--!strict
--[[
	AntiCheat.luau  (SERVER)
	MVP-level movement sanity check. Not a fortress — a speed bump that catches
	obvious speed/teleport hacks. Because money & car ownership are ALREADY
	server-authoritative (Economy validates everything), the main client attack
	surface left is movement, so we watch that.

	Approach: sample each player's HRP position periodically; if they cover
	ground implying a sustained speed above MaxPlausibleSpeed, snap them back
	to their last valid position. Generous threshold to avoid false positives
	from legit fast cars.
]]

local Players = game:GetService("Players")
local RunService = game:GetService("RunService")
local ReplicatedStorage = game:GetService("ReplicatedStorage")

local GameConfig = require(ReplicatedStorage.Shared.GameConfig)

local AntiCheat = {}

type Track = { lastPos: Vector3, lastValid: CFrame }

local tracked: { [Player]: Track } = {}
local SAMPLE_INTERVAL = 0.5

-- Server-initiated teleports (car spawn onto the pad, respawn, mode resets)
-- legitimately move a player thousands of studs in one frame, which trips the
-- speed check and snaps them straight back -- dragging their car with them.
-- Callers announce those moves via AntiCheat.grant() so the next sample is
-- accepted as the new baseline instead of treated as a hack.
local graceUntil: { [Player]: number } = {}
local GRACE_SECONDS = 2

--- Exempt a player from the speed check briefly after a legitimate teleport.
function AntiCheat.grant(player: Player, seconds: number?)
	graceUntil[player] = os.clock() + (seconds or GRACE_SECONDS)
end

local function hrpOf(player: Player): BasePart?
	local char = player.Character
	return char and char:FindFirstChild("HumanoidRootPart") :: BasePart?
end

function AntiCheat.init()
	local function watch(player: Player)
		player.CharacterAdded:Connect(function(char)
			local hrp = char:WaitForChild("HumanoidRootPart", 10) :: BasePart?
			if hrp then
				tracked[player] = { lastPos = hrp.Position, lastValid = hrp.CFrame }
			end
		end)
	end
	for _, p in ipairs(Players:GetPlayers()) do
		watch(p)
	end
	Players.PlayerAdded:Connect(watch)
	Players.PlayerRemoving:Connect(function(p)
		tracked[p] = nil
	end)

	local accum = 0
	RunService.Heartbeat:Connect(function(dt)
		accum += dt
		if accum < SAMPLE_INTERVAL then
			return
		end
		local interval = accum
		accum = 0

		for player, track in pairs(tracked) do
			local hrp = hrpOf(player)
			if not hrp then
				continue
			end
			local pos = hrp.Position
			-- Falling doesn't count (avoid flagging big drops). Only guard XZ plane.
			local horizontal = Vector3.new(pos.X - track.lastPos.X, 0, pos.Z - track.lastPos.Z).Magnitude
			local hSpeed = horizontal / interval

			local inGrace = (graceUntil[player] or 0) > os.clock()
			if hSpeed > GameConfig.MaxPlausibleSpeed and not inGrace then
				-- Snap back: likely a speed/teleport hack.
				hrp.CFrame = track.lastValid
				hrp.AssemblyLinearVelocity = Vector3.zero
			else
				track.lastValid = hrp.CFrame
			end
			track.lastPos = hrp.Position
		end
	end)
end

return AntiCheat