toms112, a C++ code which determines whether a point is inside a polygon.
The text of many ACM TOMS algorithms is available online through ACM: https://calgo.acm.org/ or NETLIB: https://www.netlib.org/toms/index.html.
The computer code and data files made available on this web page are distributed under the MIT license
toms112 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
GEOMETRY, a C++7 library which performs geometric calculations in 2, 3 and N dimensional space.
POLYGON_MONTE_CARLO, a C++ code which applies a Monte Carlo method to estimate the integral of a function over the interior of a polygon in 2D.
POLYGON_PROPERTIES, a C++ code which computes properties of an arbitrary polygon in the plane, defined by a sequence of vertices, including interior angles, area, centroid, containment of a point, convexity, diameter, distance to a point, inradius, lattice area, nearest point in set, outradius, uniform sampling.
POLYGON_TRIANGULATE, a C++ code which triangulates a possibly nonconvex polygon, and which can use gnuplot to display the external edges and internal diagonals of the triangulation.