handle the case where an author has insufficient REP to cover a negative citation

ethereum/contracts/core/Forum.sol

@@ -152,7 +152,17 @@ contract Forum is Reputation {
                 post.reputation -= uint(-amount);
             } else {
                 refundToInbound = int(post.reputation) + amount;
-                _update(post.author, address(this), post.reputation);
+                if (balanceOf(post.author) < post.reputation) {
+                    // If author has already lost reputation that was gained from this post,
+                    // that means other DAO members gained it through policing.
+                    // Here we preserve the sum of members REP but decrease the sum of posts REP.
+                    refundToInbound -= int(
+                        post.reputation - balanceOf(post.author)
+                    );
+                    _update(post.author, address(this), balanceOf(post.author));
+                } else {
+                    _update(post.author, address(this), post.reputation);
+                }
                 post.reputation = 0;
             }
         }

ethereum/test/Forum.js

@@ -233,5 +233,49 @@ describe('Forum', () => {
       expect((await dao.posts(0)).reputation).to.equal(50);
       expect(await dao.totalSupply()).to.equal(50);
     });
+
+    it('handles the case where an author has already lost reputation gained from a later-downvoted post', async () => {
+      await dao.addPost(account1, 'content-id', []);
+      await initiateValidationPool({ postIndex: 0 });
+      await dao.evaluateOutcome(0);
+      expect(await dao.balanceOf(account1)).to.equal(100);
+      expect(await dao.totalSupply()).to.equal(100);
+      expect((await dao.posts(0)).reputation).to.equal(100);
+
+      await dao.addPost(account2, 'second-content-id', []);
+      await initiateValidationPool({ postIndex: 1 });
+      await time.increase(POOL_DURATION + 1);
+      await dao.evaluateOutcome(1);
+      expect(await dao.balanceOf(account1)).to.equal(100);
+      expect(await dao.balanceOf(account2)).to.equal(100);
+      expect(await dao.totalSupply()).to.equal(200);
+      expect((await dao.posts(0)).reputation).to.equal(100);
+      expect((await dao.posts(1)).reputation).to.equal(100);
+
+      // account1 stakes and loses
+      await initiateValidationPool({ postIndex: 1 });
+      await dao.stakeOnValidationPool(2, 50, true);
+      await dao.connect(account2).stakeOnValidationPool(2, 60, false);
+      await time.increase(POOL_DURATION + 1);
+      await dao.evaluateOutcome(2);
+      expect(await dao.balanceOf(account1)).to.equal(50);
+      expect(await dao.balanceOf(account2)).to.equal(250);
+      expect(await dao.totalSupply()).to.equal(300);
+      expect((await dao.posts(0)).reputation).to.equal(100);
+      expect((await dao.posts(1)).reputation).to.equal(100);
+
+      // account1's post is later negatively referenced
+      await dao.addPost(account3, 'third-content-id', [{ weightPercent: -100, targetPostIndex: 0 }]);
+      await initiateValidationPool({ postIndex: 2, fee: 200 });
+      await time.increase(POOL_DURATION + 1);
+      await dao.evaluateOutcome(3);
+      expect(await dao.balanceOf(account1)).to.equal(0);
+      expect(await dao.balanceOf(account2)).to.equal(250);
+      expect(await dao.balanceOf(account3)).to.equal(250);
+      expect(await dao.totalSupply()).to.equal(500);
+      expect((await dao.posts(0)).reputation).to.equal(0);
+      expect((await dao.posts(1)).reputation).to.equal(100);
+      expect((await dao.posts(2)).reputation).to.equal(250);
+    });
   });
 });