cython

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import math
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def great_circle(double lon1, double lat1, double lon2, double lat2):
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    cdef double radius = 3956 # miles
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    cdef double x = math.pi/180.0
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    cdef double a, b, theta, c
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    a = (90.0 - lat1)*x
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    b = (90.0 - lat2)*x
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    theta = (lon2 - lon1)*x
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    c = math.acos(math.cos(a)*math.cos(b) + math.sin(a)*math.sin(b)*math.cos(theta))
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    return radius*c
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