fixup alt

lib/stdlib/qsort.c

@@ -1,7 +1,6 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
  * Copyright © 2017,2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
- * Copyright © 2021 Paul Dersey <pdersey@gmail.com>
  *
  * This file is part of GNU Mes.
  *
@@ -22,47 +21,78 @@
 #include <stdlib.h>
 
 void
-qswap (void* a, void* b, int size)
+qswap (void *a, void *b, size_t size)
 {
-  char buffer[size];
-  memcpy (buffer, a, size);
-  memcpy (a, b, size);
-  memcpy (b, buffer, size);
+  char *pa = a;
+  char *pb = b;
+  do
+    {
+      char tmp = *pa;
+      *pa++ = *pb;
+      *pb++ = tmp;
+    } while (--size > 0);
+}
+
+size_t
+qpart (void *base, size_t low, size_t high, size_t size,
+           int (*compare) (void const *, void const *))
+{
+  char *pbase = base;
+  // select the rightmost element as pivot
+  void *pivot = pbase + high * size;
+
+  // pointer for greater element
+  size_t i = (low - 1);
+
+  // traverse each element of the array
+  // compare them with the pivot
+  for (int j = low; j < high; j++)
+    {
+      int c = compare (pbase + j * size, pivot);
+      if (c < 0)
+        {
+          // if element smaller than pivot is found
+          // swap it with the greater element pointed by i
+          i++;
+
+          // swap element at i with element at j
+          void *p1 = pbase + i * size;
+          void *p2 = pbase + j * size;
+          qswap (p1, p2, size);
+        }
+    }
+
+  // swap the pivot element with the greater element at i
+  void *p1 = pbase + (i + 1) * size;
+  void *p2 = pbase + high * size;
+  qswap (p1, p2, size);
+
+  // return the qpart point
+  return (i + 1);
 }
 
 void
-_qsort (void *base, size_t size, size_t left, size_t right,
-        int (*compare)(void const*, void const*))
+_qsort (void *base, size_t low, size_t high, size_t size,
+        int (*compare) (void const *, void const *))
 {
-  char *pbase = base;
-  int mid = (left + right) / 2;
-  if (left >= right)
+  if (low >= high)
     return;
 
-  void *vl = pbase + (left * size);
-  void *vr = pbase + (mid * size);
-  qswap (vl, vr, size);
+  // find the pivot element such that
+  // elements smaller than pivot are on left of pivot
+  // elements greater than pivot are on right of pivot
+  int pi = qpart (base, low, high, size, compare);
 
-  int last = left;
-  for (int i = left + 1; i <= right; i++)
-    {
-      void *vt = pbase + (i * size);
-      if ((*compare)(vl, vt) > 0)
-        {
-          ++last;
-          void *v3 = pbase + (last * size);
-          qswap (vt, v3, size);
-        }
-    }
-  void *v3 = pbase + (last * size);
-  qswap (vl, v3, size);
-  _qsort (base, size, left, last - 1, compare);
-  _qsort (base, size, last + 1, right, compare);
+  // recursive call on the left of pivot
+  _qsort (base, low, pi - 1, size, compare);
+
+  // recursive call on the right of pivot
+  _qsort (base, pi + 1, high, size, compare);
 }
 
 void
 qsort (void *base, size_t count, size_t size,
-       int (*compare)(void const*, void const*))
+       int (*compare) (void const *, void const *))
 {
-  _qsort (base, size, 0, count, compare);
+  _qsort (base, 0, count, size, compare);
 }