Just another update

Task/Sorting-algorithms-Merge-sort/C/sorting-algorithms-merge-sort.c

@@ -1,102 +1,38 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#define LEN 20
-#define MAXEL 100
-
-void merge(int * list, int left_start, int left_end, int right_start, int right_end)
-{
-	/* calculate temporary list sizes */
-	int left_length = left_end - left_start;
-	int right_length = right_end - right_start;
-
-	/* declare temporary lists */
-	int left_half[left_length];
-	int right_half[right_length];
-
-	int r = 0; /* right_half index */
-	int l = 0; /* left_half index */
-	int i = 0; /* list index */
-
-	/* copy left half of list into left_half */
-	for (i = left_start; i < left_end; i++, l++)
-	{
-		left_half[l] = list[i];
-	}
-	
-	/* copy right half of list into right_half */
-	for (i = right_start; i < right_end; i++, r++)
-	{
-		right_half[r] = list[i];
-	}
-	
-	/* merge left_half and right_half back into list */
-	for ( i = left_start, r = 0, l = 0; l < left_length && r < right_length; i++)
-	{
-		if ( left_half[l] < right_half[r] ) { list[i] = left_half[l++]; }
-		else { list[i] = right_half[r++]; }
-	}
-	
-	/* Copy over leftovers of whichever temporary list hasn't finished */
-	for ( ; l < left_length; i++, l++) { list[i] = left_half[l]; }
-	for ( ; r < right_length; i++, r++) { list[i] = right_half[r]; }
+void merge (int *a, int n, int m) {
+    int i, j, k;
+    int *x = malloc(n * sizeof (int));
+    for (i = 0, j = m, k = 0; k < n; k++) {
+        x[k] = j == n      ? a[i++]
+             : i == m      ? a[j++]
+             : a[j] < a[i] ? a[j++]
+             :               a[i++];
+    }
+    for (i = 0; i < n; i++) {
+        a[i] = x[i];
+    }
+    free(x);
 }
 
-void mergesort_r(int left, int right, int * list)
-{
-	/* Base case, the list can be no simpiler */
-	if (right - left <= 1)
-	{
-		return;
-	}
-	
-	/* set up bounds to slice array into */
-	int left_start  = left;
-	int left_end    = (left+right)/2;
-	int right_start = left_end;
-	int right_end   = right;
-
-	/* sort left half */
-	mergesort_r( left_start, left_end, list);
-	/* sort right half */
-	mergesort_r( right_start, right_end, list);
-	
-	/* merge sorted halves back together */
-	merge(list, left_start, left_end, right_start, right_end);
+void merge_sort (int *a, int n) {
+    if (n < 2)
+        return;
+    int m = n / 2;
+    merge_sort(a, m);
+    merge_sort(a + m, n - m);
+    merge(a, n, m);
 }
 
-void mergesort(int * list, int length)
-{
-	mergesort_r(0, length, list);
-}
-
-void print_list(int * list, int len)
-{
-	/* Print all the ints of a list in brackets followed by a newline */
-	int i;
-	
-	printf("[ ");
-	for (i = 0; i < len; i++)
-	{
-		printf("%d ", list[i]);
-	}
-	printf("]\n");
-}
-
-int main()
-{
-	/* Set up the list */
-	int list[LEN], i;
-	for (i = 0; i < LEN; i++) { list[i] = rand() % MAXEL; }
-	
-	/* Do merge sort and print before/after */
-	printf("Before sort: ");
-	print_list(list, LEN);
-	
-	mergesort(list, LEN);
-
-	printf("After sort: ");
-	print_list(list, LEN);
-	
-	return 0;
+int main () {
+    int a[] = {4, 65, 2, -31, 0, 99, 2, 83, 782, 1};
+    int n = sizeof a / sizeof a[0];
+    int i;
+    for (i = 0; i < n; i++)
+        printf("%d%s", a[i], i == n - 1 ? "\n" : " ");
+    merge_sort(a, n);
+    for (i = 0; i < n; i++)
+        printf("%d%s", a[i], i == n - 1 ? "\n" : " ");
+    return 0;
 }