local ok, bit = pcall(require, "bit")
if not ok then
    ok, bit = pcall(require, "bit32")
    if not ok then error("no bit library") end
end

local utf8 = require("utf8")
local distribute = require("distribute")

local function ui_new(root)
    return { targets = {}, root = root }
end

local function compute(x, ...)
    if type(x) == "function" then
        return x(...)
    end
    return x
end

local function access(t, k)
    if t ~= nil then
        return t[k]
    end
end

local function align(kind, pos, needed, avail)
    if not kind then
        return pos, avail
    elseif kind == "start" then
        return pos, needed
    elseif kind == "center" then
        return pos+(avail-needed)/2, needed
    elseif kind == "end" then
        return pos+avail-needed, needed
    end
end

local function add_computed(node, attr, k, v)
    node.computed = node.computed or {}
    node.computed[attr] = node.computed[attr] or {}
    node.computed[attr][k] = v
end

local function node_compute_min_size(node, dir)
    local size = access(compute(node.size, node), dir)
    add_computed(node, "fixed_size", dir, size)

    if not size then
        size = 0
        for _, child in ipairs(node) do
            local child_size = node_compute_min_size(child, dir)
            if node.stack == dir then
                size = size + child_size
            else
                size = math.max(size, child_size)
            end
        end
    end

    add_computed(node, "min_size", dir, size)
    return size
end

local function node_compute_layout(node, dir, pos, size)
    size = node.computed.fixed_size[dir] or size
    local align_as = access(node.align, dir)

    add_computed(node, "pos", dir, pos)
    add_computed(node, "size", dir, size)

    if node.stack ~= dir then
        for _, child in ipairs(node) do
            node_compute_layout(child, dir, align(align_as, pos, child.computed.min_size[dir], size))
        end
        return
    end

    local min_size_noflex = 0
    local flex_elems = {}
    local has_flex
    for i, child in ipairs(node) do
        if child.flex then
            has_flex = true
            flex_elems[i] = {
                min = child.computed.min_size[dir],
                weight = child.flex,
            }
        else
            min_size_noflex = min_size_noflex + child.computed.min_size[dir]
        end
    end

    distribute(size-min_size_noflex, flex_elems)

    if not has_flex then
        pos = align(align_as, pos, min_size_noflex, size)
    end

    for i, child in ipairs(node) do
        local child_size = access(flex_elems[i], "receive") or child.computed.min_size[dir]
        node_compute_layout(child, dir, pos, child_size)
        pos = pos + child_size
    end
end

local function tree_compute_layout(tree, x, y, width, height)
    node_compute_min_size(tree, "x")
    node_compute_min_size(tree, "y")
    node_compute_layout(tree, "x", x, width)
    node_compute_layout(tree, "y", y, height)
end

-- render

local function update_flags(node, targets, flags)
    local new = {}
    for k, v in pairs(targets) do
        new[k] = node == v or (flags[k] and not (node.events and node.events[k]))
    end
    return new
end

local function node_render(node, targets, flags)
    flags = update_flags(node, targets, flags)

    local pos, size = node.computed.pos, node.computed.size
    if node.fill then
        love.graphics.setColor(compute(node.fill, node, flags))
        love.graphics.rectangle("fill", pos.x, pos.y, size.x, size.y)
    end
    if node.line then
        love.graphics.setColor(compute(node.line, node, flags))
        love.graphics.rectangle("line", pos.x, pos.y, size.x, size.y)
    end
    if node.text then
        love.graphics.setColor(compute(node.color, node, flags))
        love.graphics.draw(node.text, pos.x, pos.y)
    end

    for _, child in ipairs(node) do
        node_render(child, targets, flags)
    end
end

local function ui_render(ui)
    node_render(ui.root, ui.targets, {})
end

-- events

local function node_get_target(node, event, x, y)
    -- todo: maybe reverse iteration?
    for _, child in ipairs(node) do
        local hov = node_get_target(child, event, x, y)
        if hov then
            return hov
        end
    end

    if not node.events or not node.events[event] then
        return nil
    end

    local pos, size = node.computed.pos, node.computed.size
    if x >= pos.x and y >= pos.y and x < pos.x + size.x and y < pos.y + size.y then
        return node
    end
end

local function ui_get_target(ui, event)
    return node_get_target(ui.root, event, love.mouse.getPosition())
end

local function fire_event(node, event, ...)
    if node and node.events and node.events[event] then
        if type(node.events[event]) == "function" then
            node.events[event](node, ...)
        end
        return true
    end
    return false
end

local function ui_set_target(ui, event, elem)
    local old = ui.targets[event]
    if old ~= elem then
        fire_event(old, event.."_end")
        fire_event(elem, event)
    end
    ui.targets[event] = elem
end

local function ui_update_target(ui, event)
    ui_set_target(ui, event, ui_get_target(ui, event))
end

local function ui_mousepressed(ui, x, y, button)
    if button ~= 1 then
        return false
    end

    ui_update_target(ui, "press")
    ui_update_target(ui, "focus")

    local focus = ui.targets.focus
    if focus and focus.events.input then
        local pos, size = focus.computed.pos, focus.computed.size
        love.keyboard.setTextInput(true, pos.x, pos.y, size.x, size.y)
    else
        love.keyboard.setTextInput(false)
    end

    if ui.targets.press or ui.targets.focus then
        return true
    end

    return false
end

local function ui_mousereleased(ui, x, y, button)
    if button ~= 1 then
        return false
    end

    local press = ui_get_target(ui, "press", love.mouse.getPosition())
    if press == ui.targets.press then
        fire_event(press, "click")
    end
    ui_set_target(ui, "press", nil)

    if press then
        return true
    end
    return false
end

local function ui_textinput(ui, text)
    return fire_event(ui.targets.focus, "input", text)
end

local function ui_keypressed(ui, key)
    if key == "backspace" then
        return fire_event(ui.targets.focus, "input", "\b")
    elseif key == "v" and (love.keyboard.isDown("rctrl") or love.keyboard.isDown("lctrl")) then
        return fire_event(ui.targets.focus, "input", love.system.getClipboardText())
    end

    return false
end

-- update

local function ui_update(ui)
    tree_compute_layout(ui.root, 0, 0, love.graphics:getDimensions())
    ui_update_target(ui, "hover")
end

-- presets

local function color(hex)
    return { love.math.colorFromBytes(
        bit.band(bit.rshift(hex, 16), 0xff),
        bit.band(bit.rshift(hex,  8), 0xff),
        bit.band(bit.rshift(hex,  0), 0xff)
    ) }
end

local function ui_flex(n, ...)
    return { flex = n, ... }
end

local function pad(dir, size, ...)
    local p = { size = { [dir] = size } }
    local nodes = { ... }
    if #nodes > 1 then
        local x = { stack = dir }
        for i, n in ipairs(nodes) do
            table.insert(x, n)
            if i < #nodes then
                table.insert(x, p)
            end
        end
        return x
    elseif #nodes == 1 then
        return { stack = dir, p, ui_flex(1, nodes[1]), p }
    end
    return p
end

local function ui_pad_x(...)
    return pad("x", ...)
end

local function ui_pad_y(...)
    return pad("y", ...)
end

local function center_x(...)
    return { align = { x = "center" }, ... }
end

local function center_y(...)
    return { align = { y = "center" }, ... }
end

local function stack_x(...)
    return { stack = "x", ... }
end

local function stack_y(...)
    return { stack = "y", ... }
end

local function ui_text(str, size, col)
    return {
        color = col,
        text = love.graphics.newText(love.graphics.newFont(size), str),
        size = function(self) return { x = self.text:getWidth(), y = self.text:getHeight() } end,
    }
end

local function ui_button(label, click)
    return {
        fill = function(self, f)
            return f.hover and color(0x8080a0) or color(0x808080)
        end,
        line = color(0x505050),
        events = { hover = true, press = true, click = click, },
        ui_pad_y(5, center_x(ui_pad_x(10,
            ui_text(label, 25, color(0x000000))
        )))
    }
end

local function ui_input(placeholder)
    local function is_placeholder(self, focus)
        return self.input_value == "" and not focus
    end
    local function update_text(self, focus)
        self.input_text:set(is_placeholder(self, focus) and self.input_placeholder or self.input_value)
        self.input_interact_time = love.timer.getTime()
    end

    local text = ui_text(placeholder, 25, function(self, f)
        return is_placeholder(self.input_parent, f.focus) and color(0xb0b0b0) or color(0x000000)
    end)

    local marker = {
        size = { x = 1, y = text.text:getHeight() },
        fill = function(self, f)
            return (f.focus and
                math.floor(love.timer.getTime() - self.input_parent.input_interact_time) % 2 == 0)
            and color(0x000000) or color(0xffffff)
        end
    }

    -- TODO: cursor
    -- TODO: overflow as scroll
    -- TODO: paste icon (optional)

    local elem = {
        fill = color(0xffffff),
        line = function(self, f)
            return f.focus and color(0x5050ff) or color(0x505050)
        end,
        events = {
            focus = function(self)
                update_text(self, true)
            end,
            focus_end = function(self)
                update_text(self, false)
            end,
            input = function(self, text)
                if text == "\b" then
                    local byteoffset = utf8.offset(self.input_value, -1)
                    if byteoffset then
                        self.input_value = string.sub(self.input_value, 1, byteoffset - 1)
                    end
                else
                    self.input_value = self.input_value .. text
                end
                update_text(self, true)
            end,
        },
        input_value = "",
        input_text = text.text,
        input_placeholder = placeholder,
        ui_pad_y(5, ui_pad_x(10, stack_x(text, marker)))
    }

    text.input_parent = elem
    marker.input_parent = elem

    return elem
end

return {
    new = ui_new,
    render = ui_render,
    update = ui_update,
    keypressed = ui_keypressed,
    textinput = ui_textinput,
    mousepressed = ui_mousepressed,
    mousereleased = ui_mousereleased,
    color = color,
    pad_x = ui_pad_x,
    pad_y = ui_pad_y,
    center_x = center_x,
    center_y = center_y,
    stack_x = stack_x,
    stack_y = stack_y,
    flex = ui_flex,
    text = ui_text,
    button = ui_button,
    input = ui_input,
}