Nested For-Loops - Remove Both Elements i and j - lua

Question

I'm using a game-making framework implemented in Lua to keep busy in quarantine. I'm making a simple platformer with ECS/DOP, and I wanted to generate collision geometry derived from a tilemap rather than just checking for collisions with all tiles.
Each tile has a bounding box component that points to a list that contains the basic shapes. Each shape stores the edges of the bounding box as {{x1,y1},{x2,y2}}. The first step in this process is to parse a TileMap table that contains only tilenames, then insert a copy of the corresponding bounding-box translated by the row/column * grid_dimension into a table named BBOX. The next step is to delete all instances of an edge if it is a duplicate illustrated by this image
which is where I'm stuck.
The desired, basic edge-deletion algorithm looks like this:
for i = #BBOX, 1, -1 do
local edge1 = BBOX[i]
for j = i, 1, -1 do
local edge2 = BBOX[j]
same_edge = edge1 == edge2 -- Not the actual comparison, just the outcome of it
if same_edge and i ~= j then
BBOX[i] = nil
BBOX[j] = nil
end
end
end
The issue is of course that this errors when i is equal to a j that was removed earlier in the loop. I've looked around and haven't been able to find a way to remove all instances of duplicate values in lua, only solutions that care about uniqueness. Has anyone found an efficient method of doing this or something similar?

Answer 1

If you use the value unique to each key you can better detect duplicates.
we will call this unique value edge.key.
If an edge will only exist at most twice then:
BBOX[edge.key] = BBOX[edge.key] == nil and edge or nil
will do the trick.
But where an edge may exist any arbitrary number of time we can do something like this:
local duplicateEdges = {}
for i = #BBOX, 1, -1 do
if BBOX[edge_key] then
duplicateEdges[edge_key] = true
else
BBOX[edge_key] = edge
end
for j = i, 1, -1 do
if BBOX[edge_key] then
duplicateEdges[edge_key] = true
else
BBOX[edge_key] = edge
end
end
end
for k in pairs(duplicateEdges) do
BBOX[k] = nil
end