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forgejo/vendor/github.com/issue9/identicon/polygon.go
6543 57b6f83191
Update github.com/issue9/identicon from untagged to v1.0.1 (#11359)
Co-authored-by: zeripath <art27@cantab.net>
2020-05-10 06:23:17 -04:00

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// Copyright 2015 by caixw, All rights reserved.
// Use of this source code is governed by a MIT
// license that can be found in the LICENSE file.
package identicon
var (
// 4个元素分别表示 cos(0),cos(90),cos(180),cos(270)
cos = []float64{1, 0, -1, 0}
// 4个元素分别表示 sin(0),sin(90),sin(180),sin(270)
sin = []float64{0, 1, 0, -1}
)
// 将 points 中的所有点,以 x,y 为原点旋转 angle 个角度。
// angle 取值只能是 [0,1,2,3],分别表示 [090180270]
func rotate(points []float64, x, y float64, angle int) {
if angle < 0 || angle > 3 {
panic("rotate:参数angle必须0,1,2,3三值之一")
}
for i := 0; i < len(points); i += 2 {
px := points[i] - x
py := points[i+1] - y
points[i] = px*cos[angle] - py*sin[angle] + x
points[i+1] = px*sin[angle] + py*cos[angle] + y
}
}
// 判断某个点是否在多边形之内,不包含构成多边形的线和点
// x,y 需要判断的点坐标
// points 组成多边形的所顶点,每两个元素表示一点顶点,其中最后一个顶点必须与第一个顶点相同。
func pointInPolygon(x float64, y float64, points []float64) bool {
if len(points) < 8 { // 只有2个以上的点才能组成闭合多边形
return false
}
// 大致算法如下:
// 把整个平面以给定的测试点为原点分两部分:
// - y>0包含(x>0 && y==0)
// - y<0包含(x<0 && y==0)
// 依次扫描每一个点,当该点与前一个点处于不同部分时(即一个在 y>0 区,一个在 y<0 区),
// 则判断从前一点到当前点是顺时针还是逆时针(以给定的测试点为原点),如果是顺时针 r++,否则 r--。
// 结果为2==abs(r)。
r := 0
x1, y1 := points[0], points[1]
prev := (y1 > y) || ((x1 > x) && (y1 == y))
for i := 2; i < len(points); i += 2 {
x2, y2 := points[i], points[i+1]
curr := (y2 > y) || ((x2 > x) && (y2 == y))
if curr == prev {
x1, y1 = x2, y2
continue
}
mul := (x1-x)*(y2-y) - (x2-x)*(y1-y)
if mul > 0 {
r++
} else if mul < 0 {
r--
}
x1, y1 = x2, y2
prev = curr
}
return r == 2 || r == -2
}