gitech

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// Copyright 2021 The Gitea Authors. All rights reserved.
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// SPDX-License-Identifier: MIT
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// Copied and modified from https://github.com/issue9/identicon/ (MIT License)
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package identicon
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var (
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	// cos(0),cos(90),cos(180),cos(270)
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	cos = []int{1, 0, -1, 0}
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	// sin(0),sin(90),sin(180),sin(270)
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	sin = []int{0, 1, 0, -1}
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)
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// rotate the points by center point (x,y)
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// angle: [0,1,2,3] means [0，90，180，270] degree
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func rotate(points []int, x, y, angle int) {
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	// the angle is only used internally, and it has been guaranteed to be 0/1/2/3, so we do not check it again
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	for i := 0; i < len(points); i += 2 {
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		px, py := points[i]-x, points[i+1]-y
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		points[i] = px*cos[angle] - py*sin[angle] + x
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		points[i+1] = px*sin[angle] + py*cos[angle] + y
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	}
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}
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// check whether the point is inside the polygon (defined by the points)
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// the first and the last point must be the same
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func pointInPolygon(x, y int, polygonPoints []int) bool {
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	if len(polygonPoints) < 8 { // a valid polygon must have more than 2 points
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		return false
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	}
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	// reference: nonzero winding rule, https://en.wikipedia.org/wiki/Nonzero-rule
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	// split the plane into two by the check point horizontally:
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	//   y>0，includes (x>0 && y==0)
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	//   y<0，includes (x<0 && y==0)
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	//
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	// then scan every point in the polygon.
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	//
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	// if current point and previous point are in different planes (eg: curY>0 && prevY<0),
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	// check the clock-direction from previous point to current point (use check point as origin).
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	// if the direction is clockwise, then r++, otherwise then r--
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	// finally, if 2==abs(r), then the check point is inside the polygon
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	r := 0
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	prevX, prevY := polygonPoints[0], polygonPoints[1]
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	prev := (prevY > y) || ((prevX > x) && (prevY == y))
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	for i := 2; i < len(polygonPoints); i += 2 {
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		currX, currY := polygonPoints[i], polygonPoints[i+1]
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		curr := (currY > y) || ((currX > x) && (currY == y))
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		if curr == prev {
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			prevX, prevY = currX, currY
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			continue
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		}
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		if mul := (prevX-x)*(currY-y) - (currX-x)*(prevY-y); mul >= 0 {
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			r++
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		} else { // mul < 0
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			r--
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		}
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		prevX, prevY = currX, currY
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		prev = curr
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	}
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	return r == 2 || r == -2
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}
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