Implement roll_monthday, simplify SemiMonthOffset

pandas/_libs/tslibs/offsets.pyx

@@ -523,11 +523,9 @@ def shift_quarters(int64_t[:] dtindex, int quarters,
                 n = quarters
 
                 months_since = (dts.month - q1start_month) % modby
-                compare_month = dts.month - months_since
-                compare_month = compare_month or 12
                 # compare_day is only relevant for comparison in the case
                 # where months_since == 0.
-                compare_day = get_firstbday(dts.year, compare_month)
+                compare_day = get_firstbday(dts.year, dts.month)
 
                 if n <= 0 and (months_since != 0 or
                                (months_since == 0 and dts.day > compare_day)):
@@ -556,11 +554,9 @@ def shift_quarters(int64_t[:] dtindex, int quarters,
                 n = quarters
 
                 months_since = (dts.month - q1start_month) % modby
-                compare_month = dts.month - months_since
-                compare_month = compare_month or 12
                 # compare_day is only relevant for comparison in the case
                 # where months_since == 0.
-                compare_day = get_lastbday(dts.year, compare_month)
+                compare_day = get_lastbday(dts.year, dts.month)
 
                 if n <= 0 and (months_since != 0 or
                                (months_since == 0 and dts.day > compare_day)):
@@ -777,6 +773,7 @@ cpdef datetime shift_month(datetime stamp, int months, object day_opt=None):
     return stamp.replace(year=year, month=month, day=day)
 
 
+# TODO: Can we declare this so it will take datetime _or_ pandas_datetimestruct
 cpdef int get_day_of_month(datetime other, day_opt) except? -1:
     """
     Find the day in `other`'s month that satisfies a DateOffset's onOffset
@@ -827,6 +824,75 @@ cpdef int get_day_of_month(datetime other, day_opt) except? -1:
         raise ValueError(day_opt)
 
 
+cpdef int roll_monthday(other, int n, compare):
+    """
+    Possibly increment or decrement the number of periods to shift
+    based on rollforward/rollbackward conventions.
+
+    Parameters
+    ----------
+    other : datetime or Timestamp
+    n : number of periods to increment, before adjusting for rolling
+    day_opt : 'start', 'end', 'business_start', 'business_end'
+        The convention to use in finding the day in a given month against
+        which to compare for rollforward/rollbackward decisions.
+
+    Returns
+    -------
+    n : int number of periods to increment
+    """
+    # Either `other` and `compare` are _both_ datetimes or they are both
+    # integers for days in the same month.
+
+    if n > 0 and other < compare:
+        n -= 1
+    elif n <= 0 and other > compare:
+        # as if rolled forward already
+        n += 1
+    return n
+
+
+cpdef int roll_qtrday(datetime other, int n, int month, day_opt='start',
+                      int modby=3) except? -1:
+    """
+    Possibly increment or decrement the number of periods to shift
+    based on rollforward/rollbackward conventions.
+
+    Parameters
+    ----------
+    other : datetime or Timestamp
+    n : number of periods to increment, before adjusting for rolling
+    month : reference month giving the first month of the year
+    day_opt : 'start', 'end', 'business_start', 'business_end'
+        The convention to use in finding the day in a given month against
+        which to compare for rollforward/rollbackward decisions.
+
+    Returns
+    -------
+    n : int number of periods to increment
+    """
+    # TODO: type `other` as datetime-or-pandas_datetimestruct?
+    # TODO: Merge this with roll_yearday by setting modby=12 there?
+    #       code de-duplication versus perf hit?
+    # TODO: with small adjustments this could be used in shift_quarters
+    months_since = other.month % modby - month % modby
+
+    if n > 0:
+        if months_since < 0 or (months_since == 0 and
+                                other.day < get_day_of_month(other,
+                                                             day_opt)):
+            # pretend to roll back if on same month but
+            # before compare_day
+            n -= 1
+    else:
+        if (months_since > 0 or (months_since == 0 and
+                                 other.day > get_day_of_month(other,
+                                                              day_opt))):
+            # make sure to roll forward, so negate
+            n += 1
+    return n
+
+
 cpdef int roll_yearday(other, n, month, day_opt='start') except? -1:
     """
     Possibly increment or decrement the number of periods to shift
@@ -836,6 +902,7 @@ cpdef int roll_yearday(other, n, month, day_opt='start') except? -1:
     ----------
     other : datetime or Timestamp
     n : number of periods to increment, before adjusting for rolling
+    month : reference month giving the first month of the year
     day_opt : 'start', 'end'
         'start': returns 1
         'end': returns last day  of the month
@@ -888,7 +955,7 @@ cpdef int roll_yearday(other, n, month, day_opt='start') except? -1:
                                    other.day < get_day_of_month(other,
                                                                 day_opt)):
             n -= 1
-    elif n <= 0:
+    else:
         if other.month > month or (other.month == month and
                                    other.day > get_day_of_month(other,
                                                                 day_opt)):