Implement omision of zeroed-fields with + flag

example_test.go

@@ -10,11 +10,19 @@ func Example() {
 		a, b int
 	}
 	var x = []myType{{1, 2}, {3, 4}, {5, 6}}
-	fmt.Printf("%# v", pretty.Formatter(x))
+	fmt.Printf("%# v\n", pretty.Formatter(x))
+
+	var zeroedFields = []myType{{33, 0}, {a: 0, b: 34}}
+	// Note the '+' in the format
+	fmt.Printf("%# +v", pretty.Formatter(zeroedFields))
 	// output:
 	// []pretty_test.myType{
 	//     {a:1, b:2},
 	//     {a:3, b:4},
 	//     {a:5, b:6},
 	// }
+	// []pretty_test.myType{
+	//     {a:33},
+	//     {b:34},
+	// }
 }

formatter.go

@@ -26,6 +26,11 @@ type formatter struct {
 // If one of these two flags is not set, or any other verb is used, f will
 // format x according to the usual rules of package fmt.
 // In particular, if x satisfies fmt.Formatter, then x.Format will be called.
+//
+// If the "+" flag is provided, zero-valued structure fields will be omitted.
+// For example:
+//
+//     fmt.Sprintf("%# +v", Formatter(x))
 func Formatter(x interface{}) (f fmt.Formatter) {
 	return formatter{v: reflect.ValueOf(x), quote: true}
 }
@@ -55,6 +60,9 @@ func (fo formatter) Format(f fmt.State, c rune) {
 	if fo.force || c == 'v' && f.Flag('#') && f.Flag(' ') {
 		w := tabwriter.NewWriter(f, 4, 4, 1, ' ', 0)
 		p := &printer{tw: w, Writer: w, visited: make(map[visit]int)}
+		if f.Flag('+') {
+			p.skipZeroFields = true
+		}
 		p.printValue(fo.v, true, fo.quote)
 		w.Flush()
 		return
@@ -64,9 +72,10 @@ func (fo formatter) Format(f fmt.State, c rune) {
 
 type printer struct {
 	io.Writer
-	tw      *tabwriter.Writer
-	visited map[visit]int
-	depth   int
+	tw             *tabwriter.Writer
+	visited        map[visit]int
+	depth          int
+	skipZeroFields bool
 }
 
 func (p *printer) indent() *printer {
@@ -196,20 +205,35 @@ func (p *printer) printValue(v reflect.Value, showType, quote bool) {
 				writeByte(p, '\n')
 				pp = p.indent()
 			}
+			type field struct {
+				name  string
+				t     reflect.Type
+				value reflect.Value
+			}
+			fields := make([]field, 0, v.NumField())
+			// Collect fields, filtering out zero fields if needed
 			for i := 0; i < v.NumField(); i++ {
+				value := getField(v, i)
+				if p.skipZeroFields && !nonzero(value) {
+					continue
+				}
+				f := t.Field(i)
+				fields = append(fields, field{f.Name, f.Type, value})
+			}
+			for i, field := range fields {
 				showTypeInStruct := true
-				if f := t.Field(i); f.Name != "" {
-					io.WriteString(pp, f.Name)
+				if field.name != "" {
+					io.WriteString(pp, field.name)
 					writeByte(pp, ':')
 					if expand {
 						writeByte(pp, '\t')
 					}
-					showTypeInStruct = labelType(f.Type)
+					showTypeInStruct = labelType(field.t)
 				}
-				pp.printValue(getField(v, i), showTypeInStruct, true)
+				pp.printValue(field.value, showTypeInStruct, true)
 				if expand {
 					io.WriteString(pp, ",\n")
-				} else if i < v.NumField()-1 {
+				} else if i < len(fields)-1 {
 					io.WriteString(pp, ", ")
 				}
 			}

formatter_test.go

@@ -214,6 +214,124 @@ func (g *PanicGoString) GoString() string {
 	panic(g.s)
 }
 
+var gosyntaxSkipZeroFields = []test{
+	{nil, `nil`},
+	{"", `""`},
+	{"a", `"a"`},
+	{1, "int(1)"},
+	{1.0, "float64(1)"},
+	{[]int(nil), "[]int(nil)"},
+	{[0]int{}, "[0]int{}"},
+	{complex(1, 0), "(1+0i)"},
+	//{make(chan int), "(chan int)(0x1234)"},
+	{unsafe.Pointer(uintptr(unsafe.Pointer(&long))), fmt.Sprintf("unsafe.Pointer(0x%02x)", uintptr(unsafe.Pointer(&long)))},
+	{func(int) {}, "func(int) {...}"},
+	{map[string]string{"a": "a", "b": "b"}, "map[string]string{\"a\":\"a\", \"b\":\"b\"}"},
+	{map[int]int{1: 1}, "map[int]int{1:1}"},
+	{int32(1), "int32(1)"},
+	{io.EOF, `&errors.errorString{s:"EOF"}`},
+	{[]string{"a"}, `[]string{"a"}`},
+	{
+		[]string{long},
+		`[]string{"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"}`,
+	},
+	{F(5), "pretty.F(5)"},
+	{
+		SA{&T{1, 2}, T{3, 4}},
+		`pretty.SA{
+    t:  &pretty.T{x:1, y:2},
+    v:  pretty.T{x:3, y:4},
+}`,
+	},
+	{
+		map[int][]byte{1: {}},
+		`map[int][]uint8{
+    1:  {},
+}`,
+	},
+	{
+		map[int]T{1: {}},
+		`map[int]pretty.T{
+    1:  {},
+}`,
+	},
+	{
+		long,
+		`"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"`,
+	},
+	{
+		LongStructTypeName{
+			longFieldName:      LongStructTypeName{},
+			otherLongFieldName: long,
+		},
+		`pretty.LongStructTypeName{
+    otherLongFieldName: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
+}`,
+	},
+	{
+		LongStructTypeName{
+			longFieldName:      long,
+			otherLongFieldName: "",
+		},
+		`pretty.LongStructTypeName{
+    longFieldName: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
+}`,
+	},
+	{
+		LongStructTypeName{
+			longFieldName:      "",
+			otherLongFieldName: "",
+		},
+		`pretty.LongStructTypeName{}`,
+	},
+	{
+		&LongStructTypeName{
+			longFieldName:      &LongStructTypeName{},
+			otherLongFieldName: (*LongStructTypeName)(nil),
+		},
+		`&pretty.LongStructTypeName{
+    longFieldName: &pretty.LongStructTypeName{},
+}`,
+	},
+	{
+		[]LongStructTypeName{
+			{nil, nil},
+			{3, 3},
+			{long, nil},
+		},
+		`[]pretty.LongStructTypeName{
+    {},
+    {
+        longFieldName:      int(3),
+        otherLongFieldName: int(3),
+    },
+    {
+        longFieldName: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
+    },
+}`,
+	},
+	{
+		[]interface{}{
+			LongStructTypeName{nil, nil},
+			[]byte{1, 2, 3},
+			T{3, 4},
+			T{0, 4},
+			T{3, 0},
+			LongStructTypeName{long, nil},
+		},
+		`[]interface {}{
+    pretty.LongStructTypeName{},
+    []uint8{0x1, 0x2, 0x3},
+    pretty.T{x:3, y:4},
+    pretty.T{y:4},
+    pretty.T{x:3},
+    pretty.LongStructTypeName{
+        longFieldName: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
+    },
+}`,
+	},
+}
+
 func TestGoSyntax(t *testing.T) {
 	for _, tt := range gosyntax {
 		s := fmt.Sprintf("%# v", Formatter(tt.v))
@@ -226,6 +344,18 @@ func TestGoSyntax(t *testing.T) {
 	}
 }
 
+func TestGoSyntaxSkipZeroFields(t *testing.T) {
+	for _, tt := range gosyntaxSkipZeroFields {
+		s := fmt.Sprintf("%# +v", Formatter(tt.v))
+		if tt.s != s {
+			t.Errorf("expected %q", tt.s)
+			t.Errorf("got      %q", s)
+			t.Errorf("expraw\n%s", tt.s)
+			t.Errorf("gotraw\n%s", s)
+		}
+	}
+}
+
 type I struct {
 	i int
 	R interface{}