-- =====================================================================
-- lib-core.panel v0.1.0 — Generic overlay-panel framework
--
-- Provides a single-active-widget panel system with:
--   - Widget registry + lifecycle (register, open, close, toggle)
--   - Centrally-rendered overlay with configurable theme
--   - Per-frame input dispatch (mouse click + wheel, edge-detected)
--   - Context-menu (show, hit-test, auto-close)
--   - Default-trigger binding via lib-core.input (lazy-required)
--   - Optional pause-gate (pause_on_open=true widget makes is_pausing()=true)
--
-- Engine surfaces used:
--   - engine.input.get_mouse_pos()  → x, y
--   - engine.input.is_mouse_down(button) → bool
--   - engine.input.get_mouse_wheel() → number
--   - engine.input.MOUSE_LEFT / MOUSE_RIGHT
--   - engine.render.draw_rect(x, y, w, h, color)
--   - engine.render.draw_rect_lines(x, y, w, h, color, thickness)
--   - engine.render.draw_text(text, x, y, font_size, color)
--   - engine.render.measure_text(text, font_size) → w, h
--
-- Screen-size limitation: engine.render does NOT expose get_screen_size()
-- to Lua (GetScreenWidth/GetScreenHeight are C-only). v0.1 falls back to
-- 1280x720 constants matching the default Sporel window config.
--
-- DEFERRED (v0.1 non-goals):
--   - Multi-widget z-order / stacking
--   - Always-on HUD widgets (non-modal overlays)
--   - Keyboard navigation within widgets
--   - Panel animation (fade in/out)
-- =====================================================================

-- -----------------------------------------------------------------------
-- Module-level state (all local — no globals)
-- -----------------------------------------------------------------------

-- Module-table declared early so closures in module-local helpers below
-- can reference M.X functions. Method assignments (function M.x() ... end)
-- still land further down once the public surface section starts.
local M = {}

local widgets             = {}      -- widget_id (string) → widget_def table
local active              = nil     -- active widget_id or nil
local theme               = {}      -- merged DEFAULT_THEME + overrides
local ctx_menu            = nil     -- context-menu state table or nil
local trigger_action_name = nil     -- input action name for default trigger
local trigger_widget_id   = nil     -- which widget_id to toggle on trigger
local last_mouse_left     = false   -- for edge-detection (was down last frame)
local last_mouse_right    = false   -- for edge-detection (was down last frame)

-- -----------------------------------------------------------------------
-- Theme
-- -----------------------------------------------------------------------

-- 13 canonical keys. DO NOT add keys here without updating set_theme validation.
local DEFAULT_THEME = {
    bg_color           = 0x202020F0,
    border_color       = 0x808080FF,
    text_color         = 0xE0E0E0FF,
    text_color_dim     = 0xA0A0A0FF,
    selection_color    = 0x404080FF,
    context_menu_bg    = 0x303030F8,
    context_menu_hover = 0x404060FF,
    font_size_title    = 18,
    font_size_body     = 14,
    padding            = 8,
    row_height         = 24,
    panel_width_frac   = 0.5,
    panel_height_frac  = 0.7,
}

-- Init theme from DEFAULT_THEME at module load
for k, v in pairs(DEFAULT_THEME) do
    theme[k] = v
end

-- -----------------------------------------------------------------------
-- Fallback screen constants (no engine.render.get_screen_size in Lua API)
-- -----------------------------------------------------------------------
local FALLBACK_SCREEN_W = 1280
local FALLBACK_SCREEN_H = 720

-- -----------------------------------------------------------------------
-- Internal helpers
-- -----------------------------------------------------------------------

--- Retrieve the current screen dimensions. Falls back to constants because
--- engine.render does not expose a Lua-callable screen-size function.
local function get_screen_size()
    return FALLBACK_SCREEN_W, FALLBACK_SCREEN_H
end

--- Estimate menu width from action labels (rough; render can refine via
--- measure_text, but context-menu render in v0.1 uses this pre-computed w).
local function estimate_menu_width(actions, padding)
    local max_w = 0
    for _, action in ipairs(actions) do
        local chars = #(action.label or "")
        -- 8 px/char is a rough estimate for the default font; refine via
        -- engine.render.measure_text in render-phase (v0.2 deferral).
        local w = chars * 8 + 2 * padding
        if w > max_w then max_w = w end
    end
    return max_w
end

-- -----------------------------------------------------------------------
-- Internal: context-menu dispatch + render helpers
-- -----------------------------------------------------------------------

--- Build per-row hit-rects on ctx_menu and store as ctx_menu.row_rects.
--- Called before dispatch and before render to keep both in sync.
local function build_ctx_menu_row_rects()
    if not ctx_menu then return end
    local rects = {}
    for i, _ in ipairs(ctx_menu.actions) do
        local row_y = ctx_menu.y + (i - 1) * theme.row_height
        rects[i] = {
            x  = ctx_menu.x,
            y  = row_y,
            w  = ctx_menu.w,
            h  = theme.row_height,
        }
    end
    ctx_menu.row_rects = rects
end

--- Returns current mouse position safely (0,0 if not in game context).
local function safe_mouse_pos()
    if engine and engine.input and engine.input.get_mouse_pos then
        return engine.input.get_mouse_pos()
    end
    return 0, 0
end

-- -----------------------------------------------------------------------
-- Internal: event dispatch
-- -----------------------------------------------------------------------

--- _dispatch_event(event): route a synthetic or real input event.
--- event = {kind="click", x, y, button="left"|"right"} or {kind="wheel", dy}
local function _dispatch_event(event)
    -- Context-menu intercept: any click is consumed by the menu.
    if ctx_menu and event.kind == "click" then
        -- Hit-test rows
        build_ctx_menu_row_rects()
        local hit = false
        for i, rect in ipairs(ctx_menu.row_rects) do
            if event.x >= rect.x and event.x < rect.x + rect.w
            and event.y >= rect.y and event.y < rect.y + rect.h then
                hit = true
                local action = ctx_menu.actions[i]
                ctx_menu = nil  -- close menu before callback (re-entrant safety)
                if action and action.callback then
                    action.callback({ close_menu = function() ctx_menu = nil end })
                end
                break
            end
        end
        if not hit then
            -- Click outside context-menu area → auto-close
            ctx_menu = nil
        end
        return
    end

    -- Normal dispatch to active widget
    if not active or not widgets[active] then return end

    local screen_w, screen_h = get_screen_size()
    local panel_w = screen_w * theme.panel_width_frac
    local panel_h = screen_h * theme.panel_height_frac
    local panel_x = (screen_w - panel_w) / 2
    local panel_y = (screen_h - panel_h) / 2
    local padding = theme.padding
    local title_area_h = theme.font_size_title + padding * 2

    local content_bounds = {
        x = panel_x + padding,
        y = panel_y + title_area_h,
        w = panel_w - padding * 2,
        h = panel_h - title_area_h - padding,
    }

    local widget_ctx = {
        bounds     = content_bounds,
        theme      = M.get_theme(),    -- shallow copy; widget cannot mutate panel state
        is_focused = true,
    }

    widgets[active].handle_input(widget_ctx, event)
end

-- -----------------------------------------------------------------------
-- Public API (M declared at top of file so closures above can reference it)
-- -----------------------------------------------------------------------
-- -----------------------------------------------------------------------
-- Registry / Lifecycle
-- -----------------------------------------------------------------------

--- M.register(widget_id, widget_def)
--- Registers a new widget. widget_def must have:
---   .render(ctx)        — function, called each render frame when widget is active
---   .handle_input(ctx, event) — function, called for each input event
---   .title              — string, displayed in the panel title bar
--- Optional: .pause_on_open = true — makes is_pausing() return true while open.
function M.register(widget_id, widget_def)
    if type(widget_id) ~= "string" then
        error("panel.register: widget_id must be a string, got " .. type(widget_id))
    end
    if type(widget_def) ~= "table" then
        error("panel.register: widget_def must be a table, got " .. type(widget_def))
    end
    if type(widget_def.render) ~= "function" then
        error("panel.register: widget_def.render must be a function")
    end
    if type(widget_def.handle_input) ~= "function" then
        error("panel.register: widget_def.handle_input must be a function")
    end
    if type(widget_def.title) ~= "string" then
        error("panel.register: widget_def.title must be a string")
    end
    if widgets[widget_id] ~= nil then
        error("panel.register: duplicate widget_id '" .. widget_id .. "'")
    end
    widgets[widget_id] = widget_def
end

--- M.unregister(widget_id)
--- Removes a widget from the registry. If the widget is currently active,
--- closes the panel first (clears active + ctx_menu).
function M.unregister(widget_id)
    if active == widget_id then
        active   = nil
        ctx_menu = nil
    end
    widgets[widget_id] = nil
end

--- M.open(widget_id)
--- Sets widget_id as the active (displayed) widget. Loud-error if not registered.
function M.open(widget_id)
    if widgets[widget_id] == nil then
        error("panel.open: unknown widget_id '" .. tostring(widget_id) .. "' (register before open)")
    end
    active = widget_id
end

--- M.close()
--- Closes the active panel and clears any open context-menu.
function M.close()
    active   = nil
    ctx_menu = nil
end

--- M.toggle(widget_id)
--- If widget_id is currently active, closes it. Otherwise opens it.
function M.toggle(widget_id)
    if active == widget_id then
        M.close()
    else
        M.open(widget_id)
    end
end

--- M.is_open() → bool
--- Returns true if any widget is currently active.
function M.is_open()
    return active ~= nil
end

--- M.is_pausing() → bool
--- Returns true if the active widget has pause_on_open=true.
function M.is_pausing()
    return active ~= nil and widgets[active] ~= nil
        and widgets[active].pause_on_open == true
end

-- -----------------------------------------------------------------------
-- Theme
-- -----------------------------------------------------------------------

--- M.set_theme(overrides)
--- Merges overrides into the active theme. Only keys declared in DEFAULT_THEME
--- are accepted; unknown keys loud-error with "unknown theme key '<k>'".
function M.set_theme(overrides)
    if type(overrides) ~= "table" then
        error("panel.set_theme: expected table, got " .. type(overrides))
    end
    for k, v in pairs(overrides) do
        if DEFAULT_THEME[k] == nil then
            error("panel.set_theme: unknown theme key '" .. tostring(k) .. "'")
        end
        theme[k] = v
    end
end

--- M.get_theme() → table
--- Returns a shallow copy of the current theme.
function M.get_theme()
    local copy = {}
    for k, v in pairs(theme) do
        copy[k] = v
    end
    return copy
end

-- -----------------------------------------------------------------------
-- Default-trigger binding
-- -----------------------------------------------------------------------

--- M.bind_default_trigger(key, widget_id)
--- Binds a keyboard key as the default toggle trigger for a widget.
--- key defaults to "tab" when nil. Lazy-requires lib-core.input to avoid
--- module-load-time circular dependency.
--- Loud-error if widget_id is not registered ("register before bind").
function M.bind_default_trigger(key, widget_id)
    if key == nil then key = "tab" end
    if widgets[widget_id] == nil then
        error("panel.bind_default_trigger: unknown widget_id '" .. tostring(widget_id)
              .. "' (register before bind)")
    end
    local input = require("lib-core.input")  -- lazy require: avoid load-time cycle
    input.bind("panel_toggle", {key})
    trigger_action_name = "panel_toggle"
    trigger_widget_id   = widget_id
end

-- -----------------------------------------------------------------------
-- Context-menu
-- -----------------------------------------------------------------------

--- M.show_context_menu(x, y, actions)
--- Pops up a context menu at (x, y) with the given actions array.
--- actions = array of {label=string, callback=function} entries.
--- Auto-repositions to stay within screen bounds.
function M.show_context_menu(x, y, actions)
    if type(actions) ~= "table" or #actions == 0 then
        error("panel.show_context_menu: actions must be a non-empty array of {label, callback} tables")
    end
    for i, action in ipairs(actions) do
        if type(action) ~= "table" then
            error("panel.show_context_menu: action #" .. i .. " must be a table")
        end
        if type(action.label) ~= "string" then
            error("panel.show_context_menu: action #" .. i .. " must have a string label")
        end
        if type(action.callback) ~= "function" then
            error("panel.show_context_menu: action #" .. i .. " must have a callback function")
        end
    end

    local padding  = theme.padding
    local row_h    = theme.row_height
    local menu_w   = estimate_menu_width(actions, padding)
    local menu_h   = #actions * row_h

    local screen_w, screen_h = get_screen_size()

    -- Auto-reposition: clamp so menu stays on screen
    if x + menu_w > screen_w then x = screen_w - menu_w end
    if x < 0 then x = 0 end
    if y + menu_h > screen_h then y = screen_h - menu_h end
    if y < 0 then y = 0 end

    ctx_menu = {
        x       = x,
        y       = y,
        w       = menu_w,
        h       = menu_h,
        actions = actions,
        row_rects = {},  -- populated by build_ctx_menu_row_rects()
    }
    build_ctx_menu_row_rects()
end

-- -----------------------------------------------------------------------
-- Per-frame update + input dispatch
-- -----------------------------------------------------------------------

--- M.update(dt)
--- Must be called each game-update frame. Handles:
---   - Default-trigger key check (toggling bound widget)
---   - Mouse edge-detection for click events (dispatches on press, not hold)
---   - Mouse-wheel event dispatch
function M.update(dt)
    -- Check default trigger (lazy requires lib-core.input internally if bound)
    if trigger_action_name then
        local input = require("lib-core.input")
        if input.was_action_pressed(trigger_action_name) then
            M.toggle(trigger_widget_id)
        end
    end

    if not active then
        -- Still need to update last_mouse_* state even when closed so that
        -- opening mid-frame doesn't produce a spurious edge on the next frame.
        if engine and engine.input then
            last_mouse_left  = engine.input.is_mouse_down(engine.input.MOUSE_LEFT)
            last_mouse_right = engine.input.is_mouse_down(engine.input.MOUSE_RIGHT)
        end
        return
    end

    -- Read current mouse state
    local mx, my   = engine.input.get_mouse_pos()
    local cur_left  = engine.input.is_mouse_down(engine.input.MOUSE_LEFT)
    local cur_right = engine.input.is_mouse_down(engine.input.MOUSE_RIGHT)

    -- Edge-detect: pressed THIS frame (was up, now down)
    local pressed_left  = cur_left  and not last_mouse_left
    local pressed_right = cur_right and not last_mouse_right

    if pressed_left then
        _dispatch_event({ kind = "click", x = mx, y = my, button = "left" })
    end
    if pressed_right then
        _dispatch_event({ kind = "click", x = mx, y = my, button = "right" })
    end

    -- Wheel dispatch
    local wheel_amt = engine.input.get_mouse_wheel()
    if wheel_amt ~= 0 then
        _dispatch_event({ kind = "wheel", dy = wheel_amt })
    end

    -- Update last-state for next frame's edge-detection
    last_mouse_left  = cur_left
    last_mouse_right = cur_right
end

-- -----------------------------------------------------------------------
-- Render
-- -----------------------------------------------------------------------

--- M.render()
--- Must be called each render frame. Draws the active widget's panel
--- (background, border, title bar) then invokes widget_def.render(ctx).
--- If a context-menu is open, renders it on top.
--- No-op if no widget is active.
function M.render()
    if not active then return end

    local widget_def = widgets[active]
    if not widget_def then return end

    -- Screen size: fallback to 1280x720 (engine.render has no Lua-accessible
    -- get_screen_size; see module header for explanation)
    local screen_w, screen_h = get_screen_size()

    local panel_w = screen_w * theme.panel_width_frac
    local panel_h = screen_h * theme.panel_height_frac
    local panel_x = (screen_w - panel_w) / 2
    local panel_y = (screen_h - panel_h) / 2
    local padding = theme.padding

    -- Background
    engine.render.draw_rect(panel_x, panel_y, panel_w, panel_h, theme.bg_color)

    -- Border
    engine.render.draw_rect_lines(panel_x, panel_y, panel_w, panel_h,
                                  theme.border_color, 2)

    -- Title
    local title = widget_def.title
    engine.render.draw_text(title,
                            panel_x + padding,
                            panel_y + padding,
                            theme.font_size_title,
                            theme.text_color)

    -- Content area (below title bar)
    local title_area_h = theme.font_size_title + padding * 2
    local content_bounds = {
        x = panel_x + padding,
        y = panel_y + title_area_h,
        w = panel_w - padding * 2,
        h = panel_h - title_area_h - padding,
    }

    local widget_ctx = {
        bounds     = content_bounds,
        theme      = M.get_theme(),    -- shallow copy; widget cannot mutate panel state
        is_focused = true,
    }

    widget_def.render(widget_ctx)

    -- Context-menu (rendered on top of widget content)
    if ctx_menu then
        local mx, my = safe_mouse_pos()
        -- Menu background
        engine.render.draw_rect(ctx_menu.x, ctx_menu.y,
                                ctx_menu.w, ctx_menu.h,
                                theme.context_menu_bg)

        build_ctx_menu_row_rects()
        for i, action in ipairs(ctx_menu.actions) do
            local rect   = ctx_menu.row_rects[i]
            local row_bg = theme.context_menu_bg

            -- Hover highlight: mouse y within this row
            if my >= rect.y and my < rect.y + rect.h
            and mx >= rect.x and mx < rect.x + rect.w then
                row_bg = theme.context_menu_hover
                engine.render.draw_rect(rect.x, rect.y, rect.w, rect.h, row_bg)
            end

            -- Label
            engine.render.draw_text(action.label,
                                    rect.x + padding,
                                    rect.y + (theme.row_height - theme.font_size_body) / 2,
                                    theme.font_size_body,
                                    theme.text_color)
        end
    end
end

-- -----------------------------------------------------------------------
-- Test backdoor
-- -----------------------------------------------------------------------

--- M._dispatch_event_for_test(event) — for tests only.
--- Direct passthrough to the internal _dispatch_event function so that
--- panel-test can simulate input events without a running game loop.
function M._dispatch_event_for_test(event)
    _dispatch_event(event)
end

return M