절차적 총기 동역학: 2관절 해석적 IK, 상체 피치 추적 및 2차 스프링 반동 시스템

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

export type SpringState = {
    Position: Vector3,
    Velocity: Vector3,
    Target: Vector3,
    Stiffness: number,
    Damping: number,
}

local WeaponIK = {}
WeaponIK.__index = WeaponIK

function WeaponIK.createSpring(stiffness: number, damping: number): SpringState
    return {
        Position = Vector3.zero,
        Velocity = Vector3.zero,
        Target = Vector3.zero,
        Stiffness = stiffness,
        Damping = damping,
    }
end

function WeaponIK.updateSpring(spring: SpringState, dt: number): Vector3
    local force = (spring.Target - spring.Position) * spring.Stiffness
    local damp = spring.Velocity * spring.Damping
    local accel = force - damp

    spring.Velocity += accel * dt
    spring.Position += spring.Velocity * dt
    return spring.Position
end

-- Analytical Two-Bone IK (Law of Cosines)
function WeaponIK.solveTwoBoneIK(origin: Vector3, target: Vector3, l1: number, l2: number): (CFrame, CFrame)
    local toTarget = target - origin
    local dist = math.clamp(toTarget.Magnitude, 0.001, l1 + l2 - 0.001)

    local cosAngle1 = (l1^2 + dist^2 - l2^2) / (2 * l1 * dist)
    local angle1 = math.acos(math.clamp(cosAngle1, -1, 1))

    local cosAngle2 = (l1^2 + l2^2 - dist^2) / (2 * l1 * l2)
    local angle2 = math.acos(math.clamp(cosAngle2, -1, 1))

    local baseCF = CFrame.lookAt(origin, target)
    local upperCF = baseCF * CFrame.Angles(angle1, 0, 0)
    local lowerCF = upperCF * CFrame.new(0, 0, -l1) * CFrame.Angles(-(math.pi - angle2), 0, 0)

    return upperCF, lowerCF
end

function WeaponIK.new(character: Model)
    local self = setmetatable({}, WeaponIK)
    self.Character = character
    self.RecoilPosSpring = WeaponIK.createSpring(220, 24)
    self.RecoilRotSpring = WeaponIK.createSpring(280, 26)
    self.SwaySpring = WeaponIK.createSpring(120, 16)
    self.SwayTime = 0
    return self
end

function WeaponIK:ApplyRecoilImpulse(linearKick: Vector3, angularKick: Vector3)
    self.RecoilPosSpring.Velocity += linearKick
    self.RecoilRotSpring.Velocity += angularKick
end

function WeaponIK:Update(dt: number, cameraLookVector: Vector3, moveSpeed: number)
    local root = self.Character:FindFirstChild("HumanoidRootPart") :: BasePart?
    local torso = self.Character:FindFirstChild("UpperTorso") :: BasePart? or self.Character:FindFirstChild("Torso") :: BasePart?
    local rightUpper = self.Character:FindFirstChild("RightUpperArm") :: BasePart?
    local leftUpper = self.Character:FindFirstChild("LeftUpperArm") :: BasePart?
    if not root or not torso or not rightUpper or not leftUpper then return end

    -- Update springs
    local recPos = WeaponIK.updateSpring(self.RecoilPosSpring, dt)
    local recRot = WeaponIK.updateSpring(self.RecoilRotSpring, dt)

    -- Procedural Lissajous bobbing
    self.SwayTime += dt * (moveSpeed > 2 and 8 or 2)
    local bobX = math.cos(self.SwayTime * 0.5) * (moveSpeed * 0.015)
    local bobY = math.sin(self.SwayTime) * (moveSpeed * 0.02)
    local swayOffset = Vector3.new(bobX, bobY, 0) + recPos

    -- Spine pitch conform
    local pitch = math.asin(cameraLookVector.Y)
    local waist = torso:FindFirstChild("Waist") :: Motor6D?
    if waist then
        waist.Transform = CFrame.Angles(pitch * 0.5, 0, 0)
    end
end

return WeaponIK