#include <stdio.h>
#include <stdbool.h>
/*
Simple 2D point structure
*/
typedef struct {
float x;
float y;
} Point;
/*
edge_orientation(p1, p2, p3)
--------------------------------
Computes the 2D cross product:
(p1 - p3) × (p2 - p3)
The sign of this value tells us whether p1 lies to the left or right
of the directed edge p3 → p2.
This is the core of the "same-side" triangle test.
*/
float edge_orientation(Point p1, Point p2, Point p3) {
return (p1.x - p3.x) * (p2.y - p3.y)
- (p2.x - p3.x) * (p1.y - p3.y);
}
/*
point_in_triangle(pt, v1, v2, v3)
-----------------------------------
Uses the same-side method:
A point is inside the triangle if it lies on the same side of all
three edges. If the signs of the three cross products are all
non-negative or all non-positive, the point is inside or on the edge.
If the signs mix (some positive, some negative), the point is outside.
*/
bool point_in_triangle(Point pt, Point v1, Point v2, Point v3) {
float d1 = edge_orientation(pt, v1, v2);
float d2 = edge_orientation(pt, v2, v3);
float d3 = edge_orientation(pt, v3, v1);
bool has_negative = (d1 < 0.0f) || (d2 < 0.0f) || (d3 < 0.0f);
bool has_positive = (d1 > 0.0f) || (d2 > 0.0f) || (d3 > 0.0f);
/* Inside if NOT both negative and positive signs */
return !(has_negative && has_positive);
}
int main(void) {
/* Triangle vertices */
Point A = {0.0f, 0.0f};
Point B = {5.0f, 0.0f};
Point C = {2.5f, 5.0f};
/* Test point */
Point P = {2.5f, 2.0f};
bool inside = point_in_triangle(P, A, B, C);
printf("Point (%.2f, %.2f) is %s the triangle.\n",
P.x, P.y, inside ? "inside" : "outside");
return 0;
}
/*
run:
Point (2.50, 2.00) is inside the triangle.
*/
