feat!: replace split with mapAtom

src/mapAtom.luau

@@ -0,0 +1,56 @@
+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)
+
+local function mapAtom<K0, V0, K1, V1>(itemsAtom: Atom<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1?)): ReadonlyAtom<{ [K1]: V1 }>
+	local mappedAtom: ReadonlyAtom<{ [K1]: V1 }> = atom({}):readonly()
+	local mappedAtomUnmount = mappedAtom.unmount
+	local getMappedAtom = reference.weak(mappedAtom)
+	local previousMappedItems: { [K1]: V1 } = {}
+
+	local function listener(items)
+		local mappedAtom = getMappedAtom()
+
+		if not mappedAtom then
+			itemsAtom.listeners[listener] = nil
+			return
+		end
+
+		local mappedItems = table.clone(mappedAtom:get())
+		local mappedKeys = {}
+
+		for key, item in items do
+			local newItem, newKey = mapper(item, key)
+			if newKey == nil then
+				newKey = key :: any
+			end
+			if mappedItems[newKey :: K1] ~= newItem then
+				mappedItems[newKey :: K1] = newItem :: V1
+			else
+				mappedKeys[newKey] = key
+			end
+		end
+
+		for key in previousMappedItems do
+			if mappedKeys[key] == nil and mappedItems[key] == previousMappedItems[key] then
+				mappedItems[key] = nil
+			end
+		end
+
+		previousMappedItems = mappedItems
+		mappedAtom:setForced(mappedItems)
+	end
+
+	function mappedAtom:unmount()
+		mappedAtomUnmount(self)
+		itemsAtom.listeners[listener] = nil
+	end
+
+	itemsAtom.listeners[listener] = true
+	listener(itemsAtom:get())
+
+	return mappedAtom
+end
+
+return mapAtom