How to concatenate bytes in Crystal

Question

I'm testing about serialization with bytes or slices, just learning and trying. I would like to bind 3 parameters in a single 10 bytes field, but I don't now how to concatenate them in Crystal or whether it is possible. I know I can achieve this by creating arrays or tuples, but I want to try whether it is possible to mix the parameters into a single buffer.

For instance, I want a self-descriptive binary record ID mixing 3 parameters:

Type (UInt8) | Category (UInt8) | Microseconds (UInt64) = Total 80 bits - 10 bytes

type = 1_u8 # 1 byte
categ = 4_u8 # 1 byte
usec = 1527987703211000_u64 # 8 bytes (Epoch)

How do I concatenate all this variables into a continuous 10 bytes buffer?

I want to retrieve the data by the index, like:

type = buff[0,1]
categ = buff[1,1]
usec = buff[2,8].to_u64 # (Actually not possible)

Answer 1

typ = 1_u8 # 1 byte
categ = 4_u8 # 1 byte
usec = 1527987703211000_u64 # 8 bytes (Epoch)

FORMAT = IO::ByteFormat::LittleEndian

io = IO::Memory.new(10)  # Specifying the capacity is optional

io.write_bytes(typ, FORMAT)  # Specifying the format is optional
io.write_bytes(categ, FORMAT)
io.write_bytes(usec, FORMAT)

buf = io.to_slice
puts buf

# --------

io2 = IO::Memory.new(buf)

typ2 = io2.read_bytes(UInt8, FORMAT)
categ2 = io2.read_bytes(UInt8, FORMAT)
usec2 = io2.read_bytes(UInt64, FORMAT)

pp typ2, categ2, usec2

Bytes[1, 4, 248, 99, 69, 92, 178, 109, 5, 0]
typ2   # => 1_u8
categ2 # => 4_u8
usec2  # => 1527987703211000_u64

This shows an example tailored to your use case, but IO::Memory should be used for "concatenating bytes" in general -- just write to it.