Sync

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

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+#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 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 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;
+}