Allow call option writers to bid on spread

src/HookCoveredCallImplV1.sol

@@ -262,26 +262,55 @@ contract HookCoveredCallImplV1 is
     uint256 bidAmt = msg.value;
     CallOption storage call = optionParams[optionId];
 
-    // TODO(HOOK-805) allow option writer to make a bid with an deposited amount equal
-    // to the spread between the their bid and the option's strike price.
+    if (msg.sender == call.writer) {
+      _writerBid(call, optionId);
+    } else {
+      _generalBid(call);
+    }
+
+    // the new high bidder is the beneficial owner of the asset.
+    // The beneficial owner must be set here instead of with a final bid
+    // because the ability to
+    IHookERC721Vault(call.vaultAddress).setBeneficialOwner(msg.sender);
+
+    // emit event
+    emit Bid(optionId, bidAmt, msg.sender);
+  }
+
+  function _writerBid(CallOption storage call, uint256 optionId) internal {
+    uint256 bidAmt = msg.value + call.strike;
 
     require(bidAmt > call.bid, "bid - bid is lower than the current bid");
     require(bidAmt > call.strike, "bid - bid is lower than the strike price");
 
-    // return current bidder's money
-    _safeTransferETHWithFallback(call.highBidder, call.bid);
+    _returnBidToPreviousBidder(call);
 
     // set the new bidder
     call.bid = bidAmt;
     call.highBidder = msg.sender;
+  }
 
-    // the new high bidder is the beneficial owner of the asset.
-    // The beneficial owner must be set here instead of with a final bid
-    // because the ability to
-    IHookERC721Vault(call.vaultAddress).setBeneficialOwner(msg.sender);
+  function _generalBid(CallOption storage call) internal {
+    uint256 bidAmt = msg.value;
 
-    // emit event
-    emit Bid(optionId, bidAmt, msg.sender);
+    require(bidAmt > call.bid, "bid - bid is lower than the current bid");
+    require(bidAmt > call.strike, "bid - bid is lower than the strike price");
+
+    _returnBidToPreviousBidder(call);
+
+    // set the new bidder
+    call.bid = bidAmt;
+    call.highBidder = msg.sender;
+  }
+
+  function _returnBidToPreviousBidder(CallOption storage call) internal {
+    uint256 unnormalizedHighBid = call.bid;
+    if (call.highBidder == call.writer) {
+        unnormalizedHighBid -= call.strike;
+    }
+
+    // return current bidder's money
+    _safeTransferETHWithFallback(call.highBidder, unnormalizedHighBid);
   }
 
   /**
@@ -338,12 +367,15 @@ contract HookCoveredCallImplV1 is
 
     uint256 spread = call.bid - call.strike;
 
+    // If the option writer is the high bidder they don't recieve the strike because they bid on the spread.
+    if (call.highBidder != call.writer) {
+      // send option writer the strike price
+      _safeTransferETHWithFallback(call.writer, call.strike);
+    }
+
     // return send option holder their earnings
     _safeTransferETHWithFallback(ownerOf(optionId), spread);
 
-    // send option writer the strike price
-    _safeTransferETHWithFallback(call.writer, call.strike);
-
     if (returnNft) {
       IHookERC721Vault(call.vaultAddress).withdrawalAsset();
     }