suazi
Separating two integers from the upper/lower bits of a single 32 bit value from little endian
I have a bitstring:
<<3, 0, 0, 0>>
that i’m trying to split into two values like so:
<<chunk::little-unsigned-size(31), is_first_chunk::little-unsigned-size(1)>> = <<3, 0, 0, 0>>
following this specification:
chunk/isFirstChunk (upper 31bits/lowest bit)
“chunk” and “isFirstChunk” are combined into an unsigned 32bit value. Therefore it will be encoded as
uint32_t chunkXand extracted as
chunk = chunkX >> 1 isFirstChunk = chunkX & 0x1
and I’m expecting both values to be 1 but it’s actually 3 and 0 respectively.
Am I wrong?
Marked As Solved
massimo
Correct.
Elixir defaults to big endian format when manually crafting binaries.
be aware that most networking protocols are big endian, while almost any common CPU architecture is little endian.
Se if you’re decoding network streams, most probably the will be big endian (most significant byte first)
You can specify endianness in this way
iex(30)> :binary.decode_unsigned(<<3, 0, 0, 0>>, :little)
3
iex(31)> <<chunk::32-little> = <<3, 0, 0, 0>>
iex(32)> chunk
3
encoding
iex(1)> <<3::32-little>>
<<3, 0, 0, 0>>
iex(5)> <<0b0000001::32-little>>
<<1, 0, 0, 0>>
iex(6)> <<0b0000010::32-little>>
<<2, 0, 0, 0>>
iex(7)> <<0b0000011::32-little>>
<<3, 0, 0, 0>>
iex(8)> <<0b0000100::32-little>>
<<4, 0, 0, 0>>
Also Liked
massimo
it is correct.
<<3, 0, 0, 0>> is a bitstring, encoded as a 4 bytes it is 0x03 0x00 0x00 0x00 and as a 32bit integer it becomes 50331648.
50331648 & 0x01 is zero, because the last byte of the integer is not set (the fourth zero in the four bytes sequence)
0x03 0x00 0x00 0x01 (50331649) would give you 1.
iex(11)> <<chunk::little-unsigned-size(31), is_first_chunk::little-unsigned-size(1)>> = <<3, 0, 0, 1>>
<<3, 0, 0, 1>>
iex(12)> is_first_chunk
1
iex(17)> <<chunkX::32>> = <<3, 0, 0, 1>>
<<3, 0, 0, 1>>
iex(18)> require Bitwise
Bitwise
iex(19)> Bitwise.&&&(chunkX, 0x01)
1
iex(20)> chunkX
50331649
iex(21)> <<chunkX::32>> = <<3, 0, 0, 0>>
<<3, 0, 0, 0>>
iex(22)> Bitwise.&&&(chunkX, 0x01)
0
iex(23)> chunkX
50331648
iex(24)> Bitwise.>>>(chunkX, 1)
25165824 <-- chunk
EDIT: you can convert the integer back to a bitstring with
iex(25)> :binary.encode_unsigned(50331648)
<<3, 0, 0, 0>>
iex(26)> :binary.encode_unsigned(50331649)
<<3, 0, 0, 1>>
suazi
Yup, I’m workin on VelocyStream (ArangoDB).
With your insight, it became clear. To encode for an outgoing chunk:
chunk_x =
<<chunk::31, is_first_chunk::1>>
|> :binary.decode_unsigned(:little)
|> :binary.encode_unsigned(:big)
and decode from an incoming chunk:
<<chunk::31, is_first_chunk::1>> =
chunk_x
|> :binary.decode_unsigned(:big)
|> :binary.encode_unsigned(:little)
I really appreciate it, thank you.
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