Conversion of bitstring to decimal in Erlang

Question

I am trying to decode the bitstring to decimal value. For ex I have these kind of bitstrings

<<96,64,112,153,9:4>>. I want to convert them to decimal values like you take four bits as a digit (96(01100000) --> 60( first four bits is 6, next four bits is 0) , 64 --> 40 and so on. The output would be 604070999. The last 9:4 represents that you consider 4 bits to represent.

Can anyone help in doing this function erlang.

Answer 1

If you have a binary rather than a bitstring (ie, without the trailing 9:4 part), you can apply a hex conversion to each byte within a binary comprehension, then convert the resulting binary to an integer:

1> Bin = <<96,64,112,153>>.
<<96,64,112,153>>
2> binary_to_integer(<< <<(integer_to_binary(B,16))/binary>> || <<B:8>> <= Bin >>).
60407099

The same also works for your bitstring, taking 4 bits at a time instead of 8 in the comprehension:

3> Bits = <<96,64,112,153,9:4>>.
<<96,64,112,153,9:4>>
4> binary_to_integer(<< <<(integer_to_binary(B,16))/binary>> || <<B:4>> <= Bits >>).
604070999

But as @Hynek-Pichi-Vychodil points out in the comments, for the bitstring you don't need the integer_to_binary/2 call at all, but instead can convert each 4-bit digit to its corresponding character by adding $0 , the literal for the character 0 :

5> binary_to_integer(<< <<($0+B)>> || <<B:4>> <= Bits >>).
604070999