cmp/report_slices.go
func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
	assert(opts.DiffMode == diffUnknown)
	t, vx, vy := v.Type, v.ValueX, v.ValueY

	// Auto-detect the type of the data.
	var isLinedText, isText, isBinary bool
	var sx, sy string
	switch {
	case t.Kind() == reflect.String:
		sx, sy = vx.String(), vy.String()
		isText = true // Initial estimate, verify later
	case t.Kind() == reflect.Slice && t.Elem() == reflect.TypeOf(byte(0)):
		sx, sy = string(vx.Bytes()), string(vy.Bytes())
		isBinary = true // Initial estimate, verify later
	case t.Kind() == reflect.Array:
		// Arrays need to be addressable for slice operations to work.
		vx2, vy2 := reflect.New(t).Elem(), reflect.New(t).Elem()
		vx2.Set(vx)
		vy2.Set(vy)
		vx, vy = vx2, vy2
	}
	if isText || isBinary {
		var numLines, lastLineIdx, maxLineLen int
		isBinary = false
		for i, r := range sx + sy {
			if !(unicode.IsPrint(r) || unicode.IsSpace(r)) || r == utf8.RuneError {
				isBinary = true
				break
			}
			if r == '\n' {
				if maxLineLen < i-lastLineIdx {
					lastLineIdx = i - lastLineIdx
				}
				lastLineIdx = i + 1
				numLines++
			}
		}
		isText = !isBinary
		isLinedText = isText && numLines >= 4 && maxLineLen <= 256
	}

	// Format the string into printable records.
	var list textList
	var delim string
	switch {
	// If the text appears to be multi-lined text,
	// then perform differencing across individual lines.
	case isLinedText:
		ssx := strings.Split(sx, "\n")
		ssy := strings.Split(sy, "\n")
		list = opts.formatDiffSlice(
			reflect.ValueOf(ssx), reflect.ValueOf(ssy), 1, "line",
			func(v reflect.Value, d diffMode) textRecord {
				s := formatString(v.Index(0).String())
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
		delim = "\n"
	// If the text appears to be single-lined text,
	// then perform differencing in approximately fixed-sized chunks.
	// The output is printed as quoted strings.
	case isText:
		list = opts.formatDiffSlice(
			reflect.ValueOf(sx), reflect.ValueOf(sy), 64, "byte",
			func(v reflect.Value, d diffMode) textRecord {
				s := formatString(v.String())
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
		delim = ""
	// If the text appears to be binary data,
	// then perform differencing in approximately fixed-sized chunks.
	// The output is inspired by hexdump.
	case isBinary:
		list = opts.formatDiffSlice(
			reflect.ValueOf(sx), reflect.ValueOf(sy), 16, "byte",
			func(v reflect.Value, d diffMode) textRecord {
				var ss []string
				for i := 0; i < v.Len(); i++ {
					ss = append(ss, formatHex(v.Index(i).Uint()))
				}
				s := strings.Join(ss, ", ")
				comment := commentString(fmt.Sprintf("%c|%v|", d, formatASCII(v.String())))
				return textRecord{Diff: d, Value: textLine(s), Comment: comment}
			},
		)
	// For all other slices of primitive types,
	// then perform differencing in approximately fixed-sized chunks.
	// The size of each chunk depends on the width of the element kind.
	default:
		var chunkSize int
		if t.Elem().Kind() == reflect.Bool {
			chunkSize = 16
		} else {
			switch t.Elem().Bits() {
			case 8:
				chunkSize = 16
			case 16:
				chunkSize = 12
			case 32:
				chunkSize = 8
			default:
				chunkSize = 8
			}
		}
		list = opts.formatDiffSlice(
			vx, vy, chunkSize, t.Elem().Kind().String(),
			func(v reflect.Value, d diffMode) textRecord {
				var ss []string
				for i := 0; i < v.Len(); i++ {
					switch t.Elem().Kind() {
					case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
						ss = append(ss, fmt.Sprint(v.Index(i).Int()))
					case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
						ss = append(ss, formatHex(v.Index(i).Uint()))
					case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
						ss = append(ss, fmt.Sprint(v.Index(i).Interface()))
					}
				}
				s := strings.Join(ss, ", ")
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
	}

	// Wrap the output with appropriate type information.
	var out textNode = textWrap{"{", list, "}"}
	if !isText {
		// The "{...}" byte-sequence literal is not valid Go syntax for strings.
		// Emit the type for extra clarity (e.g. "string{...}").
		if t.Kind() == reflect.String {
			opts = opts.WithTypeMode(emitType)
		}
		return opts.FormatType(t, out)
	}
	switch t.Kind() {
	case reflect.String:
		out = textWrap{"strings.Join(", out, fmt.Sprintf(", %q)", delim)}
		if t != reflect.TypeOf(string("")) {
			out = opts.FormatType(t, out)
		}
	case reflect.Slice:
		out = textWrap{"bytes.Join(", out, fmt.Sprintf(", %q)", delim)}
		if t != reflect.TypeOf([]byte(nil)) {
			out = opts.FormatType(t, out)
		}
	}
	return out
}
func (opts formatOptions) FormatDiffSlice(v *valueNode) textNode {
	assert(opts.DiffMode == diffUnknown)
	t, vx, vy := v.Type, v.ValueX, v.ValueY

	// Auto-detect the type of the data.
	var isLinedText, isText, isBinary bool
	var sx, sy string
	switch {
	case t.Kind() == reflect.String:
		sx, sy = vx.String(), vy.String()
		isText = true // Initial estimate, verify later
	case t.Kind() == reflect.Slice && t.Elem() == reflect.TypeOf(byte(0)):
		sx, sy = string(vx.Bytes()), string(vy.Bytes())
		isBinary = true // Initial estimate, verify later
	case t.Kind() == reflect.Array:
		// Arrays need to be addressable for slice operations to work.
		vx2, vy2 := reflect.New(t).Elem(), reflect.New(t).Elem()
		vx2.Set(vx)
		vy2.Set(vy)
		vx, vy = vx2, vy2
	}
	if isText || isBinary {
		var numLines, lastLineIdx, maxLineLen int
		isBinary = false
		for i, r := range sx + sy {
			if !(unicode.IsPrint(r) || unicode.IsSpace(r)) || r == utf8.RuneError {
				isBinary = true
				break
			}
			if r == '\n' {
				if maxLineLen < i-lastLineIdx {
					lastLineIdx = i - lastLineIdx
				}
				lastLineIdx = i + 1
				numLines++
			}
		}
		isText = !isBinary
		isLinedText = isText && numLines >= 4 && maxLineLen <= 256
	}

	// Format the string into printable records.
	var list textList
	var delim string
	switch {
	// If the text appears to be multi-lined text,
	// then perform differencing across individual lines.
	case isLinedText:
		ssx := strings.Split(sx, "\n")
		ssy := strings.Split(sy, "\n")
		list = opts.formatDiffSlice(
			reflect.ValueOf(ssx), reflect.ValueOf(ssy), 1, "line",
			func(v reflect.Value, d diffMode) textRecord {
				s := formatString(v.Index(0).String())
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
		delim = "\n"
	// If the text appears to be single-lined text,
	// then perform differencing in approximately fixed-sized chunks.
	// The output is printed as quoted strings.
	case isText:
		list = opts.formatDiffSlice(
			reflect.ValueOf(sx), reflect.ValueOf(sy), 64, "byte",
			func(v reflect.Value, d diffMode) textRecord {
				s := formatString(v.String())
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
		delim = ""
	// If the text appears to be binary data,
	// then perform differencing in approximately fixed-sized chunks.
	// The output is inspired by hexdump.
	case isBinary:
		list = opts.formatDiffSlice(
			reflect.ValueOf(sx), reflect.ValueOf(sy), 16, "byte",
			func(v reflect.Value, d diffMode) textRecord {
				var ss []string
				for i := 0; i < v.Len(); i++ {
					ss = append(ss, formatHex(v.Index(i).Uint()))
				}
				s := strings.Join(ss, ", ")
				comment := commentString(fmt.Sprintf("%c|%v|", d, formatASCII(v.String())))
				return textRecord{Diff: d, Value: textLine(s), Comment: comment}
			},
		)
	// For all other slices of primitive types,
	// then perform differencing in approximately fixed-sized chunks.
	// The size of each chunk depends on the width of the element kind.
	default:
		var chunkSize int
		if t.Elem().Kind() == reflect.Bool {
			chunkSize = 16
		} else {
			switch t.Elem().Bits() {
			case 8:
				chunkSize = 16
			case 16:
				chunkSize = 12
			case 32:
				chunkSize = 8
			default:
				chunkSize = 8
			}
		}
		list = opts.formatDiffSlice(
			vx, vy, chunkSize, t.Elem().Kind().String(),
			func(v reflect.Value, d diffMode) textRecord {
				var ss []string
				for i := 0; i < v.Len(); i++ {
					switch t.Elem().Kind() {
					case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
						ss = append(ss, fmt.Sprint(v.Index(i).Int()))
					case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
						ss = append(ss, formatHex(v.Index(i).Uint()))
					case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
						ss = append(ss, fmt.Sprint(v.Index(i).Interface()))
					}
				}
				s := strings.Join(ss, ", ")
				return textRecord{Diff: d, Value: textLine(s)}
			},
		)
	}

	// Wrap the output with appropriate type information.
	var out textNode = textWrap{"{", list, "}"}
	if !isText {
		// The "{...}" byte-sequence literal is not valid Go syntax for strings.
		// Emit the type for extra clarity (e.g. "string{...}").
		if t.Kind() == reflect.String {
			opts = opts.WithTypeMode(emitType)
		}
		return opts.FormatType(t, out)
	}
	switch t.Kind() {
	case reflect.String:
		out = textWrap{"strings.Join(", out, fmt.Sprintf(", %q)", delim)}
		if t != reflect.TypeOf(string("")) {
			out = opts.FormatType(t, out)
		}
	case reflect.Slice:
		out = textWrap{"bytes.Join(", out, fmt.Sprintf(", %q)", delim)}
		if t != reflect.TypeOf([]byte(nil)) {
			out = opts.FormatType(t, out)
		}
	}
	return out
}
// FormatDiffSlice prints a diff for the slices (or strings) represented by v.
// This provides custom-tailored logic to make printing of differences in
// textual strings and slices of primitive kinds more readable.