98 lines
2.5 KiB
Go
98 lines
2.5 KiB
Go
// Package shatter provides boolean operations for image.Rectangle.
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package shatter
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import "image"
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// Shatter takes in a bounding rectangle, and several rectangles to be
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// subtracted from it. It returns a slice of rectangles that tile together to
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// make up the difference between them. This is intended to be used for figuring
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// out which areas of a container element's background are covered by other
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// elements so it doesn't waste CPU cycles drawing to those areas.
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func Shatter (
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glass image.Rectangle,
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rocks ...image.Rectangle,
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) (
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tiles []image.Rectangle,
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) {
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// in this function, the metaphor of throwing several rocks at a sheet
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// of glass is used to illustrate the concept.
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tiles = []image.Rectangle { glass }
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for _, rock := range rocks {
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// check each tile to see if the rock has collided with it
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tileLen := len(tiles)
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for tileIndex := 0; tileIndex < tileLen; tileIndex ++ {
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tile := tiles[tileIndex]
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if !rock.Overlaps(tile) { continue }
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newTiles, n := shatterOnce(tile, rock)
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if n > 0 {
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// the tile was shattered into one or more sub
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// tiles
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tiles[tileIndex] = newTiles[0]
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tiles = append(tiles, newTiles[1:n]...)
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} else {
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// the tile was entirely obscured by the rock
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// and must be wholly removed
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tiles = remove(tiles, tileIndex)
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tileIndex --
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tileLen --
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}
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}
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}
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return
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}
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func shatterOnce (glass, rock image.Rectangle) (tiles [4]image.Rectangle, n int) {
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rock = rock.Intersect(glass)
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// |'''''''''''|
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// | |
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// |###|'''| |
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// |###|___| |
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// | |
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// |___________|
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if rock.Min.X > glass.Min.X { tiles[n] = image.Rect (
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glass.Min.X, rock.Min.Y,
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rock.Min.X, rock.Max.Y,
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); n ++ }
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// |'''''''''''|
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// | |
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// | |'''|###|
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// | |___|###|
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// | |
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// |___________|
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if rock.Max.X < glass.Max.X { tiles[n] = image.Rect (
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rock.Max.X, rock.Min.Y,
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glass.Max.X, rock.Max.Y,
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); n ++ }
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// |###########|
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// |###########|
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// | |'''| |
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// | |___| |
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// | |
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// |___________|
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if rock.Min.Y > glass.Min.Y { tiles[n] = image.Rect (
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glass.Min.X, glass.Min.Y,
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glass.Max.X, rock.Min.Y,
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); n ++ }
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// |'''''''''''|
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// | |
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// | |'''| |
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// | |___| |
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// |###########|
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// |###########|
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if rock.Max.Y < glass.Max.Y { tiles[n] = image.Rect (
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glass.Min.X, rock.Max.Y,
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glass.Max.X, glass.Max.Y,
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); n ++ }
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return
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}
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func remove[ELEMENT any] (slice []ELEMENT, s int) []ELEMENT {
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return append(slice[:s], slice[s + 1:]...)
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}
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