lnrouter: change edge cost estimate (distance metric)

Old estimate was heavily biased towards simply minimising CLTV sum.
(fees had a too low weight; typically they were ~noise)
Now also take payment_amount into account.

electrum/lnrouter.py

@@ -131,7 +131,7 @@ class LNPathFinder(Logger):
     def _edge_cost(self, short_channel_id: bytes, start_node: bytes, end_node: bytes,
                    payment_amt_msat: int, ignore_costs=False, is_mine=False, *,
                    my_channels: Dict[ShortChannelID, 'Channel'] = None) -> Tuple[float, int]:
-        """Heuristic cost of going through a channel.
+        """Heuristic cost (distance metric) of going through a channel.
         Returns (heuristic_cost, fee_for_edge_msat).
         """
         channel_info = self.channel_db.get_channel_info(short_channel_id, my_channels=my_channels)
@@ -157,12 +157,20 @@ class LNPathFinder(Logger):
             return float('inf'), 0  # payment amount too large
         if not route_edge.is_sane_to_use(payment_amt_msat):
             return float('inf'), 0  # thanks but no thanks
-        fee_msat = route_edge.fee_for_edge(payment_amt_msat) if not ignore_costs else 0
-        # TODO revise
-        # paying 10 more satoshis ~ waiting one more block
-        fee_cost = fee_msat / 1000 / 10
-        cltv_cost = route_edge.cltv_expiry_delta if not ignore_costs else 0
-        return cltv_cost + fee_cost + 1, fee_msat
+
+        # Distance metric notes:  # TODO constants are ad-hoc
+        # ( somewhat based on https://github.com/lightningnetwork/lnd/pull/1358 )
+        # - Edges have a base cost. (more edges -> less likely none will fail)
+        # - The larger the payment amount, and the longer the CLTV,
+        #   the more irritating it is if the HTLC gets stuck.
+        # - Paying lower fees is better. :)
+        base_cost = 500  # one more edge ~ paying 500 msat more fees
+        if ignore_costs:
+            return base_cost, 0
+        fee_msat = route_edge.fee_for_edge(payment_amt_msat)
+        cltv_cost = route_edge.cltv_expiry_delta * payment_amt_msat * 15 / 1_000_000_000
+        overall_cost = base_cost + fee_msat + cltv_cost
+        return overall_cost, fee_msat
 
     @profiler
     def find_path_for_payment(self, nodeA: bytes, nodeB: bytes,

electrum/tests/test_lnrouter.py

@@ -96,9 +96,7 @@ class Test_LNRouter(TestCaseForTestnet):
         cdb.add_channel_update({'short_channel_id': bfh('0000000000000006'), 'message_flags': b'\x00', 'channel_flags': b'\x01', 'cltv_expiry_delta': o(10), 'htlc_minimum_msat': o(250), 'fee_base_msat': o(100), 'fee_proportional_millionths': o(150), 'chain_hash': BitcoinTestnet.rev_genesis_bytes(), 'timestamp': b'\x00\x00\x00\x00'})
         path = path_finder.find_path_for_payment(b'\x02aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', b'\x02eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee', 100000)
         self.assertEqual([(b'\x02bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb', b'\x00\x00\x00\x00\x00\x00\x00\x03'),
-                          (b'\x02cccccccccccccccccccccccccccccccc', b'\x00\x00\x00\x00\x00\x00\x00\x01'),
-                          (b'\x02dddddddddddddddddddddddddddddddd', b'\x00\x00\x00\x00\x00\x00\x00\x04'),
-                          (b'\x02eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee', b'\x00\x00\x00\x00\x00\x00\x00\x05')
+                          (b'\x02eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee', b'\x00\x00\x00\x00\x00\x00\x00\x02'),
                          ], path)
         start_node = b'\x02bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'
         route = path_finder.create_route_from_path(path, start_node)