feat(encode): add the InlineTables option

Assisted-by: DeepSeek V4.1 Flash

encode.go

@@ -741,7 +741,20 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 				return err
 			}
 		}
+		// An inlined sub-table is a value line, so it has to precede every
+		// header of this document: a line written after a [header] would be
+		// read back as part of that table.
+		headers := make([]entry, 0, len(tables))
 		for _, t := range tables {
+			inlined, err := e.writeInlineSubTableIfSmall(t.key, t.doc)
+			if err != nil {
+				return err
+			}
+			if !inlined {
+				headers = append(headers, t)
+			}
+		}
+		for _, t := range headers {
 			path := append(append([]string{}, prefix...), t.key)
 			e.writeBlankLine()
 			e.buf.WriteByte('[')
@@ -782,6 +795,13 @@ func (e *encoder) emitDoc(doc *tomlDoc, prefix []string) error {
 				return err
 			}
 		case entryTable:
+			inlined, err := e.writeInlineSubTableIfSmall(ent.key, ent.doc)
+			if err != nil {
+				return err
+			}
+			if inlined {
+				continue
+			}
 			path := append(append([]string{}, prefix...), ent.key)
 			e.writeBlankLine()
 			e.buf.WriteByte('[')
@@ -1021,6 +1041,107 @@ func (e *encoder) writeInlineIndent() {
 	}
 }
 
+// errInlineArrayOfTables reports an attempt to render an array of tables
+// inline, which has no form that keeps the value's type.
+var errInlineArrayOfTables = errors.New("interpres: an array of tables has no inline form")
+
+// inlinableDoc reports whether doc can be written as an inline table without
+// changing the type of any value: scalars, value arrays and further sub-tables
+// are fine, while an array of tables is not, because its inline form would
+// re-parse as a value array.
+func inlinableDoc(doc *tomlDoc) bool {
+	for _, ent := range doc.entries {
+		switch ent.kind {
+		case entryArray:
+			return false
+		case entryTable:
+			if !inlinableDoc(ent.doc) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// writeInlineDocEntry writes one "key = value" binding of an inline table,
+// without the separator that follows it.
+func (e *encoder) writeInlineDocEntry(ent entry) error {
+	if err := e.writeKey(ent.key); err != nil {
+		return err
+	}
+	e.buf.WriteString(" = ")
+	switch ent.kind {
+	case entryTable:
+		return e.writeInlineDoc(ent.doc)
+	case entryArray:
+		return errInlineArrayOfTables
+	default:
+		return e.writeValue(ent.val)
+	}
+}
+
+// writeInlineDoc renders doc as a single-line inline table in entry order, the
+// order the fields were declared in.
+func (e *encoder) writeInlineDoc(doc *tomlDoc) error {
+	e.buf.WriteByte('{')
+	for i, ent := range doc.entries {
+		if i > 0 {
+			e.buf.WriteString(", ")
+		}
+		if err := e.writeInlineDocEntry(ent); err != nil {
+			return err
+		}
+	}
+	e.buf.WriteByte('}')
+	return nil
+}
+
+// writeInlineDocMultiline renders doc 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) writeInlineDocMultiline(doc *tomlDoc) error {
+	e.buf.WriteString("{\n")
+	e.inlineDepth++
+	for _, ent := range doc.entries {
+		e.writeInlineIndent()
+		if err := e.writeInlineDocEntry(ent); err != nil {
+			return err
+		}
+		e.buf.WriteString(",\n")
+	}
+	e.inlineDepth--
+	e.writeInlineIndent()
+	e.buf.WriteByte('}')
+	return nil
+}
+
+// writeInlineSubTableIfSmall writes "key = {…}" for a sub-table whose
+// single-line rendering fits the compact threshold, and reports whether it did
+// so. An array of tables is never inlined, because its inline form would
+// re-parse as a value array and change the value's Go type.
+func (e *encoder) writeInlineSubTableIfSmall(name string, doc *tomlDoc) (bool, error) {
+	if e.opts.inlineTablesAt <= 0 || !inlinableDoc(doc) {
+		return false, nil
+	}
+	flat := e.flat()
+	if err := flat.writeInlineDoc(doc); err != nil {
+		return false, err
+	}
+	if flat.buf.Len() > e.opts.inlineTablesAt {
+		return false, nil
+	}
+	if err := e.writeKey(name); err != nil {
+		return false, err
+	}
+	e.buf.WriteString(" = ")
+	if e.column()+flat.buf.Len() <= e.limit {
+		e.buf.Write(flat.buf.Bytes())
+	} else if err := e.writeInlineDocMultiline(doc); err != nil {
+		return false, err
+	}
+	e.buf.WriteByte('\n')
+	return true, nil
+}
+
 func (e *encoder) writeStringVal(s string) error {
 	if e.opts.literalMultilineAt > 0 && strings.ContainsRune(s, '\n') &&
 		len(s) >= e.opts.literalMultilineAt && canBeLiteralMultiline(s) {