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fixed sidedness

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Alexander Munch-Hansen 2018-09-20 15:38:44 +02:00
parent 69037ec771
commit c363bb3901
1 changed files with 34 additions and 31 deletions
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15
h2/sidedness.py
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@ -14,10 +14,13 @@ class Side(Enum):
def sidedness(p1, p2, p3, eps=0.0000001): def sidedness(p1, p2, p3, eps=0.0000001):
y = p1.y * (p3.x - p2.x) # Find line from p1 to p2, ask where p3 is in relation to this
x = p1.x
a = (p3.y - p2.y) y = p3.y * (p2.x - p1.x)
b = p3.y * (p3.x - p2.x) - a * p3.x x = p3.x
a = (p2.y - p1.y)
b = p2.y * (p2.x - p1.x) - a * p2.x
if y - eps < a * x + b < y + eps: if y - eps < a * x + b < y + eps:
return Side.ON return Side.ON
@ -47,13 +50,11 @@ def genPoint():
return p_i return p_i
def graham_scan(points): def graham_scan(points):
# A funky issue where both a and b become negative in the sidedness test causes us to have to use # A funky issue where both a and b become negative in the sidedness test causes us to have to use
# Side.ABOVE for both tests, regardless of UH or LH. # Side.ABOVE for both tests, regardless of UH or LH.
sorted_points = sorted(points, key=lambda p: p.x) sorted_points = sorted(points, key=lambda p: p.x)
UH = sorted_points[:2] UH = sorted_points[:2]
@ -63,6 +64,7 @@ def graham_scan(points):
while len(UH) > 1 and (sidedness(UH[-2], UH[-1], s) != Side.ABOVE): while len(UH) > 1 and (sidedness(UH[-2], UH[-1], s) != Side.ABOVE):
del UH[-1] del UH[-1]
UH.append(s) UH.append(s)
reversed_list = list(reversed(sorted_points)) reversed_list = list(reversed(sorted_points))
reversed_list.append(UH[0]) reversed_list.append(UH[0])
LH = reversed_list[:2] LH = reversed_list[:2]
@ -75,6 +77,7 @@ def graham_scan(points):
return UH, LH return UH, LH
p = [genPoint() for _ in range(30)] p = [genPoint() for _ in range(30)]
UH, LH = graham_scan(p) UH, LH = graham_scan(p)