More things. So many things.

src/geom.c

@@ -1,5 +1,8 @@
 #include "include/geom.h"
 
+#include <stdio.h>
+#include <stdlib.h>
+
 Tri* alltris[512];
 size_t allsize = 0;
 
@@ -29,3 +32,63 @@ Pt tri_center(Tri* tri) {
     return (Pt){(tri->a.x + tri->b.x + tri->c.x) / 3.0,
                 (tri->a.y + tri->b.y + tri->c.y) / 3.0};
 }
+
+int pt_cmp(const void* a, const void* b) {
+    const Pt* p = (const Pt*)a;
+    const Pt* q = (const Pt*)b;
+
+    if (p->x != q->x) return p->x - q->x;
+    return p->y - q->y;
+}
+
+static int pt_cross(const Pt a, const Pt b, const Pt c) {
+    return ((b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x));
+}
+
+Pt* cvx_hull(Pt* area, size_t n, size_t* out_n) {
+    if (!area || n <= 0) return NULL;
+
+    if (n == 1) {
+        Pt* out = malloc(2 * sizeof(Pt));
+        if (n == 1) out[0] = area[0];
+        *out_n = 1;
+        printf("bad things are happening... %s\n", __FUNCTION__);
+        return out;
+    }
+
+    Pt* pts = malloc(n * sizeof(Pt));
+    for (size_t i = 0; i < n; i++) pts[i] = area[i];
+
+    // Sort the points.
+    qsort(pts, n, sizeof(Pt), pt_cmp);
+
+    // Max hull size is 2 * n.
+    Pt* hull = malloc(2 * n * sizeof(Pt));
+    size_t k = 0;
+
+    // Lower hull.
+    for (size_t i = 0; i < n; i++) {
+        while (k >= 2 && pt_cross(hull[k - 2], hull[k - 1], pts[i]) <= 0) k--;
+
+        hull[k++] = pts[i];
+    }
+
+    // Upper hull.
+    for (size_t i = n - 1, t = k + 1; i > 0; i--) {
+        while (k >= t && pt_cross(hull[k - 2], hull[k - 1], pts[i - 1]) <= 0)
+            k--;
+
+        hull[k++] = pts[i - 1];
+    }
+
+    // Remove duplicated end/start point.
+    k--;
+
+    Pt* out = malloc(k * sizeof(Pt));
+    for (size_t i = 0; i < k; i++) { out[i] = hull[i]; }
+
+    *out_n = k;
+    free(pts);
+    free(hull);
+    return out;
+}