package area

import (
	"fmt"
	"math"
)

const epsilon = 1e-9

func ValidateGeometry(kind, direction string, points []Point) error {
	for _, point := range points {
		if point.X < 0 || point.X > 1 || point.Y < 0 || point.Y > 1 {
			return fmt.Errorf("坐标必须位于画面范围内")
		}
	}
	switch kind {
	case KindPolygon:
		if len(points) < 3 {
			return fmt.Errorf("多边形至少需要 3 个点")
		}
		if math.Abs(polygonArea(points)) < epsilon {
			return fmt.Errorf("多边形不能退化为直线")
		}
		if selfIntersects(points, true) {
			return fmt.Errorf("多边形不能自交")
		}
	case KindLine:
		if len(points) != 2 {
			return fmt.Errorf("警戒线必须恰好包含 2 个点")
		}
		if distance(points[0], points[1]) < epsilon {
			return fmt.Errorf("警戒线两点不能重合")
		}
		if direction != DirectionAToB && direction != DirectionBToA && direction != DirectionBoth {
			return fmt.Errorf("警戒线方向不正确")
		}
	default:
		return fmt.Errorf("区域类型不正确")
	}
	return nil
}
func polygonArea(points []Point) float64 {
	sum := 0.0
	for i := range points {
		next := (i + 1) % len(points)
		sum += points[i].X*points[next].Y - points[next].X*points[i].Y
	}
	return sum / 2
}
func distance(a, b Point) float64 { dx, dy := a.X-b.X, a.Y-b.Y; return dx*dx + dy*dy }
func selfIntersects(points []Point, closed bool) bool {
	segments := len(points) - 1
	if closed {
		segments = len(points)
	}
	for i := 0; i < segments; i++ {
		a1, a2 := points[i], points[(i+1)%len(points)]
		if distance(a1, a2) < epsilon {
			return true
		}
		for j := i + 1; j < segments; j++ {
			if j == i+1 || closed && i == 0 && j == segments-1 {
				continue
			}
			b1, b2 := points[j], points[(j+1)%len(points)]
			if segmentsIntersect(a1, a2, b1, b2) {
				return true
			}
		}
	}
	return false
}
func segmentsIntersect(a, b, c, d Point) bool {
	o1, o2, o3, o4 := orientation(a, b, c), orientation(a, b, d), orientation(c, d, a), orientation(c, d, b)
	return o1*o2 < 0 && o3*o4 < 0
}
func orientation(a, b, c Point) float64 { return (b.X-a.X)*(c.Y-a.Y) - (b.Y-a.Y)*(c.X-a.X) }