Add Point in Polygon algorithm using ray casting

Author: Clear20-22

geometry/point_in_polygon.py

@@ -0,0 +1,189 @@
+"""
+Point in Polygon (PIP) Algorithm using Ray Casting (Even-Odd Rule).
+
+References:
+- https://en.wikipedia.org/wiki/Point_in_polygon
+- https://en.wikipedia.org/wiki/Even%E2%80%93odd_rule
+- Shimrat, M. (1962). "Algorithm 112: Position of point relative to polygon".
+  Communications of the ACM, 5(8), 434.
+
+The ray casting algorithm determines whether a given point in the plane lies
+inside, outside, or on the boundary of an arbitrary polygon (convex or concave).
+It works by casting a horizontal ray from the query point to infinity in the positive
+x-direction and counting how many polygon edges intersect this ray:
+- An odd number of intersections indicates the point is inside the polygon.
+- An even number of intersections indicates the point is outside the polygon.
+
+Boundary handling:
+Points lying exactly on an edge or vertex of the polygon are detected explicitly
+via segment collinearity and bounding box checks.
+"""
+
+from __future__ import annotations
+
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    """
+    A 2D point with real-valued coordinates.
+
+    >>> Point(0.0, 0.0)
+    Point(x=0.0, y=0.0)
+    >>> Point(1.5, -2.0)
+    Point(x=1.5, y=-2.0)
+    """
+
+    x: float
+    y: float
+
+
+def is_point_on_segment(
+    point: Point, seg_start: Point, seg_end: Point, tolerance: float = 1e-9
+) -> bool:
+    """
+    Determine whether a point lies on the line segment between seg_start and seg_end.
+
+    The check verifies that:
+    1. The point is collinear with the segment endpoints (cross product is near zero).
+    2. The point lies within the bounding box of the segment.
+
+    >>> is_point_on_segment(Point(1.0, 1.0), Point(0.0, 0.0), Point(2.0, 2.0))
+    True
+    >>> is_point_on_segment(Point(0.0, 0.0), Point(0.0, 0.0), Point(2.0, 2.0))
+    True
+    >>> is_point_on_segment(Point(2.0, 2.0), Point(0.0, 0.0), Point(2.0, 2.0))
+    True
+    >>> is_point_on_segment(Point(3.0, 3.0), Point(0.0, 0.0), Point(2.0, 2.0))
+    False
+    >>> is_point_on_segment(Point(1.0, 2.0), Point(0.0, 0.0), Point(2.0, 2.0))
+    False
+    >>> is_point_on_segment(Point(2.0, 0.0), Point(0.0, 0.0), Point(4.0, 0.0))
+    True
+    """
+    # Cross product of vector (seg_start -> seg_end) and (seg_start -> point)
+    cross_product = (seg_end.x - seg_start.x) * (point.y - seg_start.y) - (
+        seg_end.y - seg_start.y
+    ) * (point.x - seg_start.x)
+    if abs(cross_product) > tolerance:
+        return False
+
+    # Check bounding box
+    is_within_x_bounds = (
+        min(seg_start.x, seg_end.x) - tolerance
+        <= point.x
+        <= max(seg_start.x, seg_end.x) + tolerance
+    )
+    is_within_y_bounds = (
+        min(seg_start.y, seg_end.y) - tolerance
+        <= point.y
+        <= max(seg_start.y, seg_end.y) + tolerance
+    )
+    return is_within_x_bounds and is_within_y_bounds
+
+
+def point_in_polygon(
+    point: Point, polygon: list[Point], include_boundary: bool = True
+) -> bool:
+    """
+    Determine whether a 2D point lies inside an arbitrary polygon using ray casting.
+
+    Parameters:
+        point: The query Point(x, y).
+        polygon: A list of Point instances representing vertices of the polygon
+            in cyclic order (clockwise or counter-clockwise). Must have >= 3 vertices.
+        include_boundary: Whether points on the boundary (edges or vertices)
+            are considered inside (default True).
+
+    Returns:
+        True if the point is inside (or on the boundary if include_boundary=True),
+        False otherwise.
+
+    Raises:
+        ValueError: If the polygon has fewer than 3 vertices.
+
+    Examples:
+    >>> square = [Point(0.0, 0.0), Point(4.0, 0.0), Point(4.0, 4.0), Point(0.0, 4.0)]
+    >>> point_in_polygon(Point(2.0, 2.0), square)
+    True
+    >>> point_in_polygon(Point(5.0, 2.0), square)
+    False
+    >>> point_in_polygon(Point(-1.0, 2.0), square)
+    False
+    >>> point_in_polygon(Point(2.0, 5.0), square)
+    False
+
+    Boundary tests:
+    >>> point_in_polygon(Point(0.0, 2.0), square, include_boundary=True)
+    True
+    >>> point_in_polygon(Point(0.0, 2.0), square, include_boundary=False)
+    False
+    >>> point_in_polygon(Point(4.0, 4.0), square, include_boundary=True)
+    True
+    >>> point_in_polygon(Point(4.0, 4.0), square, include_boundary=False)
+    False
+    >>> point_in_polygon(Point(2.0, 0.0), square, include_boundary=True)
+    True
+    >>> point_in_polygon(Point(2.0, 0.0), square, include_boundary=False)
+    False
+
+    Concave (arrowhead) polygon:
+    >>> arrowhead = [Point(0.0, 0.0), Point(5.0, 2.0), Point(0.0, 4.0), Point(2.0, 2.0)]
+    >>> point_in_polygon(Point(3.0, 2.0), arrowhead)
+    True
+    >>> point_in_polygon(Point(1.0, 2.0), arrowhead)
+    False
+
+    Triangle with negative and floating point coordinates:
+    >>> triangle = [Point(-2.5, -2.5), Point(2.5, -2.5), Point(0.0, 2.5)]
+    >>> point_in_polygon(Point(0.0, 0.0), triangle)
+    True
+    >>> point_in_polygon(Point(0.0, 3.0), triangle)
+    False
+    >>> point_in_polygon(Point(-3.0, 0.0), triangle)
+    False
+
+    Invalid input (fewer than 3 vertices):
+    >>> point_in_polygon(Point(0.0, 0.0), [Point(0.0, 0.0), Point(1.0, 1.0)])
+    Traceback (most recent call last):
+        ...
+    ValueError: A polygon must have at least 3 vertices.
+    """
+    if len(polygon) < 3:
+        raise ValueError("A polygon must have at least 3 vertices.")
+
+    num_vertices = len(polygon)
+
+    # Check if point lies on any boundary edge or vertex
+    for vertex_index in range(num_vertices):
+        edge_start = polygon[vertex_index]
+        edge_end = polygon[(vertex_index + 1) % num_vertices]
+        if is_point_on_segment(point, edge_start, edge_end):
+            return include_boundary
+
+    # Ray casting: cast a horizontal ray from point towards positive x-infinity
+    is_inside = False
+    for vertex_index in range(num_vertices):
+        current_vertex = polygon[vertex_index]
+        next_vertex = polygon[(vertex_index + 1) % num_vertices]
+
+        # Check whether the edge crosses the horizontal ray.
+        # The condition (current_vertex.y > point.y) != (next_vertex.y > point.y)
+        # ensures:
+        # 1. Strictly horizontal edges (current_vertex.y == next_vertex.y) are skipped.
+        # 2. Vertices intersecting the ray are counted exactly once when crossed.
+        if (current_vertex.y > point.y) != (next_vertex.y > point.y):
+            # Compute x-coordinate of intersection with line y = point.y
+            ray_x_intersection = current_vertex.x + (point.y - current_vertex.y) * (
+                next_vertex.x - current_vertex.x
+            ) / (next_vertex.y - current_vertex.y)
+            if point.x < ray_x_intersection:
+                is_inside = not is_inside
+
+    return is_inside
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()