Generic Merge Sort Algorithm in C

Mergesort

-----
mergeAB(Item c[], Item a[], int N, Item b[], int M )
  { int i, j, k;
    for (i = 0, j = 0, k = 0; k < N+M; k++)
      {
        if (i == N) { c[k] = b[j++]; continue; }
        if (j == M) { c[k] = a[i++]; continue; }
        c[k] = (less(a[i], b[j])) ? a[i++] : b[j++];
      }
  }
-----
Item aux[maxN];
merge(Item a[], int l, int m, int r)
  { int i, j, k;
    for (i = m+1; i > l; i--) aux[i-1] = a[i-1];
    for (j = m; j < r; j++) aux[r+m-j] = a[j+1];
    for (k = l; k <= r; k++)
       if (less(aux[i], aux[j])) 
          a[k] = aux[i++]; else a[k] = aux[j--];
  }
-----
void mergesort(Item a[], int l, int r)
  { int m = (r+l)/2;
    if (r <= l) return;
    mergesort(a, l, m);  
    mergesort(a, m+1, r);
    merge(a, l, m, r);
  }
-----
#define maxN 10000
Item aux[maxN];
void mergesortABr(Item a[], Item b[], int l, int r)
  { int m = (l+r)/2;
    if (r-l <= 10) { insertion(a, l, r); return; }
    mergesortABr(b, a, l, m);  
    mergesortABr(b, a, m+1, r);
    mergeAB(a+l, b+l, m-l+1, b+m+1, r-m);
  }
void mergesortAB(Item a[], int l, int r)
  { int i;
    for (i = l; i <= r; i++) aux[i] = a[i];
    mergesortABr(a, aux, l, r);
  }
-----
#define min(A, B) (A < B) ? A : B
void mergesortBU(Item a[], int l, int r)
  { int i, m;
    for (m = 1; m < r-l; m = m+m)
      for (i = l; i <= r-m; i += m+m)
        merge(a, i, i+m-1, min(i+m+m-1, r));
  }
-----
link merge(link a, link b)
  { struct node head; link c = &head;
    while ((a != NULL) && (b != NULL))
      if (less(a->item, b->item))
        { c->next = a; c = a; a = a->next; }
      else
        { c->next = b; c = b; b = b->next; }
    c->next = (a == NULL) ? b : a;
    return head.next;
  }
-----
link merge(link a, link b);
link mergesort(link c)
  { link a, b;
    if (c->next == NULL) return c;
    a = c; b = c->next;
    while ((b != NULL) && (b->next != NULL))
      { c = c->next; b = b->next->next; }
    b = c->next; c->next = NULL;
    return merge(mergesort(a), mergesort(b));
  }
-----
link mergesort(link t)
  { link u;
    for (Qinit(); t != NULL; t = u) 
      { u = t->next; t->next = NULL; Qput(t); }
    t = Qget();     
    while (!Qempty())
      { Qput(t); t = merge(Qget(), Qget()); }
    return t;
  }