Add largestFirst coin selection algorithm

coinlib/lib/src/tx/coin_selection.dart

@@ -1,6 +1,7 @@
 import 'package:coinlib/src/common/serial.dart';
 import 'package:coinlib/src/crypto/random.dart';
 import 'package:coinlib/src/scripts/program.dart';
+import 'package:collection/collection.dart';
 import 'inputs/input.dart';
 import 'output.dart';
 import 'transaction.dart';
@@ -122,6 +123,49 @@ class CoinSelection {
 
  }
 
+ /// A simple selection algorithm that selects inputs from the [candidates]
+ /// starting from the largest value until the required amount has been
+ /// reached and then the selected inputs are given a random order.
+ factory CoinSelection.largestFirst({
+ int version = Transaction.currentVersion,
+ required Iterable<InputCandidate> candidates,
+ required Iterable<Output> recipients,
+ required Program changeProgram,
+ required BigInt feePerKb,
+ required BigInt minFee,
+ required BigInt minChange,
+ int locktime = 0,
+ }) {
+
+ final sorted = candidates.toList().sorted(
+ (a, b) => b.value.compareTo(a.value),
+ );
+
+ CoinSelection trySelection(Iterable<InputCandidate> selected)
+ => CoinSelection(
+ version: version,
+ selected: selected,
+ recipients: recipients,
+ changeProgram: changeProgram,
+ feePerKb: feePerKb,
+ minFee: minFee,
+ minChange: minChange,
+ locktime: locktime,
+ );
+
+ CoinSelection selection = trySelection([]);
+ for (int i = 0; i < sorted.length; i++) {
+ selection = trySelection(sorted.take(i+1));
+ if (selection.enoughFunds) break;
+ }
+
+ // Randomising makes it less obvious this algo is being used
+ final selected = selection.selected.toList();
+ selected.shuffle();
+ return trySelection(selected);
+
+ }
+
  /// Obtains the transaction with selected inputs and outputs including any
  /// change at a random location, ready to be signed. Throws
  /// [InsufficientFunds] if there is not enough input value to meet the output
@@ -147,7 +191,7 @@ class CoinSelection {
  bool get changeless => changeValue.compareTo(BigInt.zero) == 0;
  /// True if the resulting fully signed transaction will be too large
  bool get tooLarge => signedSize > Transaction.maxSize;
- /// True if a signable solution have been found
+ /// True if a signable solution has been found
  bool get ready => enoughFunds && !tooLarge;
 
 }

coinlib/test/tx/coin_selection_test.dart

@@ -272,6 +272,40 @@ void main() {
 
  });
 
+ void expectSelectedValues(CoinSelection selection, List<int> values) => expect(
+ selection.selected.map((candidate) => candidate.value.toInt()),
+ unorderedEquals(values),
+ );
+
+ test(".largestFirst()", () {
+
+ final candidates = [coin*4, coin, coin*3, coin, coin*2];
+
+ void expectLargestFirst(List<int> selected, int outValue) {
+ final selection = CoinSelection.largestFirst(
+ version: 1234,
+ candidates: candidates.map((value) => candidateForValue(value)),
+ recipients: [outputForValue(outValue)],
+ changeProgram: changeProgram,
+ feePerKb: feePerKb, minFee: minFee, minChange: minChange,
+ locktime: 0xabcd1234,
+ );
+ expectSelectedValues(selection, selected);
+ expect(selection.version, 1234);
+ expect(selection.locktime, 0xabcd1234);
+ }
+
+ // Can cover with single largest
+ expectLargestFirst([coin*4], coin*3);
+ // Can cover with two
+ expectLargestFirst([coin*4, coin*3], coin*4);
+ // Need all
+ expectLargestFirst(candidates, coin*10);
+ // Select all, though they aren't enough
+ expectLargestFirst(candidates, coin*12);
+
+ });
+
  });
 
 }