function_lookup_table.lua

#!/usr/bin/lua
--[[
Copyright (C) February 10, 2016, Andrew Domaszek
(MIT License)

Wrote this lua demonstrator for Joe

Show some basics on how to do index-lookup-based function indirection instead
of an if-elseif cascade and keep a table of global state. I suspect the keystate
variable could be declared global instead of manually set in _G, but I've only done
it from C-api before, and this is how I do it.
--]]

-- Replace NILs with b.
function NVL(a,b)
    if (a == nil) then
      return b
    else
      return a
    end
end

-- Demonstrate loading a table from the global table and toggling the state
function globtoggle ()
  -- Create the keystat global if it doesn't already exist.
  if rawget(_G, "keystate") == nil then
    rawset(_G, "keystate", {})
  end
  
  local LTkeystate = _G.keystate -- or if you prefer: _G["keystate"]
  
  local G1=1
  local G2=2
  local G3=3
  
  -- Using 100 as the "G-key" offset, this is so you could use different keys
  -- if, say, you were using family in the OnEvent call (logitech g-series)
  -- to switch between multiple devices with the same key nums.
  local IncomingEvent=100+G1
  
  local g1_state = NVL(LTkeystate[IncomingEvent], 0)
  
  print("g1_s: " .. g1_state)
  
  -- Toggle the state of g1
  -- Note that even though we only update the LOCAL version,
  -- the GLOBAL TABLE is pointing at the same table. lua
  -- tables are special in this regard.
  LTkeystate[IncomingEvent] = (g1_state+1) % 2
end

function globtest()
  print("--- Glob Test ---")
  
  for i=1,3 do
    globtoggle()
  end
  
  print("")
end

-- Demonstrate table initialization, value set, and access methods
function table_demonstrator()
    print("--- Table Demonstrator ---")
    
    -- Some declare-and-initialize statements
    local t1 = { apple = 1, orange = 2}
    local t2 = { ["one key"] = 1, [2] = "two", grape = false}

    -- Declare an empty local table
    local tTab = {}

    -- Can set values in tables. Normally rawset is not used.
    rawset(tTab, "dm", "dot-member style")
    rawset(tTab, "ar", "associative array style")

    -- Normal syntax for setting values
    tTab.dms = "dot-member set style"
    tTab["ars"] = "associative array set style"

    -- Note the concatenation operation with .. here:
    print("Accessor: " .. tTab.dm)
    print("Accessor: " .. tTab["dms"]) -- can access either way
    print("Accessor: " .. tTab["ar"])
    print("Accessor: " .. tTab.ars) -- can access either way
    print("")
end

-- These four functions are called by the functional_LUT_demonstrator to show
-- different functions are called. Note that all of them have the same
-- call signature; that is a limitation of this method, though the param can
-- be completely ignored or omitted. A return value, if required MUST be provided.
function G1_action(a)
  print("G1, ACTION! ("..a..")")
end

function G1m1_action(a)
  print("G1-meta1, ACTION! ("..a..")")
end

function G2_action(a)
  print("G2, ACTION! ("..a..")")
end

-- Param can be totally omitted and still work.
function G3_action()
  print("G3, ACTION! (noparam!)")
end

-- Demonstrate switching between multiple functions stored in a two-tier table
-- based on variables.
function functional_LUT_demonstrator()
    -- We only declare and use this table locally, but it
    -- could be stored in the globals table _G and referenced
    local tLUT = {
      [1] = {fn = G1_action, fn_m1 = G1m1_action},
      [2] = {fn = G2_action},
      [3] = {fn = G3_action}
      }
    
    -- Event would be like a keypress
    -- Try changing this to 1, 2, or 3
    local Event=1
    -- Meta would be like the meta state (or maybe keystate from global)
    -- Try changing this to 0 or 1
    local Meta=1
    
    local eLUT = tLUT[Event]
    local E_Action = eLUT.fn
    
    -- Only set the meta-handler if fn_m1 available, otherwise default to fn
    if Meta == 1 and not (eLUT.fn_m1 == nil) then
      -- print("meta was set")
      E_Action = eLUT.fn_m1
    end
    
    -- Can create other definitions for Meta == 2, eLUT.fn_m2, etc.
    -- Just remember to fail gracefully.
    
    -- If we have retrieved an event, we can call it. There could/should be a type
    -- check here to make sure it's actually a callable function.
    if not (E_Action == nil) then
      E_Action("Passed in stuff!")
    else
      print("E_Action is nil. Is this event handled?")
    end
end

table_demonstrator()
globtest()
functional_LUT_demonstrator()