Implement custom verbs for fmt.Printf use

The dns package implements String() for all RR types, but sometimes you will
need more flexibility. The functions Printf, Sprintf, etc. implemented formatted I/O
for the RR type.

Printing

The verbs:

Generic part of RRs:

     %N	the owner name of the RR
     %C	the class: IN, CH, CLASS15, etc.
     %D	the TTL in seconds
     %Y	the type: MX, A, etc.

The rdata of each RR differs, we allow each field to be printed as a string.

Rdata:

     %0	the first rdata field
     %1	the second rdata field
     %2	the third rdata field
     ..	...
     %9      the nineth rdata field
     %R	all rdata fields

The rdata fields remain a TODO, but will be implemented using
reflection.

dns.go

@@ -94,6 +94,7 @@
 package dns
 
 import (
+	"fmt"
 	"strconv"
 )
 
@@ -122,9 +123,11 @@ type RR interface {
 	Header() *RR_Header
 	// String returns the text representation of the resource record.
 	String() string
+	// Formatted output
+	Format(f fmt.State, c rune)
 	// copy returns a copy of the RR
 	copy() RR
-	// len returns the length (in octects) of the uncompressed RR in wire format.
+	// len returns the length (in octets) of the uncompressed RR in wire format.
 	len() int
 }
 

parse_test.go

@@ -1286,54 +1286,74 @@ func TestNewRRSpecial(t *testing.T) {
 	rr, err = NewRR("; comment")
 	expect = ""
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr != nil {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 
 	rr, err = NewRR("")
 	expect = ""
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr != nil {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 
 	rr, err = NewRR("$ORIGIN foo.")
 	expect = ""
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr != nil {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 
 	rr, err = NewRR(" ")
 	expect = ""
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr != nil {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 
 	rr, err = NewRR("\n")
 	expect = ""
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr != nil {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 
 	rr, err = NewRR("foo. A 1.1.1.1\nbar. A 2.2.2.2")
 	expect = "foo.\t3600\tIN\tA\t1.1.1.1"
 	if err != nil {
-		t.Errorf("unexpect err: %s", err)
+		t.Errorf("unexpected err: %s", err)
 	}
 	if rr == nil || rr.String() != expect {
-		t.Errorf("unexpect result: [%s] != [%s]", rr, expect)
+		t.Errorf("unexpected result: [%s] != [%s]", rr, expect)
 	}
 }
+
+func TestPrintfVerbs(t *testing.T) {
+	x, _ := NewRR("www.miek.nl. IN A 127.0.0.1")
+	if fmt.Sprintf("%N", x) != "www.miek.nl." {
+		t.Errorf("%N does return name")
+	}
+	if fmt.Sprintf("%C", x) != "IN" {
+		t.Errorf("%C does return class")
+	}
+	if fmt.Sprintf("%D", x) != "3600" {
+		t.Errorf("%D does return ttl")
+	}
+	if fmt.Sprintf("%Y", x) != "A" {
+		t.Errorf("%Y does return type")
+	}
+	if fmt.Sprintf("%Y %d", x, 5) != "A 5" {
+		t.Errorf("%N does return name")
+	}
+	// TODO(miek): RDATA
+}