REF: re-use existing conversion functions (#34625)

Author: jbrockmendel

pandas/_libs/tslibs/period.pyx

@@ -763,10 +763,9 @@ cdef int64_t get_period_ordinal(npy_datetimestruct *dts, int freq) nogil:
     period_ordinal : int64_t
     """
     cdef:
-        int64_t unix_date, seconds, delta
-        int64_t weeks
-        int64_t day_adj
+        int64_t unix_date
         int freq_group, fmonth, mdiff
+        NPY_DATETIMEUNIT unit
 
     freq_group = get_freq_group(freq)
 
@@ -789,44 +788,42 @@ cdef int64_t get_period_ordinal(npy_datetimestruct *dts, int freq) nogil:
         mdiff = dts.month - fmonth + 12
         return (dts.year - 1970) * 4 + (mdiff - 1) // 3
 
-    elif freq == FR_MTH:
-        return (dts.year - 1970) * 12 + dts.month - 1
-
-    unix_date = npy_datetimestruct_to_datetime(NPY_FR_D, dts)
-
-    if freq >= FR_SEC:
-        seconds = unix_date * 86400 + dts.hour * 3600 + dts.min * 60 + dts.sec
-
-        if freq == FR_MS:
-            return seconds * 1000 + dts.us // 1000
-
-        elif freq == FR_US:
-            return seconds * 1000000 + dts.us
-
-        elif freq == FR_NS:
-            return (seconds * 1000000000 +
-                    dts.us * 1000 + dts.ps // 1000)
+    elif freq_group == FR_WK:
+        unix_date = npy_datetimestruct_to_datetime(NPY_FR_D, dts)
+        return unix_date_to_week(unix_date, freq - FR_WK)
 
-        else:
-            return seconds
+    elif freq == FR_BUS:
+        unix_date = npy_datetimestruct_to_datetime(NPY_FR_D, dts)
+        return DtoB(dts, 0, unix_date)
 
-    elif freq == FR_MIN:
-        return unix_date * 1440 + dts.hour * 60 + dts.min
+    unit = get_unit(freq)
+    return npy_datetimestruct_to_datetime(unit, dts)
 
-    elif freq == FR_HR:
-        return unix_date * 24 + dts.hour
 
+cdef NPY_DATETIMEUNIT get_unit(int freq) nogil:
+    """
+    Convert the freq to the corresponding NPY_DATETIMEUNIT to pass
+    to npy_datetimestruct_to_datetime.
+    """
+    if freq == FR_MTH:
+        return NPY_DATETIMEUNIT.NPY_FR_M
     elif freq == FR_DAY:
-        return unix_date
-
+        return NPY_DATETIMEUNIT.NPY_FR_D
+    elif freq == FR_HR:
+        return NPY_DATETIMEUNIT.NPY_FR_h
+    elif freq == FR_MIN:
+        return NPY_DATETIMEUNIT.NPY_FR_m
+    elif freq == FR_SEC:
+        return NPY_DATETIMEUNIT.NPY_FR_s
+    elif freq == FR_MS:
+        return NPY_DATETIMEUNIT.NPY_FR_ms
+    elif freq == FR_US:
+        return NPY_DATETIMEUNIT.NPY_FR_us
+    elif freq == FR_NS:
+        return NPY_DATETIMEUNIT.NPY_FR_ns
     elif freq == FR_UND:
-        return unix_date
-
-    elif freq == FR_BUS:
-        return DtoB(dts, 0, unix_date)
-
-    elif freq_group == FR_WK:
-        return unix_date_to_week(unix_date, freq - FR_WK)
+        # Default to Day
+        return NPY_DATETIMEUNIT.NPY_FR_D
 
 
 cdef void get_date_info(int64_t ordinal, int freq, npy_datetimestruct *dts) nogil: