--[[
	Called when the original table is changed.

	This will firstly find any keys meeting any of the following criteria:

	- they were not previously present
	- a dependency used during generation of this value has changed

	It will recalculate those key pairs, storing information about any
	dependencies used in the processor callback during output key generation,
	and save the new key to the output array with the same value. If it is
	overwriting an older value, that older value will be passed to the
	destructor for cleanup.

	Finally, this function will find keys that are no longer present, and remove
	their output keys from the output table and pass them to the destructor.
]]

function class:update(): boolean
	local inputIsState = self._inputIsState
	local newInputTable = if inputIsState then self._inputTable:get(false) else self._inputTable
	local oldInputTable = self._oldInputTable
	local outputTable = self._outputTable

	local keyOIMap = self._keyOIMap
	local keyIOMap = self._keyIOMap
	local meta = self._meta

	local didChange = false


	-- clean out main dependency set
	for dependency in pairs(self.dependencySet) do
		dependency.dependentSet[self] = nil
	end

	self._oldDependencySet, self.dependencySet = self.dependencySet, self._oldDependencySet
	table.clear(self.dependencySet)

	-- if the input table is a state object, add it as a dependency
	if inputIsState then
		self._inputTable.dependentSet[self] = true
		self.dependencySet[self._inputTable] = true
	end


	-- STEP 1: find keys that changed or were not previously present
	for newInKey, value in newInputTable do
		-- get or create key data
		local keyData = self._keyData[newInKey]

		if keyData == nil then
			keyData = {
				dependencySet = setmetatable({}, utility.WEAK_KEYS_METATABLE),
				oldDependencySet = setmetatable({}, utility.WEAK_KEYS_METATABLE),
				dependencyValues = setmetatable({}, utility.WEAK_KEYS_METATABLE),
			}
			self._keyData[newInKey] = keyData
		end

		-- check if the key is new
		local shouldRecalculate = oldInputTable[newInKey] == nil

		-- check if the key's dependencies have changed
		if shouldRecalculate == false then
			for dependency, oldValue in pairs(keyData.dependencyValues) do
				if oldValue ~= dependency:get(false) then
					shouldRecalculate = true
					break
				end
			end
		end


		-- recalculate the output key if necessary
		if shouldRecalculate then
			keyData.oldDependencySet, keyData.dependencySet = keyData.dependencySet, keyData.oldDependencySet
			table.clear(keyData.dependencySet)

			local processOK, newOutKey, newMetaValue = Dependencies.captureDependencies(
				keyData.dependencySet,
				self._processor,
				newInKey
			)

			if processOK then
				if not self._destructor and (utility.needsDestruction(newOutKey) or utility.needsDestruction(newMetaValue)) then
					warn("destructorNeededForKeys")
				end

				local oldInKey = keyOIMap[newOutKey]
				local oldOutKey = keyIOMap[newInKey]

				-- check for key collision
				if oldInKey ~= newInKey and newInputTable[oldInKey] ~= nil then
					utility.parseError("forKeysKeyCollision", tostring(newOutKey), tostring(oldInKey), tostring(newOutKey))
				end

				-- check for a changed output key
				if oldOutKey ~= newOutKey and keyOIMap[oldOutKey] == newInKey then
					-- clean up the old calculated value
					local oldMetaValue = meta[oldOutKey]

					local destructOK, err = xpcall(self._destructor or utility.cleanup, utility.parseError, oldOutKey, oldMetaValue)
					if not destructOK then
						warn("forKeysDestructorError", err)
					end

					keyOIMap[oldOutKey] = nil
					outputTable[oldOutKey] = nil
					meta[oldOutKey] = nil
				end

				-- update the stored data for this key
				oldInputTable[newInKey] = value
				meta[newOutKey] = newMetaValue
				keyOIMap[newOutKey] = newInKey
				keyIOMap[newInKey] = newOutKey
				outputTable[newOutKey] = value

				-- if we had to recalculate the output, then we did change
				didChange = true
			else
				-- restore old dependencies, because the new dependencies may be corrupt
				keyData.oldDependencySet, keyData.dependencySet = keyData.dependencySet, keyData.oldDependencySet

				warn("forKeysProcessorError", newOutKey)
			end
		end


		-- save dependency values and add to main dependency set
		for dependency in pairs(keyData.dependencySet) do
			keyData.dependencyValues[dependency] = dependency:get(false)

			self.dependencySet[dependency] = true
			dependency.dependentSet[self] = true
		end
	end


	-- STEP 2: find keys that were removed
	for outputKey, inputKey in keyOIMap do
		if newInputTable[inputKey] == nil then
			-- clean up the old calculated value
			local oldMetaValue = meta[outputKey]

			local destructOK, err = xpcall(self._destructor or utility.cleanup, utility.parseError, outputKey, oldMetaValue)
			if not destructOK then
				warn("forKeysDestructorError", err)
			end

			-- remove data
			oldInputTable[inputKey] = nil
			meta[outputKey] = nil
			keyOIMap[outputKey] = nil
			keyIOMap[inputKey] = nil
			outputTable[outputKey] = nil
			self._keyData[inputKey] = nil

			-- if we removed a key, then the table/state changed
			didChange = true
		end
	end

	return didChange
end

local function ForKeys<KO, M>(
	inputTable,
	processor: (value: any) -> (KO, M?),
	destructor: (KO, M?) -> ()?
)

	local inputIsState = inputTable.type == "State" and typeof(inputTable.get) == "function"

	local self = setmetatable({
		type = "State",
		kind = "ForKeys",
		dependencySet = {},
		-- if we held strong references to the dependents, then they wouldn't be
		-- able to get garbage collected when they fall out of scope
		dependentSet = setmetatable({}, utility.WEAK_KEYS_METATABLE),
		_oldDependencySet = {},

		_processor = processor,
		_destructor = destructor,
		_inputIsState = inputIsState,

		_inputTable = inputTable,
		_oldInputTable = {},
		_outputTable = {},
		_keyOIMap = {},
		_keyIOMap = {},
		_keyData = {},
		_meta = {},
	}, CLASS_METATABLE)

	self:update()

	return self
end

return ForKeys