Perform sensible currency conversion (#1941)

It is kind of sad to see 1 GBP to be equal to 1 USD and also it looks misleading in the blotter.

client/jfx/src/main/kotlin/net/corda/client/jfx/model/ExchangeRateModel.kt

@@ -3,8 +3,12 @@ package net.corda.client.jfx.model
 import javafx.beans.property.SimpleObjectProperty
 import javafx.beans.value.ObservableValue
 import net.corda.core.contracts.Amount
+import net.corda.finance.CHF
+import net.corda.finance.EUR
+import net.corda.finance.GBP
+import net.corda.finance.USD
 import java.math.BigDecimal
-import java.math.RoundingMode
+import java.math.MathContext
 import java.util.*
 
 /**
@@ -24,9 +28,24 @@ abstract class ExchangeRate {
 /**
  * Default implementation of an exchange rate model, which uses a fixed exchange rate.
  */
+private val usdExchangeRates: Map<Currency, BigDecimal> = mapOf(
+        GBP to BigDecimal(1.31),
+        EUR to BigDecimal(1.18),
+        CHF to BigDecimal(1.01)
+)
+
+private fun safeFetchRate(currency: Currency) =
+        usdExchangeRates[currency] ?: throw IllegalArgumentException("No exchange rate for $currency")
+
 // TODO hook up an actual oracle
 class ExchangeRateModel {
     val exchangeRate: ObservableValue<ExchangeRate> = SimpleObjectProperty<ExchangeRate>(object : ExchangeRate() {
-        override fun rate(from: Currency, to: Currency) = BigDecimal.ONE
+        override fun rate(from: Currency, to: Currency): BigDecimal =
+                when {
+                    from == to -> BigDecimal.ONE
+                    USD == to -> safeFetchRate(from)
+                    USD == from -> BigDecimal.ONE.divide(safeFetchRate(to), MathContext.DECIMAL64)
+                    else -> safeFetchRate(from).divide(safeFetchRate(to), MathContext.DECIMAL64)
+                }
     })
 }

client/jfx/src/test/kotlin/net/corda/client/jfx/model/ExchangeRateModelTest.kt

@@ -0,0 +1,44 @@
+package net.corda.client.jfx.model
+
+import net.corda.core.contracts.Amount
+import net.corda.finance.CHF
+import net.corda.finance.GBP
+import net.corda.finance.RUB
+import net.corda.finance.USD
+import org.assertj.core.api.Assertions
+import org.junit.Test
+import java.math.BigDecimal
+import java.util.*
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class ExchangeRateModelTest {
+
+    companion object {
+        private val instance = ExchangeRateModel().exchangeRate.value
+
+        private fun assertEquals(one: Amount<Currency>, another: Amount<Currency>) {
+            assertEquals(one.token, another.token)
+            assertTrue("$one != $another", {(one.toDecimal() - another.toDecimal()).abs() < BigDecimal(0.01) })
+        }
+    }
+    @Test
+    fun `perform fx testing`() {
+        val tenSwissies = Amount(10, BigDecimal.ONE, CHF)
+        assertEquals(instance.exchangeAmount(tenSwissies, CHF), tenSwissies)
+
+        val tenSwissiesInUsd = Amount(101, BigDecimal.ONE.divide(BigDecimal.TEN), USD)
+        assertEquals(instance.exchangeAmount(tenSwissies, USD), tenSwissiesInUsd)
+
+        assertEquals(instance.exchangeAmount(tenSwissiesInUsd, CHF), tenSwissies)
+
+        val tenQuidInSwissies = Amount(1297, BigDecimal.ONE.divide(BigDecimal(100)), CHF)
+        val tenQuid = Amount(10, BigDecimal.ONE, GBP)
+        assertEquals(instance.exchangeAmount(tenQuid, CHF), tenQuidInSwissies)
+
+        assertEquals(instance.exchangeAmount(tenQuidInSwissies, GBP), tenQuid)
+
+        Assertions.assertThatThrownBy { instance.exchangeAmount(tenQuid, RUB) }.isInstanceOf(IllegalArgumentException::class.java)
+                .hasMessage("No exchange rate for RUB")
+    }
+}