-- Layout borrowed from CSS block flow: nesting plus box model, no cascade.
-- Sizes are pixels (>= 1), a fraction of the parent content box (< 1), or "auto".

local ui = {}

local PRESS_MS = 80  -- a fast tap must stay visible for at least this long
local SLOP = 4       -- resistive panels land a few pixels off

local function resolve(value, span, axis)
  if value == nil or value == "auto" then return nil end
  if value < 1 then
    if span == nil then
      error("fractional " .. axis .. " inside an auto-sized parent", 3)
    end
    return math.floor(value * span)
  end
  return math.floor(value)
end

local function padding(spec)
  local p = spec.pad or 0
  if type(p) == "number" then return {t = p, r = p, b = p, l = p} end
  return {t = p.t or 0, r = p.r or 0, b = p.b or 0, l = p.l or 0}
end

-- The palette set on the root reaches every descendant, so styling is one place.
local INHERITED = {"color", "bg", "size", "button_bg", "press_bg", "press_color", "radius",
                   "gradient", "press_gradient"}

local function inherit(child, parent)
  for _, key in ipairs(INHERITED) do
    if child[key] == nil then child[key] = parent[key] end
  end
end

local function contains(rect, x, y)
  return x >= rect.x - SLOP and x < rect.x + rect.w + SLOP
     and y >= rect.y - SLOP and y < rect.y + rect.h + SLOP
end

-- Components ---------------------------------------------------------------

local Component = {}
Component.__index = Component

function Component:measure(available)
  local pad = self.pad_
  local w = resolve(self.w, available.w, "width")
  local h = resolve(self.h, available.h, "height")
  local inner = {
    w = w and w - pad.l - pad.r or (available.w and available.w - pad.l - pad.r),
    h = h and h - pad.t - pad.b or nil,
  }

  local main, cross = 0, 0
  for index, child in ipairs(self.children) do
    inherit(child, self)
    local cw, ch = child:measure(inner)
    if self.row then
      main = main + cw + (index > 1 and self.gap or 0)
      cross = math.max(cross, ch)
    else
      main = main + ch + (index > 1 and self.gap or 0)
      cross = math.max(cross, cw)
    end
  end

  if self.row then
    self.mw = w or main + pad.l + pad.r
    self.mh = h or cross + pad.t + pad.b
  else
    self.mw = w or cross + pad.l + pad.r
    self.mh = h or main + pad.t + pad.b
  end
  return self.mw, self.mh
end

function Component:place(rect)
  self.rect = rect
  self.dirty = true
  local pad = self.pad_
  local content = {
    x = rect.x + pad.l,
    y = rect.y + pad.t,
    w = rect.w - pad.l - pad.r,
    h = rect.h - pad.t - pad.b,
  }

  local offset = 0
  for _, child in ipairs(self.children) do
    local cw, ch = child.mw, child.mh
    -- Cross axis fills the parent unless the child asked for a size, like CSS blocks.
    if self.row then
      if child.h == nil or child.h == "auto" then ch = content.h end
    else
      if child.w == nil or child.w == "auto" then cw = content.w end
    end

    local x, y
    if child.at then
      x = content.x + resolve(child.at.x, content.w, "x")
      y = content.y + resolve(child.at.y, content.h, "y")
    elseif self.row then
      x, y = content.x + offset, content.y
      if self.align == "center" then y = y + math.floor((content.h - ch) / 2)
      elseif self.align == "end" then y = y + content.h - ch end
      offset = offset + cw + self.gap
    else
      x, y = content.x, content.y + offset
      if self.align == "center" then x = x + math.floor((content.w - cw) / 2)
      elseif self.align == "end" then x = x + content.w - cw end
      offset = offset + ch + self.gap
    end
    child:place{x = x, y = y, w = cw, h = ch}
  end
end

function Component:draw()
  if self.dirty then
    if self.bg then gui.fillRect(self.rect.x, self.rect.y, self.rect.w, self.rect.h, self.bg) end
    if self.paint then self:paint() end
    self.dirty = false
    for _, child in ipairs(self.children) do child.dirty = true end
  end
  for _, child in ipairs(self.children) do child:draw() end
end

-- Deepest interactive component wins, so a tappable child beats its tappable parent.
function Component:hit(x, y)
  if not self.rect or not contains(self.rect, x, y) then return nil end
  for index = #self.children, 1, -1 do
    local found = self.children[index]:hit(x, y)
    if found then return found end
  end
  return self.on_press and self or nil
end

function Component:invalidate()
  self.dirty = true
end

local function component(spec)
  spec.children = spec.children or {}
  for index, child in ipairs(spec) do
    spec.children[index] = child
    spec[index] = nil
  end
  spec.gap = spec.gap or 0
  spec.align = spec.align or "start"
  spec.pad_ = padding(spec)
  return setmetatable(spec, Component)
end

ui.box = component

function ui.spacer(spec)
  return component{w = spec.w, h = spec.h}
end

function ui.text(label, spec)
  spec = spec or {}
  spec.label = label
  local node = component(spec)
  node.measure = function(self, available)
    self.mw = resolve(self.w, available.w, "width") or gui.textWidth(self.label)
    self.mh = resolve(self.h, available.h, "height") or gui.fontHeight()
    return self.mw, self.mh
  end
  node.paint = function(self)
    gui.drawText(self.label, self.rect.x, self.rect.y, self.color, self.bg)
  end
  return node
end

function ui.button(spec)
  spec.pad = spec.pad or 8
  spec.align = spec.align or "center"
  if spec.label then
    spec.children = {ui.text(spec.label)}
    spec.label = nil
  end
  local node = component(spec)
  node.paint = function(self)
    local r = self.rect
    local radius = self.radius or 6
    local fill = self.pressed and (self.press_bg or self.color) or (self.button_bg or self.bg)
    local gradient = self.pressed and self.press_gradient or (not self.pressed and self.gradient)
    if gradient then
      gui.fillRectGradient(r.x, r.y, r.w, r.h, radius, gradient[1], gradient[2])
      fill = gradient[2]  -- text blends against the bottom stop
    elseif fill then
      gui.fillRoundRect(r.x, r.y, r.w, r.h, radius, fill)
    end
    gui.drawRoundRect(r.x, r.y, r.w, r.h, radius, self.color)
    for _, child in ipairs(self.children) do
      child.bg = fill
      child.color = self.pressed and (self.press_color or self.bg) or self.color
      child.dirty = true
    end
  end
  return node
end

-- Screen -------------------------------------------------------------------

local Screen = {}
Screen.__index = Screen

function ui.screen(root, style)
  local screen = setmetatable({root = root, captured = nil, pressedAt = 0}, Screen)
  root.color = root.color or (style and style.color) or gui.color(0, 0, 0)
  root.bg = root.bg or (style and style.bg) or gui.color(255, 255, 255)
  screen:relayout()
  return screen
end

function Screen:relayout()
  local rect = {x = 0, y = 0, w = gui.width(), h = gui.height()}
  self.root:measure(rect)
  self.root:place(rect)
  gui.clear(self.root.bg)
end

function Screen:draw()
  self.root:draw()
  -- Hold the pressed look briefly so a fast tap is still perceptible.
  if self.released and sys.millis() - self.pressedAt >= PRESS_MS then
    self.released.pressed = false
    self.released:invalidate()
    self.released = nil
  end
end

function Screen:down(x, y)
  local target = self.root:hit(x, y)
  if not target then return end
  self.captured = target
  self.pressedAt = sys.millis()
  target.pressed = true
  target:invalidate()
end

function Screen:up(x, y)
  local target = self.captured
  self.captured = nil
  if not target then return end
  self.released = target
  local inside = contains(target.rect, x, y)
  if inside and target.on_press then target.on_press(target) end
end

return ui