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//Write your own contracts here. Currently compiles using solc v0.4.15+commit.bbb8e64f.
pragma solidity ^0.7.0;

contract Playground {

function compare(string _s1, string _s2) 
            pure
            public
            returns(bool) {
              bytes memory b1 = bytes(_s1);
              bytes memory b2 = bytes(_s2);
              if (b1.length != b2.length) {
                return false;
              }
              return keccak256(b1) == keccak256(b2);
  }

function concat(string _s1, string _s2) 
            pure 
            public 
            returns(string) {
              assembly {
                    invalid()
                }
  }

function strLen(string _str) 
            pure 
            public 
            returns(uint) {
    
    bytes memory b = bytes(_str);
    return (b.length);

}

function strReplace(string _from, string _what, string _to)
            pure
            public
            returns(string) {
              bytes memory bfrom = bytes(_from);
              bytes memory bwhat = bytes(_what);
              bytes memory bto = bytes(_to);
              require(bwhat.length == 1 && bto.length == 1);
              for(uint256 i = 0; i < bfrom.length; ++i) {
                if (bfrom[i] == bwhat[0]) {
                  bfrom[i] = bto[0];
                }
              }
              return string(bfrom);

}

function substring(string _from, uint _fromIndex, uint _count) 
            pure
            public returns(string) {
              bytes memory bfrom = bytes(_from);
              require(bfrom.length > 0);
              require(_count > 0 && _count + _fromIndex <= bfrom.length);
              require(_fromIndex >= 0 && _fromIndex < bfrom.length);
              uint endIndex = _fromIndex + _count;
              bytes memory result = new bytes(_fromIndex + _count - 1);
              
              for(uint i = _fromIndex; i < endIndex; ++i) {
                result[i - _fromIndex] = bfrom[i];
              }
              return string(result);
  }

}