Limit verbosity of reporter output (#215)

A common complaint is that the reporter it prints out too much
irrelevant information, resulting in a low signal-to-noise ratio.
Improve this metric by imposing a verbosity limit. For nodes that
are equal, we set the verbosity level is a lower value than when
the nodes are inequal.

Other minor changes:
* Adjust heuristic for triple-quote usage to operate on more cases.
* Elide type more aggressively for equal nodes.
* Printing the address for a slice includes the length and capacity.
* The pointed-at value for map keys are printed.

cmp/compare_test.go

@@ -159,8 +159,8 @@ func TestDiff(t *testing.T) {
 					}
 				} else {
 					wantDiff := wantDiffs[t.Name()]
-					if gotDiff != wantDiff {
-						t.Fatalf("Diff:\ngot:\n%s\nwant:\n%s\nreason: %v", gotDiff, wantDiff, tt.reason)
+					if diff := cmp.Diff(wantDiff, gotDiff); diff != "" {
+						t.Fatalf("Diff:\ngot:\n%s\nwant:\n%s\ndiff: (-want +got)\n%s\nreason: %v", gotDiff, wantDiff, diff, tt.reason)
 					}
 				}
 				gotEqual := gotDiff == ""

cmp/example_test.go

@@ -37,7 +37,7 @@ func ExampleDiff_testing() {
 	//   	SSID:      "CoffeeShopWiFi",
 	// - 	IPAddress: s"192.168.0.2",
 	// + 	IPAddress: s"192.168.0.1",
-	//   	NetMask:   net.IPMask{0xff, 0xff, 0x00, 0x00},
+	//   	NetMask:   {0xff, 0xff, 0x00, 0x00},
 	//   	Clients: []cmp_test.Client{
 	//   		... // 2 identical elements
 	//   		{Hostname: "macchiato", IPAddress: s"192.168.0.153", LastSeen: s"2009-11-10 23:39:43 +0000 UTC"},

cmp/report_compare.go

@@ -11,10 +11,6 @@ import (
 	"github.com/google/go-cmp/cmp/internal/value"
 )
 
-// TODO: Enforce limits?
-//	* Enforce maximum number of records to print per node?
-//	* Enforce maximum size in bytes allowed?
-//	* As a heuristic, use less verbosity for equal nodes than unequal nodes.
 // TODO: Enforce unique outputs?
 //	* Avoid Stringer methods if it results in same output?
 //	* Print pointer address if outputs still equal?
@@ -71,10 +67,31 @@ func (opts formatOptions) WithTypeMode(t typeMode) formatOptions {
 	opts.TypeMode = t
 	return opts
 }
+func (opts formatOptions) WithVerbosity(level int) formatOptions {
+	opts.VerbosityLevel = level
+	opts.LimitVerbosity = true
+	return opts
+}
+func (opts formatOptions) verbosity() uint {
+	switch {
+	case opts.VerbosityLevel < 0:
+		return 0
+	case opts.VerbosityLevel > 16:
+		return 16 // some reasonable maximum to avoid shift overflow
+	default:
+		return uint(opts.VerbosityLevel)
+	}
+}
 
 // FormatDiff converts a valueNode tree into a textNode tree, where the later
 // is a textual representation of the differences detected in the former.
 func (opts formatOptions) FormatDiff(v *valueNode) textNode {
+	if opts.DiffMode == diffIdentical {
+		opts = opts.WithVerbosity(1)
+	} else {
+		opts = opts.WithVerbosity(3)
+	}
+
 	// Check whether we have specialized formatting for this node.
 	// This is not necessary, but helpful for producing more readable outputs.
 	if opts.CanFormatDiffSlice(v) {
@@ -124,6 +141,8 @@ func (opts formatOptions) FormatDiff(v *valueNode) textNode {
 		}
 	}
 
+	// TODO: Print cycle reference for pointers, maps, and elements of a slice.
+
 	// Descend into the child value node.
 	if v.TransformerName != "" {
 		out := opts.WithTypeMode(emitType).FormatDiff(v.Value)
@@ -162,12 +181,27 @@ func (opts formatOptions) formatDiffList(recs []reportRecord, k reflect.Kind) te
 		formatKey = formatMapKey
 	}
 
+	maxLen := -1
+	if opts.LimitVerbosity {
+		if opts.DiffMode == diffIdentical {
+			maxLen = ((1 << opts.verbosity()) >> 1) << 2 // 0, 4, 8, 16, 32, etc...
+		} else {
+			maxLen = (1 << opts.verbosity()) << 1 // 2, 4, 8, 16, 32, 64, etc...
+		}
+		opts.VerbosityLevel--
+	}
+
 	// Handle unification.
 	switch opts.DiffMode {
 	case diffIdentical, diffRemoved, diffInserted:
 		var list textList
 		var deferredEllipsis bool // Add final "..." to indicate records were dropped
 		for _, r := range recs {
+			if len(list) == maxLen {
+				deferredEllipsis = true
+				break
+			}
+
 			// Elide struct fields that are zero value.
 			if k == reflect.Struct {
 				var isZero bool
@@ -205,11 +239,12 @@ func (opts formatOptions) formatDiffList(recs []reportRecord, k reflect.Kind) te
 	}
 
 	// Handle differencing.
+	var numDiffs int
 	var list textList
 	groups := coalesceAdjacentRecords(name, recs)
 	maxGroup := diffStats{Name: name}
 	for i, ds := range groups {
-		if len(list) >= maxDiffElements {
+		if maxLen >= 0 && numDiffs >= maxLen {
 			maxGroup = maxGroup.Append(ds)
 			continue
 		}
@@ -273,6 +308,7 @@ func (opts formatOptions) formatDiffList(recs []reportRecord, k reflect.Kind) te
 			}
 		}
 		recs = recs[ds.NumDiff():]
+		numDiffs += ds.NumDiff()
 	}
 	if maxGroup.IsZero() {
 		assert(len(recs) == 0)

cmp/report_reflect.go

@@ -21,14 +21,23 @@ type formatValueOptions struct {
 	// methods like error.Error or fmt.Stringer.String.
 	AvoidStringer bool
 
-	// ShallowPointers controls whether to avoid descending into pointers.
+	// PrintShallowPointer controls whether to print the next pointer.
 	// Useful when printing map keys, where pointer comparison is performed
 	// on the pointer address rather than the pointed-at value.
-	ShallowPointers bool
+	PrintShallowPointer bool
 
 	// PrintAddresses controls whether to print the address of all pointers,
 	// slice elements, and maps.
 	PrintAddresses bool
+
+	// VerbosityLevel controls the amount of output to produce.
+	// A higher value produces more output. A value of zero or lower produces
+	// no output (represented using an ellipsis).
+	// If LimitVerbosity is false, then the level is treated as infinite.
+	VerbosityLevel int
+
+	// LimitVerbosity specifies that formatting should respect VerbosityLevel.
+	LimitVerbosity bool
 }
 
 // FormatType prints the type as if it were wrapping s.
@@ -45,6 +54,9 @@ func (opts formatOptions) FormatType(t reflect.Type, s textNode) textNode {
 		default:
 			return s
 		}
+		if opts.DiffMode == diffIdentical {
+			return s // elide type for identical nodes
+		}
 	case elideType:
 		return s
 	}
@@ -86,11 +98,22 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 		// Avoid calling Error or String methods on nil receivers since many
 		// implementations crash when doing so.
 		if (t.Kind() != reflect.Ptr && t.Kind() != reflect.Interface) || !v.IsNil() {
+			var prefix, strVal string
 			switch v := v.Interface().(type) {
 			case error:
-				return textLine("e" + formatString(v.Error()))
+				prefix, strVal = "e", v.Error()
 			case fmt.Stringer:
-				return textLine("s" + formatString(v.String()))
+				prefix, strVal = "s", v.String()
+			}
+			if prefix != "" {
+				maxLen := len(strVal)
+				if opts.LimitVerbosity {
+					maxLen = (1 << opts.verbosity()) << 5 // 32, 64, 128, 256, etc...
+				}
+				if len(strVal) > maxLen+len(textEllipsis) {
+					return textLine(prefix + formatString(strVal[:maxLen]) + string(textEllipsis))
+				}
+				return textLine(prefix + formatString(strVal))
 			}
 		}
 	}
@@ -123,17 +146,33 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 	case reflect.Complex64, reflect.Complex128:
 		return textLine(fmt.Sprint(v.Complex()))
 	case reflect.String:
+		maxLen := v.Len()
+		if opts.LimitVerbosity {
+			maxLen = (1 << opts.verbosity()) << 5 // 32, 64, 128, 256, etc...
+		}
+		if v.Len() > maxLen+len(textEllipsis) {
+			return textLine(formatString(v.String()[:maxLen]) + string(textEllipsis))
+		}
 		return textLine(formatString(v.String()))
 	case reflect.UnsafePointer, reflect.Chan, reflect.Func:
 		return textLine(formatPointer(v))
 	case reflect.Struct:
 		var list textList
 		v := makeAddressable(v) // needed for retrieveUnexportedField
+		maxLen := v.NumField()
+		if opts.LimitVerbosity {
+			maxLen = ((1 << opts.verbosity()) >> 1) << 2 // 0, 4, 8, 16, 32, etc...
+			opts.VerbosityLevel--
+		}
 		for i := 0; i < v.NumField(); i++ {
 			vv := v.Field(i)
 			if value.IsZero(vv) {
 				continue // Elide fields with zero values
 			}
+			if len(list) == maxLen {
+				list.AppendEllipsis(diffStats{})
+				break
+			}
 			sf := t.Field(i)
 			if supportExporters && !isExported(sf.Name) {
 				vv = retrieveUnexportedField(v, sf, true)
@@ -147,12 +186,21 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 			return textNil
 		}
 		if opts.PrintAddresses {
-			ptr = formatPointer(v)
+			ptr = fmt.Sprintf("⟪ptr:0x%x, len:%d, cap:%d⟫", pointerValue(v), v.Len(), v.Cap())
 		}
 		fallthrough
 	case reflect.Array:
+		maxLen := v.Len()
+		if opts.LimitVerbosity {
+			maxLen = ((1 << opts.verbosity()) >> 1) << 2 // 0, 4, 8, 16, 32, etc...
+			opts.VerbosityLevel--
+		}
 		var list textList
 		for i := 0; i < v.Len(); i++ {
+			if len(list) == maxLen {
+				list.AppendEllipsis(diffStats{})
+				break
+			}
 			vi := v.Index(i)
 			if vi.CanAddr() { // Check for cyclic elements
 				p := vi.Addr()
@@ -177,8 +225,17 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 			return textLine(formatPointer(v))
 		}
 
+		maxLen := v.Len()
+		if opts.LimitVerbosity {
+			maxLen = ((1 << opts.verbosity()) >> 1) << 2 // 0, 4, 8, 16, 32, etc...
+			opts.VerbosityLevel--
+		}
 		var list textList
 		for _, k := range value.SortKeys(v.MapKeys()) {
+			if len(list) == maxLen {
+				list.AppendEllipsis(diffStats{})
+				break
+			}
 			sk := formatMapKey(k)
 			sv := opts.WithTypeMode(elideType).FormatValue(v.MapIndex(k), false, m)
 			list = append(list, textRecord{Key: sk, Value: sv})
@@ -191,11 +248,12 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 		if v.IsNil() {
 			return textNil
 		}
-		if m.Visit(v) || opts.ShallowPointers {
+		if m.Visit(v) {
 			return textLine(formatPointer(v))
 		}
-		if opts.PrintAddresses {
+		if opts.PrintAddresses || opts.PrintShallowPointer {
 			ptr = formatPointer(v)
+			opts.PrintShallowPointer = false
 		}
 		skipType = true // Let the underlying value print the type instead
 		return textWrap{"&" + ptr, opts.FormatValue(v.Elem(), false, m), ""}
@@ -217,7 +275,7 @@ func (opts formatOptions) FormatValue(v reflect.Value, withinSlice bool, m visit
 func formatMapKey(v reflect.Value) string {
 	var opts formatOptions
 	opts.TypeMode = elideType
-	opts.ShallowPointers = true
+	opts.PrintShallowPointer = true
 	s := opts.FormatValue(v, false, visitedPointers{}).String()
 	return strings.TrimSpace(s)
 }
@@ -268,11 +326,14 @@ func formatHex(u uint64) string {
 
 // formatPointer prints the address of the pointer.
 func formatPointer(v reflect.Value) string {
+	return fmt.Sprintf("⟪0x%x⟫", pointerValue(v))
+}
+func pointerValue(v reflect.Value) uintptr {
 	p := v.Pointer()
 	if flags.Deterministic {
 		p = 0xdeadf00f // Only used for stable testing purposes
 	}
-	return fmt.Sprintf("⟪0x%x⟫", p)
+	return p
 }
 
 type visitedPointers map[value.Pointer]struct{}

cmp/report_slices.go

@@ -16,10 +16,6 @@ import (
 	"github.com/google/go-cmp/cmp/internal/diff"
 )
 
-// maxDiffElements is the maximum number of difference elements to format
-// before the remaining differences are coalesced together.
-const maxDiffElements = 32
-
 // CanFormatDiffSlice reports whether we support custom formatting for nodes
 // that are slices of primitive kinds or strings.
 func (opts formatOptions) CanFormatDiffSlice(v *valueNode) bool {
@@ -155,7 +151,8 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 		//	+	BAZ
 		//		"""
 		isTripleQuoted := true
-		prevDiffLines := map[string]bool{}
+		prevRemoveLines := map[string]bool{}
+		prevInsertLines := map[string]bool{}
 		var list2 textList
 		list2 = append(list2, textRecord{Value: textLine(`"""`), ElideComma: true})
 		for _, r := range list {
@@ -171,20 +168,24 @@ func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
 				isPrintable := func(r rune) bool {
 					return unicode.IsPrint(r) || r == '\t' // specially treat tab as printable
 				}
-				isTripleQuoted = isTripleQuoted &&
-					!strings.HasPrefix(line, `"""`) &&
-					!strings.HasPrefix(line, "...") &&
-					strings.TrimFunc(line, isPrintable) == "" &&
-					(r.Diff == 0 || !prevDiffLines[normLine])
+				isTripleQuoted = !strings.HasPrefix(line, `"""`) && !strings.HasPrefix(line, "...") && strings.TrimFunc(line, isPrintable) == ""
+				switch r.Diff {
+				case diffRemoved:
+					isTripleQuoted = isTripleQuoted && !prevInsertLines[normLine]
+					prevRemoveLines[normLine] = true
+				case diffInserted:
+					isTripleQuoted = isTripleQuoted && !prevRemoveLines[normLine]
+					prevInsertLines[normLine] = true
+				}
 				if !isTripleQuoted {
 					break
 				}
 				r.Value = textLine(line)
 				r.ElideComma = true
-				prevDiffLines[normLine] = true
 			}
-			if r.Diff == 0 {
-				prevDiffLines = map[string]bool{} // start a new non-adjacent difference group
+			if !(r.Diff == diffRemoved || r.Diff == diffInserted) { // start a new non-adjacent difference group
+				prevRemoveLines = map[string]bool{}
+				prevInsertLines = map[string]bool{}
 			}
 			list2 = append(list2, r)
 		}
@@ -337,11 +338,18 @@ func (opts formatOptions) formatDiffSlice(
 		return n0 - v.Len()
 	}
 
+	var numDiffs int
+	maxLen := -1
+	if opts.LimitVerbosity {
+		maxLen = (1 << opts.verbosity()) << 2 // 4, 8, 16, 32, 64, etc...
+		opts.VerbosityLevel--
+	}
+
 	groups := coalesceAdjacentEdits(name, es)
 	groups = coalesceInterveningIdentical(groups, chunkSize/4)
 	maxGroup := diffStats{Name: name}
 	for i, ds := range groups {
-		if len(list) >= maxDiffElements {
+		if maxLen >= 0 && numDiffs >= maxLen {
 			maxGroup = maxGroup.Append(ds)
 			continue
 		}
@@ -374,10 +382,12 @@ func (opts formatOptions) formatDiffSlice(
 		}
 
 		// Print unequal.
+		len0 := len(list)
 		nx := appendChunks(vx.Slice(0, ds.NumIdentical+ds.NumRemoved+ds.NumModified), diffRemoved)
 		vx = vx.Slice(nx, vx.Len())
 		ny := appendChunks(vy.Slice(0, ds.NumIdentical+ds.NumInserted+ds.NumModified), diffInserted)
 		vy = vy.Slice(ny, vy.Len())
+		numDiffs += len(list) - len0
 	}
 	if maxGroup.IsZero() {
 		assert(vx.Len() == 0 && vy.Len() == 0)