function comma_value(amount)
  local formatted = amount
  while true do  
    formatted, k = string.gsub(formatted, "^(-?%d+)(%d%d%d)", '%1.%2')
    if (k==0) then
      break
    end
  end
  return formatted
end


function round(val, decimal)
  if (decimal) then
    return math.floor( (val * 10^decimal) + 0.5) / (10^decimal)
  else
    return math.floor(val+0.5)
  end
end


function format_num(amount, decimal, prefix, neg_prefix)
  local str_amount,  formatted, famount, remain

  decimal = decimal or 2  
  neg_prefix = neg_prefix or "-" 

  famount = math.abs(round(amount,decimal))
  famount = math.floor(famount)

  remain = round(math.abs(amount) - famount, decimal)

  formatted = comma_value(famount)

  if (decimal > 0) then
    remain = string.sub(tostring(remain),3)
    formatted = formatted .. "." .. remain ..
                string.rep("0", decimal - string.len(remain))
  end

  formatted =  formatted ..  (prefix or "")

  if (amount<0) then
    if (neg_prefix=="()") then
      formatted = "("..formatted ..")"
    else
      formatted = neg_prefix .. formatted 
    end
  end

  return formatted
end