Fix the gregorian date <-> julian day calculations in QDate

The old code is just plain wrong for negative julian days. Replaced
with plain math from The Calendar FAQ [1], which is correct for all
julian days, provided you use mathematical integer division (round to
negative infinity) rather than c++11 integer division (round to zero).

[1] http://www.tondering.dk/claus/cal/julperiod.php

While the conversion code works for up to around JD +/- (2^63/4), we
only use an int for the year in the API, so this patch limits minJd()
and maxJd() to 1 Jan (2^31) BC and 31 Dec (2^31-1) AD, respectively.

Note that while the new conversion code looks like it would be more
expensive than the old, gcc will in fact be able to optimize it to be
slightly faster (probably because x86 hardware implements round to
negative infinity, and so GCC manages to optimize floordiv to a single
instruction, compared to the three instuctions needed for operator/).

In the following test application, run with a release mode Qt and
redirecting stderr to /dev/null, I measured an improvement from
6.81s +/- 0.08s to 6.26s +/- 0.16s user time over five runs on an
otherwise idle x86_64 system.

int main(int, char *[])
{
    int year, month, day;
    qint64 jd;
    for (qint64 i = Q_INT64_C(-1048576) ; i < Q_INT64_C(1048576); ++i) {
        QDate::fromJulianDay(i).getDate(&year, &month, &day);
        jd = QDate(year, month, day).toJulianDay();
        qDebug() << jd << year << month << day;
    }
}

Change-Id: Ifd0dd01f0027f260401f7f9b4f1201d2b7a3b087
Reviewed-by: David Faure (KDE) <faure@kde.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

1 files changed, 23 insertions, 11 deletions

diff --git a/tests/auto/corelib/tools/qdate/tst_qdate.cpp b/tests/auto/corelib/tools/qdate/tst_qdate.cpp
index 01805e1271..9978a252bf 100644
--- a/tests/auto/corelib/tools/qdate/tst_qdate.cpp
+++ b/tests/auto/corelib/tools/qdate/tst_qdate.cpp
@@ -117,8 +117,8 @@ void tst_QDate::isNull_data()
     QTest::addColumn<qint64>("jd");
     QTest::addColumn<bool>("null");
 
-    qint64 minJd = std::numeric_limits<qint64>::min() / 2;
-    qint64 maxJd = std::numeric_limits<qint64>::max() / 2;
+    qint64 minJd = Q_INT64_C(-784350574879);
+    qint64 maxJd = Q_INT64_C( 784354017364);
 
     QTest::newRow("qint64 min") << std::numeric_limits<qint64>::min() << true;
     QTest::newRow("minJd - 1")  << minJd - 1                          << true;
@@ -448,8 +448,8 @@ void tst_QDate::julianDaysLimits()
 {
     qint64 min = std::numeric_limits<qint64>::min();
     qint64 max = std::numeric_limits<qint64>::max();
-    qint64 minJd = std::numeric_limits<qint64>::min() / 2;
-    qint64 maxJd = std::numeric_limits<qint64>::max() / 2;
+    qint64 minJd = Q_INT64_C(-784350574879);
+    qint64 maxJd = Q_INT64_C( 784354017364);
 
     QDate maxDate = QDate::fromJulianDay(maxJd);
     QDate minDate = QDate::fromJulianDay(minJd);
@@ -492,7 +492,7 @@ void tst_QDate::julianDaysLimits()
     dt = minDate.addDays(min);
     QCOMPARE(dt.isValid(), false);
     dt = minDate.addDays(max);
-    QCOMPARE(dt.isValid(), true);
+    QCOMPARE(dt.isValid(), false);
 
     dt = zeroDate.addDays(-1);
     QCOMPARE(dt.isValid(), true);
@@ -664,8 +664,8 @@ void tst_QDate::addYears_data()
 
 void tst_QDate::daysTo()
 {
-    qint64 minJd = std::numeric_limits<qint64>::min() / 2;
-    qint64 maxJd = std::numeric_limits<qint64>::max() / 2;
+    qint64 minJd = Q_INT64_C(-784350574879);
+    qint64 maxJd = Q_INT64_C( 784354017364);
 
     QDate dt1(2000, 1, 1);
     QDate dt2(2000, 1, 5);
@@ -1356,9 +1356,10 @@ void tst_QDate::roundtrip() const
     // year(), month(), day(), julianDayFromDate(), and getDateFromJulianDay()
     // to ensure they are internally consistent (but doesn't guarantee correct)
 
-    // Test Julian round trip around JD 0 and current low end of valid range
+    // Test Julian round trip around JD 0 and the c++ integer division rounding
+    // problem point (eg. negative numbers) in the conversion functions.
     QDate testDate;
-    QDate loopDate = QDate::fromJulianDay(-31738); // 1 Jan 4800 BC
+    QDate loopDate = QDate::fromJulianDay(-50001); // 1 Jan 4850 BC
     while (loopDate.toJulianDay() <= 5150) {     // 31 Dec 4700 BC
         testDate.setDate(loopDate.year(), loopDate.month(), loopDate.day());
         QCOMPARE(loopDate.toJulianDay(), testDate.toJulianDay());
@@ -1389,9 +1390,20 @@ void tst_QDate::roundtrip() const
         loopDate = loopDate.addDays(1);
     }
 
+    qint64 minJd = Q_INT64_C(-784350574879);
+    qint64 maxJd = Q_INT64_C( 784354017364);
+
     // Test Gregorian round trip at top end of conversion range
-    loopDate = QDate::fromJulianDay(513024036);     //  1 Jan 1399900 AD
-    while (loopDate.toJulianDay() <= 513060925) { // 31 Dec 1400000 AD
+    loopDate = QDate::fromJulianDay(maxJd);
+    while (loopDate.toJulianDay() >= maxJd - 146397) {
+        testDate.setDate(loopDate.year(), loopDate.month(), loopDate.day());
+        QCOMPARE(loopDate.toJulianDay(), testDate.toJulianDay());
+        loopDate = loopDate.addDays(-1);
+    }
+
+    // Test Gregorian round trip at low end of conversion range
+    loopDate = QDate::fromJulianDay(minJd);
+    while (loopDate.toJulianDay() <= minJd + 146397) {
         testDate.setDate(loopDate.year(), loopDate.month(), loopDate.day());
         QCOMPARE(loopDate.toJulianDay(), testDate.toJulianDay());
         loopDate = loopDate.addDays(1);