refactor: embrace scope cleanup, restructure code

pkgs/charm-sync/__benchmarks__/patch.bench.luau

@@ -0,0 +1,54 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+
+local patch = CharmSync.patch
+
+local USERS = 1000
+
+local function generate()
+	local users = {}
+
+	for user = 1, USERS do
+		users[user] = {
+			coins = math.random(1, 2),
+			gems = math.random(1, 2),
+		}
+	end
+
+	return { users = users }
+end
+
+local function update(state)
+	state = table.clone(state)
+	state.users = table.clone(state.users)
+
+	for user = 1, USERS do
+		local rng = math.random()
+
+		if rng < 1 / 3 then
+			continue
+		elseif rng < 2 / 3 then
+			state.users[user] = nil
+		else
+			state.users[user] = {
+				coins = math.random(1, 2),
+				gems = math.random(1, 2),
+			}
+		end
+	end
+
+	return state
+end
+
+return {
+	ParameterGenerator = function()
+		local state = generate()
+		return state, update(state)
+	end,
+	Functions = {
+		diff = function(_, a, b)
+			patch.diff(a, b)
+		end,
+	},
+}

pkgs/charm-sync/__tests__/client.spec.luau

@@ -0,0 +1,125 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local subscribe = Charm.subscribe
+local client = CharmSync.client
+local test = suite.test
+
+test("should sync state payload", function()
+	local a = atom(1)
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = 2 },
+	})
+
+	assert(a() == 2)
+end)
+
+test("should sync state patch", function()
+	local a = atom({ b = 1, c = 2 })
+	local previous = a()
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	assert(a() == previous)
+
+	client:sync({
+		type = "patch",
+		data = { a = { b = 3 } },
+	})
+
+	assert(a() ~= previous)
+	assert(a().b == 3)
+	assert(a().c == 2)
+end)
+
+test("should fix sparse array patches", function()
+	local a = atom({ 1 })
+	local client = client({ atoms = { a = a } })
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	-- This patch would be the result of a diff between { 1 } and { 1, 2 } that
+	-- was JSON-serialized and deserialized. The client should be able to catch
+	-- this and convert the string key back to a number.
+	client:sync({
+		type = "patch",
+		data = { a = { ["2"] = 2 } },
+	})
+
+	assert(a()["2" :: any] == nil)
+	assert(a()[2] == 2)
+end)
+
+test("should apply multiple patches", function()
+	local a = atom(1)
+	local client = client({ atoms = { a = a } })
+	local calls = {}
+
+	subscribe(a, function(value)
+		table.insert(calls, value)
+	end)
+
+	client:sync({
+		type = "init",
+		data = { a = a() },
+	})
+
+	client:sync({
+		type = "patch",
+		data = { a = 2 },
+	}, {
+		type = "patch",
+		data = { a = 3 },
+	})
+
+	assert(a() == 3)
+	assert(calls[1] == 2)
+	assert(calls[2] == 3)
+end)
+
+test("should batch patches", function()
+	local a = atom(0)
+	local b = atom(0)
+	local client = client({ atoms = { a = a, b = b } })
+	local states = {}
+
+	subscribe(a, function()
+		states.b = b()
+	end)
+
+	subscribe(b, function()
+		states.a = a()
+	end)
+
+	client:sync({
+		type = "init",
+		data = { a = a(), b = b() },
+	})
+
+	client:sync({
+		type = "patch",
+		data = { a = 1, b = 2 },
+	})
+
+	assert(a() == 1)
+	assert(b() == 2)
+	assert(states.a == 1)
+	assert(states.b == 2)
+end)
+
+return {}

pkgs/charm-sync/__tests__/flatten.spec.luau

@@ -0,0 +1,28 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local flatten = CharmSync.flatten
+local test = suite.test
+
+test("should flatten atoms", function()
+	local input = {
+		a = atom(0),
+		b = { c = atom(0) },
+		d = { e = { f = atom(0) } },
+	}
+	local result = flatten(input)
+
+	assert(result.a == input.a)
+	assert(result["b/c"] == input.b.c)
+	assert(result["d/e/f"] == input.d.e.f)
+
+	assert(result.b == nil)
+	assert(result.d == nil)
+end)
+
+return {}

pkgs/charm-sync/__tests__/patch.spec.luau

@@ -0,0 +1,129 @@
+local HttpService = game:GetService("HttpService")
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require("../suite")
+
+local patch = CharmSync.patch
+local test = suite.test
+
+local function serdes(value: { [any]: any })
+	return HttpService:JSONDecode(HttpService:JSONEncode(value))
+end
+
+test("returns a patch", function()
+	local prevState = { foo = "bar", baz = "qux" }
+	local nextState = { foo = "bar", baz = "quux" }
+	local patches = patch.diff(prevState, nextState)
+
+	assert(patches ~= nil)
+	assert(patches.baz == "quux")
+	assert(patches.foo == nil)
+end)
+
+test("applies a patch", function()
+	local prevState = { foo = "bar", baz = "qux" }
+	local patches = { baz = "quux", qaz = "qux" }
+	local applied = patch.apply(prevState, patches)
+
+	assert(applied ~= nil)
+	assert(applied.baz == "quux")
+	assert(applied.foo == "bar")
+	assert(applied.qaz == "qux")
+end)
+
+test("allows nested patches", function()
+	local prevState = { foo = { bar = "baz" } }
+	local nextState = { foo = { bar = "qux" } }
+	local patches = patch.diff(prevState, nextState)
+	local applied = patch.apply(prevState, patches)
+
+	assert(applied.foo.bar == "qux")
+end)
+
+test("preserves array density when applying", function()
+	local prevState = { "a", "b", "c", "d" }
+	local nextState = { ["2"] = "B", ["4"] = "D" }
+	local applied = patch.apply(prevState, nextState)
+
+	assert(applied[1] == "a")
+	assert(applied[2] == "B")
+	assert(applied[3] == "c")
+	assert(applied[4] == "D")
+
+	assert(applied["2"] == nil)
+	assert(applied["4"] == nil)
+end)
+
+test("preserves array density after serdes", function()
+	-- This particular change results in a patch that cannot be serialized
+	-- without losing information. Charm resolves this issue internally.
+	local prevState = { "a", "b", "c", "d", "e" }
+	local nextState = { "A", "b", "c", "D" }
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	for index in next, prevState do
+		assert(applied[index] == nextState[index])
+	end
+
+	for index in next, applied do
+		assert(applied[index] == nextState[index])
+	end
+end)
+
+test("preserves large gaps between keys", function()
+	local prevState = table.create(10000, "foo")
+	local nextState = table.move(prevState, 2, 9998, 2, { [1] = "bar", [9999] = "bar" })
+
+	local start = os.clock()
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	assert(os.clock() - start < 10e-3, `Performance is not acceptable ({(os.clock() - start) * 1e3}ms)`)
+
+	for index in next, prevState do
+		assert(applied[index] == nextState[index])
+	end
+end)
+
+test("cannot preserve sparse arrays", function()
+	local prevState = {}
+	local nextState = { [2] = "foo" }
+
+	local patches = serdes(patch.diff(prevState, nextState))
+	local applied = patch.apply(prevState, patches)
+
+	-- Not enough information is available to determine whether 'patches'
+	-- should keep the string keys added during remote event serialization.
+	assert(applied["2"] ~= nil)
+	assert(applied[2] == nil)
+end)
+
+test("optionally fixes sparse arrays", function()
+	for index in next, patch.diff({ 1, 1, 1, 1, 1 }, { 2, 1, 1, 2 }, true) do
+		assert(type(index) == "string")
+	end
+
+	for index in next, patch.diff({ 1, 1, 1, 1, 1 }, { 2, 1, 1, 2 }, false) do
+		assert(type(index) == "number")
+	end
+end)
+
+test("optionally validates state", function()
+	local success = pcall(function()
+		patch.diff({}, { [0.1] = true }, true)
+	end)
+
+	assert(not success)
+
+	success = pcall(function()
+		patch.diff({}, { [0.1] = true }, false)
+	end)
+
+	assert(success)
+end)
+
+return {}

pkgs/charm-sync/__tests__/server.spec.luau

@@ -0,0 +1,102 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+type SyncPayload = CharmSync.SyncPayload
+
+local atom = Charm.atom
+local server = CharmSync.server
+local patch = CharmSync.patch
+local test = suite.test
+
+local target: Player = {} :: any
+
+test("sends state to new players", function()
+	local a = atom(1)
+	local server = server({ atoms = { a = a }, interval = -1 })
+	local player: Player, payload: SyncPayload
+
+	local disconnect = server:connect(function(...)
+		player, payload = ...
+	end)
+
+	server:hydrate(target)
+
+	assert(player == target)
+	assert(payload.type == "init")
+	assert(payload.data.a == 1)
+
+	disconnect()
+end)
+
+test("sends state patches", function()
+	local a = atom({ b = 1, c = 2 } :: { [string]: number? })
+	local server = server({ atoms = { a = a }, interval = -1 })
+	local player: Player, payload: SyncPayload
+
+	local disconnect = server:connect(function(...)
+		player, payload = ...
+	end)
+
+	a({ b = 1, c = 5 })
+	server:_sendPatch(target)
+
+	assert(player == target)
+	assert(payload.type == "patch")
+	assert(payload.data.a.b == nil)
+	assert(payload.data.a.c == 5)
+
+	a({ b = 1, d = 1 })
+	server:_sendPatch(target)
+
+	assert(payload.data.a.b == nil)
+	assert(patch.isNone(payload.data.a.c) == true)
+	assert(payload.data.a.d == 1)
+
+	disconnect()
+end)
+
+test("preserves history", function()
+	local a = atom(1)
+	local b = atom(1)
+	local payloads: { SyncPayload }
+
+	local server = server({
+		atoms = { a = a, b = b },
+		interval = -1,
+		preserveHistory = true,
+	})
+
+	local disconnect = server:connect(function(_, ...)
+		payloads = { ... }
+	end)
+
+	a(2)
+	b(2)
+	server:_sendPatch(target)
+
+	assert(#payloads == 1)
+	assert(payloads[1].type == "patch")
+	assert(payloads[1].data.a == 2)
+	assert(payloads[1].data.b == 2)
+
+	a(3)
+	a(4)
+	b(3)
+	server:_sendPatch(target)
+
+	assert(#payloads == 2)
+	assert(payloads[1].type == "patch")
+	assert(payloads[1].data.a == 3)
+	assert(payloads[1].data.b == nil)
+	assert(payloads[2].type == "patch")
+	assert(payloads[2].data.a == 4)
+	assert(payloads[2].data.b == 3)
+
+	disconnect()
+end)
+
+return {}

pkgs/charm-sync/__tests__/spec.luau

@@ -0,0 +1,19 @@
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local suite = require(ServerScriptService.Tests.suite)
+
+local describe = suite.describe
+
+local function spec(name: string)
+	describe(name, function()
+		return require(`./{name}.spec`) :: any
+	end)
+end
+
+spec("client")
+spec("server")
+spec("flatten")
+spec("patch")
+spec("validate")
+
+return {}

pkgs/charm-sync/__tests__/validate.spec.luau

@@ -0,0 +1,38 @@
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+local ServerScriptService = game:GetService("ServerScriptService")
+
+local Charm = require(ReplicatedStorage.Packages.Charm)
+local CharmSync = require(ReplicatedStorage.Packages.CharmSync)
+local suite = require(ServerScriptService.Tests.suite)
+
+local atom = Charm.atom
+local validate = CharmSync.validate
+local test = suite.test
+
+test("validates inputs", function()
+	local cases: { [string]: { value: any, expected: any } } = {
+		array = { value = { 1, 2, 3 }, expected = true },
+		array_sparse = { value = { 1, nil :: any, 3 }, expected = true },
+		array_sparse_invalid = { value = { [1] = 0, [3] = 0 }, expected = false },
+
+		map_decimal_key = { value = { [1.5] = 0 }, expected = false },
+		map_infinity_key = { value = { [math.huge] = true }, expected = false },
+		map_mixed_key = { value = { [1] = 0, a = 0 }, expected = false },
+
+		nested_atom = { value = atom(0), expected = false },
+	}
+
+	for key, case in next, cases do
+		local success, result = pcall(function()
+			validate(case.value, key)
+		end)
+
+		if case.expected and not success then
+			error(`Expected {key} to be valid, threw {result}`)
+		elseif not case.expected and success then
+			error(`Expected {key} to be invalid, but it was valid`)
+		end
+	end
+end)
+
+return {}

pkgs/charm-sync/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "CharmSync",
+	"tree": {
+		"$path": "src"
+	}
+}

pkgs/charm-sync/src/client.luau

@@ -0,0 +1,84 @@
+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

pkgs/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

pkgs/charm-sync/src/init.luau

@@ -0,0 +1,21 @@
+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
+
+--[=[
+	Synchronizes state between the client and server. The server sends patches
+	to the client, which applies them to its local state.
+]=]
+return {
+	client = client,
+	server = server,
+	flatten = flatten,
+	validate = validate,
+	patch = patch,
+	isNone = patch.isNone,
+}

pkgs/charm-sync/src/interval.luau

@@ -0,0 +1,30 @@
+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

pkgs/charm-sync/src/patch.luau

@@ -0,0 +1,117 @@
+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,
+}

pkgs/charm-sync/src/server.luau

@@ -0,0 +1,268 @@
+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

pkgs/charm-sync/src/types.luau

@@ -0,0 +1,21 @@
+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

pkgs/charm-sync/src/validate.luau

@@ -0,0 +1,68 @@
+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

pkgs/charm-sync/wally.toml

@@ -0,0 +1,9 @@
+[package]
+name = "littensy/charm-sync"
+version = "0.3.0"
+registry = "https://github.com/UpliftGames/wally-index"
+realm = "shared"
+exclude = ["Charm.luau"]
+
+[dependencies]
+Charm = "littensy/charm@^0.10"