Revert "Revert "initial commit""

This reverts commit c6d2f76.

Author: CSchank

example_constant.c

@@ -0,0 +1,59 @@
+/*
+ * C library computing the partial sums of a 2D matrix in O(m*n) time.
+ * Query operations run in constant time.
+ *
+ * Author: Christopher W. Schankula
+ * Last updated: December 16th, 2017
+ * Code is licensed under the CC BY 4.0 license.
+ *
+ * THE CODE PROVIDED HEREIN IS PROVIDED "AS-IS"
+ * THE AUTHOR ASSUMES NO RESPONSBILITY OR WARRANTY
+ * FOR USE OF THE CODE PROVIDED HEREIN
+ * EXTENSIVE TESTING OF THIS CODE IS REQUIRED
+ * TO ENSURE ITS CORRECTNESS AS PART OF THE INTENDED
+ * APPLICATION THEREOF
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "partialsums.h"
+
+#define W 10
+#define H 30
+#define V 5
+
+#define X1 0
+#define Y1 0
+#define X2 4
+#define Y2 4
+
+int main(){
+	unsigned int A[W*H];
+
+	for (int i = 0; i < W * H; i++)
+		A[i] = V;
+
+	unsigned long int *res = generate_lookup(A, H, W);
+
+	printf("Original Matrix\n");
+	for (int j = 0; j < H; j++){
+		for (int i = 0; i < W; i++)
+			printf("%5d", A[j*W + i]);
+		printf("\n");
+	}
+
+	printf("\n");
+
+	printf("Sum Lookup Matrix\n");
+	for (int j = 0; j < H; j++){
+		for (int i = 0; i < W; i++)
+			printf("%5ld", res[j*W + i]);
+		printf("\n");
+	}
+	printf("\n");
+
+	printf("Sum of region with size %d x %d: %ld\n", X2 - X1 + 1, Y2 - Y1 + 1, query_sum(res, W, X1, Y1, X2, Y2));
+
+	free(res);
+	return 0;
+}

example_mean_filt.c

@@ -0,0 +1,70 @@
+/*
+ * C library computing the partial sums of a 2D matrix in O(m*n) time.
+ * Query operations run in constant time.
+ *
+ * Author: Christopher W. Schankula
+ * Last updated: December 16th, 2017
+ * Code is licensed under the CC BY 4.0 license.
+ *
+ * THE CODE PROVIDED HEREIN IS PROVIDED "AS-IS"
+ * THE AUTHOR ASSUMES NO RESPONSBILITY OR WARRANTY
+ * FOR USE OF THE CODE PROVIDED HEREIN
+ * EXTENSIVE TESTING OF THIS CODE IS REQUIRED
+ * TO ENSURE ITS CORRECTNESS AS PART OF THE INTENDED
+ * APPLICATION THEREOF
+ */
+
+#include <stdio.h>
+#include "partialsums.h"
+#include <stdlib.h>
+
+#define H 8
+#define W 8
+#define S 3
+
+unsigned int picture[64] = 
+	{
+ 	 255,255,255,255,255,255,255,255
+	,255,255,255,255,255,255,255,255
+	,255, 0 ,255,255,255,255, 0 ,255
+	,255,255,255,255,255,255,255,255
+	,255,255,255,255,255,255,255,255
+	,255, 0 ,255,255,255,255, 0 ,255
+	,255,255, 0 , 0 , 0 , 0 ,255,255
+	,255,255,255,255,255,255,255,255
+	};	
+
+int main(){
+	unsigned long int *sum_lookup = generate_lookup(picture, 8, 8);
+	unsigned int *new_picture = malloc(H * W * sizeof(int));
+
+	printf("Original picture\n");
+	for (int j = 0; j < H; j++){
+		for (int i = 0; i < W; i++)
+			printf("%5d", picture[j*W + i]);
+		printf("\n");
+	}
+
+	for (int j = 0; j < H; j++){
+		for (int i = 0; i < W; i++){
+			/* Handle edge cases */
+			int x1 = i - S / 2 > 0 ? i - S / 2 : 0;
+			int y1 = j - S / 2 > 0 ? j - S / 2 : 0;
+			int x2 = i + S / 2 < W ? i + S / 2 : W - 1;
+			int y2 = j + S / 2 < H ? j + S / 2 : H - 1;
+
+			*(new_picture + j * W + i) = query_sum(sum_lookup, W, x1, y1, x2, y2) / ((x2 - x1 + 1) * (y2 - y1 + 1));
+		}
+	}
+
+	printf("New picture\n");
+	for (int j = 0; j < H; j++){
+		for (int i = 0; i < W; i++)
+			printf("%5d", new_picture[j*W + i]);
+		printf("\n");
+	}
+
+	free(new_picture);
+	free(sum_lookup);
+	return 0;
+}

makefile

@@ -0,0 +1,13 @@
+CFLAGS = -Wall -O2 -std=c99 -pedantic
+all: example_constant example_mean_filt
+
+example_constant: example_constant.o partialsums.o
+	$(CC) $(CFLAGS) -o $@ $?
+
+example_mean_filt: example_mean_filt.o partialsums.o
+	$(CC) $(CFLAGS) -o $@ $?
+
+c: clean
+
+clean:
+	rm *.o example_constant example_mean_filt

partialsums.c

@@ -0,0 +1,56 @@
+/*
+ * C library computing the partial sums of a 2D matrix in O(m*n) time.
+ * Query operations run in constant time.
+ *
+ * Author: Christopher W. Schankula
+ * Last updated: December 16th, 2017
+ * Code is licensed under the CC BY 4.0 license.
+ *
+ * THE CODE PROVIDED HEREIN IS PROVIDED "AS-IS"
+ * THE AUTHOR ASSUMES NO RESPONSBILITY OR WARRANTY
+ * FOR USE OF THE CODE PROVIDED HEREIN
+ * EXTENSIVE TESTING OF THIS CODE IS REQUIRED
+ * TO ENSURE ITS CORRECTNESS AS PART OF THE INTENDED
+ * APPLICATION THEREOF
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include "partialsums.h"
+
+/* 
+ * Generates the lookup table for a given matrix with m rows and n columns. 
+ * Adapted from user rolfl's O(n) algorithm: 
+ * https://codereview.stackexchange.com/questions/42906/partial-sums-of-two-dimensional-array
+ */
+unsigned long int *generate_lookup(unsigned int *A, int m, int n){
+	unsigned long int *B = malloc(m*n*sizeof(unsigned long int));
+	for (int j = 0; j < m; j++){
+		for (int i = 0; i < n; i++){
+			unsigned long int abovesum = j > 0 ? *(B + (j - 1) * n + i) : 0;
+	   		unsigned long int leftsum = i > 0 ? *(B + j * n + i - 1) : 0;
+			unsigned long int aboveleftsum = (i > 0 && j > 0) ? *(B + (j - 1) * n + i - 1) : 0;
+			int val = *(A + j * n + i);
+
+			*(B + j * n + i) = val 
+							 + aboveleftsum 
+							 + (leftsum - aboveleftsum)
+							 + (abovesum - aboveleftsum);
+		}
+	}
+	return B;
+}
+
+/* 
+ * Queries a 2D partial-sum lookup table, returning the sum of the area enclosed by 
+ * the rectangle (x1,y1,x2,y2) where (x1,y1) is the top-left corner and (x2,y2) is the
+ * bottom-right corner.
+ *
+*/
+unsigned long int query_sum(unsigned long int *A, int n, int x1, int y1, int x2, int y2){
+	unsigned long int r1 = *(A + y2 * n + x2);
+	unsigned long int r2 = x1 > 0 ? *(A + y2 * n + x1 - 1) : 0;
+	unsigned long int r3 = y1 > 0 ? *(A + (y1 - 1) * n + x2) : 0;
+	unsigned long int r4 = x1 > 0 && y1 > 0 ? * (A + (y1 - 1) * n + x1 - 1) : 0;
+	return r1 - r2 - r3 + r4;
+}

partialsums.h

@@ -0,0 +1,18 @@
+/*
+ * C library computing the partial sums of a 2D matrix in O(m*n) time.
+ * Query operations run in constant time.
+ *
+ * Author: Christopher W. Schankula
+ * Last updated: December 16th, 2017
+ * Code is licensed under the CC BY 4.0 license.
+ *
+ * THE CODE PROVIDED HEREIN IS PROVIDED "AS-IS"
+ * THE AUTHOR ASSUMES NO RESPONSBILITY OR WARRANTY
+ * FOR USE OF THE CODE PROVIDED HEREIN
+ * EXTENSIVE TESTING OF THIS CODE IS REQUIRED
+ * TO ENSURE ITS CORRECTNESS AS PART OF THE INTENDED
+ * APPLICATION THEREOF
+ */
+
+unsigned long int *generate_lookup(unsigned int *A, int m, int n);
+unsigned long int query_sum(unsigned long int *A, int n, int x1, int y1, int x2, int y2);