wip: use unchecked addition where overflow is not possible

Cargo.toml

@@ -15,7 +15,7 @@ keywords = ["crypto", "bitcoin"]
 readme = "README.md"
 
 [dependencies]
-bitcoin = { git="https://github.com/yancyribbens/rust-bitcoin", branch = "tmp/mark-txin-base-weight-public" }
+bitcoin = { git="https://github.com/yancyribbens/rust-bitcoin", rev="254fafdf71b30ae9156b43d57b336cf4a251ed33" }
 rand = {version = "0.8.5", default-features = false, optional = true}
 
 [dev-dependencies]
@@ -24,10 +24,10 @@ rust-bitcoin-coin-selection = {path = ".", features = ["rand"]}
 rand = "0.8.5"
 
 [patch.crates-io]
-bitcoin_hashes = { git = "https://github.com/yancyribbens/rust-bitcoin", branch = "tmp/mark-txin-base-weight-public" }
-bitcoin-io = { git = "https://github.com/yancyribbens/rust-bitcoin", branch = "tmp/mark-txin-base-weight-public" }
-bitcoin-units = { git = "https://github.com/yancyribbens/rust-bitcoin", branch = "tmp/mark-txin-base-weight-public" }
-bitcoin-internals = { git = "https://github.com/yancyribbens/rust-bitcoin", branch = "tmp/mark-txin-base-weight-public" }
+bitcoin_hashes = { git = "https://github.com/yancyribbens/rust-bitcoin", rev="254fafdf71b30ae9156b43d57b336cf4a251ed33" }
+bitcoin-io = { git = "https://github.com/yancyribbens/rust-bitcoin", rev="254fafdf71b30ae9156b43d57b336cf4a251ed33" }
+bitcoin-units = { git = "https://github.com/yancyribbens/rust-bitcoin", rev="254fafdf71b30ae9156b43d57b336cf4a251ed33" }
+bitcoin-internals = { git = "https://github.com/yancyribbens/rust-bitcoin", rev="254fafdf71b30ae9156b43d57b336cf4a251ed33" }
 
 [[bench]]
 name = "coin_selection"

src/branch_and_bound.rs

@@ -214,7 +214,11 @@ pub fn select_coins_bnb(
 
         // * not enough value to make it to the target.
         //   Therefore, explore a new new subtree.
-        if available_value + value < target {
+        //
+        // unchecked_add is used here for performance.  Before entering the search loop, all
+        // utxos are summed and checked for overflow.  Since there was no overflow then, any
+        // subset of addition will not overflow.
+        if available_value.unchecked_add(value) < target {
             backtrack_subtree = true;
         }
         // This optimization provides an upper bound on the amount of waste that is acceptable.
@@ -233,7 +237,9 @@ pub fn select_coins_bnb(
         // it's high fee_rate environment.  During low fee environments, a utxo may
         // have negative waste, therefore adding more utxos in such an environment
         // may still result in reduced waste.
-        else if value > target + cost_of_change
+        else if value > target.unchecked_add(cost_of_change) //TODO we can check this for overflow
+                                                             //before the loop begins by adding
+                                                             //target to utxo_pool sum.
             || current_waste > best_waste && fee_rate > long_term_fee_rate
         {
             backtrack = true;
@@ -285,9 +291,11 @@ pub fn select_coins_bnb(
 
             // The available value of the next iteration.  This should never overflow
             // since the value is always less than the last available_value calculation.
+            // TODO - can this be use sum() in Amount instead.
+            // TODO - Amount should have unchecked_sum() for this case.
             available_value =
                 w_utxos[index + 1..].iter().fold(Amount::ZERO, |mut s, &(v, _, _)| {
-                    s += v;
+                    s = s.unchecked_add(v);
                     s
                 });
 
@@ -306,7 +314,10 @@ pub fn select_coins_bnb(
             current_waste += utxo_waste;
 
             index_selection.push(index);
-            value += eff_value;
+
+            // unchecked add is used her for performance.  since the sum of all utxo values
+            // did not overflow, then any positive subset of the sum will not overflow.
+            value = value.unchecked_add(eff_value);
 
             available_value -= eff_value;
         }