feat(文件拆分): 拆分字体图像基础能力

internal/base/font/file.go

@@ -0,0 +1,154 @@
+// 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 font
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"strings"
+	"unicode/utf8"
+)
+
+// Face 字体文件中的字体名称与零起始索引，不依赖文件名
+type Face struct {
+	Index          int      `json:"index"`
+	FullName       string   `json:"fullName"`
+	Family         string   `json:"family"`
+	Style          string   `json:"style"`
+	PostScriptName string   `json:"postscriptName"`
+	Names          []string `json:"names"`
+}
+
+// FaceInfo 从名称表读取字体信息
+// 入参: data 名称表数据, index 零起始字体索引
+// 返回: Face 字体信息
+func FaceInfo(data []byte, index int) Face {
+	records, values := fontNameValues(data)
+	name := func(ids ...uint16) string {
+		for _, id := range ids {
+			for _, record := range records {
+				if record.Name == id {
+					if value := strings.TrimSpace(values[[4]uint16{record.Platform, record.Encoding, record.Language, id}]); value != "" {
+						return value
+					}
+				}
+			}
+		}
+		return ""
+	}
+	return Face{Index: index, FullName: name(4, 6, 16, 1), Family: name(16, 1), Style: name(17, 2), PostScriptName: name(6), Names: NamesFromTable(data)}
+}
+
+// FileCount 校验字体或集合头并读取字体数量
+// 入参: data OpenType数据
+// 返回: int 字体数量, error 格式错误
+func FileCount(data []byte) (int, error) {
+	if len(data) < 12 {
+		return 0, fmt.Errorf("invalid font header")
+	}
+	switch string(data[:4]) {
+	case "\x00\x01\x00\x00", "OTTO", "true":
+		return 1, nil
+	case "ttcf":
+		version := binary.BigEndian.Uint32(data[4:])
+		count := uint64(binary.BigEndian.Uint32(data[8:]))
+		if (version != 0x00010000 && version != 0x00020000) || count == 0 || count > uint64(len(data)-12)/4 {
+			return 0, fmt.Errorf("invalid font collection header")
+		}
+		return int(count), nil
+	default:
+		return 0, fmt.Errorf("invalid OpenType header")
+	}
+}
+
+// ReadFaces 读取已解封装字体或集合的名称信息
+// 入参: data OpenType数据
+// 返回: []Face 字体列表, error 格式或名称表错误
+func ReadFaces(data []byte) ([]Face, error) {
+	count, err := FileCount(data)
+	if err != nil {
+		return nil, err
+	}
+	faces := make([]Face, 0, count)
+	for index := 0; index < count; index++ {
+		tables, err := FileTables(data, index)
+		if err != nil {
+			return nil, err
+		}
+		face := FaceInfo(tables["name"], index)
+		if face.FullName == "" {
+			return nil, fmt.Errorf("font %d has no usable name", index)
+		}
+		faces = append(faces, face)
+	}
+	return faces, nil
+}
+
+// ExtractFace 提取已解封装字体中的集合项，规范字符映射并保留原始表
+// 入参: data OpenType数据, index 零起始字体索引
+// 返回: []byte 独立字体数据, error 格式或索引错误
+func ExtractFace(data []byte, index int) ([]byte, error) {
+	var err error
+	if bytes.HasPrefix(data, []byte("ttcf")) {
+		data, err = ExtractCollectionFont(data, index)
+		if err != nil {
+			return nil, err
+		}
+		index = 0
+	}
+	tables, err := FileTables(data, index)
+	if err != nil {
+		return nil, err
+	}
+	cmap, err := NormalizeCmap(tables["cmap"])
+	if err != nil {
+		return nil, err
+	}
+	if !bytes.Equal(cmap, tables["cmap"]) {
+		tables["cmap"] = cmap
+		return SerializeOTF(tables)
+	}
+	return bytes.Clone(data), nil
+}
+
+// MissingGlyphs 检查独立字体的字符覆盖，忽略段落换行并按首次出现顺序去重
+// 入参: data 独立OpenType数据, text 待检查文字
+// 返回: string 缺失字符, error 字体或文字格式错误
+func MissingGlyphs(data []byte, text string) (string, error) {
+	if !utf8.ValidString(text) {
+		return "", fmt.Errorf("text must be UTF-8")
+	}
+	data, err := DefaultCFF2Font(data)
+	if err != nil {
+		return "", err
+	}
+	sfnt, err := ParseSFNT(data)
+	if err != nil {
+		return "", err
+	}
+	seen := make(map[rune]bool)
+	var missing strings.Builder
+	for _, char := range text {
+		if char == '\r' || char == '\n' || seen[char] {
+			continue
+		}
+		seen[char] = true
+		if sfnt.GlyphIndex(char) == 0 {
+			missing.WriteRune(char)
+		}
+	}
+	return missing.String(), nil
+}