Small Useful Functions

[ivan]

Code: Select all

function intersect(x1, y1, x2, y2, x3, y3, x4, y4)
    local x21, x43 = x2 - x1, x4 - x3
    local y21, y43 = y2 - y1, y4 - y3
    local d = x21 * y43 - y21 * x43
    if d == 0 then return false end
    local xy34 = x3 * y4 - y3 * x4
    local xy12 = x1 * y2 - y1 * x2
    local a = xy34 * x21 - xy12 * x43
    local b = xy34 * y21 - xy12 * y43
    return a / d, b / d
end

This method is used in the 'path' library by Cosmin Apreutesei.
I added the last line to calculate the 'point of intersection' in addition to the intersection times:

Code: Select all

function intersect(x1, y1, x2, y2, x3, y3, x4, y4)
  local dx1, dy1 = x2 - x1, y2 - y1
  local dx2, dy2 = x4 - x3, y4 - y3
  local dx3, dy3 = x1 - x3, y1 - y3
  local d = dx1*dy2 - dy1*dx2
  if d == 0 then
    return -- collinear
  end
  local t1 = (dx2*dy3 - dy2*dx3)/d
  local t2 = (dx1*dy3 - dy1*dx3)/d
  return x1 + t1*dx1, y1 + t1*dy1, t1, t2
end

Very easy to modify it and support segments:

Code: Select all

function intersectSegment(x1, y1, x2, y2, x3, y3, x4, y4)
  local dx1, dy1 = x2 - x1, y2 - y1
  local dx2, dy2 = x4 - x3, y4 - y3
  local dx3, dy3 = x1 - x3, y1 - y3
  local d = dx1*dy2 - dy1*dx2
  if d == 0 then
    return -- collinear
  end
  local t1 = (dx2*dy3 - dy2*dx3)/d
  if (t1 < 0) or (t1 > 1) then
    return -- non-intersecting segment 
  end
  local t2 = (dx1*dy3 - dy1*dx3)/d
  if (t2 < 0) or (t2 > 1) then
    return -- non-intersecting segment
  end
  return x1 + t1*dx1, y1 + t1*dy1, t1, t2
end

Slightly faster than Nixola's version because it bails out as soon as the t1/t2 checks fail.

[bakpakin]

Perfect ivan. I remember the solution had something to do with checking if a parametric t was between 0 and 1. I actually originally got the algorithm from wikipedia https://en.wikipedia.org/wiki/Line%E2%8 ... tersection

Also, here's OO in like 10 lines. I don't really like using class frameworks in lua because I like being able to play with metatables, but classes are pretty useful sometimes. Here are two functions for simple lua classes with constructors, inheritance, and callable classes to create instances. I just stick this at the top of a file if I want to use classes in a module.

Code: Select all

local function new(c, ...)
    local inst, ctor = setmetatable({}, c), c.init
    if ctor then ctor(inst, ...) end
    return inst
end

local class = function (super)
    local class = setmetatable({ super = super }, { __index = super, __call = new })
    class.__index = class
    return class
end

This has been done a thousand times already I'm sure, but here's my own two cents. Classes are created like

Code: Select all

local myClass = class(mySuperClass)

and instances can be created via

Code: Select all

local instance = new(myClass, ...)

or

Code: Select all

local instance = myClass(...)

Super classes are optional, and passing nil to the class function works fine as well.

[georgeprosser]

Haven't seen this one posted. Maybe that's because it's so obvious and simple, but I use it all the time. This function returns true with the supplied probability.

Code: Select all

function chance(p)
    return math.random() < p
end

Means you can write neat stuff like this:

Code: Select all

if chance(0.5) then
    print('heads')
else
    print('tails')
end

[airstruck]

Maybe this has been posted before, but this little snippet has helped me catch a few mistakes since I started using it.

Code: Select all

-- lock global table

setmetatable(_G, {
    __index = function () error('referenced an undefined variable', 2) end,
    __newindex = function () error('new global variables disabled', 2) end
})

Drop it in at the top of main.lua to catch accidental global declarations and references. If you want to declare some globals, do it above this snippet.

[zorg]

...

Or, you know, there's always strict.lua

[georgeprosser]

Here's another while I'm sharing. This function gives you the angular distance between two angles.

Code: Select all

function angularDistance(a, b)
    return math.abs(math.atan2(math.sin(a - b), math.cos(a - b)))
end

[airstruck]

Or, you know, there's always strict.lua

I'm aware of strict.lua. IMO the debug module is overkill for this sort of thing (and probably has no place in production code at all other than in something like a test framework). In fact I think strict.lua on the whole is overkill. YMMV.

[Davidobot]

Code to normalize (find the difference between) two angles:

Code: Select all

function angleDifference(t1, t2)
   return (t1 - t2 + math.pi) % (math.pi * 2) - math.pi
end

[airstruck]

Code to normalize (find the difference between) two angles:

Code: Select all

function angleDifference(t1, t2)
   return (t1 - t2 + math.pi) % (math.pi * 2) - math.pi
end

Yoink

[Davidobot]

...

No problem, man! You shouldn't really thank me though, Jasoco first sent me this code.