up

internal/mailutil/codec.go

@@ -0,0 +1,74 @@
+// Package mailutil provides utilities for email processing:
+// charset conversion and RFC 2047 address formatting.
+package mailutil
+
+import (
+	"fmt"
+	"strings"
+	"unicode/utf8"
+
+	asgomail "github.com/emersion/go-message/mail"
+	"golang.org/x/text/encoding/htmlindex"
+)
+
+// DecodeCharset converts bytes from the given charset to a UTF-8 string.
+// If charset is empty, "utf-8", or "us-ascii", it returns string(buf) directly.
+// If the bytes are already valid UTF-8, no conversion is attempted.
+func DecodeCharset(buf []byte, charset string) string {
+	cs := strings.ToLower(strings.Trim(charset, `"'`))
+	if cs == "" || cs == "utf-8" || cs == "us-ascii" {
+		return string(buf)
+	}
+
+	// 已是合法 UTF-8，无需转换
+	if utf8.Valid(buf) {
+		return string(buf)
+	}
+
+	enc, err := htmlindex.Get(cs)
+	if err != nil {
+		// 未知字符集，原样返回
+		return string(buf)
+	}
+
+	decoded, err := enc.NewDecoder().Bytes(buf)
+	if err != nil {
+		// 解码失败，原样返回
+		return string(buf)
+	}
+
+	return string(decoded)
+}
+
+// FormatAddressList 解析 RFC 2047 编码的地址列表头并格式化为可读字符串。
+// 例如: "=?gb2312?B?zuIgzsS35Q==?= <a@b.com>" → "吴文锋 <a@b.com>"
+func FormatAddressList(header *asgomail.Header, key string) string {
+	addrs, err := header.AddressList(key)
+	if err != nil || len(addrs) == 0 {
+		// 降级：返回原始值
+		return header.Get(key)
+	}
+
+	var parts []string
+	for _, addr := range addrs {
+		if addr.Name != "" {
+			parts = append(parts, fmt.Sprintf("%s <%s>", addr.Name, addr.Address))
+		} else {
+			parts = append(parts, addr.Address)
+		}
+	}
+	return strings.Join(parts, ", ")
+}
+
+// ExtractCharset 从 Content-Type 头值中提取 charset 参数。
+// 例如: "text/plain; charset=gb2312" → "gb2312"
+func ExtractCharset(contentType string) string {
+	parts := strings.Split(contentType, ";")
+	for _, part := range parts[1:] {
+		part = strings.TrimSpace(part)
+		if strings.HasPrefix(strings.ToLower(part), "charset=") {
+			return strings.Trim(strings.TrimPrefix(part, "charset="), `"'`)
+		}
+	}
+	return ""
+}