chore: housekeeping

packages/charm-sync/src/client.luau

@@ -0,0 +1,76 @@
+local Charm = require(script.Parent.Parent.Charm)
+local patch = require(script.Parent.patch)
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+type SyncPayload = types.SyncPayload
+
+type ClientOptions = {
+	--- The atoms to synchronize with the server.
+	atoms: AtomMap,
+	--- Whether to ignore patches sent before the client has been hydrated.
+	--- Default is `true`.
+	ignoreUnhydrated: boolean?,
+}
+
+type ClientSyncer = {
+	--- Applies a patch or initializes the state of the atoms with the given
+	--- payload from the server.
+	---
+	--- @param ...payloads The patches or hydration payloads to apply.
+	sync: (self: ClientSyncer, ...SyncPayload) -> (),
+}
+
+--- Creates a `ClientSyncer` object that receives patches from the server and
+--- applies them to the local state.
+---
+--- @param options The atoms to synchronize with the server.
+--- @return A `ClientSyncer` object.
+local function client(options: ClientOptions): ClientSyncer
+	local atoms = options.atoms
+	local ignoreUnhydrated = options.ignoreUnhydrated ~= false
+
+	local self = {} :: ClientSyncer
+	local hydrated = false
+
+	local function hydrate(state: { [string | number]: any })
+		hydrated = true
+
+		for key, value in next, state do
+			local atom = atoms[key]
+			atom(value)
+		end
+	end
+
+	local function apply(data: { [string | number]: any })
+		local target = {}
+
+		for key, atom in next, atoms do
+			target[key] = atom()
+		end
+
+		target = patch.apply(target, data)
+
+		for key, atom in next, atoms do
+			atom(target[key])
+		end
+	end
+
+	function self:sync(...)
+		for index = 1, select("#", ...) do
+			local payload: SyncPayload = select(index, ...)
+
+			Charm.batched(function()
+				if payload.type == "init" then
+					hydrate(payload.data)
+				elseif not ignoreUnhydrated or hydrated then
+					apply(payload.data)
+				end
+			end)
+		end
+	end
+
+	return self
+end
+
+return client

packages/charm-sync/src/flatten.luau

@@ -0,0 +1,29 @@
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+
+type NestedAtomMap = {
+	[string]: NestedAtomMap | () -> (),
+}
+
+local function flatten(atoms: NestedAtomMap): AtomMap
+	local result: AtomMap = {}
+
+	local function visit(node: NestedAtomMap, path: string)
+		for key, value in node do
+			local location = if path == "" then key else path .. "/" .. key
+
+			if type(value) == "table" then
+				visit(value, location)
+			else
+				result[location] = value
+			end
+		end
+	end
+
+	visit(atoms, "")
+
+	return result
+end
+
+return flatten

packages/charm-sync/src/init.luau

@@ -0,0 +1,17 @@
+local client = require(script.client)
+local flatten = require(script.flatten)
+local patch = require(script.patch)
+local server = require(script.server)
+local types = require(script.types)
+local validate = require(script.validate)
+
+export type SyncPayload = types.SyncPayload
+
+return {
+	client = client,
+	server = server,
+	flatten = flatten,
+	validate = validate,
+	patch = patch,
+	isNone = patch.isNone,
+}

packages/charm-sync/src/interval.luau

@@ -0,0 +1,28 @@
+local RunService = game:GetService("RunService")
+
+--- Schedules a function to be called at a fixed interval of `seconds`.
+---
+--- @param callback The function to call.
+--- @param seconds The interval in seconds.
+--- @return A function that cancels the interval.
+local function interval(callback: () -> (), seconds: number): () -> ()
+	if seconds < 0 then
+		return function() end
+	end
+
+	local clock = 0
+
+	local connection = RunService.Heartbeat:Connect(function(delta)
+		clock += delta
+		if clock >= seconds then
+			clock = 0
+			callback()
+		end
+	end)
+
+	return function()
+		connection:Disconnect()
+	end
+end
+
+return interval

packages/charm-sync/src/patch.luau

@@ -0,0 +1,109 @@
+local validate = require(script.Parent.validate)
+
+local __DEV__ = _G.__DEV__
+
+--- A special symbol that denotes the absence of a value. Used to represent
+--- deleted values in patches.
+local None = { __none = "__none" }
+
+local function isNone(value: any): boolean
+	return type(value) == "table" and value.__none == "__none"
+end
+
+--- JSON serialization can drop the last values of a sparse array. For example,
+--- `{ [1] = 1, [3] = 3 }` becomes `[1]` in some cases. Ambiguity in the
+--- serialization of sparse arrays can be resolved by converting them to
+--- dictionaries with string keys.
+---
+--- The `apply` function will convert these dictionaries back into arrays as
+--- long as the target array is not sparse. This function ensures that the diff
+--- function produces a patch that can be correctly applied.
+local function stringifySparseArray(object: { any }): { [string | number]: any }
+	local maxn = table.maxn(object)
+
+	if maxn == 0 or maxn == #object then
+		-- The object is either not an array or a dense array
+		return object
+	end
+
+	local dictionary = {}
+
+	for index, value in next, object do
+		dictionary[tostring(index)] = value
+	end
+
+	return dictionary
+end
+
+--- Compares two states and returns a patch that can be applied to the previous
+--- state to produce the next state.
+---
+--- @param prevState The previous state.
+--- @param nextState The next state.
+--- @param serialize Whether to convert sparse arrays into dictionaries. Defaults to `true`.
+--- @returns A patch that can be applied to the previous state.
+local function diff(prevState: { [any]: any }, nextState: { [any]: any }, serialize: boolean?)
+	serialize = serialize ~= false
+
+	local patches = table.clone(nextState)
+
+	for key, previous in next, prevState do
+		local current = nextState[key]
+
+		if previous == current then
+			patches[key] = nil
+		elseif current == nil then
+			patches[key] = None
+		elseif type(previous) == "table" and type(current) == "table" then
+			patches[key] = diff(previous, current, serialize)
+		end
+	end
+
+	-- Coerce sparse array patches into dictionaries
+	if serialize and (prevState[1] ~= nil or nextState[1] ~= nil) then
+		patches = stringifySparseArray(patches)
+	end
+
+	if serialize and __DEV__ then
+		for key, value in next, patches do
+			validate(value, key)
+		end
+	end
+
+	return patches
+end
+
+--- Applies a patch to a state and returns the next state.
+---
+--- @param state The current state.
+--- @param patches The patches to apply.
+--- @returns The next state.
+local function apply(state: any, patches: any): any
+	if type(patches) == "table" and patches.__none == "__none" then
+		return nil
+	elseif type(state) ~= "table" or type(patches) ~= "table" then
+		return patches
+	end
+
+	local nextState = table.clone(state)
+	local stateIsArray = state[1] ~= nil
+
+	for key, patch in next, patches do
+		-- Diff-checking an array produces a sparse array, which will not be
+		-- preserved when converted to JSON. To prevent this, we turn string
+		-- keys back into numeric keys.
+		if stateIsArray and type(key) == "string" then
+			key = tonumber(key) or key
+		end
+
+		nextState[key] = apply(nextState[key], patch)
+	end
+
+	return nextState
+end
+
+return {
+	isNone = isNone,
+	diff = diff,
+	apply = apply,
+}

packages/charm-sync/src/server.luau

@@ -0,0 +1,252 @@
+local Players = game:GetService("Players")
+
+local Charm = require(script.Parent.Parent.Charm)
+local interval = require(script.Parent.interval)
+local patch = require(script.Parent.patch)
+local types = require(script.Parent.types)
+
+type AtomMap = types.AtomMap
+type SyncPayload = types.SyncPayload
+
+type ServerOptions = {
+	--- The atoms to synchronize with the client.
+	atoms: AtomMap,
+	--- The interval at which to send patches to the client, in seconds.
+	--- Defaults to `0` (patches are sent up to once per frame). Set to a
+	--- negative value to disable automatic syncing.
+	interval: number?,
+	--- Whether the history of state changes since the client's last update
+	--- should be preserved. This is useful for values that change multiple times
+	--- per frame, where each individual change is important. Defaults to `false`.
+	preserveHistory: boolean?,
+	--- When `true`, Charm will apply validation and serialize unsafe arrays
+	--- to address remote event argument limitations. Defaults to `true`.
+	---
+	--- This option should be disabled if your network library uses a custom
+	--- serialization method (i.e. Zap, ByteNet) to prevent interference.
+	autoSerialize: boolean?,
+}
+
+type ServerSyncer = {
+	--- Sets up a subscription to each atom that schedules a patch to be sent to
+	--- the client whenever the state changes. When a change occurs, the `callback`
+	--- is called with the player and the payload to send.
+	---
+	--- Note that the `payload` object should not be mutated. If you need to
+	--- modify the payload, apply the changes to a copy of the object.
+	---
+	--- @param callback The function to call when the state changes.
+	--- @return A cleanup function that unsubscribes all listeners.
+	connect: (self: ServerSyncer, callback: (player: Player, ...SyncPayload) -> ()) -> () -> (),
+	--- Hydrates the client's state with the server's state. This should be
+	--- called when a player joins the game and requires the server's state.
+	---
+	--- @param player The player to hydrate.
+	hydrate: (self: ServerSyncer, player: Player) -> (),
+	--- @deprecated For internal use only.
+	_sendPatch: (self: ServerSyncer, player: Player) -> (),
+}
+
+--- Creates a `ServerSyncer` object that sends patches to the client and
+--- hydrates the client's state.
+---
+--- @param options The atoms to synchronize with the client.
+--- @return A `ServerSyncer` object.
+local function server(options: ServerOptions): ServerSyncer
+	local atoms = options.atoms
+	local autoSerialize = options.autoSerialize ~= false
+	local preserveHistory = options.preserveHistory
+	local syncRate = options.interval or 0
+
+	local self = {} :: ServerSyncer
+	local sync: (player: Player, payload: SyncPayload) -> ()
+
+	local function createSnapshot()
+		local snapshot = {}
+
+		for key, atom in next, atoms do
+			snapshot[key] = atom()
+		end
+
+		return snapshot
+	end
+
+	-- Send the initial state to a player when they join the server.
+	function self:hydrate(player)
+		assert(sync, "connect() must be called before hydrate()")
+
+		sync(player, {
+			type = "init",
+			data = createSnapshot(),
+		})
+	end
+
+	-- If the history of state changes should be preserved, keep a list of
+	-- previous snapshots and diff-check each one from the last.
+	if preserveHistory then
+		local snapshots = { createSnapshot() }
+		local changed = false
+
+		function self:connect(callback)
+			local subscriptions = {}
+
+			sync = callback
+
+			local function pushSnapshot(key: string | number, current: any, previous: any)
+				local lastSnapshot = snapshots[#snapshots]
+				local previousSnapshot = snapshots[#snapshots - 1]
+
+				-- Optimize snapshots by updating the most recent snapshot if the
+				-- previous and current values are the same, since this allows us
+				-- to group multiple changes into a single snapshot.
+				if previousSnapshot and previousSnapshot[key] == previous and lastSnapshot[key] == previous then
+					lastSnapshot[key] = current
+				else
+					local nextSnapshot = table.clone(lastSnapshot)
+					nextSnapshot[key] = current
+					table.insert(snapshots, nextSnapshot)
+				end
+			end
+
+			-- Populate the snapshot with the initial state of each atom.
+			-- Subscribe to each atom and update the state when it changes.
+			for key, atom in next, atoms do
+				subscriptions[key] = Charm.subscribe(atom, function(current, previous)
+					pushSnapshot(key, current, previous)
+					changed = true
+				end)
+			end
+
+			local disconnect = interval(function()
+				if not changed then
+					return
+				end
+
+				local payloads: { SyncPayload } = {}
+				local lastSnapshot
+
+				for index, snapshot in next, snapshots do
+					lastSnapshot = snapshot
+
+					if index == 1 then
+						continue
+					end
+
+					table.insert(payloads, {
+						type = "patch",
+						data = patch.diff(snapshots[index - 1], snapshot, autoSerialize),
+					})
+				end
+
+				snapshots = { lastSnapshot }
+				changed = false
+
+				for _, player in next, Players:GetPlayers() do
+					callback(player, unpack(payloads))
+				end
+			end, syncRate)
+
+			return function()
+				disconnect()
+
+				for _, unsubscribe in next, subscriptions do
+					unsubscribe()
+				end
+			end
+		end
+
+		function self:_sendPatch(player: Player)
+			assert(sync, "connect() must be called before _sendPatch()")
+
+			if not changed then
+				return
+			end
+
+			local payloads: { SyncPayload } = {}
+
+			for index, snapshot in next, snapshots do
+				if index == 1 then
+					continue
+				end
+
+				table.insert(payloads, {
+					type = "patch",
+					data = patch.diff(snapshots[index - 1], snapshot, autoSerialize),
+				})
+			end
+
+			snapshots = { snapshots[#snapshots] }
+			changed = false
+
+			sync(player, unpack(payloads))
+		end
+
+		return self
+	end
+
+	local previousSnapshot = createSnapshot()
+	local currentSnapshot = table.clone(previousSnapshot)
+	local changed = false
+
+	function self:connect(callback)
+		local subscriptions = {}
+
+		sync = callback
+
+		-- Subscribe to each atom and update the state when one changes.
+		for key, atom in next, atoms do
+			subscriptions[key] = Charm.subscribe(atom, function(state)
+				currentSnapshot[key] = state
+				changed = true
+			end)
+		end
+
+		local disconnect = interval(function()
+			if not changed then
+				return
+			end
+
+			local payload: SyncPayload = {
+				type = "patch",
+				data = patch.diff(previousSnapshot, currentSnapshot, autoSerialize),
+			}
+
+			previousSnapshot = table.clone(currentSnapshot)
+			changed = false
+
+			for _, player in next, Players:GetPlayers() do
+				callback(player, payload)
+			end
+		end, syncRate)
+
+		return function()
+			disconnect()
+
+			for _, unsubscribe in next, subscriptions do
+				unsubscribe()
+			end
+		end
+	end
+
+	function self:_sendPatch(player)
+		assert(sync, "connect() must be called before _sendPatch()")
+
+		if not changed then
+			return
+		end
+
+		local payload: SyncPayload = {
+			type = "patch",
+			data = patch.diff(previousSnapshot, currentSnapshot, autoSerialize),
+		}
+
+		previousSnapshot = table.clone(currentSnapshot)
+		changed = false
+
+		sync(player, payload)
+	end
+
+	return self
+end
+
+return server

packages/charm-sync/src/types.luau

@@ -0,0 +1,19 @@
+local Charm = require(script.Parent.Parent.Charm)
+
+type Atom<T> = Charm.Atom<T>
+
+--- A payload that can be sent from the server to the client to synchronize
+--- state between the two.
+export type SyncPayload = {
+	type: "init",
+	data: { [string | number]: any },
+} | {
+	type: "patch",
+	data: { [string | number]: any },
+}
+
+export type AtomMap = {
+	[string | number]: Atom<any>,
+}
+
+return nil

packages/charm-sync/src/validate.luau

@@ -0,0 +1,66 @@
+local SAFE_KEYS = { string = true, number = true }
+local UNSAFE_VALUES = { ["function"] = true, thread = true }
+
+local function isTableUnsafe(object: { [any]: any })
+	local keyType = nil
+	local objectSize = 0
+
+	-- All keys must have the same type
+	for key in next, object do
+		local currentType = type(key)
+
+		if not keyType and SAFE_KEYS[currentType] then
+			keyType = currentType
+		elseif keyType ~= currentType then
+			return true
+		end
+
+		objectSize += 1
+	end
+
+	-- If there are more keys than the length of the array, it's an array with
+	-- non-sequential keys.
+	if objectSize > #object and keyType == "number" then
+		return true
+	end
+
+	return false
+end
+
+--- Validates a value to ensure it can be synced over a remote event.
+---
+--- @param value The value to validate.
+--- @param key The key of the value in the table.
+--- @error Throws an error if the value cannot be synced.
+local function validate(value: any, key: any)
+	local typeOfKey = type(key)
+	local typeOfValue = type(value)
+
+	if not SAFE_KEYS[typeOfKey] then
+		error(`Invalid key type '{typeOfKey}' at key '{key}'`)
+	elseif UNSAFE_VALUES[typeOfValue] then
+		error(`Invalid value type '{typeOfValue}' at key '{key}'`)
+	elseif typeOfValue == "table" then
+		if getmetatable(value) ~= nil then
+			error(`Cannot sync tables with metatables! Got {value} at key '{key}'`)
+		elseif isTableUnsafe(value) then
+			error(
+				`Cannot sync tables unsupported by remote events! The value has the key '{key}'.\n\n`
+					.. "This can be for the following reasons:\n"
+					.. "1. The object is an array with non-sequential keys\n"
+					.. "2. The object is a dictionary with mixed key types (e.g. string and number)\n\n"
+					.. "Read more: https://create.roblox.com/docs/scripting/events/remote#argument-limitations"
+			)
+		end
+	end
+
+	if typeOfKey == "number" then
+		if key == math.huge or key == -math.huge then
+			error("Cannot sync infinity as key")
+		elseif key ~= math.floor(key) then
+			error("Cannot sync non-integer number as key")
+		end
+	end
+end
+
+return validate