refactor: simplify calculator dispatch with variadic Set methods

README.md

@@ -10,9 +10,9 @@
 
 Two things in one repo:
 
-- **`engine`** — a generic RPN calculator library: stack, registers, TVM, cash
+- **`engine`**: a generic RPN calculator library: stack, registers, TVM, cash
   flows, statistics, depreciation, date math. No GUI dependency.
-- **App** — a cross-platform financial calculator built with
+- **App**: a cross-platform financial calculator built with
   [Wails v3](https://v3.wails.io) on top of the engine. macOS, Windows, Linux,
   iOS, Android.
 
@@ -45,13 +45,19 @@ c.Snapshot()               // EngineState for persistence
 c.Restore(state)
 ```
 
+Set methods are variadic. Omit the argument to store the current X value:
+
+```go
+c.X = 8; c.SetI()          // equivalent to c.SetI(8)
+```
+
 ### Methods
 
 | Category     | Methods                                                                                        |
 | ------------ | ---------------------------------------------------------------------------------------------- |
 | Stack        | `Enter`, `Clx`, `Chs`, `XY`, `RollDown`, `RollUp`, `LastXRecall`                               |
 | Arithmetic   | `Add`, `Sub`, `Mul`, `Div`, `YPowX`                                                            |
-| TVM store    | `SetN(n)`, `SetI(i)`, `SetPV(pv)`, `SetPMT(pmt)`, `SetFV(fv)`                                  |
+| TVM store    | `SetN(n?)`, `SetI(i?)`, `SetPV(pv?)`, `SetPMT(pmt?)`, `SetFV(fv?)`                             |
 | TVM solve    | `SolveN`, `SolveI`, `SolvePV`, `SolvePMT`, `SolveFV`                                           |
 | Cash flow    | `NPV`, `IRR`                                                                                   |
 | Amortization | `Amortize`                                                                                     |
@@ -68,6 +74,9 @@ c.Restore(state)
 | State        | `Snapshot`, `Restore`                                                                          |
 | Clear        | `ClearFin`, `ClearReg`, `ClearStats`, `ClearPgm`                                               |
 
+`SetN/SetI/SetPV/SetPMT/SetFV` are variadic: omit the argument to store the
+current X register value (`e.SetN()` is shorthand for `e.SetN(e.X)`).
+
 ### Fields
 
 ```go

calculator.go

@@ -163,15 +163,15 @@ func (c *CalcService) PressKey(input KeyInput) KeyResult {
 func (c *CalcService) storeTVM(op string) {
 	switch op {
 	case "n":
-		c.e.SetN(c.e.X)
+		c.e.SetN()
 	case "i":
-		c.e.SetI(c.e.X)
+		c.e.SetI()
 	case "PV":
-		c.e.SetPV(c.e.X)
+		c.e.SetPV()
 	case "PMT":
-		c.e.SetPMT(c.e.X)
+		c.e.SetPMT()
 	case "FV":
-		c.e.SetFV(c.e.X)
+		c.e.SetFV()
 	}
 	c.e.Flags.StackLift = false
 }

engine/financial.go

@@ -2,11 +2,41 @@ package engine
 
 import "math"
 
-func (e *Engine) SetN(n float64)     { e.FinN = n }
-func (e *Engine) SetI(i float64)     { e.FinI = i }
-func (e *Engine) SetPV(pv float64)   { e.FinPV = pv }
-func (e *Engine) SetPMT(pmt float64) { e.FinPMT = pmt }
-func (e *Engine) SetFV(fv float64)   { e.FinFV = fv }
+func (e *Engine) SetN(v ...float64) {
+	if len(v) > 0 {
+		e.FinN = v[0]
+	} else {
+		e.FinN = e.X
+	}
+}
+func (e *Engine) SetI(v ...float64) {
+	if len(v) > 0 {
+		e.FinI = v[0]
+	} else {
+		e.FinI = e.X
+	}
+}
+func (e *Engine) SetPV(v ...float64) {
+	if len(v) > 0 {
+		e.FinPV = v[0]
+	} else {
+		e.FinPV = e.X
+	}
+}
+func (e *Engine) SetPMT(v ...float64) {
+	if len(v) > 0 {
+		e.FinPMT = v[0]
+	} else {
+		e.FinPMT = e.X
+	}
+}
+func (e *Engine) SetFV(v ...float64) {
+	if len(v) > 0 {
+		e.FinFV = v[0]
+	} else {
+		e.FinFV = e.X
+	}
+}
 
 func (e *Engine) SolveN() { e.solve(math.NaN(), e.FinI/100, e.FinPV, e.FinPMT, e.FinFV) }
 func (e *Engine) SolveI() {