Keep the previous versions of a project when saving it

Saving replaced the project file with nothing kept of what it was: one
bad save -- a fault in QElectroTech, a full disk, a wrong edit saved by
mistake -- and the last good version was gone. AutoCAD keeps a .bak on
every save, KiCad several backups.

Before a project is saved over, the file as it was is now copied to a
folder beside it, "<name>-backups", named by the time that version was
saved ("industrial 2026-10-16 08-30-12.qetz"); only the newest are kept,
5 by default (Settings > General > Projects, 0 for none). A copy is
never a move, so a save that fails leaves the file in place; a copy that
fails only warns, the save goes on. The same bytes as the newest version
kept are not kept twice.

The prompt on opening ("Create a backup copy?"), which writes one dated
copy beside the project and never removes any, is left as it is.

New tst_previousversions (newest kept, same bytes once, same second,
nothing for a new file or a count of 0, a failed copy said); it fails
with the oldest never removed. Checked with the program: two scripted
saves of a .qetz leave the original and the first save in
industrial-backups, the project holds the second. ctest 107/107.

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

sources/qetproject.cpp

@@ -48,6 +48,7 @@
 #include "qetinformation.h"
 #include "qetversion.h"
 #include "undocommand/adddiagramcommand.h"
+#include "utils/previousversions.h"
 
 #include <QCryptographicHash>
 #include <QElapsedTimer>
@@ -2759,6 +2760,13 @@ QETResult QETProject::write()
 			return(QString("the file %1 was opened read-only and thus will not be written").arg(m_file_path));
 	}
 
+		//The file as it was goes among its previous versions first
+		//(Settings > General > Projects); a copy that fails does not stop
+		//the save
+	QString backup_error;
+	if (!PreviousVersions::keep(m_file_path, PreviousVersions::count(), &backup_error))
+		qWarning().noquote() << "Previous version of" << m_file_path << "not kept:" << backup_error;
+
 	QString error_message;
 	if (isContainerPath(m_file_path))
 	{

sources/ui/configpage/generalconfigurationpage.cpp

@@ -26,6 +26,7 @@
 #include "../../cablelist/cabletypelist.h"
 #include "../../utils/qetsettings.h"
 #include "../../utils/qetutils.h"
+#include "../../utils/previousversions.h"
 #include "../../qetmessagebox.h"
 #include "../../textgrid.h"
 #include "../../wiringrules.h"
@@ -190,6 +191,7 @@ GeneralConfigurationPage::GeneralConfigurationPage(QWidget *parent) :
 	ui->m_use_folio_label->setChecked(settings.value("genericpanel/folio", true).toBool());
 	ui->m_border_0->setChecked(settings.value("border-columns_0", false).toBool());
 	ui->m_autosave_sb->setValue(settings.value("diagrameditor/autosave-interval", 0).toInt());
+	ui->m_previous_versions_sb->setValue(PreviousVersions::count());
 	
 	QString fontInfos = settings.value("diagramitemfont", "Liberation Sans").toString() + " " +
 			settings.value("diagramitemsize", "9").toString() + " (" +
@@ -417,6 +419,7 @@ void GeneralConfigurationPage::applyConf()
 		}
 	}
 	settings.setValue("diagrameditor/autosave-interval", ui->m_autosave_sb->value());
+	PreviousVersions::setCount(ui->m_previous_versions_sb->value());
 
 	settings.setValue("diagrameditor/grid_display_startup", ui->grid_startup_cb->isChecked());
 	settings.setValue("diagrameditor/guides_display_startup", ui->guides_startup_cb->isChecked());

sources/ui/configpage/generalconfigurationpage.ui

@@ -365,6 +365,29 @@
         </widget>
        </item>
        <item row="7" column="0">
+        <widget class="QLabel" name="m_previous_versions_label">
+         <property name="text">
+          <string>Previous versions kept when saving a project</string>
+         </property>
+         <property name="toolTip">
+          <string>Before a project is saved, the file as it was is copied to a folder beside it, named after the project with "-backups". Only the newest are kept.</string>
+         </property>
+        </widget>
+       </item>
+       <item row="7" column="2">
+        <widget class="QSpinBox" name="m_previous_versions_sb">
+         <property name="toolTip">
+          <string>Before a project is saved, the file as it was is copied to a folder beside it, named after the project with "-backups". Only the newest are kept.</string>
+         </property>
+         <property name="specialValueText">
+          <string>None</string>
+         </property>
+         <property name="maximum">
+          <number>100</number>
+         </property>
+        </widget>
+       </item>
+       <item row="8" column="0">
         <spacer name="verticalSpacer_2">
          <property name="orientation">
           <enum>Qt::Vertical</enum>
@@ -1534,6 +1557,7 @@
   <tabstop>m_use_folio_label</tabstop>
   <tabstop>m_border_0</tabstop>
   <tabstop>m_autosave_sb</tabstop>
+  <tabstop>m_previous_versions_sb</tabstop>
   <tabstop>m_common_elmt_path_cb</tabstop>
   <tabstop>m_custom_elmt_path_cb</tabstop>
   <tabstop>m_custom_tbt_path_cb</tabstop>

sources/utils/previousversions.cpp

@@ -0,0 +1,113 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech 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 QElectroTech. If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "previousversions.h"
+
+#include <QCryptographicHash>
+#include <QDateTime>
+#include <QDir>
+#include <QFile>
+#include <QFileInfo>
+#include <QSettings>
+
+namespace {
+const QString CountKey = QStringLiteral("diagrameditor/previous-versions");
+constexpr int DefaultCount = 5;
+
+	//What a version of @p info is named, without the extension: its base
+	//name and a time
+QString versionPrefix(const QFileInfo &info)
+{
+	return info.completeBaseName() + QLatin1Char(' ');
+}
+
+QByteArray digest(const QString &path)
+{
+	QFile file(path);
+	if (!file.open(QIODevice::ReadOnly)) return {};
+	QCryptographicHash hash(QCryptographicHash::Sha256);
+	hash.addData(&file);
+	return hash.result();
+}
+}
+
+int PreviousVersions::count()
+{
+	return qMax(0, QSettings().value(CountKey, DefaultCount).toInt());
+}
+
+void PreviousVersions::setCount(int count)
+{
+	QSettings().setValue(CountKey, qMax(0, count));
+}
+
+QString PreviousVersions::folder(const QString &file_path)
+{
+	const QFileInfo info(file_path);
+	return info.absoluteDir().filePath(info.completeBaseName() + QStringLiteral("-backups"));
+}
+
+QStringList PreviousVersions::kept(const QString &file_path)
+{
+	const QFileInfo info(file_path);
+	const QDir dir(folder(file_path));
+		//Each copy has the time its version was saved (keep()); a .qet and
+		//a .qetz of the same project are versions of it alike
+	QStringList names = dir.entryList({versionPrefix(info) + QStringLiteral("????-??-?? ??-??-??*.qet"),
+									   versionPrefix(info) + QStringLiteral("????-??-?? ??-??-??*.qetz")},
+									  QDir::Files, QDir::Time);
+	QStringList paths;
+	for (const QString &name : std::as_const(names))
+		paths << dir.filePath(name);
+	return paths;
+}
+
+bool PreviousVersions::keep(const QString &file_path, int count, QString *error)
+{
+	const QFileInfo info(file_path);
+	if (count <= 0 || !info.isFile()) return true;
+
+	const QStringList versions = kept(file_path);
+	if (!versions.isEmpty() && QFileInfo(versions.first()).size() == info.size()
+			&& digest(versions.first()) == digest(file_path))
+		return true;
+
+	QDir dir(folder(file_path));
+	if (!dir.exists() && !QDir().mkpath(dir.path())) {
+		if (error) *error = QStringLiteral("cannot create the folder %1").arg(dir.path());
+		return false;
+	}
+		//Named by the time this version was saved, made unique if another
+		//was saved the same second
+	const QDateTime saved = info.lastModified();
+	const QString stem = versionPrefix(info) + saved.toString(QStringLiteral("yyyy-MM-dd HH-mm-ss"));
+	QString target = dir.filePath(stem + QLatin1Char('.') + info.suffix());
+	for (int n = 2 ; QFileInfo::exists(target) ; ++n)
+		target = dir.filePath(stem + QStringLiteral(" (%1).").arg(n) + info.suffix());
+	if (!QFile::copy(file_path, target)) {
+		if (error) *error = QStringLiteral("cannot copy %1 to %2").arg(file_path, target);
+		return false;
+	}
+	QFile copy(target);
+	if (copy.open(QIODevice::ReadWrite))
+		copy.setFileTime(saved, QFileDevice::FileModificationTime);
+
+	const QStringList all = kept(file_path);
+	for (int i = count ; i < all.size() ; ++i)
+		QFile::remove(all.at(i));
+	return true;
+}

sources/utils/previousversions.h

@@ -0,0 +1,49 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech 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 QElectroTech. If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef PREVIOUSVERSIONS_H
+#define PREVIOUSVERSIONS_H
+
+#include <QString>
+#include <QStringList>
+
+/**
+	@brief The PreviousVersions namespace
+	The versions of a project file kept before it is saved over, as KiCad
+	and AutoCAD keep theirs: a copy of the file as it was goes in a folder
+	beside it, "<name>-backups", named by the time that version was saved
+	("industrial 2026-10-16 08-30-12.qetz"), and only the newest are kept.
+	The file is copied, never moved: a save that fails leaves it in place.
+*/
+namespace PreviousVersions
+{
+		/// How many versions to keep (Settings > General > Projects); 0 keeps none
+	int count();
+	void setCount(int count);
+		/// The folder the versions of @p file_path go in
+	QString folder(const QString &file_path);
+		/// The versions kept of @p file_path, newest first
+	QStringList kept(const QString &file_path);
+		/// Copy @p file_path, about to be saved over, among its versions and
+		/// keep only the @p count newest. Nothing is copied when the file does
+		/// not exist, when @p count is 0, or when the newest version kept
+		/// holds the same bytes. @return false, with @p error, when the copy
+		/// could not be made: the save goes on all the same.
+	bool keep(const QString &file_path, int count, QString *error = nullptr);
+}
+
+#endif // PREVIOUSVERSIONS_H