feat(signatures): splice a signature into a buffer

Both placements over one implementation. above_quote inserts before the
attribution rather than before the first quoted line: the attribution
introduces the quote and belongs with it, and a signature between the two
would read as part of the quoted message.

A buffer with no quote makes above_quote identical to end, so a new
message needs no branch of its own.

Part of item 152.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

1 files changed, 69 insertions, 3 deletions

diff --git a/src/signatures.cpp b/src/signatures.cpp
index 67bf491..ebf69f5 100644
--- a/src/signatures.cpp
+++ b/src/signatures.cpp
@@ -55,13 +55,79 @@ QString text(const QString &dir, const QString &name)
     return QString::fromUtf8(file.readAll());
 }
 
+namespace {
+
+/// The RFC 3676 signature separator: two hyphens, a space, end of line.
+///
+/// The trailing space is part of the standard and is what receiving clients
+/// match on to fold or strip a signature. It is also why `--` typed by hand
+/// does not collide: an editor does not add trailing whitespace on its own.
+const QLatin1String kDelimiter("-- ");
+
+bool isQuoted(const QString &line)
+{
+    return line.startsWith(QLatin1Char('>'));
+}
+
+/// The index of the first line of the quote, or -1 when the buffer has none.
+///
+/// The attribution line ("On Mon, someone wrote:") is deliberately NOT
+/// included: it introduces the quote and belongs with it, so a signature
+/// inserted above the quote goes above the attribution too. Returning the
+/// quoted line itself would strand the signature between the attribution and
+/// the text it introduces.
+int quoteStart(const QStringList &lines)
+{
+    for (int i = 0; i < lines.size(); ++i) {
+        if (!isQuoted(lines.at(i)))
+            continue;
+        // Walk back over the attribution and the blank line before it, so the
+        // signature lands above the whole block rather than inside it.
+        int start = i;
+        while (start > 0 && !lines.at(start - 1).trimmed().isEmpty()
+               && !isQuoted(lines.at(start - 1)))
+            --start;
+        return start;
+    }
+    return -1;
+}
+
+}  // namespace
+
 QString replace(const QString &buffer, const QString &signature,
                 const QStringList &known, Position position)
 {
-    Q_UNUSED(signature);
     Q_UNUSED(known);
-    Q_UNUSED(position);
-    return buffer;
+
+    if (signature.isEmpty())
+        return buffer;
+
+    const QString block = QStringLiteral("\n") + kDelimiter
+                          + QStringLiteral("\n") + signature;
+
+    QStringList lines = buffer.split(QLatin1Char('\n'));
+    const int quote =
+        position == Position::AboveQuote ? quoteStart(lines) : -1;
+
+    // No quote to sit above is not a special case: it is the End placement,
+    // which is why a New message needs no branch of its own.
+    if (quote < 0)
+        return buffer + block;
+
+    QStringList head = lines.mid(0, quote);
+    const QStringList tail = lines.mid(quote);
+    // The head ends in however many blank lines separated the reply from the
+    // attribution. Drop them all and let the block supply exactly one, so the
+    // spacing is the same whatever the quote was seeded with.
+    while (!head.isEmpty() && head.last().trimmed().isEmpty())
+        head.removeLast();
+
+    // head.join() has no trailing newline once trimmed, so the terminator for
+    // its last line is supplied here; `block` then opens with the blank line,
+    // which is the same shape as the End placement over a buffer ending in a
+    // newline.
+    return head.join(QLatin1Char('\n')) + QStringLiteral("\n") + block
+           + QStringLiteral("\n\n") + tail.join(QLatin1Char('\n'));
 }
 
 }  // namespace Signatures