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On 10/19/07, Steven Johnson <temujin98007@yahoo.com> wrote:
Alright, here's a usage example. A guy with a spear versus a dragon,
and a simulation built up around that.
Untested and full of nonexistent functions:
 
+++++++++++++++++++++++++++++++++++++++++++++++++
 
-- Object-object hit test, two parameters. By default, objects don't collide.
local Hit = class.New("Multimethod", 2)
 
Hit:Define(function()
    return false
end, nil, nil)    -- nil: accept anything   
 
-- Post-hit response, ditto. No-op by default.
local Response = class.New("Multimethod", 2)
 
Response:Define(function() end)    -- omitting the nils
 
-- Hit test for a thrown spear and any kind of sphere (I dunno, floating rocks or something). If there's
-- an intersection, it also returns an information packet.
Hit:Define(function(sphere, spear)
    -- Sphere-ray intersection at time t: (c - (p + t * v)) . (c - (p + t * v)) = r^2
    local diff = sphere.c - spear.p
    local t = SolveQuadratic(spear.v * spear.v, -2 * diff * spear.v, diff * diff - sphere.r * sphere.r)
    if t then
        local point = spear.p + t * spear.v
        return true, { p = point, n = Normalize(point - sphere.c), t = t }
    end
    return false
end, "Sphere", "Spear")    -- if not nil, name or unique id used to instantiate class, returned by Type (see below)
 
-- Response to the above. Note that since the multimethod only knows about two parameters, "how",
-- the collision info from the hit test, will be ignored when deciding the method dispatch.
Response:Define(function(sphere, spear, how)
    spear:BounceOff(how.p, how.n, sphere:SurfaceProperties())
end, "Sphere", "Spear")
 
-- Bubbles? In this case, "SquishySphere" is derived from "Sphere", so this gets called instead when
-- our spear meets the squishier sort of sphere.
Response:Define(function(Sphere, Spear, how)
    PlaySound("squish")
    Spear:GetStuck(how.p, how.n)
end, "SquishySphere", "Spear")
 
-- If anything else ran into a sphere (normal, squishy, other), blow it up!
Response:Define(function(Sphere, OtherThing)
    Sphere:Explode()
end, "Sphere", nil)
 
-- Hit test against a composite thing, in this case a dragon. If it hits a part, include
-- that part along with the usual collision info.
Hit:Define(function(dragon, spear)
    for part in dragon:GetParts() do
        local bHit, arg = Hit(part, spear)
        if bHit then
            return true, { part = part, info = arg }
        end
    end
end, "Dragon", "Spear")
 
-- Another composite case, in this case with one of the built-in types as a parameter.
Hit:Define(function(terrain_things, spear)
    for _, thing in ipairs(terrain_things) do
        local bHit, arg = Hit(thing, spear)
        if bHit then
            return true, arg
        end
    end
    return false
end, "table", "Spear")
 
-- The spear struck a dragon.
Response:Define(function(dragon, spear, how)
    local name = how.part:GetName()
 
    -- Spear to the head: dragon flinches and takes damage.
    if name == "head" then
        how.part:KnockBack(spear.v)
        dragon:Hurt(spear:GetPower())
 
    -- Cripple the wing if it's not already in tatters.
    elseif name == "wing" then
        if how.part:IsIntact() then
            how.part:Maim()
        end
 
    -- Thick scales everywhere else.
    else
       spear:BounceOff(how.info.p, how.info.n, how.part:SurfaceProperties())    
    end
end, "Dragon", "Spear")
 
-- If the spear hits something else, it just drops dead.
Response:Define(function(object, spear)
    PlaySound("clank")
    spear.v = Vec3(0, 0, 0)
end, nil, "Spear")
 
--
-- etc. etc. etc. All kinds of other interactions
--
 
-- Updates objects, and updates any that run into each other. Owing to laziness, objects are assumed
-- to be ordered so that the arguments are always passed in the order listed in the above signatures.
function RunDynamics (objects, dt)
    UpdateObjects(objects, dt)
    for i = 1, #objects do
        for j = i + 1, #objects do
            local bHit, how = Hit(objects[i], objects[j])
            if bHit then
                Response(objects[i], objects[j], how)
            end
        end
    end
end
 
+++++++++++++++++++++++++++++++++++++++++++++++++
 
Comments on the implementation itself:
 
callops.IsCallable is just this:
 
    int IsCallable (lua_State * L)
    {
        lua_pushboolean(L, lua_isfunction(L, 1) || luaL_getmetafield(L, index, "__call") != 0);
 
        return 1;
    }
 
Right now, this only supports single inheritance, so class.Supers is actually only returning one value here... I'll probably generalize that
eventually with an iterator up the hierarchy linearization. The three class functions just have to behave like these:
 
    function IsType (item, what)
        assert(type ~= nil)
        local itype = item ~= nil and WeakTableOfClassInstances[item] or type(item)
        repeat
            if itype == what then
                return true
            end
            itype = Superclasses[itype]
        until itype == nil
        return false
    end
 
    function Supers (type)
        assert(type ~= nil)
        return Superclasses[type]
    end
 
    function Type (item)
        if item ~= nil then
            local itype = WeakTableOfClassInstances[item]
 
            -- Is an instance.
            if itype ~= nil then
                return itype, true
            end
        end
 
        -- Not an instance.
        return type(item), false
    end


Thanks for sharing. I will study this code.
--
Regards,
Ryan
RJP Computing