feat(encode): write the TOML 1.1 output form

Assisted-by: DeepSeek V4.1 Flash

encode.go

@@ -28,18 +28,50 @@ var (
 	textMarshalerType = reflect.TypeFor[encoding.TextMarshaler]()
 )
 
+// inlineLimit is the column past which an inline table is written across
+// lines. TOML 1.1 lets an inline table carry newlines and a trailing comma, so
+// a long one stays readable instead of running off the line.
+const inlineLimit = 100
+
+// noInlineBreak is the limit a measuring encoder carries, high enough that the
+// form it renders is always the single-line one.
+const noInlineBreak = 1 << 30
+
 // encoder produces a TOML document from a Go value via a small intermediate
 // representation that preserves the order in which fields were declared.
 type encoder struct {
 	buf  bytes.Buffer
 	ctx  context.Context
 	opts Encoder
+
+	// inlineDepth is the nesting level inside inline tables, which decides
+	// their indentation.
+	inlineDepth int
+
+	// limit is the column at which an inline table is broken; only a
+	// measuring encoder raises it.
+	limit int
 }
 
-func newEncoder() *encoder { return &encoder{} }
+func newEncoder() *encoder { return &encoder{limit: inlineLimit} }
+
+// flat returns an encoder that measures a value by rendering it on one line,
+// so a caller can decide which form to write before writing it.
+func (e *encoder) flat() *encoder {
+	return &encoder{ctx: e.ctx, opts: e.opts, limit: noInlineBreak}
+}
 
 func (e *encoder) bytes() []byte { return e.buf.Bytes() }
 
+// column reports how many bytes the current line already holds, so a form can
+// be measured against the limit before it is written.
+func (e *encoder) column() int {
+	if i := bytes.LastIndexByte(e.buf.Bytes(), '\n'); i >= 0 {
+		return e.buf.Len() - i - 1
+	}
+	return e.buf.Len()
+}
+
 func (e *encoder) checkCtx() error {
 	if e.ctx == nil {
 		return nil
@@ -891,7 +923,7 @@ func (e *encoder) writeValue(val any) error {
 	case float64:
 		return e.writeFloat(v)
 	case time.Time:
-		e.buf.WriteString(v.Format(time.RFC3339Nano))
+		e.buf.WriteString(offsetString(v))
 		return nil
 	case LocalDateTime:
 		e.buf.WriteString(v.String())
@@ -915,7 +947,7 @@ func (e *encoder) writeValue(val any) error {
 		e.buf.WriteByte(']')
 		return nil
 	case map[string]any:
-		return e.writeInlineTable(v)
+		return e.writeInlineMap(v)
 	case nil:
 		return fmt.Errorf("interpres: cannot encode nil value")
 	default:
@@ -923,10 +955,24 @@ func (e *encoder) writeValue(val any) error {
 	}
 }
 
-// writeInlineTable renders m as a TOML inline table with sorted keys, the
-// order buildMapDoc uses for header tables. It backs the table elements of a
-// value array, where the [[header]] form is not available.
-func (e *encoder) writeInlineTable(m map[string]any) error {
+// writeInlineMap renders m as a TOML inline table, on one line when it fits
+// there and across lines when it does not.
+func (e *encoder) writeInlineMap(m map[string]any) error {
+	flat := e.flat()
+	if err := flat.writeInlineMapFlat(m); err != nil {
+		return err
+	}
+	if e.column()+flat.buf.Len() <= e.limit {
+		e.buf.Write(flat.buf.Bytes())
+		return nil
+	}
+	return e.writeInlineMapMultiline(m)
+}
+
+// writeInlineMapFlat renders m as a single-line inline table with sorted keys,
+// the order buildMapDoc uses for header tables. It backs the table elements of
+// a value array, where the [[header]] form is not available.
+func (e *encoder) writeInlineMapFlat(m map[string]any) error {
 	keys := slices.Sorted(maps.Keys(m))
 	e.buf.WriteByte('{')
 	for i, k := range keys {
@@ -945,6 +991,36 @@ func (e *encoder) writeInlineTable(m map[string]any) error {
 	return nil
 }
 
+// writeInlineMapMultiline renders m with one entry per line and a trailing
+// comma, the form TOML 1.1 allows for an inline table too long for one line.
+func (e *encoder) writeInlineMapMultiline(m map[string]any) error {
+	keys := slices.Sorted(maps.Keys(m))
+	e.buf.WriteString("{\n")
+	e.inlineDepth++
+	for _, k := range keys {
+		e.writeInlineIndent()
+		if err := e.writeKey(k); err != nil {
+			return err
+		}
+		e.buf.WriteString(" = ")
+		if err := e.writeValue(m[k]); err != nil {
+			return err
+		}
+		e.buf.WriteString(",\n")
+	}
+	e.inlineDepth--
+	e.writeInlineIndent()
+	e.buf.WriteByte('}')
+	return nil
+}
+
+// writeInlineIndent writes one tab per inline-table nesting level.
+func (e *encoder) writeInlineIndent() {
+	for range e.inlineDepth {
+		e.buf.WriteByte('\t')
+	}
+}
+
 func (e *encoder) writeStringVal(s string) error {
 	if e.opts.literalMultilineAt > 0 && strings.ContainsRune(s, '\n') &&
 		len(s) >= e.opts.literalMultilineAt && canBeLiteralMultiline(s) {