Task: Determine if two triangles overlap
C++: #include <vector>
#include <iostream>
#include <stdexcept>
using namespace std;

typedef std::pair<double, double> TriPoint;

inline double Det2D(TriPoint &p1, TriPoint &p2, TriPoint &p3) 
{
	return +p1.first*(p2.second-p3.second)
		+p2.first*(p3.second-p1.second)
		+p3.first*(p1.second-p2.second);
}

void CheckTriWinding(TriPoint &p1, TriPoint &p2, TriPoint &p3, bool allowReversed)
{
	double detTri = Det2D(p1, p2, p3);
	if(detTri < 0.0)
	{
		if (allowReversed)
		{
			TriPoint a = p3;
			p3 = p2;
			p2 = a;
		}
		else throw std::runtime_error("triangle has wrong winding direction");
	}
}

bool BoundaryCollideChk(TriPoint &p1, TriPoint &p2, TriPoint &p3, double eps)
{
	return Det2D(p1, p2, p3) < eps;
}

bool BoundaryDoesntCollideChk(TriPoint &p1, TriPoint &p2, TriPoint &p3, double eps)
{
	return Det2D(p1, p2, p3) <= eps;
}

bool TriTri2D(TriPoint *t1,
	TriPoint *t2,
	double eps = 0.0, bool allowReversed = false, bool onBoundary = true)
{
	
	CheckTriWinding(t1[0], t1[1], t1[2], allowReversed);
	CheckTriWinding(t2[0], t2[1], t2[2], allowReversed);

	bool (*chkEdge)(TriPoint &, TriPoint &, TriPoint &, double) = NULL;
	if(onBoundary) 
		chkEdge = BoundaryCollideChk;
	else 
		chkEdge = BoundaryDoesntCollideChk;

	
	for(int 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(int 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()
{
	{TriPoint t1[] = {TriPoint(0,0),TriPoint(5,0),TriPoint(0,5)};
	TriPoint t2[] = {TriPoint(0,0),TriPoint(5,0),TriPoint(0,6)};
	cout << TriTri2D(t1, t2) << "," << true << endl;}

	{TriPoint t1[] = {TriPoint(0,0),TriPoint(0,5),TriPoint(5,0)};
	TriPoint t2[] = {TriPoint(0,0),TriPoint(0,5),TriPoint(5,0)};
	cout << TriTri2D(t1, t2, 0.0, true) << "," << true << endl;}

	{TriPoint t1[] = {TriPoint(0,0),TriPoint(5,0),TriPoint(0,5)};
	TriPoint t2[] = {TriPoint(-10,0),TriPoint(-5,0),TriPoint(-1,6)};
	cout << TriTri2D(t1, t2) << "," << false << endl;}

	{TriPoint t1[] = {TriPoint(0,0),TriPoint(5,0),TriPoint(2.5,5)};
	TriPoint t2[] = {TriPoint(0,4),TriPoint(2.5,-1),TriPoint(5,4)};
	cout << TriTri2D(t1, t2) << "," << true << endl;}

	{TriPoint t1[] = {TriPoint(0,0),TriPoint(1,1),TriPoint(0,2)};
	TriPoint t2[] = {TriPoint(2,1),TriPoint(3,0),TriPoint(3,2)};
	cout << TriTri2D(t1, t2) << "," << false << endl;}

	{TriPoint t1[] = {TriPoint(0,0),TriPoint(1,1),TriPoint(0,2)};
	TriPoint t2[] = {TriPoint(2,1),TriPoint(3,-2),TriPoint(3,4)};
	cout << TriTri2D(t1, t2) << "," << false << endl;}

	
	{TriPoint t1[] = {TriPoint(0,0),TriPoint(1,0),TriPoint(0,1)};
	TriPoint t2[] = {TriPoint(1,0),TriPoint(2,0),TriPoint(1,1)};
	cout << TriTri2D(t1, t2, 0.0, false, true) << "," << true << endl;}

	
	{TriPoint t1[] = {TriPoint(0,0),TriPoint(1,0),TriPoint(0,1)};
	TriPoint t2[] = {TriPoint(1,0),TriPoint(2,0),TriPoint(1,1)};
	cout << TriTri2D(t1, t2, 0.0, false, false) << "," << false << endl;}

}

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"))
}