message: try to detect body text language

Try to detect the language of the e-mail body and make it searchable.

lib/message/mu-fields.cc

@@ -165,7 +165,6 @@ test_field_from_name()
 		      Field::Id::Bcc);
 }
 
-
 static void
 test_xapian_term()
 {

lib/message/mu-fields.hh

@@ -52,6 +52,7 @@ struct Field {
 		File,		/**< Filename */
 		Flags,		/**< Message flags */
 		From,		/**< Message sender */
+		Language,	/**< Body language */
 		Maildir,	/**< Maildir path */
 		MailingList,	/**< Mailing list */
 		MessageId,	/**< Message Id */
@@ -252,7 +253,6 @@ static constexpr std::array<Field, Field::id_size()>
 		Field::Flag::IncludeInSexp |
 		Field::Flag::IndexableTerm,
 	    },
-
 	    {
 		Field::Id::Changed,
 		Field::Type::TimeT,
@@ -316,6 +316,17 @@ static constexpr std::array<Field, Field::id_size()>
 		Field::Flag::IncludeInSexp |
 		Field::Flag::IndexableTerm,
 	    },
+	    {
+		Field::Id::Language,
+		Field::Type::String,
+		"language", "lang",
+		"ISO 639-1 language code for body",
+		"lang:nl",
+		'a',
+		Field::Flag::BooleanTerm |
+		Field::Flag::Value |
+		Field::Flag::IncludeInSexp
+	    },
 	    {
 		Field::Id::Maildir,
 		Field::Type::String,

lib/message/mu-message.cc

@@ -29,6 +29,7 @@
 #include <utils/mu-utils.hh>
 #include <utils/mu-error.hh>
 #include <utils/mu-option.hh>
+#include <utils/mu-lang-detector.hh>
 
 #include <atomic>
 #include <mutex>
@@ -67,6 +68,8 @@ struct Message::Private {
 	Option<std::string> body_txt;
 	Option<std::string> body_html;
 	Option<std::string> embedded;
+
+	Option<std::string> language; /* body ISO language code */
 };
 
 
@@ -531,6 +534,14 @@ process_message(const MimeMessage& mime_msg, const std::string& path,
 	info.mailing_list = get_mailing_list(mime_msg);
 	if (info.mailing_list)
 		info.flags |= Flags::MailingList;
+
+	if (info.body_txt) { /* attempt to get the body-language */
+		if (const auto lang{detect_language(info.body_txt.value())}; lang) {
+			info.language = lang->code;
+			g_debug("detected language: %s", lang->code);
+		} else
+			g_debug("could not detect language");
+	}
 }
 
 static Mu::Result<std::string>
@@ -586,8 +597,6 @@ fake_message_id(const std::string& path)
  * based on a field. So we add them here.
  */
 
-
-
 static void
 doc_add_list_post(Document& doc, const MimeMessage& mime_msg)
 {
@@ -643,7 +652,7 @@ fill_document(Message::Private& priv)
 	doc_add_reply_to(doc, mime_msg);  /* only in sexp */
 
 	field_for_each([&](auto&& field) {
-		/* insist on expliclity handling each */
+		/* insist on explicitly handling each */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic error "-Wswitch"
 		switch(field.id) {
@@ -652,7 +661,6 @@ fill_document(Message::Private& priv)
 			break;
 		case Field::Id::BodyText:
 			doc.add(field.id, priv.body_txt);
-
 			break;
 		case Field::Id::Cc:
 			doc.add(field.id, mime_msg.contacts(Contact::Type::Cc));
@@ -676,6 +684,9 @@ fill_document(Message::Private& priv)
 		case Field::Id::From:
 			doc.add(field.id, mime_msg.contacts(Contact::Type::From));
 			break;
+		case Field::Id::Language:
+			doc.add(field.id, priv.language);
+			break;
 		case Field::Id::Maildir: /* already */
 			break;
 		case Field::Id::MailingList:

lib/message/mu-message.hh

@@ -110,7 +110,6 @@ public:
 	}
 	/* LCOV_EXCL_STOP */
 
-
 	/**
 	 * Construct a message from a string. This is mostly useful for testing.
 	 *
@@ -406,8 +405,8 @@ public:
 	const std::vector<Part>& parts() const;
 
 	/**
-	 * Get the path to a cche directory for this message, which
-	 * is useful for temporarily saving attachments
+	 * Get the path to a cache directory for this message, which is useful
+	 * for temporarily saving attachments
 	 *
 	 * @param index optionally, create <cache-path>/<index> instead;
 	 * this is useful for having part-specific subdirectories.