feat(能力完善): 完善转换保真，修复交互缺陷

ofdgo_format.go

@@ -0,0 +1,64 @@
+// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ofdgo
+
+import (
+	"archive/zip"
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// DetectFormat 按文件内容识别PDF或OFD，不读取压缩内容或关闭输入
+// 识别结果不代表文档完整有效，仍需通过对应阅读器解析
+// 入参: source 随机读取源, size 文件字节数
+// 返回: string 格式标识pdf或ofd, error 读取错误或不支持的格式
+func DetectFormat(source io.ReaderAt, size int64) (string, error) {
+	if source == nil || size <= 0 {
+		return "", fmt.Errorf("empty document source")
+	}
+	source = io.NewSectionReader(source, 0, size)
+	var header [1024]byte
+	data := header[:min(size, int64(len(header)))]
+	n, err := source.ReadAt(data, 0)
+	if err != nil && err != io.EOF {
+		return "", err
+	}
+	if n != len(data) {
+		return "", io.ErrUnexpectedEOF
+	}
+	if bytes.HasPrefix(data, []byte("%PDF-")) {
+		return "pdf", nil
+	}
+	archive, err := zip.NewReader(source, size)
+	if err == nil {
+		reader := &Reader{Zip: archive}
+		if err := reader.indexPackage(); err != nil {
+			return "", err
+		}
+		if reader.fileIndexFold["ofd.xml"] != nil || reader.fileIndexFold["encryptions.xml"] != nil {
+			return "ofd", nil
+		}
+	} else {
+		if !errors.Is(err, zip.ErrFormat) {
+			return "", err
+		}
+		if bytes.Contains(data, []byte("%PDF-")) {
+			return "pdf", nil
+		}
+	}
+	return "", fmt.Errorf("unsupported input format")
+}