Improve the contracts DSL, make the Cash contract extend the DSL in a small way to make working with arrays of cash outputs easier.

src/contracts/Cash.kt

@@ -1,3 +1,5 @@
+package contracts
+
 import core.*
 import java.security.PublicKey
 
@@ -11,7 +13,7 @@ val CASH_PROGRAM_ID = SecureHash.sha256("cash")
 /** A state representing a claim on the cash reserves of some institution */
 data class CashState(
     /** Where the underlying currency backing this ledger entry can be found (propagated) */
-    val deposit: DepositPointer,
+    val deposit: InstitutionReference,
 
     val amount: Amount,
 
@@ -35,6 +37,9 @@ class ExitCashCommand(val amount: Amount) : Command {
 
 class InsufficientBalanceException(val amountMissing: Amount) : Exception()
 
+// Small DSL extension.
+fun Iterable<ContractState>.sumCashBy(owner: PublicKey) = this.filterIsInstance<CashState>().filter { it.owner == owner }.map { it.amount }.sum()
+
 /**
  * A cash transaction may split and merge money represented by a set of (issuer, depositRef) pairs, across multiple
  * input and output states. Imagine a Bitcoin transaction but in which all UTXOs had a colour
@@ -50,7 +55,7 @@ class InsufficientBalanceException(val amountMissing: Amount) : Exception()
  */
 object CashContract : Contract {
     /** This is the function EVERYONE runs */
-    override fun verify(inStates: List<ContractState>, outStates: List<ContractState>, args: List<VerifiedSignedCommand>) {
+    override fun verify(inStates: List<ContractState>, outStates: List<ContractState>, args: List<VerifiedSigned<Command>>) {
         val cashInputs = inStates.filterIsInstance<CashState>()
 
         requireThat {
@@ -77,13 +82,8 @@ object CashContract : Contract {
             val inputAmount = inputs.map { it.amount }.sum()
             val outputAmount = outputs.map { it.amount }.sumOrZero(currency)
 
-            val issuerCommand = args.
-                    filter { it.signingInstitution == deposit.institution }.
-                    // TODO: this map+filterNotNull pattern will become a single function in the next Kotlin beta.
-                    map { it.command as? ExitCashCommand }.
-                    filterNotNull().
-                    singleOrNull()
-            val amountExitingLedger = issuerCommand?.amount ?: Amount(0, inputAmount.currency)
+            val issuerCommand = args.select<ExitCashCommand>(institution = deposit.institution).singleOrNull()
+            val amountExitingLedger = issuerCommand?.value?.amount ?: Amount(0, inputAmount.currency)
 
             requireThat {
                 "for deposit ${deposit.reference} at issuer ${deposit.institution.name} the amounts balance" by (inputAmount == outputAmount + amountExitingLedger)
@@ -96,7 +96,7 @@ object CashContract : Contract {
         // see a signature from each of those keys. The actual signatures have been verified against the transaction
         // data by the platform before execution.
         val owningPubKeys  = cashInputs.map  { it.owner }.toSortedSet()
-        val keysThatSigned = args.filter { it.command is MoveCashCommand }.map { it.signer }.toSortedSet()
+        val keysThatSigned = args.select<MoveCashCommand>().map { it.signer }.toSortedSet()
         requireThat {
             "the owning keys are the same as the signing keys" by (owningPubKeys == keysThatSigned)
         }
@@ -167,7 +167,7 @@ object CashContract : Contract {
         } else states
 
         // Finally, generate the commands. Pretend to sign here, real signatures aren't done yet.
-        val commands = keysUsed.map { VerifiedSignedCommand(it, null, MoveCashCommand()) }
+        val commands = keysUsed.map { VerifiedSigned(it, null, MoveCashCommand()) }
 
         return TransactionForTest(gathered.toArrayList(), outputs.toArrayList(), commands.toArrayList())
     }

src/core/ContractsDSL.kt

@@ -1,6 +1,7 @@
 package core
 
 import java.math.BigDecimal
+import java.security.PublicKey
 import java.util.*
 import kotlin.math.div
 
@@ -14,6 +15,17 @@ import kotlin.math.div
 
 // TODO: Look into replacing Currency and Amount with CurrencyUnit and MonetaryAmount from the javax.money API (JSR 354)
 
+// region Misc
+inline fun <reified T : Command> List<VerifiedSigned<Command>>.select(signer: PublicKey? = null, institution: Institution? = null) =
+        filter { it.value is T }.
+        filter { if (signer == null) true else signer == it.signer }.
+        filter { if (institution == null) true else institution == it.signingInstitution }.
+        map { VerifiedSigned<T>(it.signer, it.signingInstitution, it.value as T) }
+
+inline fun <reified T : Command> List<VerifiedSigned<Command>>.requireSingleCommand() = select<T>().single()
+
+// endregion
+
 // region Currencies
 ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 fun currency(code: String) = Currency.getInstance(code)
@@ -46,7 +58,6 @@ fun requireThat(body: Requirements.() -> Unit) {
 // endregion
 
 
-
 // region Amounts
 ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 

src/core/Crypto.kt

@@ -36,3 +36,10 @@ sealed class DigitalSignature(bits: ByteArray, val covering: Int) : OpaqueBytes(
     class LegallyIdentifiable(val signer: Institution, bits: ByteArray, covering: Int) : WithKey(signer.owningKey, bits, covering)
 }
 
+object NullPublicKey : PublicKey, Comparable<PublicKey> {
+    override fun getAlgorithm() = "NULL"
+    override fun getEncoded() = byteArrayOf(0)
+    override fun getFormat() = "NULL"
+    override fun compareTo(other: PublicKey): Int = if (other == NullPublicKey) 0 else -1
+    override fun toString() = "NULL_KEY"
+}

src/core/Structures.kt

@@ -76,11 +76,11 @@ data class SignedCommand(
 )
 
 /** Obtained from a [SignedCommand], deserialised and signature checked */
-data class VerifiedSignedCommand(
+data class VerifiedSigned<out T : Command>(
     val signer: PublicKey,
     /** If the public key was recognised, the looked up institution is available here, otherwise it's null */
     val signingInstitution: Institution?,
-    val command: Command
+    val value: T
 )
 
 /**
@@ -90,13 +90,13 @@ data class VerifiedSignedCommand(
  */
 interface Contract {
     /** Must throw an exception if there's a problem that should prevent state transition. */
-    fun verify(inStates: List<ContractState>, outStates: List<ContractState>, args: List<VerifiedSignedCommand>)
+    fun verify(inStates: List<ContractState>, outStates: List<ContractState>, args: List<VerifiedSigned<Command>>)
 }
 
 /**
- * Reference to some money being stored by an institution e.g. in a vault or (more likely) on their normal ledger.
- * The deposit reference is intended to be encrypted so it's meaningless to anyone other than the institution.
+ * Reference to something being stored or issued by an institution e.g. in a vault or (more likely) on their normal
+ * ledger. The reference is intended to be encrypted so it's meaningless to anyone other than the institution.
  */
-data class DepositPointer(val institution: Institution, val reference: OpaqueBytes) {
+data class InstitutionReference(val institution: Institution, val reference: OpaqueBytes) {
     override fun toString() = "${institution.name}$reference"
 }

src/core/TestUtils.kt

@@ -50,11 +50,11 @@ val TEST_KEYS_TO_CORP_MAP: Map<PublicKey, Institution> = mapOf(
 data class TransactionForTest(
     private val inStates: MutableList<ContractState> = arrayListOf(),
     private val outStates: MutableList<ContractState> = arrayListOf(),
-    private val args: MutableList<VerifiedSignedCommand> = arrayListOf()
+    private val args: MutableList<VerifiedSigned<Command>> = arrayListOf()
 ) {
     fun input(s: () -> ContractState) = inStates.add(s())
     fun output(s: () -> ContractState) = outStates.add(s())
-    fun arg(key: PublicKey, c: () -> Command) = args.add(VerifiedSignedCommand(key, TEST_KEYS_TO_CORP_MAP[key], c()))
+    fun arg(key: PublicKey, c: () -> Command) = args.add(VerifiedSigned(key, TEST_KEYS_TO_CORP_MAP[key], c()))
 
     infix fun Contract.`fails requirement`(msg: String) {
         try {

tests/contracts/CashTests.kt

@@ -1,3 +1,4 @@
+import contracts.*
 import core.*
 import org.junit.Test
 import kotlin.test.assertEquals
@@ -9,7 +10,7 @@ import kotlin.test.assertFailsWith
 
 class CashTests {
     val inState = CashState(
-            deposit = DepositPointer(MEGA_CORP, OpaqueBytes.of(1)),
+            deposit = InstitutionReference(MEGA_CORP, OpaqueBytes.of(1)),
             amount = 1000.DOLLARS,
             owner = DUMMY_PUBKEY_1
     )
@@ -222,10 +223,10 @@ class CashTests {
     val OUR_PUBKEY_1 = DUMMY_PUBKEY_1
     val THEIR_PUBKEY_1 = DUMMY_PUBKEY_2
     val WALLET = listOf(
-            CashState(DepositPointer(MEGA_CORP, OpaqueBytes.of(1)), 100.DOLLARS, OUR_PUBKEY_1),
-            CashState(DepositPointer(MEGA_CORP, OpaqueBytes.of(1)), 400.DOLLARS, OUR_PUBKEY_1),
-            CashState(DepositPointer(MINI_CORP, OpaqueBytes.of(1)), 80.DOLLARS, OUR_PUBKEY_1),
-            CashState(DepositPointer(MINI_CORP, OpaqueBytes.of(2)), 80.SWISS_FRANCS, OUR_PUBKEY_1)
+            CashState(InstitutionReference(MEGA_CORP, OpaqueBytes.of(1)), 100.DOLLARS, OUR_PUBKEY_1),
+            CashState(InstitutionReference(MEGA_CORP, OpaqueBytes.of(1)), 400.DOLLARS, OUR_PUBKEY_1),
+            CashState(InstitutionReference(MINI_CORP, OpaqueBytes.of(1)), 80.DOLLARS, OUR_PUBKEY_1),
+            CashState(InstitutionReference(MINI_CORP, OpaqueBytes.of(2)), 80.SWISS_FRANCS, OUR_PUBKEY_1)
     )
 
     @Test