local ui = require('openmw.ui')
local util = require('openmw.util')
local self = require('openmw.self')

local MODE = ui._getAllUiModes()
local WINDOW = ui._getAllWindowIds()

local replacedWindows = {}
local hiddenWindows = {}
local modeStack = {}

local function registerWindow(window, showFn, hideFn)
    if not WINDOW[window] then
        error('At the moment it is only possible to override existing windows. Window "'..
              tostring(window)..'" not found.')
    end
    ui._setWindowDisabled(window, true)
    if replacedWindows[window] then
        replacedWindows[window].hideFn()
    end
    replacedWindows[window] = {showFn = showFn, hideFn = hideFn, visible = false}
    hiddenWindows[window] = nil
end

local function updateHidden(mode, options)
    local toHide = {}
    if options and options.windows then
        for _, w in pairs(ui._getAllowedWindows(mode)) do
            toHide[w] = true
        end
        for _, w in pairs(options.windows) do
            toHide[w] = nil
        end
    end
    for w, _ in pairs(hiddenWindows) do
        if toHide[w] then
            toHide[w] = nil
        else
            hiddenWindows[w] = nil
            if not replacedWindows[w] then
                ui._setWindowDisabled(w, false)
            end
        end
    end
    for w, _ in pairs(toHide) do
        hiddenWindows[w] = true
        if not replacedWindows[w] then
            ui._setWindowDisabled(w, true)
        end
    end
end

local function setMode(mode, options)
    local function impl()
        updateHidden(mode, options)
        ui._setUiModeStack({mode}, options and options.target)
    end
    if mode then
        if not pcall(impl) then
            error('Invalid mode: ' .. tostring(mode))
        end
    else
        ui._setUiModeStack({})
    end
end

local function addMode(mode, options)
    local function impl()
        updateHidden(mode, options)
        ui._setUiModeStack(modeStack, options and options.target)
    end
    modeStack[#modeStack + 1] = mode
    if not pcall(impl) then
        modeStack[#modeStack] = nil
        error('Invalid mode: ' .. tostring(mode))
    end
end

local function removeMode(mode)
    local sizeBefore = #modeStack
    local j = 1
    for i = 1, sizeBefore do
        if modeStack[i] ~= mode then
            modeStack[j] = modeStack[i]
            j = j + 1
        end
    end
    for i = j, sizeBefore do modeStack[i] = nil end
    if sizeBefore > #modeStack then
        ui._setUiModeStack(modeStack)
    end
end

local function onUiModeChanged(arg)
    local newStack = ui._getUiModeStack()
    for i = 1, math.max(#modeStack, #newStack) do
        modeStack[i] = newStack[i]
    end
    for w, state in pairs(replacedWindows) do
        if state.visible then
            state.hideFn()
            state.visible = false
        end
    end
    local mode = newStack[#newStack]
    if mode then
        for _, w in pairs(ui._getAllowedWindows(mode)) do
            local state = replacedWindows[w]
            if state and not hiddenWindows[w] then
                state.showFn(arg)
                state.visible = true
            end
        end
    end
    self:sendEvent('UiModeChanged', {mode = mode, arg = arg})
end

return {
    interfaceName = 'UI',
    ---
    -- @module UI
    -- @usage require('openmw.interfaces').UI
    interface = {
        --- Interface version
        -- @field [parent=#UI] #number version
        version = 0,

        --- All available UI modes.
        -- Use `view(I.UI.MODE)` in `luap` console mode to see the list.
        -- @field [parent=#UI] #table MODE
        MODE = util.makeStrictReadOnly(MODE),

        --- All windows.
        -- Use `view(I.UI.WINDOW)` in `luap` console mode to see the list.
        -- @field [parent=#UI] #table WINDOW
        WINDOW = util.makeStrictReadOnly(WINDOW),

        --- Register new implementation for the window with given name; overrides previous implementation.
        -- Adding new windows is not supported yet. At the moment it is only possible to override built-in windows.
        -- @function [parent=#UI] registerWindow
        -- @param #string windowName
        -- @param #function showFn Callback that will be called when the window should become visible
        -- @param #function hideFn Callback that will be called when the window should be hidden
        registerWindow = registerWindow,

        --- Returns windows that can be shown in given mode.
        -- @function [parent=#UI] getWindowsForMode
        -- @param #string mode
        -- @return #table
        getWindowsForMode = ui._getAllowedWindows,

        --- Stack of currently active modes
        -- @field [parent=#UI] modes
        modes = util.makeReadOnly(modeStack),

        --- Get current mode (nil if all windows are closed), equivalent to `I.UI.modes[#I.UI.modes]`
        -- @function [parent=#UI] getMode
        -- @return #string
        getMode = function() return modeStack[#modeStack] end,

        --- Drop all active modes and set mode.
        -- @function [parent=#UI] setMode
        -- @param #string mode (optional) New mode
        -- @param #table options (optional) Table with keys 'windows' and/or 'target' (see example).
        -- @usage I.UI.setMode() -- drop all modes
        -- @usage I.UI.setMode('Interface') -- drop all modes and open interface
        -- @usage -- Drop all modes, open interface, but show only the map window.
        -- I.UI.setMode('Interface', {windows = {'Map'}})
        setMode = setMode,

        --- Add mode to stack without dropping other active modes.
        -- @function [parent=#UI] addMode
        -- @param #string mode New mode
        -- @param #table options (optional) Table with keys 'windows' and/or 'target' (see example).
        -- @usage I.UI.addMode('Journal') -- open journal without dropping active modes.
        -- @usage -- Open barter with an NPC
        -- I.UI.addMode('Barter', {target = actor})
        addMode = addMode,

        --- Remove the specified mode from active modes.
        -- @function [parent=#UI] removeMode
        -- @param #string mode Mode to drop
        removeMode = removeMode,

        -- TODO
        -- registerHudElement = function(name, showFn, hideFn) end,
        -- showHud = function(bool) end,
        -- isHudVisible = function() end,
        -- showHudElement = function(name, bool) end,
        -- hudElements,  -- map from element name to its visibility
    },
    engineHandlers = {
        _onUiModeChanged = onUiModeChanged,
    },
}