NodeMCU RF receiver and decoder

rfrecv.lua

local BUFFER_SIZE = 512
local timings = { }
local write = 0
local read = 0
local max = 0

local function count(pos)
  if pos == read then
    return 0
  elseif pos > read then
    return pos - read
  else
    return BUFFER_SIZE + pos - read;
  end
end

local prev_ts = 0

local function capture(level, ts)
  local len = math.abs(prev_ts - ts)
  prev_ts = ts

  if count(write) < BUFFER_SIZE - 2 then
    write = (write + 1) % BUFFER_SIZE
    timings[write] = len
  end
end

local STATE_PREAMBLE = 0
local STATE_BIT_HIGH = 1
local STATE_BIT_LOW  = 2

local state  = 0
local preamb = 0
local long   = 0
local short  = 0
local bits   = 24
local nibble = 0
local bit    = 0
local code   = 0
local callback = function(code, bits, preamble, short, long) end

local function process()
  repeat
    local pos = write
    while count(pos) > 0 do
      read = (read + 1) % BUFFER_SIZE
      if state == STATE_PREAMBLE then
        if timings[read] > 6000 then
          preamble = timings[read]
          long     = (preamble * 3) / 31
          short    = (preamble * 1) / 31
          bit      = 0
          code     = 0
          state    = STATE_BIT_HIGH
        end
      elseif state == STATE_BIT_HIGH then
        if math.abs(timings[read] - long) < (long * 10) / 25 then
          nibble = 30
          state  = STATE_BIT_LOW
        elseif math.abs(timings[read] - short) < (short * 10) / 15 then
          nibble = 10
          state = STATE_BIT_LOW
        else
          state = STATE_PREAMBLE
        end
      elseif state == STATE_BIT_LOW then
        if math.abs(timings[read] - long) < (long * 10) / 25 then
          nibble = nibble + 3
          state  = STATE_BIT_HIGH
        elseif math.abs(timings[read] - short) < (short * 10) / 15 then
          nibble = nibble + 1
          state = STATE_BIT_HIGH
        else
          state = STATE_PREAMBLE
        end

        if state == STATE_BIT_HIGH then
          if nibble == 13 then
            code = code * 2
            bit = bit + 1
          elseif nibble == 31 then
            code = code * 2 + 1
            bit = bit + 1
          else
            state = STATE_PREAMBLE
          end
        end

        if state == STATE_BIT_HIGH and bit == bits then
          callback(code, bits, preamble, short, long)
          state = STATE_PREAMBLE
        end
      else
        state = STATE_PREAMBLE
      end
    end
  until read == write
end

local function init(pin, led, cb)
  for i = 0, BUFFER_SIZE - 1 do
    timings[i] = 0
  end

  callback = cb

  gpio.mode(pin, gpio.INT, gpio.PULLUP)
  gpio.trig(pin, "both", capture)

  tmr.alarm(5, 100, tmr.ALARM_AUTO, process)

  gpio.mode(led, gpio.OUTPUT)
  gpio.write(led, 0)
end

rfrecv = {
  init = init
}