Amount: split out of Structures.kt and into a separate file, add operator overloads.

src/Amounts.kt

@@ -0,0 +1,49 @@
+import java.math.BigDecimal
+import java.util.*
+import kotlin.math.div
+
+/**
+ * Amount represents a positive quantity of currency, measured in pennies, which are the smallest representable units.
+ * Note that "pennies" are not necessarily 1/100ths of a currency unit, but are the actual smallest amount used in
+ * whatever currency the amount represents.
+ *
+ * Amounts of different currencies *do not mix* and attempting to add or subtract two amounts of different currencies
+ * will throw [IllegalArgumentException]. Amounts may not be negative.
+ */
+data class Amount(val pennies: Int, val currency: Currency) : Comparable<Amount> {
+    init {
+        // Negative amounts are of course a vital part of any ledger, but negative values are only valid in certain
+        // contexts: you cannot send a negative amount of cash, but you can (sometimes) have a negative balance.
+        require(pennies >= 0) { "Negative amounts are not allowed: $pennies" }
+    }
+
+    operator fun plus(other: Amount): Amount {
+        checkCurrency(other)
+        return Amount(pennies + other.pennies, currency)
+    }
+
+    operator fun minus(other: Amount): Amount {
+        checkCurrency(other)
+        return Amount(pennies - other.pennies, currency)
+    }
+
+    private fun checkCurrency(other: Amount) {
+        require(other.currency == currency) { "Currency mismatch: ${other.currency} vs $currency" }
+    }
+
+    operator fun div(other: Int): Amount = Amount(pennies / other, currency)
+    operator fun times(other: Int): Amount = Amount(pennies * other, currency)
+
+    override fun toString(): String = currency.currencyCode + " " + (BigDecimal(pennies) / BigDecimal(100)).toPlainString()
+
+    override fun compareTo(other: Amount): Int {
+        checkCurrency(other)
+        return pennies.compareTo(other.pennies)
+    }
+}
+
+// Note: this will throw an exception if the iterable is empty.
+fun Iterable<Amount>.sum() = reduce { left, right -> left + right }
+fun Iterable<Amount>.sumOrZero(currency: Currency) = if (iterator().hasNext()) sum() else Amount(0, currency)
+
+// TODO: Think about how positive-only vs positive-or-negative amounts can be represented in the type system.

src/Structures.kt

@@ -2,25 +2,6 @@ import java.security.PublicKey
 import java.security.Timestamp
 import java.util.*
 
-data class Amount(val pennies: Int, val currency: Currency) {
-    init {
-        // Negative amounts are of course a vital part of any ledger, but negative values are only valid in certain
-        // contexts: you cannot send a negative amount of cash, but you can (sometimes) have a negative balance.
-        // TODO: Think about how positive-only vs positive-or-negative amounts can be represented in the type system.
-        require(pennies >= 0) { "Negative amounts are not allowed: $pennies" }
-    }
-
-    operator fun plus(other: Amount): Amount {
-        require(other.currency == currency)
-        return Amount(pennies + other.pennies, currency)
-    }
-
-    operator fun minus(other: Amount): Amount {
-        require(other.currency == currency)
-        return Amount(pennies - other.pennies, currency)
-    }
-}
-
 /**
  * A contract state (or just "state") contains opaque data used by a contract program. It can be thought of as a disk
  * file that the program can use to persist data across transactions. States are immutable: once created they are never