로블록스 파티클 물리 아키텍처: 베를레 적분, 거리 제약 완화 및 절차적 케이블 제작

--!strict
local RunService = game:GetService("RunService")
local Workspace = game:GetService("Workspace")

export type Particle = {
    Position: Vector3,
    PrevPosition: Vector3,
    Acceleration: Vector3,
    IsPinned: boolean,
    PinCFrame: CFrame?,
}

export type Constraint = {
    P1Index: number,
    P2Index: number,
    RestLength: number,
}

local VerletRope = {}
VerletRope.__index = VerletRope

function VerletRope.new(segmentCount: number, totalLength: number, startPos: Vector3, endPos: Vector3)
    local self = setmetatable({}, VerletRope)
    self.Particles = {} :: {Particle}
    self.Constraints = {} :: {Constraint}
    self.Damping = 0.985
    self.Gravity = Vector3.new(0, -98.2, 0)
    self.Iterations = 5

    local segmentLength = totalLength / segmentCount
    local stepVector = (endPos - startPos) / segmentCount

    for i = 0, segmentCount do
        local pos = startPos + stepVector * i
        table.insert(self.Particles, {
            Position = pos,
            PrevPosition = pos,
            Acceleration = Vector3.zero,
            IsPinned = (i == 0 or i == segmentCount),
            PinCFrame = (i == 0 and CFrame.new(startPos)) or (i == segmentCount and CFrame.new(endPos)) or nil,
        })
    end

    for i = 1, segmentCount do
        table.insert(self.Constraints, {
            P1Index = i,
            P2Index = i + 1,
            RestLength = segmentLength,
        })
    end

    return self
end

function VerletRope:Update(dt: number)
    -- Step 1: 베를레 위치 적분
    for _, p in ipairs(self.Particles) do
        if not p.IsPinned then
            local velocity = (p.Position - p.PrevPosition) * self.Damping
            p.PrevPosition = p.Position
            p.Position = p.Position + velocity + (self.Gravity + p.Acceleration) * (dt * dt)
            p.Acceleration = Vector3.zero
        elseif p.PinCFrame then
            p.PrevPosition = p.Position
            p.Position = p.PinCFrame.Position
        end
    end

    -- Step 2: 거리 제약 완화 패스
    for _ = 1, self.Iterations do
        for _, c in ipairs(self.Constraints) do
            local p1 = self.Particles[c.P1Index]
            local p2 = self.Particles[c.P2Index]

            local delta = p2.Position - p1.Position
            local currentDist = delta.Magnitude
            if currentDist > 1e-4 then
                local diff = (currentDist - c.RestLength) / currentDist
                local correction = delta * (0.5 * diff)

                if not p1.IsPinned and not p2.IsPinned then
                    p1.Position = p1.Position + correction
                    p2.Position = p2.Position - correction
                elseif not p1.IsPinned and p2.IsPinned then
                    p1.Position = p1.Position + correction * 2
                elseif p1.IsPinned and not p2.IsPinned then
                    p2.Position = p2.Position - correction * 2
                end
            end
        end
    end

    -- Step 3: 지형 및 메시 충돌 레이캐스트
    local raycastParams = RaycastParams.new()
    raycastParams.FilterType = RaycastFilterType.Exclude

    for i = 1, #self.Particles - 1 do
        local p1 = self.Particles[i]
        local p2 = self.Particles[i + 1]
        local segmentVector = p2.Position - p1.Position
        local hit = Workspace:Raycast(p1.Position, segmentVector, raycastParams)
        if hit then
            local penetration = hit.Position + hit.Normal * 0.1
            if not p2.IsPinned then
                p2.Position = penetration
            end
        end
    end
end

return VerletRope