engine displacement

3 files changed, 85 insertions, 9 deletions

diff --git a/main.go b/main.go
index aca48bb..d36ebc8 100644
--- a/main.go
+++ b/main.go
@@ -16,18 +16,17 @@ func check(err error) {
 	}
 }
 
+var (
+	displacement = volume{2000, cubicCentimetre}
+	// selectedVolumeUnit is used to index volumeUnitStrings.
+	selectedVolumeUnit = defaultVolumeUnitIndex
+)
+
 var engineSpeed = [numPoints]int32{2000, 3000, 4000, 5000, 6000, 7000}
 
 var volumetricEfficiency = [numPoints]int32{100, 100, 100, 100, 100, 100}
 
 var (
-	manifoldPressure [numPoints]pressure
-
-	// selectedPressureUnit is used to index pressureUnits
-	selectedPressureUnit int32
-)
-
-func init() {
 	manifoldPressure = [numPoints]pressure{
 		newPressure(),
 		newPressure(),
@@ -37,12 +36,13 @@ func init() {
 		newPressure(),
 	}
 
-	// selectedPressureUnit is used to index pressureUnitStrings
+	// selectedPressureUnit is used to index pressureUnitStrings.
 	selectedPressureUnit = defaultPressureUnitIndex
-}
+)
 
 func loop() {
 	g.SingleWindow().Layout(
+		engineDisplacementRow(),
 		g.Table().
 			Rows(
 				engineSpeedRow(),
diff --git a/ui.go b/ui.go
index 67f31ca..dd12b06 100644
--- a/ui.go
+++ b/ui.go
@@ -2,6 +2,28 @@ package main
 
 import g "github.com/AllenDang/giu"
 
+func engineDisplacementRow() *g.RowWidget {
+	return g.Row(
+		g.Label("Engine Displacement"),
+		g.InputFloat(&displacement.val).Format("%.2f"),
+		g.Combo(
+			"",
+			volumeUnitStrings()[selectedVolumeUnit],
+			volumeUnitStrings(),
+			&selectedVolumeUnit,
+		).
+			OnChange(func() {
+				s := volumeUnitStrings()[selectedVolumeUnit]
+				u, err := volumeUnitFromString(s)
+				check(err)
+				displacement = volume{
+					displacement.asUnit(u),
+					u,
+				}
+			}),
+	)
+}
+
 func engineSpeedRow() *g.TableRowWidget {
 	return g.TableRow(
 		g.Label("Engine Speed"),
diff --git a/volume.go b/volume.go
new file mode 100644
index 0000000..cb7098a
--- /dev/null
+++ b/volume.go
@@ -0,0 +1,54 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+)
+
+type volumeUnit float32
+
+const (
+	cubicCentimetre volumeUnit = 1
+	litre           volumeUnit = 1_000
+	cubicMetre      volumeUnit = 1_000_000
+	cubicInch       volumeUnit = 16.38706
+)
+
+// volumeUnitStrings returns a slice of strings, each representing a
+// volumeUnit.
+// This is necessary because giu.Combo only works with strings.
+func volumeUnitStrings() []string {
+	return []string{"cc", "L", "m³", "in³"}
+}
+
+const (
+	defaultVolumeUnit volumeUnit = cubicCentimetre
+	// Used to index volumeUnitStrings
+	defaultVolumeUnitIndex int32 = 0 // cc
+)
+
+func volumeUnitFromString(s string) (volumeUnit, error) {
+	// Each case corresponds to a value in volumeUnitStrings
+	switch s {
+	case "cc":
+		return cubicCentimetre, nil
+	case "L":
+		return litre, nil
+	case "m³":
+		return cubicMetre, nil
+	case "in³":
+		return cubicInch, nil
+	default:
+		return *new(volumeUnit), errors.New(fmt.Sprintf("invalid volumeUnit: '%s'", s))
+	}
+}
+
+type volume struct {
+	val  float32
+	unit volumeUnit
+}
+
+func (v volume) asUnit(u volumeUnit) float32 {
+	cc := v.val * float32(v.unit) // Convert to cubic centimetres.
+	return cc / float32(u)        // Convert to desired unit.
+}