feat: lay out the calendar's month grid without a painter

5 files changed, 221 insertions, 0 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 8f32fcc..7e3508b 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -48,6 +48,7 @@ add_library(qtmaildir_lib STATIC
     calendarstore.cpp
     calendarwriter.cpp
     calendarsync.cpp
+    monthlayout.cpp
 )
 
 target_include_directories(qtmaildir_lib
diff --git a/src/monthlayout.cpp b/src/monthlayout.cpp
new file mode 100644
index 0000000..4801fe2
--- /dev/null
+++ b/src/monthlayout.cpp
@@ -0,0 +1,81 @@
+/*
+ * 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.
+ */
+
+#include "monthlayout.h"
+
+MonthLayout::MonthLayout(const QRect &area, int year, int month, Qt::DayOfWeek firstDay,
+                         int dayNumberHeight, int chipHeight)
+    : m_area(area), m_start(gridStart(year, month, firstDay)),
+      m_dayNumberHeight(dayNumberHeight), m_chipHeight(chipHeight)
+{
+}
+
+QDate MonthLayout::gridStart(int year, int month, Qt::DayOfWeek firstDay)
+{
+    const QDate first(year, month, 1);
+    const int back = (first.dayOfWeek() - firstDay + 7) % 7;
+    return first.addDays(-back);
+}
+
+QDate MonthLayout::dateAt(int cell) const { return m_start.addDays(cell); }
+
+int MonthLayout::cellOf(const QDate &date) const
+{
+    const qint64 cell = m_start.daysTo(date);
+    return cell >= 0 && cell < kCells ? int(cell) : -1;
+}
+
+QRect MonthLayout::cellRect(int cell) const
+{
+    // Edges computed from the area by proportion, so the rounding remainder
+    // is spread across cells and the last edge lands exactly on the area's.
+    const int col = cell % 7, row = cell / 7;
+    const int left = m_area.left() + m_area.width() * col / 7;
+    const int right = m_area.left() + m_area.width() * (col + 1) / 7;  // exclusive
+    const int top = m_area.top() + m_area.height() * row / 6;
+    const int bottom = m_area.top() + m_area.height() * (row + 1) / 6;
+    return QRect(QPoint(left, top), QPoint(right - 1, bottom - 1));
+}
+
+int MonthLayout::cellAt(const QPoint &point) const
+{
+    if (!m_area.contains(point))
+        return -1;
+    const int col = (point.x() - m_area.left()) * 7 / m_area.width();
+    const int row = (point.y() - m_area.top()) * 6 / m_area.height();
+    return row * 7 + col;
+}
+
+int MonthLayout::capacity() const
+{
+    const int height = m_area.height() / 6 - m_dayNumberHeight;
+    return m_chipHeight > 0 ? qMax(0, height / m_chipHeight) : 0;
+}
+
+int MonthLayout::visibleChips(int total) const
+{
+    const int cap = capacity();
+    return total <= cap ? total : qMax(0, cap - 1);
+}
+
+QRect MonthLayout::chipRect(int cell, int slot) const
+{
+    const QRect c = cellRect(cell);
+    return QRect(c.left() + 2, c.top() + m_dayNumberHeight + slot * m_chipHeight,
+                 c.width() - 4, m_chipHeight - 1);
+}
diff --git a/src/monthlayout.h b/src/monthlayout.h
new file mode 100644
index 0000000..0aad3fe
--- /dev/null
+++ b/src/monthlayout.h
@@ -0,0 +1,57 @@
+/*
+ * 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 <QDate>
+#include <QPoint>
+#include <QRect>
+
+/// Where everything in the month grid goes, with no painter and no widget.
+///
+/// The CardLayout precedent: a geometric claim is tested as a function call,
+/// because AGENTS.md records how rendering probes lie. Six rows of seven
+/// always, so the grid does not jump in height between months.
+class MonthLayout
+{
+public:
+    static constexpr int kCells = 42;
+
+    MonthLayout(const QRect &area, int year, int month, Qt::DayOfWeek firstDay,
+                int dayNumberHeight, int chipHeight);
+
+    static QDate gridStart(int year, int month, Qt::DayOfWeek firstDay);
+
+    QDate dateAt(int cell) const;
+    int cellOf(const QDate &date) const;  ///< -1 when not on the grid.
+    QRect cellRect(int cell) const;
+    int cellAt(const QPoint &point) const;  ///< -1 outside the grid.
+
+    /// How many chip slots fit under a cell's day number.
+    int capacity() const;
+    /// How many of `total` chips are drawn; when fewer than `total`, the
+    /// next slot holds "+N more".
+    int visibleChips(int total) const;
+    QRect chipRect(int cell, int slot) const;
+
+private:
+    QRect m_area;
+    QDate m_start;
+    int m_dayNumberHeight;
+    int m_chipHeight;
+};
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 6721340..bea6a31 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -76,6 +76,7 @@ add_qtmaildir_test(repeatrule)
 add_qtmaildir_test(calendarstore)
 add_qtmaildir_test(calendarwriter)
 add_qtmaildir_test(calendarsync)
+add_qtmaildir_test(monthlayout)
 add_qtmaildir_test(signatures)
 add_qtmaildir_test(busyindicator)
 add_qtmaildir_test(tagstrip)
diff --git a/tests/test_monthlayout.cpp b/tests/test_monthlayout.cpp
new file mode 100644
index 0000000..fbeb219
--- /dev/null
+++ b/tests/test_monthlayout.cpp
@@ -0,0 +1,81 @@
+/*
+ * 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.
+ */
+
+#include <QtTest>
+
+#include "monthlayout.h"
+
+class TestMonthLayout : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void theGridStartsOnTheLocalesFirstDay();
+    void cellsTileTheAreaWithNoGapOrOverlap();
+    void chipsThatDoNotFitBecomeMore();
+    void aPointFindsItsCell();
+};
+
+void TestMonthLayout::theGridStartsOnTheLocalesFirstDay()
+{
+    // 2026-09-01 is a Tuesday.
+    QCOMPARE(MonthLayout::gridStart(2026, 9, Qt::Monday), QDate(2026, 8, 31));
+    QCOMPARE(MonthLayout::gridStart(2026, 9, Qt::Sunday), QDate(2026, 8, 30));
+    // A month starting ON the first weekday starts on its own first day.
+    QCOMPARE(MonthLayout::gridStart(2026, 6, Qt::Monday), QDate(2026, 6, 1));
+    const MonthLayout layout(QRect(0, 0, 700, 600), 2026, 9, Qt::Monday, 20, 16);
+    QCOMPARE(layout.dateAt(0), QDate(2026, 8, 31));
+    QCOMPARE(layout.dateAt(41), QDate(2026, 10, 11));
+}
+
+void TestMonthLayout::cellsTileTheAreaWithNoGapOrOverlap()
+{
+    // 703 does not divide by 7: the remainder must go somewhere, not vanish
+    // as a gap down the right edge.
+    const QRect area(0, 0, 703, 605);
+    const MonthLayout layout(area, 2026, 9, Qt::Monday, 20, 16);
+    QCOMPARE(layout.cellRect(0).left(), area.left());
+    QCOMPARE(layout.cellRect(6).right(), area.right());
+    QCOMPARE(layout.cellRect(41).bottom(), area.bottom());
+    for (int i = 0; i < 6; ++i)
+        QCOMPARE(layout.cellRect(i).right() + 1, layout.cellRect(i + 1).left());
+    QCOMPARE(layout.cellRect(0).bottom() + 1, layout.cellRect(7).top());
+}
+
+void TestMonthLayout::chipsThatDoNotFitBecomeMore()
+{
+    // 600 / 6 rows = 100 px a cell; 20 px of day number; 16 px chips: 5 fit.
+    const MonthLayout layout(QRect(0, 0, 700, 600), 2026, 9, Qt::Monday, 20, 16);
+    QCOMPARE(layout.capacity(), 5);
+    QCOMPARE(layout.visibleChips(3), 3);
+    QCOMPARE(layout.visibleChips(5), 5);
+    // Six do not fit: four chips and a "+2 more" line in the fifth slot.
+    QCOMPARE(layout.visibleChips(6), 4);
+    QCOMPARE(layout.chipRect(0, 0).top(), layout.cellRect(0).top() + 20);
+    QVERIFY(layout.cellRect(0).contains(layout.chipRect(0, 4)));
+}
+
+void TestMonthLayout::aPointFindsItsCell()
+{
+    const MonthLayout layout(QRect(0, 0, 700, 600), 2026, 9, Qt::Monday, 20, 16);
+    QCOMPARE(layout.cellAt(layout.cellRect(17).center()), 17);
+    QCOMPARE(layout.cellAt(QPoint(-5, 10)), -1);
+}
+
+QTEST_MAIN(TestMonthLayout)
+#include "test_monthlayout.moc"