add: calculator example to x-tba

x-tba/foundations/calc/calc-scanner/calculations.txt

@@ -0,0 +1,4 @@
+4 + 5
+6.5 + 2
+5 - 3
+q

x-tba/foundations/calc/calc-scanner/main.go

@@ -0,0 +1,66 @@
+/*
+go run main.go < ./calculations.txt
+
+If you're not on Linux or OS X, etc but on Windows,
+Then, use Windows PowerShell to do the same thing.
+*/
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// After functions, you'll see that how we're going to refactor this
+// into a more readable version.
+
+func main() {
+	const (
+		promptChar     = "> "
+		errWrongOp     = "%s operation is not supported\n"
+		errWrongFormat = "operation is not recognized\n"
+		usage          = `
+usage: number operation number
+quit : type q to quit
+examples:
+  3 + 5
+  5 - 3
+`
+	)
+
+	fmt.Println(strings.TrimSpace(usage))
+
+	for s := bufio.NewScanner(os.Stdin); ; {
+		var (
+			a, b, res float64
+			op        string
+		)
+
+		fmt.Print(promptChar, " ")
+		if !s.Scan() {
+			break
+		}
+
+		_, err := fmt.Sscanf(s.Text(), "%f %s %f", &a, &op, &b)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, errWrongFormat)
+			continue
+		}
+
+		switch op {
+		case "+":
+			res = a + b
+		case "-":
+			res = a - b
+		default:
+			fmt.Printf(errWrongOp, op)
+			continue
+		}
+
+		fmt.Printf("%g %s %g = %g\n", a, op, b, res)
+	}
+
+	fmt.Println("bye.")
+}