[Ethereum] Casting a uint to a smaller uint

solidity

I'm looking through a contract and it casts a blockhash into a uint32, then divides by 42949673 in order to attempt to get a random number between 0 and 100. I assume a blockhash is equivalent to a uint256, but would casting into a uint32 take the most significant 32 bits or the least significant 32 bits? It would make the number smaller either way, but is it chopping off the larger amount or effectively dividing by enough to make it fit into a uint32?

I ask because every block I look at has the least significant bits as zeros, after being converted into binary. I haven't looked at many, but none of the blocks I have looked at have had anything other than a stream of zeros in the least significant bits. On top of this, the contract has generated a low number each time I've rolled, which again isn't a whole lot but the odds that this happens at random is decreasing with each roll. Is this as ineffective at generating a random number as it seems, or am I just beating the odds?

Best Answer

I didn't get what do you try to do. the blockhash is a bytes32. However to cast uint256 into uint32 use the following function 

function convert (uint256 _a) returns (uint32) 
{
    return uint32(_a);
}

In your case:

function get_hash_block_uint (uint256 _a) public returns (uint32) 
{
    return uint32(block.blockhash(_a));
}

Results:

  • bytes32: 0x97f5d27188791a619a9a12388f67169476222e646acde42ce6f1c5ef57db05bd"

  • uint32 after casting: "0x0000000000000000000000000000000000000000000000000000000057db05bd"

So you could conclude that the conversion preserves the least significant 4 bytes.

Update Solidity ^0.5.0

The blockhash function is no longer part of the block namespace in solidity versions 5 and above:

function get_hash_block_uint (uint256 _a) public returns (uint32) 
{
    return uint32(blockhash(_a));
}
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