Finish first pass of migration contract

contracts/Migrator.sol

@@ -1,16 +1,38 @@
 pragma solidity 0.6.12;
 
+import "./uniswapv2/interfaces/IUniswapV2ERC20.sol";
+import "./uniswapv2/interfaces/IUniswapV2Pair.sol";
+import "./uniswapv2/interfaces/IUniswapV2Factory.sol";
+
 
 contract Migrator {
     address public chef;
     address public factory;
+    uint256 public notBeforeBlock;
+    uint256 public desiredLiquidity = uint256(-1);
 
-    constructor(address _chef) public {
+    constructor(address _chef, address _factory, uint256 _notBeforeBlock) public {
         chef = _chef;
+        factory = _factory;
+        notBeforeBlock = _notBeforeBlock;
     }
 
-    function migrate(address orig) public {
-        require(msg.sender == chef);
-        // TODO
+    function migrate(address orig) public returns (address) {
+        require(msg.sender == chef, "not the chef");
+        require(block.number >= notBeforeBlock, "too early to migrate");
+        address token0 = IUniswapV2Pair(orig).token0();
+        address token1 = IUniswapV2Pair(orig).token1();
+        address pair = IUniswapV2Factory(factory).getPair(token0, token1);
+        if (pair == address(0)) {
+            pair = IUniswapV2Factory(factory).createPair(token0, token1);
+        }
+        uint256 lp = IUniswapV2ERC20(orig).balanceOf(msg.sender);
+        if (lp == 0) return pair;
+        desiredLiquidity = lp;
+        IUniswapV2ERC20(orig).transferFrom(msg.sender, orig, lp);
+        IUniswapV2Pair(orig).burn(pair);
+        IUniswapV2Pair(pair).mint(msg.sender);
+        desiredLiquidity = uint256(-1);
+        return pair;
     }
 }

contracts/uniswapv2/UniswapV2Pair.sol

@@ -125,7 +125,7 @@ contract UniswapV2Pair is UniswapV2ERC20 {
             address migrator = IUniswapV2Factory(factory).migrator();
             if (msg.sender == migrator) {
                 liquidity = Migrator(migrator).desiredLiquidity();
-                require(liquidity > 0, "Bad desired liquidity");
+                require(liquidity > 0 && liquidity != uint256(-1), "Bad desired liquidity");
             } else {
                 require(migrator == address(0), "Must not have migrator");
                 liquidity = Math.sqrt(amount0.mul(amount1)).sub(MINIMUM_LIQUIDITY);

contracts/uniswapv2/interfaces/IUniswapV2ERC20.sol

@@ -0,0 +1,23 @@
+pragma solidity >=0.5.0;
+
+interface IUniswapV2ERC20 {
+    event Approval(address indexed owner, address indexed spender, uint value);
+    event Transfer(address indexed from, address indexed to, uint value);
+
+    function name() external pure returns (string memory);
+    function symbol() external pure returns (string memory);
+    function decimals() external pure returns (uint8);
+    function totalSupply() external view returns (uint);
+    function balanceOf(address owner) external view returns (uint);
+    function allowance(address owner, address spender) external view returns (uint);
+
+    function approve(address spender, uint value) external returns (bool);
+    function transfer(address to, uint value) external returns (bool);
+    function transferFrom(address from, address to, uint value) external returns (bool);
+
+    function DOMAIN_SEPARATOR() external view returns (bytes32);
+    function PERMIT_TYPEHASH() external pure returns (bytes32);
+    function nonces(address owner) external view returns (uint);
+
+    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external;
+}

contracts/uniswapv2/interfaces/IUniswapV2Pair.sol

@@ -0,0 +1,52 @@
+pragma solidity >=0.5.0;
+
+interface IUniswapV2Pair {
+    event Approval(address indexed owner, address indexed spender, uint value);
+    event Transfer(address indexed from, address indexed to, uint value);
+
+    function name() external pure returns (string memory);
+    function symbol() external pure returns (string memory);
+    function decimals() external pure returns (uint8);
+    function totalSupply() external view returns (uint);
+    function balanceOf(address owner) external view returns (uint);
+    function allowance(address owner, address spender) external view returns (uint);
+
+    function approve(address spender, uint value) external returns (bool);
+    function transfer(address to, uint value) external returns (bool);
+    function transferFrom(address from, address to, uint value) external returns (bool);
+
+    function DOMAIN_SEPARATOR() external view returns (bytes32);
+    function PERMIT_TYPEHASH() external pure returns (bytes32);
+    function nonces(address owner) external view returns (uint);
+
+    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external;
+
+    event Mint(address indexed sender, uint amount0, uint amount1);
+    event Burn(address indexed sender, uint amount0, uint amount1, address indexed to);
+    event Swap(
+        address indexed sender,
+        uint amount0In,
+        uint amount1In,
+        uint amount0Out,
+        uint amount1Out,
+        address indexed to
+    );
+    event Sync(uint112 reserve0, uint112 reserve1);
+
+    function MINIMUM_LIQUIDITY() external pure returns (uint);
+    function factory() external view returns (address);
+    function token0() external view returns (address);
+    function token1() external view returns (address);
+    function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);
+    function price0CumulativeLast() external view returns (uint);
+    function price1CumulativeLast() external view returns (uint);
+    function kLast() external view returns (uint);
+
+    function mint(address to) external returns (uint liquidity);
+    function burn(address to) external returns (uint amount0, uint amount1);
+    function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external;
+    function skim(address to) external;
+    function sync() external;
+
+    function initialize(address, address) external;
+}