Add a link table to the project database

One row per element and element it is linked to -- a coil and its
contacts, a pair of folio reports -- from each side, with the contact
group the element saved for it. Nothing reads it yet: cross-references
are the main thing one folio needs from another, and this is what lets
them be answered without building every folio.

Filled like the other tables: from the document when it can be read
exactly (a link only one side lists, between kinds that cannot link, or
giving a contact a second coil is left to the folios), from the folios
otherwise. A link made or undone afterwards rewrites that element's rows
when the database is next read, as drawing items are, since the contact
group is set after the link is made.

Test: tst_databasefromdocument compares the link table both ways on
every example; linksAreListed checks a coil and contact (with a group)
in m_000.qet, oneSidedLinkFallsBack the refusal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Wga9B92BNurDEtxxtCDxf

tests/qttest/tst_databasefromdocument.cpp

@@ -15,7 +15,7 @@
 // what the same database filled from the built folios holds. Every example
 // is saved once (so that it carries the uuids a current QElectroTech
 // writes), then opened twice through the real binary's --run: once as is,
-// once with QET_DATABASE_FROM_FOLIOS=1, and the six tables compared.
+// once with QET_DATABASE_FROM_FOLIOS=1, and the seven tables compared.
 class tst_databasefromdocument : public QObject
 {
 	Q_OBJECT
@@ -50,7 +50,7 @@ class tst_databasefromdocument : public QObject
 		return QString::fromUtf8(proc.readAllStandardOutput() + proc.readAllStandardError());
 	}
 
-	// The six tables, each as a sorted list of its rows, and which way the
+	// The seven tables, each as a sorted list of its rows, and which way the
 	// database was filled.
 	QJsonObject dump(const QString &project, bool from_folios, QString *how)
 	{
@@ -102,7 +102,7 @@ class tst_databasefromdocument : public QObject
 		QVERIFY2(how_document == QLatin1String("Project database filled from the document"),
 				 qPrintable(how_document));
 		QVERIFY2(how_folios.contains(QStringLiteral("QET_DATABASE_FROM_FOLIOS")), qPrintable(how_folios));
-		QCOMPARE(document.keys().size(), 6);
+		QCOMPARE(document.keys().size(), 7);
 		for (const QString &table : folios.keys()) {
 			const QJsonArray a = document.value(table).toArray(), b = folios.value(table).toArray();
 			QVERIFY2(a == b, qPrintable(QStringLiteral("%1: %2 rows from the document, %3 from the folios")
@@ -121,7 +121,7 @@ private slots:
 		QFile js(m_dir.filePath(QStringLiteral("dump.js")));
 		QVERIFY(js.open(QIODevice::WriteOnly));
 		js.write("var out = {};\n"
-				 "['diagram', 'diagram_info', 'element', 'element_info', 'terminal', 'conductor']"
+				 "['diagram', 'diagram_info', 'element', 'element_info', 'terminal', 'conductor', 'link']"
 				 ".forEach(function (t) { out[t] = qet.query('SELECT * FROM ' + t); });\n"
 				 "qet.log('DUMP ' + JSON.stringify(out));\n");
 	}
@@ -280,6 +280,71 @@ private slots:
 									 "two elements on a folio number their terminals alike"));
 	}
 
+	// m_000.qet saved once, as a document to change
+	QDomDocument resavedWithLinks(const QString &saved)
+	{
+		run({QStringLiteral("--resave"), QStringLiteral(QET_EXAMPLES_DIR "/m_000.qet"), saved});
+		QFile file(saved);
+		QDomDocument document;
+		if (file.open(QIODevice::ReadOnly))
+			document.setContent(&file);
+		return document;
+	}
+
+	// A coil and its contacts are in the link table, from each side, with
+	// both fills: m_000.qet's coil b02216df and its contact 998190fa.
+	void linksAreListed()
+	{
+		const QString saved = m_dir.filePath(QStringLiteral("links.qet"));
+		QDomDocument document = resavedWithLinks(saved);
+		const QString coil = QStringLiteral("{b02216df-0851-4732-893b-901fea80703e}"),
+				contact = QStringLiteral("{998190fa-5b4c-4be3-bc67-8a828eaab2b4}");
+			//The coil puts this contact in its contact group 1.
+		bool grouped = false;
+		const QDomNodeList elements = document.elementsByTagName(QStringLiteral("element"));
+		for (int i = 0 ; i < elements.size() ; ++i) {
+			const QDomElement e = elements.at(i).toElement();
+			if (e.attribute(QStringLiteral("uuid")) != coil) continue;
+			for (QDomElement l = e.firstChildElement(QStringLiteral("links_uuids")).firstChildElement(QStringLiteral("link_uuid")) ;
+				 !l.isNull() ; l = l.nextSiblingElement(QStringLiteral("link_uuid"))) {
+				if (l.attribute(QStringLiteral("uuid")) == contact) {
+					l.setAttribute(QStringLiteral("group_index"), 1);
+					grouped = true;
+				}
+			}
+		}
+		QVERIFY(grouped);
+		QVERIFY(write(saved, document));
+		compareBothWays(saved);
+		bool coil_side = false, contact_side = false;
+		for (const QJsonValue &row : m_last.value(QStringLiteral("link")).toArray()) {
+			const QJsonObject link = QJsonDocument::fromJson(row.toString().toUtf8()).object();
+			const QString from = link.value(QStringLiteral("element_uuid")).toString(),
+					to = link.value(QStringLiteral("linked_uuid")).toString();
+			coil_side |= from == coil && to == contact
+					&& link.value(QStringLiteral("group_index")).toVariant().toInt() == 1;
+			contact_side |= from == contact && to == coil;
+		}
+		QVERIFY2(coil_side, "the coil's link to its contact, in group 1, is missing");
+		QVERIFY2(contact_side, "the contact's link to its coil is missing");
+	}
+
+	// A link only one side lists: the folios decide what it becomes.
+	void oneSidedLinkFallsBack()
+	{
+		const QString saved = m_dir.filePath(QStringLiteral("onesided.qet"));
+		QDomDocument document = resavedWithLinks(saved);
+		const QDomNodeList links = document.elementsByTagName(QStringLiteral("link_uuid"));
+		QVERIFY(links.size() > 0);
+		QDomNode link = links.at(0);
+		link.parentNode().removeChild(link);
+		QVERIFY(write(saved, document));
+		QString how;
+		dump(saved, false, &how);
+		QCOMPARE(how, QStringLiteral("Project database filled from the folios: "
+									 "a link is not one the folios would make as saved"));
+	}
+
 	// A file whose items carry no saved uuid is filled from the folios,
 	// which work those uuids out as they are built, and says why.
 	void olderFileFallsBack()