coinchooser: improve performance significantly

existing code was n^2 in number of UTXOs
this is now mostly linear
(linear if shortcut is hit; otherwise in rare cases still quadratic)

tested using wallet with 800 UTXOs, most of which were needed to make payment
coinchooser.make_tx() went from 18 sec to 0.8 sec

electrum/coinchooser.py

@@ -80,12 +80,17 @@ class Bucket(NamedTuple):
 
 def strip_unneeded(bkts, sufficient_funds):
     '''Remove buckets that are unnecessary in achieving the spend amount'''
-    bkts = sorted(bkts, key = lambda bkt: bkt.value)
+    if sufficient_funds([], bucket_value_sum=0):
+        # none of the buckets are needed
+        return []
+    bkts = sorted(bkts, key=lambda bkt: bkt.value, reverse=True)
+    bucket_value_sum = 0
     for i in range(len(bkts)):
-        if not sufficient_funds(bkts[i + 1:]):
-            return bkts[i:]
-    # none of the buckets are needed
-    return []
+        bucket_value_sum += (bkts[i]).value
+        if sufficient_funds(bkts[:i+1], bucket_value_sum=bucket_value_sum):
+            return bkts[:i+1]
+    raise Exception("keeping all buckets is still not enough")
+
 
 class CoinChooserBase(PrintError):
 
@@ -230,10 +235,15 @@ class CoinChooserBase(PrintError):
 
             return total_weight
 
-        def sufficient_funds(buckets):
+        def sufficient_funds(buckets, *, bucket_value_sum):
             '''Given a list of buckets, return True if it has enough
             value to pay for the transaction'''
-            total_input = input_value + sum(bucket.value for bucket in buckets)
+            # assert bucket_value_sum == sum(bucket.value for bucket in buckets)  # expensive!
+            total_input = input_value + bucket_value_sum
+            if total_input < spent_amount:  # shortcut for performance
+                return False
+            # note re performance: so far this was constant time
+            # what follows is linear in len(buckets)
             total_weight = get_tx_weight(buckets)
             return total_input >= spent_amount + fee_estimator_w(total_weight)
 
@@ -278,7 +288,7 @@ class CoinChooserRandom(CoinChooserBase):
 
         # Add all singletons
         for n, bucket in enumerate(buckets):
-            if sufficient_funds([bucket]):
+            if sufficient_funds([bucket], bucket_value_sum=bucket.value):
                 candidates.add((n, ))
 
         # And now some random ones
@@ -289,9 +299,12 @@ class CoinChooserRandom(CoinChooserBase):
             # incrementally combine buckets until sufficient
             self.p.shuffle(permutation)
             bkts = []
+            bucket_value_sum = 0
             for count, index in enumerate(permutation):
-                bkts.append(buckets[index])
-                if sufficient_funds(bkts):
+                bucket = buckets[index]
+                bkts.append(bucket)
+                bucket_value_sum += bucket.value
+                if sufficient_funds(bkts, bucket_value_sum=bucket_value_sum):
                     candidates.add(tuple(sorted(permutation[:count + 1])))
                     break
             else:
@@ -320,16 +333,20 @@ class CoinChooserRandom(CoinChooserBase):
 
         bucket_sets = [conf_buckets, unconf_buckets, other_buckets]
         already_selected_buckets = []
+        already_selected_buckets_value_sum = 0
 
         for bkts_choose_from in bucket_sets:
             try:
-                def sfunds(bkts):
-                    return sufficient_funds(already_selected_buckets + bkts)
+                def sfunds(bkts, *, bucket_value_sum):
+                    bucket_value_sum += already_selected_buckets_value_sum
+                    return sufficient_funds(already_selected_buckets + bkts,
+                                            bucket_value_sum=bucket_value_sum)
 
                 candidates = self.bucket_candidates_any(bkts_choose_from, sfunds)
                 break
             except NotEnoughFunds:
                 already_selected_buckets += bkts_choose_from
+                already_selected_buckets_value_sum += sum(bucket.value for bucket in bkts_choose_from)
         else:
             raise NotEnoughFunds()