feat: initial commit

src/split.luau

@@ -0,0 +1,75 @@
+local atom = require(script.Parent.atom)
+type Atom<T> = atom.Atom<T>
+type ReadonlyAtom<T> = atom.ReadonlyAtom<T>
+
+local reference = require(script.Parent.utils.reference)
+
+type Split =
+	(<Item, Key>(atom: Atom<{ [Key]: Item }>) -> ReadonlyAtom<{ [Key]: Atom<Item> }>)
+	& (<Item, Key>(atom: Atom<{ [any]: Item }>, keyExtractor: (Item) -> Key) -> ReadonlyAtom<{ [Key]: Atom<Item> }>)
+
+local function split<Item, Key>(
+	itemsAtom: Atom<{ [Key]: Item }>,
+	keyExtractor: ((Item) -> Key)?
+): ReadonlyAtom<{ [Key]: Atom<Item> }>
+	local splitAtom: ReadonlyAtom<{ [Key]: Atom<Item> }> = atom({}):readonly()
+	local getSplitAtom = reference.weak(splitAtom)
+
+	local previousAtoms: { [Key]: Atom<Item> } = {}
+	local previousItems = itemsAtom:get()
+	local listener: ({ [Key]: Item }) -> ()
+
+	local function cleanup()
+		itemsAtom.listeners[listener] = nil
+		for _, atom in previousAtoms do
+			atom:unmount()
+		end
+	end
+
+	function listener(items)
+		local splitAtom = getSplitAtom()
+
+		if not splitAtom then
+			return cleanup()
+		end
+
+		local atoms = table.clone(splitAtom:get())
+		local itemsByKeys: { [Key]: Item } = {}
+
+		for key, item in items do
+			local id = if keyExtractor then keyExtractor(item) else key
+
+			itemsByKeys[id] = item
+
+			if atoms[id] then
+				-- Force set the atom to avoid redundantly updating itemsAtom
+				atoms[id]:setForced(item)
+			else
+				atoms[id] = atom(item):writable(function(update)
+					local nextItems = table.clone(itemsAtom:get())
+					nextItems[key] = update
+					itemsAtom:set(nextItems)
+				end)
+			end
+		end
+
+		for key, item in previousItems do
+			local id = if keyExtractor then keyExtractor(item) else key
+
+			if itemsByKeys[id] == nil then
+				atoms[id]:unmount()
+				atoms[id] = nil
+			end
+		end
+
+		previousAtoms, previousItems = atoms, items
+		splitAtom:setForced(atoms)
+	end
+
+	itemsAtom.listeners[listener] = true
+	listener(itemsAtom:get())
+
+	return splitAtom
+end
+
+return (split :: any) :: Split