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articles/collision-logic.md

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+Abstract:
+
+The goal of this pseudocode is to create a collision system that brings several
+colliding bodies to a near-contact state (no space between the bodies, but not
+overlapping), while preserving the motion of said bodies. For simplicity, I've
+decided to use a square box (BoundingBox, or BBox for short) for the collidable
+bodies.
+
+Example:
+
+My current game has a tiled-map, where the tiles are arranged on a 2D grid. The
+tiles each have a flag indicating if they are solid (i.e. collidable) or not. I
+also have characters walking around on these tile maps, that must not intersect
+with solid tiles. For stylistic reasons, I want characters moving at an angle
+to "slide" along these walls, and continue along with their original velocity
+when they've circumvented the obstacles.
+
+I'd also like to keep the possibility of non-grid collisions using this logic
+open (for objects like trees, etc.) if the code allows it.
+
+Obviously, there are many in depth issues that I will need to  take into
+account when writing this logic, that have been glossed over or omitted in this
+article.
+
+-------------------------
+
+```
+	velocity = motion + speed
+	if (collision(position + velocity)) then
+		if (collision(position + {velocity.x, 0})) then
+			velocity.x = 0
+		end
+		if (collision(position + {0, velocity.y})) then
+			velocity.y = 0
+		end
+	end
+	position = position + velocity
+```
+
+This code is a basic outline for a collision system that preserves the object's
+motion, but it still leaves several pixels of space between the bounding boxes.
+Notably, it also treats "collision" as an abstract concept, rather than as an
+event that could happen multiple times per frame.
+
+-------------------------
+
+```
+	velocity = motion + speed
+	if (collisionSimple(BOXSET, position + velocity)) then
+		velocity.x = collisionX(BOXSET, velocity.x)
+		velocity.y = collisionY(BOXSET, velocity.y)
+	end
+	position = position + velocity
+```
+
+Here, collisions are still abstract, but "BOXSET" is defined externally
+(probably as a set of solid boxes, and their positions). This does require more
+in depth calculations, as well as three specialized utility functions, but the
+results might be what I'm looking for.
+
+If there are any collisions between the player object and the given box set,
+then collisionX() and collisionY() are called to calculate the new distance
+that the character will move.
+
+-------------------------
+
+```
+	bool collisionSimple(BOXSET, newPos):
+		for_each box in BOXSET do
+			if (box.overlap(PLAYER.box + newPos)) then
+				return true
+			end
+		end
+		return false
+	end
+```
+
+collisionSimple() first runs through the BOXSET, checking if any of the given
+bounding boxes would collide with the player object's new position (just
+pretend PLAYER is accessible). In this case, non-tile bounding boxes can be
+included as part of BOXSET; they're treated just the same. The optimal outcome
+is that there are no collisions.
+
+A possible, but flawed, optimization that could be preformed here is to remove
+any elements from BOXSET that do not collide with newPos, and let the other
+utility functions operate only on what remains. However, if there are any
+collisions, than newPos is not the algorithm's final result, therefore any
+final result that the algorithm would calculate based on the remaining elements
+would not have been checked against the removed elements.
+
+Just something to note.
+
+-------------------------
+
+```
+	var collisionX(BOXSET, velocityX):
+		var ret = velocityX
+
+		for_each box in BOXSET do
+			if (box.overlap(PLAYER.box + PLAYER.position + {velocityX,0})) then
+				if (velocityX > 0) then
+					ret = min(ret, box.west - PLAYER.position.x)
+				else
+					ret = max(ret, box.east - PLAYER.position.x)
+				end
+			end
+		end
+
+		return ret
+	end
+```
+
+Two things: 1. collisionX() and collisionY() should be identical except for the
+axis of operation and 2. if a player object is "sliding" along a wall (or
+stuck), then these functions will be called every frame.
+
+collisionX() and collisionY() check the sides of the elements in BOXSET, and if
+there's a box that the player would collide with, given the current distance to
+move, than the distance is reduced, based on if the character is moving left or
+right (or up or down).
+
+An unfortunate bug I can already see is that this logic doesn't check corners;
+it might be possible to get stuck on a corner of a wall, but if this becomes an
+issue in my implementation I will update this article with that information,
+and you can promptly ignore it.