Stamp and LTV

project/build.properties

@@ -1 +1 @@
-sbt.version=1.9.2
+sbt.version=1.9.9

project/plugins.sbt

@@ -1 +1 @@
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.13.2")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.14.0")

src/main/scala/example/Main.scala

@@ -7,6 +7,9 @@ import org.scalajs.dom
 import scala.scalajs.js.annotation.JSImport
 
 import scalajs.js
+import java.text.DecimalFormat
+import org.scalajs.dom.intl.NumberFormat
+import com.raquo.airstream.core.Observable
 
 trait ChartDataset extends js.Any:
   var label: String
@@ -42,10 +45,45 @@ case class Settings(
     val effectiveMonthlyRate = rate / 100 / 12
     val thing                = math.pow(1 + effectiveMonthlyRate, months)
     loanAmount * effectiveMonthlyRate * thing / (thing - 1)
+
+  def stampDuty: Double =
+    val firstTimeBuyerRelief = housePrice <= 625_000
+    val bands =
+      if !firstTimeBuyerRelief then
+        List(
+          250_000     -> 0.0,
+          925_000     -> 0.05,
+          1_500_000   -> 0.10,
+          100_000_000 -> 0.12
+        )
+      else List(425_000 -> 0.0, 625_000 -> 0.05)
+
+    val augmentedBands =
+      bands
+        .prepended(0 -> 0.0)
+        .zip(bands)
+        .map:
+          case ((st, _), (ed, rate)) =>
+            (st, ed) -> rate
+
+    augmentedBands
+      .map:
+        case ((from, to), rate) =>
+          if from < housePrice then
+            val length   = to - from
+            val baseline = (housePrice - from)
+            if baseline > length then length * rate
+            else baseline * rate
+          else 0.0
+      .sum
+
+  end stampDuty
+
+  def LTVPercent = 100 * (loanAmount / housePrice)
 end Settings
 
 def app =
-  val housePrice    = Var(400_000.0)
+  val housePrice    = Var(800_000.0)
   val deposit       = Var(50_000.0)
   val durationYears = Var(25)
   val rate          = Var(6.0)
@@ -55,8 +93,12 @@ def app =
   val calculation =
     combined.map(_.monthlyPayment)
 
-  inline def poundValue(source: Source[Double]) =
-    span(color.maroon, child.text <-- source.toObservable.map(d => f"£$d%.2f"))
+  val numberFormat = NumberFormat("en-GB")
+
+  inline def poundValue(source: Observable[Double]) =
+    val nn =
+      source.toObservable.map(value => "£" + numberFormat.format(value.floor))
+    span(color.maroon, child.text <-- nn)
 
   val sliders = div(
     width := "50%",
@@ -71,7 +113,7 @@ def app =
       _.showTickmarks := true,
       _.events.onInput.map(_.target.value) --> rate
     ),
-    h1("House price: ", poundValue(housePrice)),
+    h1("House price: ", poundValue(housePrice.signal)),
     Slider(
       _.min           := 100_000,
       _.max           := 1_200_000,
@@ -82,7 +124,7 @@ def app =
       _.showTickmarks := true,
       _.events.onInput.map(_.target.value) --> housePrice
     ),
-    h1("Deposit: ", poundValue(deposit)),
+    h1("Deposit: ", poundValue(deposit.signal)),
     Slider(
       _.min           := 10_000,
       _.max           := 500_000,
@@ -117,6 +159,14 @@ def app =
   )
   val chart = div(
     width := "50%",
+    h2(
+      b("LTV: "),
+      child.text <-- combined.map(_.LTVPercent.toString() + "%")
+    ),
+    h2(
+      b("Stamp duty: "),
+      poundValue(combined.map(_.stampDuty))
+    ),
     canvasTag( // onMountUnmount callback to bridge the Laminar world and the Chart.js world
       onMountUnmountCallback(
         // on mount, create the `Chart` instance and store it in optChart
@@ -149,6 +199,7 @@ def app =
       }
     )
   )
+
   div(
     h1(
       "The lolno calculator - why you will never afford a mortgage in the UK"