fix: return 0 from length() when excluded boundaries leave a period empty

Excluding the boundaries of a one or two unit long period pushes the
included start past the included end. diff()->days is unsigned and the
HOUR/MINUTE/SECOND branch applied abs() outright, so that backwards range
was measured as a positive distance and the unconditional +1 inflated it
further: a one day period reported 1, 2, 2 and 3 for EXCLUDE_NONE, START,
END and ALL, while iterating the same objects yielded nothing.

Return 0 up front when the included start is past the included end. This
matches what the MONTH and YEAR precisions already returned through
iterator_count(), and lets the now unreachable abs() go.

src/PeriodTraits/PeriodGetters.php

@@ -66,6 +66,13 @@ trait PeriodGetters
 
     public function length(): int
     {
+        // Excluding the boundaries of a period that is only a few units long can push the
+        // included start past the included end. Such a period contains nothing, which is what
+        // iterating over it yields, so its length is 0 rather than a distance measured backwards.
+        if ($this->includedStart() > $this->includedEnd()) {
+            return 0;
+        }
+
         // Length of month and year are not fixed, so we can't predict the length without iterate
         // TODO: maybe we can use cal_days_in_month ?
         if ($this->precision->equals(Precision::MONTH(), Precision::YEAR())) {
@@ -73,7 +80,7 @@ trait PeriodGetters
         }
 
         if ($this->precision->equals(Precision::HOUR(), Precision::MINUTE(), Precision::SECOND())) {
-            $length = abs($this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp());
+            $length = $this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp();
 
             if ($this->precision->equals(Precision::SECOND())) {
                 return $length + 1;

tests/BoundaryTest.php

@@ -35,6 +35,11 @@ it('length with boundaries', function ($expectedAmount, Period $period) {
     expect($period->length())->toEqual($expectedAmount);
 })->with('periods_with_amounts_of_included_dates');
 
+it('length is zero when excluding boundaries leaves nothing', function (Period $period) {
+    expect(iterator_count($period))->toEqual(0);
+    expect($period->length())->toEqual(0);
+})->with('periods_left_empty_by_excluded_boundaries');
+
 it('overlap with excluded boundaries', function () {
     $a = Period::make('2018-01-01', '2018-01-05', boundaries: Boundaries::EXCLUDE_END());
     $b = Period::make('2018-01-05', '2018-01-10');

tests/Datasets/Dates.php

@@ -81,6 +81,38 @@ dataset('periods_with_amounts_of_included_dates', function () {
     yield [86362, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_START())];
     yield [86362, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_END())];
     yield [86361, Period::make('2018-01-01 00:00:00', '2018-01-01 23:59:22', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+
+    // Excluding a boundary of a one unit long period leaves nothing behind.
+    yield [1, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_NONE())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+
+    yield [1, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_NONE())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_START())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_ALL())];
+
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::MINUTE(), Boundaries::EXCLUDE_ALL())];
+    yield [0, Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+
+    // A two unit long period keeps exactly one unit when one of its boundaries is excluded.
+    yield [2, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_NONE())];
+    yield [1, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield [1, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield [0, Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+});
+
+dataset('periods_left_empty_by_excluded_boundaries', function () {
+    yield 'day' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_START())];
+    yield 'day, end excluded' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_END())];
+    yield 'day, all excluded' => [Period::make('2021-01-01', '2021-01-01', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+    yield 'two days, all excluded' => [Period::make('2021-01-01', '2021-01-02', Precision::DAY(), Boundaries::EXCLUDE_ALL())];
+    yield 'hour' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::HOUR(), Boundaries::EXCLUDE_ALL())];
+    yield 'minute' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::MINUTE(), Boundaries::EXCLUDE_ALL())];
+    yield 'second' => [Period::make('2021-01-01 00:00:00', '2021-01-01 00:00:00', Precision::SECOND(), Boundaries::EXCLUDE_ALL())];
+    yield 'month' => [Period::make('2021-01-01', '2021-01-01', Precision::MONTH(), Boundaries::EXCLUDE_ALL())];
+    yield 'year' => [Period::make('2021-01-01', '2021-01-01', Precision::YEAR(), Boundaries::EXCLUDE_ALL())];
 });
 
 dataset('ceiling_dates', function () {