Solidity How to return id from hash value

Question

I have a solidity contract with a mapping table. I am mapping an id(uint) to hash values. I am able to return hash value from id but now I wanna do it the other way around, i.e., get id from hash value. Here's the code that I am using:

pragma solidity ^0.4.18;

contract Hash {
  bytes32 comphash;

  struct hashstruct {   
    bytes32 fhash;
  }

  mapping (uint => hashstruct) hashstructs;
  uint[] public hashAccts;

  function setinstructor(uint _uint,string _fhash) public {
    var a = hashstructs[_uint];

    a.fhash = sha256(_fhash);  

    hashAccts.push(_uint) -1;              
  }

  function getInstructor(uint ins) view public returns (bytes32) {
    return (hashstructs[ins].fhash);
  }

  function count() view public returns (uint) {
    return hashAccts.length;
  }            
}

How can i do it?

Answer 1

You can do it by storing the hash of your struct into another mapping like this:

mapping(bytes32 => uint) _map;

function setinstructor(uint _uint,string _fhash) public {
  var a = hashstructs[_uint];

  a.fhash = sha256(_fhash);  

  hashAccts.push(_uint) -1;              

  _map[keccak256(a.fhash)] = _uint; // Can pass in other struct members as well
}