I am working on a protocol encoder & decoder. It has the quirk that if you wish to encode the maximum possible 64-bit floating point value then you don’t serialize it as all other values (namely the non-scientific full string representation) but only as a singular byte with the value zero.

Poking around on the net, and double-checking with Rust, I found that the value representing the maximum f64 has the byte representation of 0x7FEFFFFFFFFFFFFF. So we can decode it like so:

iex(1)> <<f::float-big-64>> = <<0x7FEFFFFFFFFFFFFF::unsigned-big-integer-64>>
<<127, 239, 255, 255, 255, 255, 255, 255>>
iex(2)> f
1.7976931348623157e308

Which looks (near-)identical to what Rust prints when you execute println!("{}", f64::MAX) in the Rust playground. So far, so good. But now I want to have an encoding function that recognizes the special value in its head and I can’t achieve it.

Obviously this does NOT work:

defmodule Codec do
  def encode_double(0x7FEFFFFFFFFFFFFF), do: <<0>>

  def encode_double(x) when is_float(x) do
    textual_float = Decimal.from_float(x) |> Decimal.to_string(:normal)
    textual_float <> <<0>>
  end
end

And this works just fine:

defmodule Codec do
  def encode_double(x) when is_float(x) do
    case <<x::float-big-64>> do
      # If the value is f64::MAX i.e. `0x7FEFFFFFFFFFFFFF`
      <<127, 239, 255, 255, 255, 255, 255, 255>> ->
        <<0>>

      _ ->
        textual_float = Decimal.from_float(x) |> Decimal.to_string(:normal)
        textual_float <> <<0>>
    end
  end
end

In the REPL:

iex(3)> Codec.encode_double f
<<0>>
iex(4)> Codec.encode_double 3.14
<<51, 46, 49, 52, 0>>

Is there a way to match on that special floating-point value in the function head? Mind you, that value must pass the is_float guard; if it wasn’t for that requirement I could have just used the byte array value in the function head.