Merge pull request algorithm-archivists#106 from Gathros/CGrahamPR

Adding graham.c

Author: june128

chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c

@@ -0,0 +1,114 @@
+#include <math.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct point {
+    double x, y;
+};
+
+int cmp_points(const void *a, const void *b) {
+    struct point* pa = (struct point*) a;
+    struct point* pb = (struct point*) b;
+
+    if (pa->y > pb->y) {
+        return 1;
+    } else if (pa->y < pb->y) {
+        return -1;
+    } else {
+        return 0;
+    }
+}
+
+double ccw(struct point a, struct point b, struct point c) {
+    return (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
+}
+
+double polar_angle(struct point origin, struct point p) {
+    return atan2(p.y - origin.y, p.x - origin.x);
+}
+
+void polar_angles_sort(struct point *points, struct point origin, size_t size) {
+    if (size < 2) {
+        return;
+    }
+
+    double pivot_angle = polar_angle(origin, points[size / 2]);
+
+    int i = 0;
+    int j = size - 1;
+    while (1) {
+        while (polar_angle(origin, points[i]) < pivot_angle) {
+            i++;
+        }
+        while (polar_angle(origin, points[j]) > pivot_angle) {
+            j--;
+        }
+
+        if (i >= j) {
+            break;
+        }
+
+        struct point tmp = points[i];
+        points[i] = points[j];
+        points[j] = tmp;
+
+        i++;
+        j--;
+    }
+
+    polar_angles_sort(points, origin, i);
+    polar_angles_sort(points + i, origin, size - i);
+}
+
+size_t graham_scan(struct point *points, size_t size) {
+    qsort(points, size, sizeof(struct point), cmp_points);
+    polar_angles_sort(points, points[0], size);
+
+    struct point tmp_points[size + 1];
+    memcpy(tmp_points + 1, points, size * sizeof(struct point));
+    tmp_points[0] = tmp_points[size];
+
+    size_t m = 1;
+    for (size_t i = 2; i <= size; ++i) {
+        while (ccw(tmp_points[m - 1], tmp_points[m], tmp_points[i]) <= 0) {
+            if (m > 1) {
+                m--;
+                continue;
+            } else if (i == size) {
+                break;
+            } else {
+                i++;
+            }
+        }
+
+        m++;
+        struct point tmp = tmp_points[i];
+        tmp_points[i] = tmp_points[m];
+        tmp_points[m] = tmp;
+    }
+
+    memcpy(points, tmp_points + 1, size * sizeof(struct point));
+
+    return m;
+}
+
+int main() {
+    struct point points[] = {{2.0, 1.9}, {1.0, 1.0}, {2.0, 4.0}, {3.0, 1.0},
+                                {2.0, 0.0}};
+
+    printf("Points:\n");
+    for (size_t i = 0; i < 5; ++i) {
+        printf("(%f,%f)\n", points[i].x, points[i].y);
+    }
+
+    size_t hull_size = graham_scan(points, 5);
+
+    printf("\nHull:\n");
+    for (size_t i = 0; i < hull_size; ++i) {
+        printf("(%f,%f)\n", points[i].x, points[i].y);
+    }
+
+    return 0;
+}

chapters/computational_geometry/gift_wrapping/graham_scan/graham_scan.md

@@ -14,6 +14,8 @@ We can find whether a rotation is counter-clockwise with trigonometric functions
 {% method %}
 {% sample lang="jl" %}
 [import:30-32, lang:"julia"](code/julia/graham.jl)
+{% sample lang="c" %}
+[import:24-26, lang:"c_cpp"](code/c/graham.c)
 {% endmethod %}
 
 If the output of this function is 0, the points are collinear.
@@ -30,6 +32,8 @@ In the end, the code should look something like this:
 {% method %}
 {% sample lang="jl" %}
 [import:34-69, lang:"julia"](code/julia/graham.jl)
+{% sample lang="c" %}
+[import:65-95, lang:"c_cpp"](code/c/graham.c)
 {% endmethod %}
 
 ### Bibliography
@@ -41,6 +45,8 @@ In the end, the code should look something like this:
 {% method %}
 {% sample lang="jl" %}
 [import, lang:"julia"](code/julia/graham.jl)
+{% sample lang="c" %}
+[import, lang:"c_cpp"](code/c/graham.c)
 {% endmethod %}
 
 <script>