math.Min(remaining, size) - 1;
        Sort(entries1, entries2, left, right);
      }

      Merge(entries1, entries2, first, last);
      Array.Copy(entries2, first, entries1, first, length);
    }
    #endregion

    #region Merge Methods
    public void Merge(T[] entries1, T[] entries2, Int32 first, Int32 last) {
      Array.Clear(Positions, 0, Merges);
      
      for (var index = first; index <= last; index++)
        entries2[index] = remove(entries1, first, last);
    }

    private T remove(T[] entries, Int32 first, Int32 last) {
      T entry = default;
      Int32? found = default;
      var length = last + 1 - first;

      var index = 0;
      var left = first;
      var size = ceiling(length, Merges);
      for (var remaining = length; remaining > 0; remaining -= size, left += size, index++) {
        var position = Positions[index];
        if (position < Math.Min(remaining, size)) {
          var next = entries[left + position];
          if (!found.HasValue || entry.CompareTo(next) > 0) {
            found = index;
            entry = next;
          }
        }
      }

      
      Positions[found.Value]++;
      return entry;
    }
    #endregion

    #region Math Methods
    private static Int32 ceiling(Int32 numerator, Int32 denominator) {
      return (numerator + denominator - 1) / denominator;
    }
    #endregion
  }

  #region Insertion Sort
  static class InsertionSort<T> where T : IComparable {
    public static void Sort(T[] entries, Int32 first, Int32 last) {
      for (var next = first + 1; next <= last; next++)
        insert(entries, first, next);
    }

    
    private static void insert(T[] entries, Int32 first, Int32 next) {
      var entry = entries[next];
      while (next > first && entries[next - 1].CompareTo(entry) > 0)
        entries[next] = entries[--next];
      entries[next] = entry;
    }
  }
  #endregion
}

Go: package main

import "fmt"

var a = []int{170, 45, 75, -90, -802, 24, 2, 66}
var s = make([]int, len(a)/2+1) 

func main() {
    fmt.Println("before:", a)
    mergeSort(a)
    fmt.Println("after: ", a)
}

func mergeSort(a []int) {
    if len(a) < 2 {
        return
    }
    mid := len(a) / 2
    mergeSort(a[:mid])
    mergeSort(a[mid:])
    if a[mid-1] <= a[mid] {
        return
    }
    
    copy(s, a[:mid])
    l, r := 0, mid
    for i := 0; ; i++ {
        if s[l] <= a[r] {
            a[i] = s[l]
            l++
            if l == mid {
                break
            }
        } else {
            a[i] = a[r]
            r++
            if r == len(a) {
                copy(a[i+1:], s[l:mid])
                break
            }
        }
    }
    return
}