From 17d0bf258b40064f35189ca3cf62b3179ca65c72 Mon Sep 17 00:00:00 2001
From: Edward Welbourne <edward.welbourne@qt.io>
Date: Thu, 2 Mar 2023 11:06:53 +0100
Subject: [PATCH] Fix counting of number of months from one date to another

QQuickCalendarModelPrivate::getCount() was kludgey and wrong.
Rewrite it to use QCalendar::partsFromDate(), thereby bypassing repeat
calls to calendrical calculations, document what it's meant to be
doing and do that.

Fixes: QTBUG-111634
Pick-to: 6.5 6.4
Change-Id: I3bdf0233fc1b170eaeb1948ba74e93f636a29bd7
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
---
 src/quicktemplates/qquickcalendarmodel.cpp    | 18 +++++++------
 .../controls/data/tst_calendarmodel.qml       | 25 +++++++++++++------
 2 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/src/quicktemplates/qquickcalendarmodel.cpp b/src/quicktemplates/qquickcalendarmodel.cpp
index 8e2cfe3178..a1605ba77a 100644
--- a/src/quicktemplates/qquickcalendarmodel.cpp
+++ b/src/quicktemplates/qquickcalendarmodel.cpp
@@ -52,21 +52,23 @@ public:
     int count;
 };
 
+// Returns the number of months we need to display for both from and to to be shown,
+// or zero if from is in a later month than to, or either is invalid.
 int QQuickCalendarModelPrivate::getCount(QDate from, QDate to)
 {
     if (!from.isValid() || !to.isValid())
         return 0;
 
-    QDate f(from.year(), from.month(), 1);
-    QDate t(to.year(), to.month(), to.daysInMonth());
-    int days = f.daysTo(t);
-    if (days < 0)
+    const QCalendar gregorian;
+    Q_ASSERT(gregorian.isGregorian());
+    const QCalendar::YearMonthDay &f = gregorian.partsFromDate(from);
+    const QCalendar::YearMonthDay &t = gregorian.partsFromDate(to);
+    Q_ASSERT(f.isValid() && t.isValid()); // ... because from and to are valid.
+    if (f.year > t.year || (f.year == t.year && f.month > t.month))
         return 0;
 
-    QDate r = QDate(1, 1, 1).addDays(days);
-    int years = r.year() - 1;
-    int months = r.month() - 1;
-    return 12 * years + months + (r.day() / t.day());
+    // Count from's month and every subsequent month until to's:
+    return 1 + t.month + 12 * (t.year - f.year) - f.month;
 }
 
 void QQuickCalendarModelPrivate::populate(QDate f, QDate t, bool force)
diff --git a/tests/auto/quickcontrols/controls/data/tst_calendarmodel.qml b/tests/auto/quickcontrols/controls/data/tst_calendarmodel.qml
index 19070e074d..a2f843678f 100644
--- a/tests/auto/quickcontrols/controls/data/tst_calendarmodel.qml
+++ b/tests/auto/quickcontrols/controls/data/tst_calendarmodel.qml
@@ -31,23 +31,32 @@ TestCase {
 
     function test_indices_data() {
         return [
+            // "from" and "to" must currently be in the same year.
             { tag: "2013", from: "2013-01-01", to: "2013-12-31", count: 12 },
-            { tag: "2016", from: "2016-01-01", to: "2016-03-31", count: 3 }
+            { tag: "2016", from: "2016-01-01", to: "2016-03-31", count: 3 },
+            { tag: "2016-02-01 to 2016-12-31", from: "2016-02-01", to: "2016-12-31", count: 11 },
+            { tag: "2014-11-30 to 2016-01-01", from: "2014-11-30", to: "2016-01-01", count: 15 }
         ]
     }
 
     function test_indices(data) {
-        var model = calendarModel.createObject(testCase, {from: data.from, to: data.to})
+        let model = calendarModel.createObject(testCase, {from: data.from, to: data.to})
         verify(model)
 
         compare(model.count, data.count)
 
-        var y = parseInt(data.tag)
-        for (var m = 0; m < 12; ++m) {
-            compare(model.yearAt(m), y)
-            compare(model.indexOf(y, m), m)
-            compare(model.indexOf(new Date(y, m, 1)), m)
-            compare(model.monthAt(m), m)
+        const from = new Date(data.from)
+        const to = new Date(data.to)
+        let index = 0
+        for (let date = from; date <= to; date.setMonth(date.getMonth() + 1, 28), ++index) {
+            compare(model.yearAt(index), date.getFullYear(),
+                `yearAt(${index}) returned incorrect value`)
+            compare(model.indexOf(date.getFullYear(), date.getMonth()), index,
+                `indexOf(${date.getFullYear()}, ${date.getMonth()}) returned incorrect value`)
+            compare(model.indexOf(date), index,
+                `indexOf(${date}) returned incorrect value`)
+            compare(model.monthAt(index), date.getMonth(),
+                `monthAt(${index}) returned incorrect value`)
         }
 
         model.destroy()