Task: Determine if two triangles overlap
C: #include <errno.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    double x, y;
} Point;

double det2D(const Point * const p1, const Point * const p2, const Point * const p3) {
    return p1->x * (p2->y - p3->y)
        + p2->x * (p3->y - p1->y)
        + p3->x * (p1->y - p2->y);
}

void checkTriWinding(Point * p1, Point * p2, Point * p3, bool allowReversed) {
    double detTri = det2D(p1, p2, p3);
    if (detTri < 0.0) {
        if (allowReversed) {
            double t = p3->x;
            p3->x = p2->x;
            p2->x = t;

            t = p3->y;
            p3->y = p2->y;
            p2->y = t;
        } else {
            errno = 1;
        }
    }
}

bool boundaryCollideChk(const Point *p1, const Point *p2, const Point *p3, double eps) {
    return det2D(p1, p2, p3) < eps;
}

bool boundaryDoesntCollideChk(const Point *p1, const Point *p2, const Point *p3, double eps) {
    return det2D(p1, p2, p3) <= eps;
}

bool triTri2D(Point t1[], Point t2[], double eps, bool allowReversed, bool onBoundary) {
    bool(*chkEdge)(Point*, Point*, Point*, double);
    int i;

    
    checkTriWinding(&t1[0], &t1[1], &t1[2], allowReversed);
    if (errno != 0) {
        return false;
    }
    checkTriWinding(&t2[0], &t2[1], &t2[2], allowReversed);
    if (errno != 0) {
        return false;
    }

    if (onBoundary) {
        
        chkEdge = boundaryCollideChk;
    } else {
        
        chkEdge = boundaryDoesntCollideChk;
    }

    
    for (i = 0; i < 3; ++i) {
        int j = (i + 1) % 3;

        
        
        if (chkEdge(&t1[i], &t1[j], &t2[0], eps) &&
            chkEdge(&t1[i], &t1[j], &t2[1], eps) &&
            chkEdge(&t1[i], &t1[j], &t2[2], eps)) {
            return false;
        }
    }

    
    for (i = 0; i < 3; i++) {
        int j = (i + 1) % 3;

        
        
        if (chkEdge(&t2[i], &t2[j], &t1[0], eps) &&
            chkEdge(&t2[i], &t2[j], &t1[1], eps) &&
            chkEdge(&t2[i], &t2[j], &t1[2], eps))
            return false;
    }

    
    return true;
}

int main() {
    {
        Point t1[] = { {0, 0}, {5, 0}, {0, 5} };
        Point t2[] = { {0, 0}, {5, 0}, {0, 6} };
        printf("%d,true\n", triTri2D(t1, t2, 0.0, false, true));
    }

    {
        Point t1[] = { {0, 0}, {0, 5}, {5, 0} };
        Point t2[] = { {0, 0}, {0, 5}, {5, 0} };
        printf("%d,true\n", triTri2D(t1, t2, 0.0, true, true));
    }

    {
        Point t1[] = { {0, 0}, {5, 0}, {0, 5} };
        Point t2[] = { {-10, 0}, {-5, 0}, {-1, 6} };
        printf("%d,false\n", triTri2D(t1, t2, 0.0, false, true));
    }

    {
        Point t1[] = { {0, 0}, {5, 0}, {2.5, 5} };
        Point t2[] = { {0, 4}, {2.5, -1}, {5, 4} };
        printf("%d,true\n", triTri2D(t1, t2, 0.0, false, true));
    }

    {
        Point t1[] = { {0, 0}, {1, 1}, {0, 2} };
        Point t2[] = { {2, 1}, {3, 0}, {3, 2} };
        printf("%d,false\n", triTri2D(t1, t2, 0.0, false, true));
    }

    {
        Point t1[] = { {0, 0}, {1, 1}, {0, 2} };
        Point t2[] = { {2, 1}, {3, -2}, {3, 4} };
        printf("%d,false\n", triTri2D(t1, t2, 0.0, false, true));
    }

    
    {
        Point t1[] = { {0, 0}, {1, 0}, {0, 1} };
        Point t2[] = { {1, 0}, {2, 0}, {1, 1} };
        printf("%d,true\n", triTri2D(t1, t2, 0.0, false, true));
    }

    
    {
        Point t1[] = { {0, 0}, {1, 0}, {0, 1} };
        Point t2[] = { {1, 0}, {2, 0}, {1, 1} };
        printf("%d,false\n", triTri2D(t1, t2, 0.0, false, false));
    }

    return EXIT_SUCCESS;
}

Go: package main

import "fmt"

type point struct {
    x, y float64
}

func (p point) String() string {
    return fmt.Sprintf("(%.1f, %.1f)", p.x, p.y)
}

type triangle struct {
    p1, p2, p3 point
}

func (t *triangle) String() string {
    return fmt.Sprintf("Triangle %s, %s, %s", t.p1, t.p2, t.p3)
}

func (t *triangle) det2D() float64 {
    return t.p1.x * (t.p2.y - t.p3.y) +
           t.p2.x * (t.p3.y - t.p1.y) +
           t.p3.x * (t.p1.y - t.p2.y) 
}

func (t *triangle) checkTriWinding(allowReversed bool) {
    detTri := t.det2D()
    if detTri < 0.0 {
        if allowReversed {
            a := t.p3
            t.p3 = t.p2
            t.p2 = a
        } else {
            panic("Triangle has wrong winding direction.")
        }
    }
}

func boundaryCollideChk(t *triangle, eps float64) bool {
    return t.det2D() < eps
}

func boundaryDoesntCollideChk(t *triangle, eps float64) bool {
    return t.det2D() <= eps
}

func triTri2D(t1, t2 *triangle, eps float64, allowReversed, onBoundary bool) bool {
    
    t1.checkTriWinding(allowReversed)
    t2.checkTriWinding(allowReversed)

    
    var chkEdge func (*triangle, float64) bool
    if onBoundary {
        chkEdge = boundaryCollideChk
    } else {
        chkEdge = boundaryDoesntCollideChk
    }
    lp1 := [3]point{t1.p1, t1.p2, t1.p3}
    lp2 := [3]point{t2.p1, t2.p2, t2.p3}

    
    for i := 0; i < 3; i++ {
        j := (i + 1) % 3
        
        
        tri1 := &triangle{lp1[i], lp1[j], lp2[0]}
        tri2 := &triangle{lp1[i], lp1[j], lp2[1]}
        tri3 := &triangle{lp1[i], lp1[j], lp2[2]}
        if chkEdge(tri1, eps) && chkEdge(tri2, eps) && chkEdge(tri3, eps) {
            return false
        }
    }

    
    for i := 0; i < 3; i++ {
        j := (i + 1) % 3
        
        
        tri1 := &triangle{lp2[i], lp2[j], lp1[0]}
        tri2 := &triangle{lp2[i], lp2[j], lp1[1]}
        tri3 := &triangle{lp2[i], lp2[j], lp1[2]}
        if chkEdge(tri1, eps) && chkEdge(tri2, eps) && chkEdge(tri3, eps) {
            return false
        }
    }

    
    return true
}

func iff(cond bool, s1, s2 string) string {
    if cond {
        return s1
    }
    return s2
}

func main() {
    t1 := &triangle{point{0.0, 0.0}, point{5.0, 0.0}, point{0.0, 5.0}}
    t2 := &triangle{point{0.0, 0.0}, point{5.0, 0.0}, point{0.0, 6.0}}
    fmt.Printf("%s and\n%s\n", t1, t2)
    overlapping := triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    
    t1 = &triangle{point{0.0, 0.0}, point{0.0, 5.0}, point{5.0, 0.0}}
    t2 = t1
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    overlapping = triTri2D(t1, t2, 0.0, true, true)
    fmt.Println(iff(overlapping, "overlap (reversed)", "do not overlap"))

    t1 = &triangle{point{0.0, 0.0}, point{5.0, 0.0}, point{0.0, 5.0}}
    t2 = &triangle{point{-10.0, 0.0}, point{-5.0, 0.0}, point{-1.0, 6.0}}
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    overlapping = triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    t1.p3 = point{2.5, 5.0}
    t2 = &triangle{point{0.0, 4.0}, point{2.5, -1.0}, point{5.0, 4.0}}
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    overlapping = triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    t1 = &triangle{point{0.0, 0.0}, point{1.0, 1.0}, point{0.0, 2.0}}
    t2 = &triangle{point{2.0, 1.0}, point{3.0, 0.0}, point{3.0, 2.0}}
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    overlapping = triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    t2 = &triangle{point{2.0, 1.0}, point{3.0, -2.0}, point{3.0, 4.0}}
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    overlapping = triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    t1 = &triangle{point{0.0, 0.0}, point{1.0, 0.0}, point{0.0, 1.0}}
    t2 = &triangle{point{1.0, 0.0}, point{2.0, 0.0}, point{1.0, 1.1}}
    fmt.Printf("\n%s and\n%s\n", t1, t2)
    println("which have only a single corner in contact, if boundary points collide")
    overlapping = triTri2D(t1, t2, 0.0, false, true)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))

    fmt.Printf("\n%s and\n%s\n", t1, t2)
    fmt.Println("which have only a single corner in contact, if boundary points do not collide")
    overlapping = triTri2D(t1, t2, 0.0, false, false)
    fmt.Println(iff(overlapping, "overlap", "do not overlap"))
}