From a22414ece40c1622588813e92a527ab47a79a389 Mon Sep 17 00:00:00 2001
From: Gathros <6323830+Gathros@users.noreply.github.com>
Date: Fri, 11 May 2018 18:45:18 +0100
Subject: [PATCH 1/4] Adding graham.c

---
 .../gift_wrapping/graham_scan/code/c/graham.c | 114 ++++++++++++++++++
 .../gift_wrapping/graham_scan/graham_scan.md  |   6 +
 2 files changed, 120 insertions(+)
 create mode 100644 chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c

diff --git a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
new file mode 100644
index 000000000..3beb3cd51
--- /dev/null
+++ b/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 point1 = *(struct point*)a;
+    struct point point2 = *(struct point*)b;
+
+    if (point1.y > point2.y) {
+        return 1;
+    } else if (point1.y < point2.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;
+}
diff --git a/chapters/computational_geometry/gift_wrapping/graham_scan/graham_scan.md b/chapters/computational_geometry/gift_wrapping/graham_scan/graham_scan.md
index fa7ce31e2..879bf63fb 100644
--- a/chapters/computational_geometry/gift_wrapping/graham_scan/graham_scan.md
+++ b/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>

From 61bde3924252fbd2ff74aeb254be3e50e8766996 Mon Sep 17 00:00:00 2001
From: Gathros <6323830+Gathros@users.noreply.github.com>
Date: Sat, 12 May 2018 22:14:26 +0100
Subject: [PATCH 2/4] maded small changes to graham.c

---
 .../gift_wrapping/graham_scan/code/c/graham.c      | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
index 3beb3cd51..2233edbdf 100644
--- a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
+++ b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
@@ -1,4 +1,5 @@
 #include <math.h>
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -9,20 +10,17 @@ struct point {
 };
 
 int cmp_points(const void *a, const void *b) {
-    struct point point1 = *(struct point*)a;
-    struct point point2 = *(struct point*)b;
-
-    if (point1.y > point2.y) {
+    if (((struct point*)a)->y > ((struct point*)b)->y) {
         return 1;
-    } else if (point1.y < point2.y) {
+    } else if (((struct point*)a)->y < ((struct point*)b)->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);
+bool is_left_of(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) {
@@ -72,7 +70,7 @@ size_t graham_scan(struct point *points, size_t 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) {
+        while (is_left_of(tmp_points[m - 1], tmp_points[m], tmp_points[i])) {
             if (m > 1) {
                 m--;
                 continue;

From a6f6d9e650ddb63747374410d20dc6c902f8f167 Mon Sep 17 00:00:00 2001
From: Gathros <6323830+Gathros@users.noreply.github.com>
Date: Sat, 12 May 2018 22:39:24 +0100
Subject: [PATCH 3/4] adding variables to cmp_points in graham.c

---
 .../gift_wrapping/graham_scan/code/c/graham.c              | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
index 2233edbdf..c1914d80c 100644
--- a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
+++ b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
@@ -10,9 +10,12 @@ struct point {
 };
 
 int cmp_points(const void *a, const void *b) {
-    if (((struct point*)a)->y > ((struct point*)b)->y) {
+    struct point* pa = (struct point*) a;
+    struct point* pb = (struct point*) b;
+
+    if (pa->y > pb->y) {
         return 1;
-    } else if (((struct point*)a)->y < ((struct point*)b)->y) {
+    } else if (pa->y < pb->y) {
         return -1;
     } else {
         return 0;

From 65e069f644ffd280eaa65d53e1edc7bec2cd6a9b Mon Sep 17 00:00:00 2001
From: Gathros <6323830+Gathros@users.noreply.github.com>
Date: Mon, 14 May 2018 21:56:38 +0100
Subject: [PATCH 4/4] Changing back to the standard

---
 .../gift_wrapping/graham_scan/code/c/graham.c              | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
index c1914d80c..69e93077d 100644
--- a/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
+++ b/chapters/computational_geometry/gift_wrapping/graham_scan/code/c/graham.c
@@ -1,5 +1,4 @@
 #include <math.h>
-#include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -22,8 +21,8 @@ int cmp_points(const void *a, const void *b) {
     }
 }
 
-bool is_left_of(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 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) {
@@ -73,7 +72,7 @@ size_t graham_scan(struct point *points, size_t size) {
 
     size_t m = 1;
     for (size_t i = 2; i <= size; ++i) {
-        while (is_left_of(tmp_points[m - 1], tmp_points[m], tmp_points[i])) {
+        while (ccw(tmp_points[m - 1], tmp_points[m], tmp_points[i]) <= 0) {
             if (m > 1) {
                 m--;
                 continue;