feat(拆分文件): 下沉PDF计算并精简根包

internal/codec/pdf/scratch.go

@@ -0,0 +1,97 @@
+// 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 "image"
+
+// CompositeTileSize 限定单次合成块的像素边长
+const CompositeTileSize = 256
+
+// CompositeBuffers 在一次局部合成中保留至多八个块容量内的空闲缓冲
+type CompositeBuffers[T any] [8][]T
+
+// RasterBuffers 保存按像素尺寸复用的独占覆盖画布
+type RasterBuffers [8]*image.RGBA
+
+// CoverageRow 读取完整范围内的原生覆盖行，其他类型沿用逐点采样
+// 入参: coverage 覆盖图像, y 行坐标, left 起始列, right 结束列
+// 返回: []byte RGBA像素或空值
+func CoverageRow(coverage image.Image, y, left, right int) []byte {
+	buffer, ok := coverage.(*image.RGBA)
+	if !ok || y < buffer.Rect.Min.Y || y >= buffer.Rect.Max.Y || left < buffer.Rect.Min.X || right > buffer.Rect.Max.X || left >= right {
+		return nil
+	}
+	start := buffer.PixOffset(left, y)
+	return buffer.Pix[start : start+4*(right-left)]
+}
+
+// Acquire 借用尚未初始化的独立缓冲，嵌套调用不复用活动缓冲
+// 入参: count 所需元素数量
+// 返回: []T 可由当前调用独占的缓冲
+func (s *CompositeBuffers[T]) Acquire(count int) []T {
+	selected := -1
+	for i, buffer := range s {
+		if cap(buffer) >= count && (selected < 0 || cap(buffer) < cap(s[selected])) {
+			selected = i
+		}
+	}
+	if selected < 0 {
+		return make([]T, count)
+	}
+	buffer := s[selected]
+	s[selected] = nil
+	return buffer[:count]
+}
+
+// Release 归还已不再访问的缓冲，超出块容量或空闲槽数量时直接释放
+// 入参: buffer 当前调用借用的缓冲
+func (s *CompositeBuffers[T]) Release(buffer []T) {
+	if len(buffer) == 0 || cap(buffer) > CompositeTileSize*CompositeTileSize {
+		return
+	}
+	for i, stored := range s {
+		if stored == nil {
+			s[i] = buffer
+			return
+		}
+	}
+}
+
+// Acquire 借用尺寸相同的独占画布，不清空已有像素
+// 入参: rect 输出像素范围
+// 返回: *image.RGBA 当前调用独占的画布
+func (s *RasterBuffers) Acquire(rect image.Rectangle) *image.RGBA {
+	for i, stored := range s {
+		if stored != nil && stored.Rect == rect {
+			s[i] = nil
+			return stored
+		}
+	}
+	return image.NewRGBA(rect)
+}
+
+// Release 归还块容量内的画布，超出容量或空闲槽数量时直接释放
+// 入参: buffer 当前调用借用的画布
+func (s *RasterBuffers) Release(buffer *image.RGBA) {
+	if buffer == nil || cap(buffer.Pix) > 4*CompositeTileSize*CompositeTileSize {
+		return
+	}
+	for i, stored := range s {
+		if stored == nil {
+			s[i] = buffer
+			return
+		}
+	}
+}