get range of bytes into fixed-size type

[nmushegian]

Is there an idiomatic way to put the first 32 bytes of a bytes into a bytes32?

[chriseth]

There is no way yet. You can use this snippet:

contract c {
    function toBytes32(bytes data, uint index) returns (bytes32) {
        uint val;
        for (uint i = index; i < index + 32; i++)  {
            val *= 256;
            if (i < data.length)
                val |= uint8(data[i]);
        }
        return bytes32(val);
    }
}

[augustoteixeira]

Any news on this?

This would be very useful for the following rationale:

• accessing storage data is very expensive, while accessing contract code is very cheap;
• this gives a great gas advantage for the use of constants, because they live in the contract's code;
• value-type constants are not good for storing larger amounts of data;
• bytes32[20] constant is considered a constant of non-value type;
• pointers to data defined through bytes constant table = hex"398ba..." are not accessible in assembly (TypeError: Only direct number constants and references to such constants are supported by inline assembly.`);
• the solution of copying every byte through a loop reduces the gas gains obtained by avoiding storage.

Here are possible solutions:

• allow the code to get a range of bytes into fixed-size types, like this issue suggests;
• make bytes32[20] constants possible, like bytes constants currently are;
• make a pointer to a bytes constant accessible in assembly;

Is there a chance of any of this solutions be made available in the near future?

Is there another solution that I am not seeing?

[cameel]

If you mean the initial bytes like the original requester, this is now possible with a conversion. See #9170.

If you mean arbitrary slices, then not yet but it's on our roadmap and will be possible eventually. See #7423.