feat(completion): add the query cursor-context tokenizer

Prefix completion only so far: the token under the cursor, bounded by
whitespace or an opening parenthesis rather than by the start of the line.

1 files changed, 52 insertions, 0 deletions

diff --git a/src/querycompleter.h b/src/querycompleter.h
new file mode 100644
index 0000000..d9635c0
--- /dev/null
+++ b/src/querycompleter.h
@@ -0,0 +1,52 @@
+/*
+ * 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>
+
+/// Where the cursor sits in a query, and therefore what should be offered.
+///
+/// A plain value type produced by a pure function so the parsing rules can be
+/// tested without a widget or a database.
+struct CompletionContext
+{
+    enum Kind {
+        None,    ///< Complete nothing: inside a quoted literal, for instance.
+        Prefix,  ///< Complete a query keyword: tag:, date:, and, or, not.
+        Value,   ///< Complete a value for `prefix`.
+    };
+
+    Kind kind = None;
+
+    /// For Value, the keyword left of ':', lowercased. Empty for Prefix.
+    QString prefix;
+
+    /// The text being matched against the candidates.
+    QString stem;
+
+    /// The exact span an accepted completion overwrites. Covers only the text
+    /// being completed, so accepting never disturbs neighbouring text.
+    int replaceFrom = 0;
+    int replaceLength = 0;
+};
+
+/// Decides what the cursor position implies about completion.
+///
+/// `cursor` is an offset into `text`, as QLineEdit::cursorPosition() returns.
+CompletionContext completionContext(const QString &text, int cursor);