-- 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

-- Theme --------------------------------------------------------------------
-- Inheritance carries context (what surface am I on); the theme carries constants
-- (what is the accent). Roles are derived from three seeds so a new component costs
-- no theme keys, and so a theme cannot state its own contrast wrongly.

local BLACK, WHITE = {0, 0, 0}, {255, 255, 255}
local FALLBACK = {bg = WHITE, fg = BLACK, accent = {0, 120, 255}}
local PINNED = {muted = "color", accent_fg = "color", face = "pair", face_pressed = "pair", radius = "number"}

local function channel(value)
  return math.max(0, math.min(math.floor(value + 0.5), 255))
end

local function mix(a, b, amount)
  local blended = {}
  for i = 1, 3 do blended[i] = channel(a[i] + (b[i] - a[i]) * amount) end
  return blended
end

-- Perceived brightness, so text on the accent is chosen rather than configured.
local function luminance(c)
  return (c[1] * 299 + c[2] * 587 + c[3] * 114) / 1000
end

local function rgb(c)
  return gui.color(c[1], c[2], c[3])
end

local function palette(seed)
  local bg, fg, accent = seed.bg, seed.fg, seed.accent
  local theme = {
    bg = rgb(bg),
    fg = rgb(fg),
    muted = rgb(mix(fg, bg, 0.45)),
    accent = rgb(accent),
    accent_fg = rgb(luminance(accent) > 140 and BLACK or WHITE),
    face = {rgb(bg), rgb(mix(bg, fg, 0.08))},
    face_pressed = {rgb(mix(accent, WHITE, 0.12)), rgb(mix(accent, BLACK, 0.12))},
    radius = 6,
  }
  for role, kind in pairs(PINNED) do
    local pin = seed[role]
    if pin ~= nil then
      if kind == "color" then theme[role] = rgb(pin)
      elseif kind == "pair" then theme[role] = {rgb(pin[1]), rgb(pin[2])}
      else theme[role] = pin end
    end
  end
  return theme
end

local function themes()
  local ok, loaded = pcall(require, "theme")
  return ok and type(loaded) == "table" and loaded or {}
end

function ui.themeNames()
  local names = {}
  for name in pairs(themes()) do names[#names + 1] = name end
  table.sort(names)
  return names
end

-- Re-read the saved theme. Called at load, and again by the settings app so a
-- changed theme takes effect without relaunching.
function ui.reloadTheme()
  ui.themeName = sys.getTheme and sys.getTheme() or "light"
  local available = themes()
  ui.theme = palette(available[ui.themeName] or available.light or FALLBACK)
  return ui.theme
end

ui.reloadTheme()

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,
  }

  -- Main-axis distribution, CSS justify-content minus the modes nothing here asks for.
  -- Absolutely placed children take no part in the flow, so they are excluded.
  local flowing, used = 0, 0
  for _, child in ipairs(self.children) do
    if not child.at then
      flowing = flowing + 1
      used = used + (self.row and child.mw or child.mh)
    end
  end
  used = used + math.max(0, flowing - 1) * self.gap
  local free = math.max(0, (self.row and content.w or content.h) - used)

  local offset, spread = 0, 0
  if self.justify == "end" then offset = free
  elseif self.justify == "center" then offset = math.floor(free / 2)
  elseif self.justify == "between" and flowing > 1 then spread = math.floor(free / (flowing - 1))
  end

  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 + spread
    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 + spread
    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 gradient = self.pressed and self.press_gradient or (not self.pressed and self.gradient)
    local top, bottom
    if gradient then
      top, bottom = gradient[1], gradient[2]
    else
      top = self.pressed and (self.press_bg or self.color) or self.button_bg
      bottom = top
    end
    -- self.bg is the surface this button sits on, which the anti-aliased edge blends into.
    gui.roundRect(r.x, r.y, r.w, r.h, self.radius or ui.theme.radius, self.bg, top, bottom, self.color)
    for _, child in ipairs(self.children) do
      child.bg = bottom or self.bg  -- text blends against the bottom stop
      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

-- The root seeds the tree, so an app that names no colors is themed by inheritance.
local ROOT_STYLE = {
  color = "fg", bg = "bg", press_bg = "accent", press_color = "accent_fg",
  gradient = "face", press_gradient = "face_pressed", radius = "radius",
}

function ui.screen(root, style)
  local screen = setmetatable({root = root, captured = nil, pressedAt = 0}, Screen)
  for key, role in pairs(ROOT_STYLE) do
    if root[key] == nil then root[key] = (style and style[key]) or ui.theme[role] end
  end
  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