Re: Formatting numbers without precision loss

[Thijs Schreijer <thijs@...>]

> 
> In short, the options so far;
> 
> - use “%a”, but it is ugly and not always available
> - determine the number of decimals for each number (Pallene approach)
> - determine once the number of significant decimals for the current system, and use every time with a "%g” modifier

So here's what I ended up with;


local num2string do
  -- will not handle NaN, infinity and the likes.
  local format = string.format

  if pcall(function() format("%a", 1.1) end) then
    -- the "%a" specifier is available
    num2string = function(value) return format("%a", value) end

  else
    -- the "%a" specifier is unavailable
    -- dynamically test the available number of decimals for the %g specifier
    local max_dec = 1
    while true do  -- testing showed 99 locally...
      if not pcall(function() string.format("%."..max_dec.."g", 1.1) end) then
        max_dec = max_dec - 1
        break
      end
      max_dec = max_dec + 1
    end
    local MAX_FLOAT_FMT = "%."..tostring(max_dec).."g"
    --print(MAX_FLOAT_FMT)

    -- 17 digits should be enough to represent a double
    -- See https://stackoverflow.com/a/21162120 and DBL_DECIMAL_DIG in float.h
    -- But we don't know how the Lua engine was compiled, so let's test it dynamically
    -- use a known value 1/9; 0.1111111111... to detect relevant number of decimals
    local relevant_size = #(MAX_FLOAT_FMT):format(1/9):match("%.(1+)") + 1
    local FLOAT_FMT = "%."..tostring(relevant_size).."g" -- local test resulted in 17
    --print(FLOAT_FMT)

    num2string = function(value) return format(FLOAT_FMT, value) end
  end
end