[RFC][UN-13487] Fix diff in months (#43)

* Fix diff in months counting months twice

* Add carbon php tests

* Fix logic all test suite passes

* Add negative date tests

* Refactor diff in months

carbon.go

@@ -1181,46 +1181,54 @@ func (c *Carbon) DiffInMonths(carb *Carbon, abs bool) int64 {
 		return 0
 	}
 
-	diffHr := c.DiffInHours(carb, abs)
-	hrLastMonth := int64(c.DaysInMonth() * hoursPerDay)
-
-	if (diffHr - hrLastMonth) >= 0 {
-		var m int64
-		if c.Year() < carb.Year() {
-			m = (int64(monthsPerYear) - int64(c.In(time.UTC).Month())) + (int64(carb.In(time.UTC).Month()) - 1)
-			totalHr := int64(c.DaysInMonth() * hoursPerDay)
-			cHr := c.StartOfMonth().DiffInHours(c, abs)
-			remainHr := totalHr - cHr
-			spentInHr := carb.StartOfMonth().DiffInHours(carb, abs)
-			if (remainHr + spentInHr) >= totalHr {
-				m = m + 1
-			}
-		} else if c.Year() > carb.Year() {
-			m = (int64(monthsPerYear) - int64(carb.In(time.UTC).Month())) + (int64(c.In(time.UTC).Month()) - 1)
-			totalHr := int64(carb.DaysInMonth() * hoursPerDay)
-			carbHr := carb.StartOfMonth().DiffInHours(carb, abs)
-			remainHr := totalHr - carbHr
-			spentInHr := c.StartOfMonth().DiffInHours(c, abs)
-			if (remainHr + spentInHr) >= totalHr {
-				m = m + 1
-			}
-		} else {
-			m = int64(carb.In(time.UTC).Month() - c.In(time.UTC).Month())
+	if c.Month() != carb.Month() && c.Year() == carb.Year() {
+		diffInMonths := int64(carb.In(time.UTC).Month() - c.In(time.UTC).Month())
+		remainingTime := int(carb.DiffInHours(c, true))
+
+		if remainingTime < c.DaysInMonth()*hoursPerDay {
+			return 0
 		}
 
-		diffYr := c.Year() - carb.Year()
-		if math.Abs(float64(diffYr)) > 1 {
-			diff := c.DiffInYears(carb, abs)*monthsPerYear + m
+		return absValue(abs, diffInMonths)
+	}
+
+	m := monthsPerYear - c.In(time.UTC).Month() + carb.In(time.UTC).Month() - 1
+	if c.Year() < carb.Year() && c.hasRemainingHours(carb) {
+		m = m + 1
+	}
+
+	if c.Year() > carb.Year() {
+		m = monthsPerYear - carb.In(time.UTC).Month() + c.In(time.UTC).Month() - 1
 
-			return absValue(abs, diff)
+		if carb.hasRemainingHours(c) {
+			m = m + 1
 		}
+	}
 
-		diff := m
+	diffYr := c.Year() - carb.Year()
+	if math.Abs(float64(diffYr)) > 1 {
+		dateWithoutMonths := c.AddMonths(int(m))
+		diff := dateWithoutMonths.DiffInYears(carb, abs)*monthsPerYear + int64(m)
 
 		return absValue(abs, diff)
 	}
 
-	return 0
+	diff := int64(m)
+
+	if c.GreaterThan(carb) {
+		diff = -diff
+	}
+
+	return absValue(abs, diff)
+}
+
+func (c *Carbon) hasRemainingHours(carb *Carbon) bool {
+	totalHr := int64(c.DaysInMonth() * hoursPerDay)
+	cHr := c.StartOfMonth().DiffInHours(c, false)
+	remainHr := totalHr - cHr
+	spentInHr := carb.StartOfMonth().DiffInHours(carb, false)
+
+	return remainHr+spentInHr >= totalHr
 }
 
 // DiffDurationInString returns the duration difference in string format

carbon_test.go

@@ -1993,6 +1993,76 @@ func TestDiffInMonthsSameMonth(t *testing.T) {
 	assert.EqualValues(t, 0, t1.DiffInMonths(t2, true))
 }
 
+func TestDiffInMonthsDifferentYears(t *testing.T) {
+	t1, _ := Create(2018, time.May, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2020, time.June, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 25, t1.DiffInMonths(t2, true))
+}
+
+func TestDiffInMonthsPositive(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2019, time.February, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 13, t1.DiffInMonths(t2, true))
+}
+
+func TestDiffInMonthsNegativeWithSign(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.February, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, -11, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeWithSignOneHourLess(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.January, 1, 1, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, -11, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeWithSignWithMonthsLess(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.July, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, -6, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeWithSignWithOneMonthLess(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.December, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, -1, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeWithSignWithOneMonthAndOneHourLess(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.December, 1, 1, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 0, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeWithFewDaysLeft(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.December, 20, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 0, t1.DiffInMonths(t2, false))
+}
+
+func TestDiffInMonthsNegativeNoSign(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2017, time.February, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 11, t1.DiffInMonths(t2, true))
+}
+
+func TestDiffInMonthsEnsureIsTruncated(t *testing.T) {
+	t1, _ := Create(2018, time.January, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2018, time.February, 17, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 1, t1.DiffInMonths(t2, true))
+}
+
 func TestDiffInString(t *testing.T) {
 	t1, _ := Create(2016, time.August, 10, 10, 0, 0, 0, "UTC")
 	t2, _ := Create(2016, time.August, 1, 23, 0, 0, 0, "UTC")