Menu API

-- Menu API
--
-- Written By: GhastTearz
-- Version: 2.0.0
--
-- This program is a simple and flexible way to get
-- menus in your program.
--
-- The format of menu strings is very simple. Each
-- line of the string is a line in the menu. To add
-- a "link" to a function or screen just as a tag
-- somewhere in the line e.g. "@tag". See the demo
-- program for more information at
-- https://pastebin.com/ZNYdQzaR
--
-- KEYBINDINGS
-- w, k, up           =  move up the menu
-- a, h, left         =  go to previous screen
-- s, j, down         =  move down the menu
-- d, l, right, enter = select an item
-- q, x = quit
-- page up = go up a page
-- page down = go down a page
--
---------------------------------------------------

function addFunc(menu, tag, func, ...)

  if menu == nil or type(menu) ~= "table" then
    error("Expected a menu table.")
  end
  if tag == nil or type(tag) ~= "string" then
    error("Expected a tag string.")
  end
  if func == nil or type(func) ~= "function" then
    error("Expected a function.")
  end

  if menu[tag] ~= nil then
    error("The tag "..tag.." is already in use.")
  end

  local args = ...
  menu[tag] = function() func(args) end

end

function addScreen(menu, tag, screen)

  if menu == nil or type(menu) ~= "table" then
    error("Expected a menu table.")
  end
  if tag == nil or type(tag) ~= "string" then
    error("Expected a tag string.")
  end
  if screen == nil or type(screen) ~= "string" then
    error("Expected a screen string.")
  end

  if menu[tag] ~= nil then
    error("The tag "..tag.." is already in use.")
  end

  menu[tag] = {}

  -- parse the screen string into a table
  local line = 1
  local text = ""
  local ref  = ""

  local i = 1
  while i < #screen do

    if screen:sub(i,i) == '@' then
      if screen:sub(i+1,i+1) == '@' then
      --escape two @s by skipping a char
        i = i + 1
      else
        if ref ~= "" then
          error("On input line "..line
                .." for screen "..tag
                .." there is more than one tag.")
        end

        i = i + 1
        while screen:sub(i,i) ~= "" and
              screen:sub(i,i) ~= ' ' and
              screen:sub(i,i) ~= '\t' and
              screen:sub(i,i) ~= '\n' do

          ref = ref..screen:sub(i,i)
          i = i + 1
        end

        if ref == "" then
          error("On input line "..line
                .." for screen "..tag
                .." the tag is empty.")
        end
      end
    end

    if screen:sub(i,i) == '\n' or
       screen:sub(i,i) == "" then

      menu[tag][line] = {}
      menu[tag][line].text = text
      menu[tag][line].ref  = ref
      text = ""
      ref  = ""
      line = line + 1

    else
      text = text..screen:sub(i,i)

    end

    i = i + 1

  end
end

function displayScreen(menu, tag)

  if menu == nil or type(menu) ~= "table" then
    error("Expected a menu table.")
  end
  if tag == nil or type(tag) ~= "string" then
    error("Expected a tag string.")
  end
  if menu[tag] == nil then
    error("The tag "..tag.." does not exist.")
  end
  if type(menu[tag]) ~= "table" then
    error("The tag "..tag.." is not a screen.")
  end

  --Check that all the tags refer to something
  for k, v in pairs(menu) do
    if type(menu[k]) == "table" then
      for l = 1, #menu[k] do
        local r = menu[k][l].ref
        if r ~= "" and menu[r] == nil then
          error("The tag "..r.." is not defined.")
        end
      end
    end
  end


  local currentScreen = menu[tag]

  local screenStack = {}
  screenStack[1] = menu[tag]

  local width, height = term.getSize()


  -- These are indexes into the screen.
  -- Top is the first line to be printed.
  -- Bottom is the last line to be printed.
  -- Selection is the line the user might select.
  local top = 1
  local bottom = height
  local selection = 0

  local function printScreen()

    term.clear()
    term.setCursorPos(1,1)

    for i = top, bottom do
      if i > #currentScreen then
        break
      end

      local y = i % height
      if y == 0 then
        y = height
      end

      term.setCursorPos(1, y)
      if i == selection then
        write("->"..currentScreen[i].text)
      else
        write("  "..currentScreen[i].text)
      end
    end
  end

  local function setSelection()
    selection = 0
    for i = top, bottom do
      if i > #currentScreen then
        break
      end
      if currentScreen[i].ref ~= "" then
        selection = i
        break
      end
    end
  end

  local function pageUp()
    if top > 1 then
      top = top - height
      bottom = bottom - height
      setSelection()
    end
  end

  local function pageDown()
    if #currentScreen > bottom then
      top = top + height
      bottom = bottom + height
      setSelection()
   end
  end


  setSelection()
  printScreen()

  while true do

    local e, key = os.pullEvent("key")

    -- go up the screen
    if key == 17 or
       key == 37 or
       key == 200 then

      -- find prev item with a ref
      local prevItem
      for i = selection - 1, 1, -1 do
        if currentScreen[i].ref ~= "" then
          prevItem = i
          break
        end
      end

      if prevItem == nil then
        pageUp()
      else
        selection = prevItem
        if prevItem < top then
          pageUp()
        end
      end

    end

    -- go down the screen
    if key == 31 or
       key == 36 or
       key == 208 then

      -- find the next item with a ref
      local nextItem
      for i = selection + 1, #currentScreen do
        if currentScreen[i].ref ~= "" then
          nextItem = i
          break
        end
      end

      if nextItem == nil then
        pageDown()
      else
        selection = nextItem
        if nextItem > bottom then
          pageDown()
        end
      end

    end

    -- select the current item
    if (key == 32 or
       key == 38 or
       key == 205 or
       key == 28 ) and
       currentScreen[selection] ~= nil then

      local r = currentScreen[selection].ref

      if type(menu[r]) == "function" then
        term.clear()
        term.setCursorPos(1,1)
        return menu[r]
      else
        screenStack[#screenStack + 1] = menu[r]
        currentScreen = menu[r]
        top = 1
        bottom = height
        setSelection()
      end

    end

    -- Go back to previous screen
    if (key == 30 or
       key == 35 or
       key == 203) and
       #screenStack > 1 then

      screenStack[#screenStack] = nil
      currentScreen = screenStack[#screenStack]
      top = 1
      bottom = height
      setSelection()
    end

    -- quit the menu
    if key == 16 or
       key == 45 then
      term.clear()
      term.setCursorPos(1,1)
      return
    end

    if key == 201 then
      pageUp()
    end

    if key == 209 then
      pageDown()
    end

    printScreen()

  end

end


function new()
  local t = {}
  t.addFunc       = addFunc
  t.addScreen     = addScreen
  t.displayScreen = displayScreen
  return t
end