95 lines
1.8 KiB
Python
95 lines
1.8 KiB
Python
import random
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from collections import namedtuple
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from enum import Enum, auto
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import matplotlib.pyplot as plt
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Point = namedtuple('Point', ['x', 'y'])
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class Side(Enum):
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ON = auto()
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ABOVE = auto()
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BELOW = auto()
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def sidedness(p1, p2, p3, eps=0.0000001):
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y = p1.y * (p3.x - p2.x)
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x = p1.x
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a = (p3.y - p2.y)
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b = p3.y * (p3.x - p2.x) - a * p3.x
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if y - eps < a * x + b < y + eps:
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return Side.ON
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elif y > a * x + b:
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return Side.ABOVE
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return Side.BELOW
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# test
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p1 = Point(4, 4)
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p2 = Point(0, 0)
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p3 = Point(5, 2)
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# print(sidedness(p1, p2, p3))
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def genPoint():
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a = random.uniform(1, 5)
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b = random.uniform(1, 5)
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x_i = random.uniform(1, 5)
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p_i = Point(x_i, a * x_i + b)
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return p_i
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def graham_scan(points):
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# A funky issue where both a and b become negative in the sidedness test causes us to have to use
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# Side.ABOVE for both tests, regardless of UH or LH.
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sorted_points = sorted(points, key=lambda p: p.x)
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UH = sorted_points[:2]
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#del sorted_points[0]
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for s in sorted_points[2:]:
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while len(UH) > 1 and (sidedness(UH[-2], UH[-1], s) != Side.ABOVE):
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del UH[-1]
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UH.append(s)
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reversed_list = list(reversed(sorted_points))
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reversed_list.append(UH[0])
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LH = reversed_list[:2]
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#del reversed_list[0]
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for s in reversed_list[2:]:
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while len(LH) > 1 and (sidedness(LH[-2], LH[-1], s) != Side.ABOVE):
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del LH[-1]
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LH.append(s)
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return UH, LH
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p = [genPoint() for _ in range(30)]
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UH, LH = graham_scan(p)
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x = [point.x for point in p]
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y = [point.y for point in p]
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UH_x = [point.x for point in UH]
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UH_y = [point.y for point in UH]
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LH_x = [point.x for point in LH]
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LH_y = [point.y for point in LH]
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plt.plot(UH_x, UH_y, 'ro-')
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plt.plot(LH_x, LH_y, 'ro-')
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plt.scatter(x,y)
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plt.show()
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