feat(后端架构): 完善后端扩展与字体资源解耦，修复元数据保存和缩略图底边

ofdgo_font_resource.go

@@ -0,0 +1,131 @@
+// 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 (
+	"fmt"
+	"slices"
+
+	"github.com/tdewolff/font"
+)
+
+// FontResource 保存独立字体数据、标准元信息与只读度量，不包含绘图库类型
+type FontResource struct {
+	FontMetrics
+	Data      []byte
+	Font      Font
+	Extension string
+	CanSubset bool
+}
+
+// FontUsage 描述全包已确认的字体引用，Unsafe禁止裁剪
+type FontUsage struct {
+	Characters []rune
+	Glyphs     []uint16
+	Unsafe     bool
+}
+
+// FontResourceBackend 解析和裁剪保存资源，独立于显示字体后端
+// SubsetFont的preserve为true时不得改变字形编号，SubsetSourceFont无安全收益时返回nil
+type FontResourceBackend interface {
+	Backend
+	OpenFontResource(file FontFile, index int) (*FontResource, error)
+	SubsetFont(data []byte, glyphs []uint16, preserve bool) ([]byte, error)
+	SubsetSourceFont(data []byte, usage FontUsage) ([]byte, error)
+}
+
+// SFNTBackend 提供默认字体资源处理，不依赖Canvas绘图库
+type SFNTBackend struct{}
+
+// Name 返回资源后端标识
+// 返回: string 后端标识
+func (SFNTBackend) Name() string { return "sfnt" }
+
+// OpenFontResource 提取独立字体并读取名称、样式和裁剪能力
+// 入参: file 字体文件, index 集合索引
+// 返回: *FontResource 字体资源, error 解析错误
+func (SFNTBackend) OpenFontResource(file FontFile, index int) (*FontResource, error) {
+	data, err := file.Face(index)
+	if err != nil {
+		return nil, err
+	}
+	sfnt, err := font.ParseSFNT(data, 0)
+	if err != nil {
+		return nil, err
+	}
+	name := editorFontName(sfnt, font.NamePostScript, font.NameFull, font.NameFontFamily)
+	if name == "" {
+		return nil, fmt.Errorf("font has no name")
+	}
+	extension := ".ttf"
+	if sfnt.IsCFF {
+		extension = ".otf"
+	}
+	return &FontResource{
+		FontMetrics: sfntResourceMetrics{sfnt}, Data: data, Extension: extension,
+		Font:      Font{FontName: name, FamilyName: editorFontName(sfnt, font.NamePreferredFamily, font.NameFontFamily), Charset: "unicode", Bold: sfnt.Head.MacStyle[0], Italic: sfnt.Head.MacStyle[1], FixedWidth: sfnt.Post.IsFixedPitch != 0},
+		CanSubset: sfntSubsettable(sfnt),
+	}, nil
+}
+
+// sfntSubsettable 判断默认资源后端能否无损裁剪该字体
+// 入参: sfnt 已解析字体
+// 返回: bool 是否支持裁剪
+func sfntSubsettable(sfnt *font.SFNT) bool {
+	return sfnt.IsTrueType && !slices.ContainsFunc([]string{"fvar", "COLR", "CBDT", "sbix", "SVG "}, func(tag string) bool { return sfnt.Tables[tag] != nil })
+}
+
+// sfntResourceMetrics 隔离资源解析器的度量实现
+type sfntResourceMetrics struct{ *font.SFNT }
+
+// Write 返回未改动元信息的字体数据
+// 返回: []byte 字体数据
+func (f sfntResourceMetrics) Write() []byte { return fontSFNTData(f.SFNT) }
+
+// SubsetFont 裁剪新建字体并按要求保留显式字形编号
+// 入参: data 字体数据, glyphs 用字, preserve 是否保留编号
+// 返回: []byte 子集, error 裁剪错误
+func (SFNTBackend) SubsetFont(data []byte, glyphs []uint16, preserve bool) ([]byte, error) {
+	return subsetEditorFont(data, glyphs, preserve)
+}
+
+// SubsetSourceFont 按全包引用裁剪原字体，不确定时保留原资源
+// 入参: data 字体数据, usage 用字信息
+// 返回: []byte 子集, error 裁剪错误
+func (SFNTBackend) SubsetSourceFont(data []byte, usage FontUsage) ([]byte, error) {
+	u := newEditorFontUsage()
+	u.unsafe = usage.Unsafe
+	for _, char := range usage.Characters {
+		u.chars[char] = true
+	}
+	for _, glyph := range usage.Glyphs {
+		u.glyphs[glyph] = true
+	}
+	return subsetSourceFont(data, u), nil
+}
+
+// editorFontName 获取字体中的名称
+// 入参: sfnt 字体, names 名称类型，按顺序查找
+// 返回: string 字体名称
+func editorFontName(sfnt *font.SFNT, names ...font.NameID) string {
+	for _, name := range names {
+		for _, record := range sfnt.Name.Get(name) {
+			if value := record.String(); value != "" {
+				return value
+			}
+		}
+	}
+	return ""
+}