func encode()

in lib/json/json.go [84:227]
116 lines of code
31 McCabe index (conditional complexity)

func encode(thread *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
	var x starlark.Value
	if err := starlark.UnpackPositionalArgs(b.Name(), args, kwargs, 1, &x); err != nil {
		return nil, err
	}

	buf := new(bytes.Buffer)

	var quoteSpace [128]byte
	quote := func(s string) {
		// Non-trivial escaping is handled by Go's encoding/json.
		if isPrintableASCII(s) {
			buf.Write(strconv.AppendQuote(quoteSpace[:0], s))
		} else {
			// TODO(adonovan): opt: RFC 8259 mandates UTF-8 for JSON.
			// Can we avoid this call?
			data, _ := json.Marshal(s)
			buf.Write(data)
		}
	}

	path := make([]unsafe.Pointer, 0, 8)

	var emit func(x starlark.Value) error
	emit = func(x starlark.Value) error {

		// It is only necessary to push/pop the item when it might contain
		// itself (i.e. the last three switch cases), but omitting it in the other
		// cases did not show significant improvement on the benchmarks.
		if ptr := pointer(x); ptr != nil {
			if pathContains(path, ptr) {
				return fmt.Errorf("cycle in JSON structure")
			}

			path = append(path, ptr)
			defer func() { path = path[0 : len(path)-1] }()
		}

		switch x := x.(type) {
		case json.Marshaler:
			// Application-defined starlark.Value types
			// may define their own JSON encoding.
			data, err := x.MarshalJSON()
			if err != nil {
				return err
			}
			buf.Write(data)

		case starlark.NoneType:
			buf.WriteString("null")

		case starlark.Bool:
			if x {
				buf.WriteString("true")
			} else {
				buf.WriteString("false")
			}

		case starlark.Int:
			fmt.Fprint(buf, x)

		case starlark.Float:
			if !isFinite(float64(x)) {
				return fmt.Errorf("cannot encode non-finite float %v", x)
			}
			fmt.Fprintf(buf, "%g", x) // always contains a decimal point

		case starlark.String:
			quote(string(x))

		case starlark.IterableMapping:
			// e.g. dict (must have string keys)
			buf.WriteByte('{')
			items := x.Items()
			for _, item := range items {
				if _, ok := item[0].(starlark.String); !ok {
					return fmt.Errorf("%s has %s key, want string", x.Type(), item[0].Type())
				}
			}
			sort.Slice(items, func(i, j int) bool {
				return items[i][0].(starlark.String) < items[j][0].(starlark.String)
			})
			for i, item := range items {
				if i > 0 {
					buf.WriteByte(',')
				}
				k, _ := starlark.AsString(item[0])
				quote(k)
				buf.WriteByte(':')
				if err := emit(item[1]); err != nil {
					return fmt.Errorf("in %s key %s: %v", x.Type(), item[0], err)
				}
			}
			buf.WriteByte('}')

		case starlark.Iterable:
			// e.g. tuple, list
			buf.WriteByte('[')
			iter := x.Iterate()
			defer iter.Done()
			var elem starlark.Value
			for i := 0; iter.Next(&elem); i++ {
				if i > 0 {
					buf.WriteByte(',')
				}
				if err := emit(elem); err != nil {
					return fmt.Errorf("at %s index %d: %v", x.Type(), i, err)
				}
			}
			buf.WriteByte(']')

		case starlark.HasAttrs:
			// e.g. struct
			buf.WriteByte('{')
			var names []string
			names = append(names, x.AttrNames()...)
			sort.Strings(names)
			for i, name := range names {
				v, err := x.Attr(name)
				if err != nil || v == nil {
					log.Fatalf("internal error: dir(%s) includes %q but value has no .%s field", x.Type(), name, name)
				}
				if i > 0 {
					buf.WriteByte(',')
				}
				quote(name)
				buf.WriteByte(':')
				if err := emit(v); err != nil {
					return fmt.Errorf("in field .%s: %v", name, err)
				}
			}
			buf.WriteByte('}')

		default:
			return fmt.Errorf("cannot encode %s as JSON", x.Type())
		}
		return nil
	}

	if err := emit(x); err != nil {
		return nil, fmt.Errorf("%s: %v", b.Name(), err)
	}
	return starlark.String(buf.String()), nil
}