feat(文件拆分): 拆分PDF格式适配模块

internal/codec/pdf/image_cache.go

@@ -0,0 +1,106 @@
+// 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 pdf
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"reflect"
+
+	"github.com/xiaoqidun/pdfgo"
+)
+
+// imageValueBudget 限制图像描述的检查深度和保留字节数
+type imageValueBudget struct {
+	nodes int
+	bytes int
+}
+
+// ImageDigest 对无间接依赖的小图像生成编码摘要，超预算时不参与内容缓存
+// 入参: ctx 取消上下文, source 图像描述
+// 返回: [32]byte 编码摘要, bool 是否可复用, error 取消错误
+func ImageDigest(ctx context.Context, source *pdfgo.Image) ([32]byte, bool, error) {
+	if source == nil || source.Stream == nil || len(source.Stream.Data) > 1<<20 {
+		return [32]byte{}, false, nil
+	}
+	budget := imageValueBudget{nodes: 256, bytes: 65536}
+	for _, value := range []pdfgo.Object{source.Stream.Dictionary, source.ColorSpace, source.Decode, source.Mask, source.SoftMask} {
+		if !imageDirectValue(value, &budget) {
+			return [32]byte{}, false, ctx.Err()
+		}
+	}
+	hash := sha256.New()
+	data := source.Stream.Data
+	for len(data) != 0 {
+		if err := ctx.Err(); err != nil {
+			return [32]byte{}, false, err
+		}
+		n := min(len(data), 65536)
+		hash.Write(data[:n])
+		data = data[n:]
+	}
+	var result [32]byte
+	hash.Sum(result[:0])
+	return result, true, ctx.Err()
+}
+
+// SameImage 核对完整编码与有效图像属性，不以摘要相同代替内容相同
+// 入参: left 已缓存图像, right 待复用图像，二者须包含有效编码流
+// 返回: bool 是否可使用同一图像资源
+func SameImage(left, right *pdfgo.Image) bool {
+	return left.Width == right.Width && left.Height == right.Height && left.BitsPerComponent == right.BitsPerComponent &&
+		left.ImageMask == right.ImageMask && left.Interpolate == right.Interpolate &&
+		bytes.Equal(left.Stream.Data, right.Stream.Data) && reflect.DeepEqual(left.Stream.Dictionary, right.Stream.Dictionary) &&
+		reflect.DeepEqual(left.ColorSpace, right.ColorSpace) && reflect.DeepEqual(left.Decode, right.Decode) &&
+		reflect.DeepEqual(left.Mask, right.Mask) && reflect.DeepEqual(left.SoftMask, right.SoftMask)
+}
+
+// imageDirectValue 检查图像描述是否完全独立，拒绝间接引用和嵌套流
+// 入参: value 描述值, budget 描述检查预算
+// 返回: bool 是否不依赖阅读器资源
+func imageDirectValue(value pdfgo.Object, budget *imageValueBudget) bool {
+	if budget.nodes <= 0 || budget.bytes < 64 {
+		return false
+	}
+	budget.nodes--
+	budget.bytes -= 64
+	switch v := value.(type) {
+	case nil, pdfgo.Boolean, pdfgo.Integer, pdfgo.Real:
+		return true
+	case pdfgo.Name:
+		budget.bytes -= len(v)
+		return budget.bytes >= 0
+	case pdfgo.String:
+		budget.bytes -= len(v)
+		return budget.bytes >= 0
+	case pdfgo.Array:
+		for _, child := range v {
+			if !imageDirectValue(child, budget) {
+				return false
+			}
+		}
+		return true
+	case pdfgo.Dictionary:
+		for key, child := range v {
+			budget.bytes -= len(key)
+			if !imageDirectValue(child, budget) {
+				return false
+			}
+		}
+		return true
+	}
+	return false
+}