Add burn and burnFrom (#2)

* burn, burnFrom

Co-authored-by: Brandon Iles <brandon@ampleforth.org>

Author: brandoniles

contracts/Forth.sol

@@ -131,6 +131,49 @@ contract Forth {
         _moveDelegates(address(0), delegates[dst], amount);
     }
 
+    /**
+     * @notice Destroys `amount` tokens from the caller
+     * @param rawAmount The number of tokens to burn
+     */
+    function burn(uint256 rawAmount) external {
+        uint96 amount = safe96(rawAmount, "Forth::burn: rawAmount exceeds 96 bits");
+        _burn(msg.sender, amount);
+    }
+
+    /**
+     * @notice Destroys `amount` tokens from `account`, deducting from the caller's allowance
+     * @param account The address of the account to burn from
+     * @param rawAmount The number of tokens to burn
+     */
+    function burnFrom(address account, uint256 rawAmount) external {
+        uint96 amount = safe96(rawAmount, "Forth::burnFrom: rawAmount exceeds 96 bits");
+
+        uint96 decreasedAllowance =
+            sub96(allowances[account][msg.sender], amount, "Forth::burnFrom: amount exceeds allowance");
+        allowances[account][msg.sender] = decreasedAllowance;
+        emit Approval(account, msg.sender, decreasedAllowance);
+
+        _burn(account, amount);
+    }
+
+    /**
+     * @notice Destroys `amount` tokens from `account`, reducing the total supply
+     * @param account The address of the account to burn from
+     * @param amount The number of tokens to burn
+     */
+    function _burn(address account, uint96 amount) internal {
+        require(account != address(0), "Forth::_burn: burn from the zero address");
+
+        uint96 supply = safe96(totalSupply, "Forth::_burn: old supply exceeds 96 bits");
+        totalSupply = sub96(supply, amount, "Forth::_burn: amount exceeds totalSupply");
+
+        balances[account] = sub96(balances[account], amount, "Forth::_burn: amount exceeds balance");
+        emit Transfer(account, address(0), amount);
+
+        // move delegates
+        _moveDelegates(delegates[account], address(0), amount);
+    }
+
     /**
      * @notice Get the number of tokens `spender` is approved to spend on behalf of `account`
      * @param account The address of the account holding the funds

test/Forth.spec.ts

@@ -126,4 +126,76 @@ describe('Forth', () => {
       'Forth::mint: exceeded mint cap'
     )
   })
+
+  it('burn', async () => {
+    const { timestamp: now } = await provider.getBlock('latest')
+    const forth = await deployContract(wallet, Forth, [wallet.address, wallet.address, now + 60 * 60])
+    const supply = await forth.totalSupply()
+
+    // burn 0
+    let balanceBefore = await forth.balanceOf(wallet.address)
+    await forth.connect(wallet).burn(0)
+    expect(await forth.balanceOf(wallet.address)).to.be.eq(balanceBefore)
+    expect(await forth.totalSupply()).to.be.eq(supply)
+
+    // burn non-zero
+    await forth.connect(wallet).burn(1)
+    expect(await forth.balanceOf(wallet.address)).to.be.eq(balanceBefore.sub(1))
+    expect(await forth.totalSupply()).to.be.eq(supply.sub(1))
+
+    // burn > totalSupply
+    await expect(forth.connect(wallet).burn(supply + 2)).to.be.revertedWith('Forth::_burn: amount exceeds totalSupply')
+
+    // burn > balance
+    await forth.connect(wallet).transfer(other0.address, 100)
+    balanceBefore = await forth.balanceOf(wallet.address)
+    await expect(forth.connect(wallet).burn(balanceBefore.add(1))).to.be.revertedWith(
+      'Forth::_burn: amount exceeds balance'
+    )
+  })
+
+  it('burnFrom', async () => {
+    const { timestamp: now } = await provider.getBlock('latest')
+    const forth = await deployContract(wallet, Forth, [wallet.address, wallet.address, now + 60 * 60])
+    const supply = await forth.totalSupply()
+
+    // burn 0
+    let balanceBefore = await forth.balanceOf(wallet.address)
+    await forth.connect(other0).burnFrom(wallet.address, 0)
+    expect(await forth.balanceOf(wallet.address)).to.be.eq(balanceBefore)
+    expect(await forth.totalSupply()).to.be.eq(supply)
+
+    // burn non-zero
+    await forth.connect(wallet).approve(other0.address, 100)
+    await forth.connect(other0).burnFrom(wallet.address, 1)
+    expect(await forth.balanceOf(wallet.address)).to.be.eq(balanceBefore.sub(1))
+    expect(await forth.totalSupply()).to.be.eq(supply.sub(1))
+
+    // burn > approval
+    balanceBefore = await forth.balanceOf(wallet.address)
+    await forth.connect(wallet).approve(other0.address, 100)
+    await expect(forth.connect(other0).burnFrom(wallet.address, 101)).to.be.revertedWith(
+      'Forth::burnFrom: amount exceeds allowance'
+    )
+
+    // burn > totalSupply
+    balanceBefore = await forth.balanceOf(wallet.address)
+    await forth.connect(wallet).approve(other0.address, balanceBefore.add(1))
+    await expect(forth.connect(other0).burnFrom(wallet.address, balanceBefore.add(1))).to.be.revertedWith(
+      'Forth::_burn: amount exceeds totalSupply'
+    )
+
+    // burn > balance
+    await forth.connect(wallet).transfer(other0.address, 100)
+    balanceBefore = await forth.balanceOf(wallet.address)
+    await forth.connect(wallet).approve(other0.address, balanceBefore.add(1))
+    await expect(forth.connect(other0).burnFrom(wallet.address, balanceBefore.add(1))).to.be.revertedWith(
+      'Forth::_burn: amount exceeds balance'
+    )
+
+    // Zero Address
+    await expect(forth.connect(wallet).burnFrom('0x0000000000000000000000000000000000000000', 0)).to.be.revertedWith(
+      'Forth::_burn: burn from the zero address'
+    )
+  })
 })