feat: build the query for what a tag change would move

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 35 insertions, 0 deletions

diff --git a/src/searchterm.cpp b/src/searchterm.cpp
index 16936df..06d03be 100644
--- a/src/searchterm.cpp
+++ b/src/searchterm.cpp
@@ -18,6 +18,8 @@
 
 #include "searchterm.h"
 
+#include "config.h"
+
 #include <algorithm>
 
 namespace SearchTerm {
@@ -108,4 +110,23 @@ QString exclude(const QString &existing, const QString &addition)
     return QStringLiteral("(%1) AND NOT (%2)").arg(left, right);
 }
 
+QString changes(const QString &query, const QStringList &add,
+                const QStringList &remove)
+{
+    QStringList terms;
+    for (const QString &name : add) {
+        const QString term = tag(name);
+        if (!term.isEmpty())
+            terms.append(QStringLiteral("not ") + term);
+    }
+    for (const QString &name : remove) {
+        const QString term = tag(name);
+        if (!term.isEmpty())
+            terms.append(term);
+    }
+    if (query.trimmed().isEmpty() || terms.isEmpty())
+        return Config::matchNothingQuery();
+    return extend(query, terms.join(QStringLiteral(" or ")));
+}
+
 }  // namespace SearchTerm
diff --git a/src/searchterm.h b/src/searchterm.h
index 8ca67e4..dd43e99 100644
--- a/src/searchterm.h
+++ b/src/searchterm.h
@@ -111,6 +111,20 @@ QString extend(const QString &existing, const QString &addition);
 /// An empty `addition` leaves `existing` untouched.
 QString exclude(const QString &existing, const QString &addition);
 
+/// The messages among `query` that `+add -remove` would actually change:
+/// `(query) AND (not tag:a1 or ... or tag:r1 ...)`.
+///
+/// Used to count and to write a rule applied to existing mail (item 212), so
+/// the number in the confirmation is the number of messages touched, not the
+/// number matched. Built on extend(), so a disjunction in the rule cannot
+/// escape the scope.
+///
+/// **Never empty.** An empty query or an empty change yields
+/// Config::matchNothingQuery(), because notmuch reads an empty query as
+/// "match everything" and this string is written to.
+QString changes(const QString &query, const QStringList &add,
+                const QStringList &remove);
+
 }  // namespace SearchTerm
 
 /// One entry a context menu can offer: a finished query and the text naming it.