refactor: wrap.py

gift_wrapping/wrap.py

@@ -0,0 +1,59 @@
+# Gift wrapping algorithm
+# https://en.wikipedia.org/wiki/Gift_wrapping_algorithm
+# Date: 2021-11-16
+
+import random
+import matplotlib.pyplot as plt
+
+
+def generate_points(n=100):
+    X = []
+    Y = []
+    for _i in range(n):
+        X.append(random.random())
+        Y.append(random.random())
+    plt.plot(X, Y, ".")
+    return [(X[i], Y[i]) for i in range(n)]
+
+
+def cross_product(u, v):
+    x1, y1 = u
+    x2, y2 = v
+    return x1 * y2 - x2 * y1
+
+
+def gift_wrapping(points):
+    convex_hull = []
+    x_sorted = sorted(points, key=lambda point: point[0])
+    left_most = x_sorted[0]
+    point_on_hull = left_most
+    endpoint = None
+    while endpoint != left_most:
+        convex_hull.append(point_on_hull)
+        endpoint = points[0]
+        for point in points:
+            cp = cross_product(
+                (endpoint[0] - point_on_hull[0], endpoint[1] - point_on_hull[1]),
+                (point[0] - point_on_hull[0], point[1] - point_on_hull[1]),
+            )
+            if endpoint == point_on_hull or cp < 0:
+                endpoint = point
+        point_on_hull = endpoint
+    return convex_hull
+
+
+def draw_points(points, with_edge=False):
+    X, Y = [], []
+    for p in points:
+        X.append(p[0])
+        Y.append(p[1])
+    plt.plot(X, Y, "." if not with_edge else "")
+
+
+pts = generate_points(50)
+
+print(pts)
+draw_points(pts)
+hull = gift_wrapping(pts)
+draw_points(hull + [hull[0]], True)
+plt.show()