Out of gas exception when calculating Keccak256 inline solidity assembly

Question

I am trying to write a simple function which should return me Keccak256 of an incoming uint using Solidity inline assembly function, however it returns me out of gas exception when trying to execute the function on remix, can someone help me fix the issue here ?

   pragma solidity ^0.4.18;


contract HashedTimelock {


        address sender = 0xca35b7d915458ef540ade6068dfe2f44e8fa733c;
        address receiver = 0x14723a09acff6d2a60dcdf7aa4aff308fddc160c;
        bytes32 hashlock = 0x731dc163f73d31d8c68f9917ce4ff967753939f70432973c04fd2c2a48148607; // sha-2 sha256 hash
        uint  timelock = 1522660821; // UNIX timestamp seconds - locked UNTIL this time

   function HashedTimelock() payable{}


    function withdraw(bytes32 _preimage)
        external returns(bytes32 sample)
    {
        uint  timenow = now;
        assembly {
            // first check if the pre image matches with internal hashlockMatches
            // check the timelock
            // use selfdestruct to transfer the funds
            if eq(1,lt(timenow, sload(timelock_slot))){
               let selfptr := mload(add(_preimage, 0x20))
               sample := keccak256(selfptr, 32)

            }
        }

    }



}

Answer 1

Here's the solution I came up with.

function withdraw(bytes32 _preimage)
    external view returns(bytes32 sample)
{
    uint  timenow = now;
    assembly {
        let freemem_pointer := mload(0x40)
        mstore(add(freemem_pointer,0x00), _preimage)
        sample := keccak256(freemem_pointer, 32)
        }

}

Firstly, I changed the function to a view because, for now, you don't change the contract's state.

Secondly, about the code itself, keccak256 has to be used on data in memory, according to the documentation here (you can find the same information in the yellow paper). So, we need to write the data in memory first. This post tells you more about where you can write it.

Once it's done, it's over. You just have to assign to sample the result of keccak256.

P-S : Why use assembly for this since the keccak256 function is implemented in Solidity already?