Get all shapes that intersect a line

I was working on some personal projects with HardonCollider , and I came across a situation where I wanted something like a raycast or a linecast.

I knew Hardon used a spatial hash, so after a quick Google search I found this page,

[Ray casting in a Spatial Hash with DDA](http://www.playchilla.com/ray-casting-spatial-hash-dda)

and after a few days of tinkering, I got something working.

The Lua file below demonstrates the "shapesOnLine" function.

shapesOnLine takes four arguments,
 
x1, y2, x2,and y2, 
 
which are the coordinates of the two endpoints of the line.

Using the DDA algorithm, the function looks at each cell the line passes through and calls intersectsRay on each object in each cell. only the cells the line passes through are checked. IntersectsRay dose not register a ht if the ray originates inside a shape, so shapesAt is used to get the shapes at the origin point.

The return value of the function is a table containing all the shapes the line passed through. They are not in any guaranteed order, and currently it may be possible to have duplicates in the table.

Run it with love using my fork of Hardon to see it in action.

I tried to move the DDA algorithm into the spatialhash class (and rename it something like cellsOnLine), but for some reason it wasn't working right and the part that was working was really slow. Also, I need to rewrite the DDA algorithm so using the vector stuff and the methods in spatialhash (another reason I tried to move it there). It will be a little cleaner after that I fix that.

````lua
--main.lua
HC = require 'hardoncollider'

function love.load()
  Collider = HC(25)
  
  --default endpoints of the LineCast
  originX = 350
  originY = 350
  endX = 350
  endY = 450
  
  --set of rectangle shapes for the LineCast to hit
  blocks = {}
  for i=0,39,1 do
    blocks[i]={}
    for j=0,29,1 do
      blocks[i][j] = Collider:addRectangle(20*i+5,20*j+5,10,10);
    end 
  end
end

function love.update(dt)
  --position the endpoints of the line based on mouse position
  if love.mouse.isDown("r") then
    endX = love.mouse.getX()
    endY = love.mouse.getY()
  elseif love.mouse.isDown("l") then
    originX = love.mouse.getX()
    originY = love.mouse.getY()
  end
end

function love.draw()
  --draw outline of all blocks
  for _,table in pairs(blocks) do
    for _,object in pairs(table)do
      object:draw("line")
    end
  end
  
  --get table of all shapes under the line
  hits = Collider:shapesOnLine(originX,originY,endX,endY)
  
  --fill in all the shapes under the line
  for k,v in pairs(hits) do
    v:draw("fill")
  end
  
  --draw tooltips
  love.graphics.setColor(0,100,255,200)
  love.graphics.rectangle("fill",endX,endY,100,20)
  love.graphics.rectangle("fill",originX,originY,100,20)
  love.graphics.setColor(255,0,0,255)
  love.graphics.print(" RMB to move",endX,endY)
  love.graphics.print(" LMB to move",originX,originY)
  
  --draw the line
  love.graphics.line(originX,originY,endX,endY)
  love.graphics.setColor(255,255,255,255)
end
````

By the way, Hardon really is an awesome library. I cant imagine working in love without it!