feat: yfi buyback

contracts/Buyback.vy

@@ -0,0 +1,165 @@
+# @version 0.3.10
+"""
+@title YFI Buyer
+@license MIT
+@author banteg, 0xkorin
+@notice
+    Buy YFI for ETH at the current Chainlink price.
+"""
+from vyper.interfaces import ERC20
+
+YFI: constant(address) = 0x0bc529c00C6401aEF6D220BE8C6Ea1667F6Ad93e
+YFI_ETH: constant(address) = 0x3EbEACa272Ce4f60E800f6C5EE678f50D2882fd4
+
+STALE_AFTER: constant(uint256) = 3600
+EPOCH_LENGTH: constant(uint256) = 14 * 24 * 60 * 60
+PRICE_SCALE: constant(uint256) = 10**18
+
+admin: public(address)
+treasury: public(address)
+
+available: public(uint256)
+epoch_end: public(uint256)
+last_update: public(uint256)
+rate: public(uint256)
+
+struct ChainlinkRound:
+    roundId: uint80
+    answer: int256
+    startedAt: uint256
+    updatedAt: uint256
+    answeredInRound: uint80
+
+interface Chainlink:
+    def latestRoundData() -> ChainlinkRound: view
+
+event Buyback:
+    buyer: indexed(address)
+    yfi: uint256
+    eth: uint256
+
+event UpdateAdmin:
+    admin: indexed(address)
+
+event UpdateTreasury:
+    treasury: indexed(address)
+
+
+@external
+def __init__(_epoch_end: uint256):
+    assert _epoch_end > block.timestamp
+
+    self.admin = msg.sender
+    self.treasury = msg.sender
+    self.last_update = block.timestamp
+    self.epoch_end = _epoch_end
+
+    log UpdateAdmin(msg.sender)
+    log UpdateTreasury(msg.sender)
+
+
+@external
+@payable
+@nonreentrant("buy")
+def __default__():
+    self._update()
+
+
+@external
+@nonreentrant("buy")
+def buy_eth(yfi_amount: uint256, receiver: address = msg.sender):
+    self._update()
+
+    oracle: ChainlinkRound = Chainlink(YFI_ETH).latestRoundData()
+    assert oracle.updatedAt + STALE_AFTER > block.timestamp  # dev: stale oracle
+
+    eth_amount: uint256 = convert(oracle.answer, uint256) * yfi_amount / PRICE_SCALE
+    self.available -= eth_amount
+
+    assert ERC20(YFI).transferFrom(msg.sender, self.treasury, yfi_amount, default_return_value=True) # dev: no allowance
+    raw_call(receiver, b"", value=eth_amount)
+
+    log Buyback(msg.sender, yfi_amount, eth_amount)
+
+
+@view
+@external
+def price() -> uint256:
+    oracle: ChainlinkRound = Chainlink(YFI_ETH).latestRoundData()
+    return convert(oracle.answer, uint256)
+
+
+@view
+@external
+def total_eth() -> uint256:
+    return self._available(0)[0]
+
+
+@view
+@external
+def max_amount() -> uint256:
+    available: uint256 = self._available(0)[0]
+    oracle: ChainlinkRound = Chainlink(YFI_ETH).latestRoundData()
+    return available * PRICE_SCALE / convert(oracle.answer, uint256)
+
+
+@internal
+@payable
+def _update():
+    self.available, self.rate, self.epoch_end = self._available(msg.value)
+    self.last_update = block.timestamp
+
+
+@internal
+@view
+def _available(_value: uint256) -> (uint256, uint256, uint256):
+    ts: uint256 = block.timestamp
+    end: uint256 = self.epoch_end
+    if block.timestamp > end:
+        ts = end
+    available: uint256 = self.available
+    rate: uint256 = self.rate
+    available += (ts - self.last_update) * rate
+
+    if block.timestamp >= end:
+        amount: uint256 = self.balance - _value - available
+        epochs: uint256 = (end - block.timestamp) / EPOCH_LENGTH
+        end += (epochs + 1) * EPOCH_LENGTH
+        if epochs == 0:
+            rate = amount / EPOCH_LENGTH
+            available += (block.timestamp + EPOCH_LENGTH - end) * rate
+        else:
+            rate = 0
+            available += amount
+
+    return available, rate, end
+
+
+@external
+def sweep(token: address, amount: uint256 = max_value(uint256)):
+    assert msg.sender == self.admin
+    value: uint256 = amount
+    if token == empty(address):
+        if value == max_value(uint256):
+            value = self.balance
+        raw_call(self.admin, b"", value=value)
+    else:
+        if value == max_value(uint256):
+            value = ERC20(token).balanceOf(self)
+        assert ERC20(token).transfer(self.admin, value, default_return_value=True)
+
+
+@external
+def set_admin(proposed_admin: address):
+    assert msg.sender == self.admin
+    self.admin = proposed_admin
+
+    log UpdateAdmin(proposed_admin)
+
+
+@external
+def set_treasury(new_treasury: address):
+    assert msg.sender == self.admin
+    self.treasury = new_treasury
+
+    log UpdateTreasury(new_treasury)