package main

import "fmt"

type xy struct {
    x, y float64
}

type closedPoly struct {
    name string
    vert []xy
}

func inside(pt xy, pg closedPoly) bool {
    if len(pg.vert) < 3 {
        return false
    }
    in := rayIntersectsSegment(pt, pg.vert[len(pg.vert)-1], pg.vert[0])
    for i := 1; i < len(pg.vert); i++ {
        if rayIntersectsSegment(pt, pg.vert[i-1], pg.vert[i]) {
            in = !in
        }
    }
    return in
}

func rayIntersectsSegment(p, a, b xy) bool {
    return (a.y > p.y) != (b.y > p.y) &&
        p.x < (b.x-a.x)*(p.y-a.y)/(b.y-a.y)+a.x
}

var tpg = []closedPoly{
    {"square", []xy{{0, 0}, {10, 0}, {10, 10}, {0, 10}}},
    {"square hole", []xy{{0, 0}, {10, 0}, {10, 10}, {0, 10}, {0, 0},
        {2.5, 2.5}, {7.5, 2.5}, {7.5, 7.5}, {2.5, 7.5}, {2.5, 2.5}}},
    {"strange", []xy{{0, 0}, {2.5, 2.5}, {0, 10}, {2.5, 7.5}, {7.5, 7.5},
        {10, 10}, {10, 0}, {2.5, 2.5}}},
    {"exagon", []xy{{3, 0}, {7, 0}, {10, 5}, {7, 10}, {3, 10}, {0, 5}}},
}

var tpt = []xy{{1, 2}, {2, 1}}

func main() {
    for _, pg := range tpg {
        fmt.Printf("%s:\n", pg.name)
        for _, pt := range tpt {
            fmt.Println(pt, inside(pt, pg))
        }
    }
}