Make code testable, add test against native xxd

xxd.go

@@ -8,15 +8,7 @@ import (
 	"unicode/utf8"
 )
 
-const (
-	byteOffsetInit = 8
-	charOffsetInt  = 39
-	line_length    = 50
-)
-
 func main() {
-	line_offset := 0
-
 	if len(os.Args) != 2 {
 		fmt.Fprintf(os.Stderr, "Usage: %s [file]\n", os.Args[0])
 		os.Exit(1)
@@ -27,8 +19,21 @@ func main() {
 		panic(err)
 	}
 	defer f.Close()
+	if err := XXD(f, os.Stdout); err != nil {
+		panic(err)
+	}
+}
+
+const (
+	byteOffsetInit = 8
+	charOffsetInt  = 39
+	line_length    = 50
+)
+
+func XXD(r io.Reader, w io.Writer) error {
+	line_offset := 0
 
-	r := bufio.NewReader(f)
+	r = bufio.NewReader(r)
 	buf := make([]byte, 16)
 	for {
 		n, err := r.Read(buf)
@@ -37,41 +42,42 @@ func main() {
 		}
 
 		// Line offset
-		fmt.Printf("%06x0: ", line_offset)
+		fmt.Fprintf(w, "%06x0: ", line_offset)
 		line_offset++
 
 		// Hex values
 		for i := 0; i < n; i++ {
-			fmt.Printf("%02x", buf[i])
+			fmt.Fprintf(w, "%02x", buf[i])
 
 			if i%2 == 1 {
-				fmt.Print(" ")
+				fmt.Fprint(w, " ")
 			}
 		}
 		if n < len(buf) {
 			for i := n; i < len(buf); i++ {
-				fmt.Printf("  ")
+				fmt.Fprintf(w, "  ")
 				if i%2 == 1 {
-					fmt.Print(" ")
+					fmt.Fprint(w, " ")
 				}
 			}
 		}
 
-		fmt.Printf(" ")
+		fmt.Fprintf(w, " ")
 
 		// Character values
 		b := buf[:n]
 		for len(b) > 0 {
 			r, size := utf8.DecodeRune(b)
 
 			if int(r) > 0x1f && int(r) < 0x7f {
-				fmt.Printf("%v", string(r))
+				fmt.Fprintf(w, "%v", string(r))
 			} else {
-				fmt.Printf(".")
+				fmt.Fprintf(w, ".")
 			}
 			b = b[size:]
 		}
 
-		fmt.Printf("\n")
+		fmt.Fprintf(w, "\n")
 	}
+	return nil
 }

xxd_test.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"bytes"
+	"flag"
+	"io"
+	"io/ioutil"
+	"os/exec"
+	"strings"
+	"testing"
+	"testing/quick"
+)
+
+var xxdFile = flag.String("xxdFile", "", "File to test against.")
+
+func TestXXD(t *testing.T) {
+	if *xxdFile == "" {
+		t.Skip("-xxdFile argument not given")
+	}
+	data, err := ioutil.ReadFile(*xxdFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	test := func(fn func(r io.Reader, w io.Writer) error) func(n uint64) []string {
+		return func(n uint64) []string {
+			size := n % uint64(len(data))
+			var out bytes.Buffer
+			if err := fn(bytes.NewBuffer(data[0:size]), &out); err != nil {
+				return []string{err.Error()}
+			}
+			return strings.Split(out.String(), "\n")
+		}
+	}
+	if err := quick.CheckEqual(test(XXD), test(xxdNative), nil); err != nil {
+		cErr := err.(*quick.CheckEqualError)
+		size := cErr.In[0].(uint64) % uint64(len(data))
+		for i := range cErr.Out1[0].([]string) {
+			got := cErr.Out1[0].([]string)[i]
+			want := cErr.Out2[0].([]string)[i]
+			if got != want {
+				t.Errorf("size: %d\n\ngot : %s\nwant: %s\n", size, got, want)
+				break
+			}
+		}
+	}
+}
+
+func xxdNative(r io.Reader, w io.Writer) error {
+	xxd := exec.Command("xxd", "-")
+	xxd.Stdin = r
+	xxd.Stdout = w
+	xxd.Stderr = w
+	return xxd.Run()
+}