Polish notation parser in Lua

polish-notation.lua

-- Operations definitions
local operations = {
  ["+"] = {
    Arguments = 2,
    Operation = function(a, b) return a + b end
  },
  ["-"] = {
    Arguments = 2,
    Operation = function(a, b) return a - b end
  },
  ["*"] = {
    Arguments = 2,
    Operation = function(a, b) return a * b end
  },
  ["/"] = {
    Arguments = 2,
    Operation = function(a, b) return a / b end
  },
  ["^"] = {
    Arguments = 2,
    Operation = function(a, b) return a ^ b end
  },
  ["%"] = {
    Arguments = 2,
    Operation = function(a, b) return a % b end
  },
  ["<<"] = {
    Arguments = 2,
    Operation = function(a, b) return a * 2^b end
  },
  [">>"] = {
    Arguments = 2,
    Operation = function(a, b) return math.floor(a / 2^b) end
  }
}

function evaluatePN(str)
  local symbols = {}

  for symbol in str:gmatch("%S+") do
    table.insert(symbols, symbol)
  end

  while #symbols > 1 do
    for i=#symbols,1,-1 do
      local symbol = symbols[i]

      if operations[symbol] then
        local op = operations[symbol]
        local args = {}

        for j=i+1, i+op.Arguments do
          local arg = symbols[j]

          table.insert(args, tonumber(arg))
        end

        assert(#args == op.Arguments, ("Operations requires %d arguments, got %d"):format(op.Arguments, #args))

        symbols[i] = op.Operation(unpack(args))

        -- Shift symbols array left by argument count
        for j=i+1,#symbols do
          symbols[j] = symbols[j+op.Arguments]
        end
      end
    end
  end

  return symbols[1]
end