REF: implement periods_per_day (pandas-dev#46462)

Author: jbrockmendel

pandas/_libs/tslibs/dtypes.pxd

@@ -1,8 +1,11 @@
+from numpy cimport int64_t
+
 from pandas._libs.tslibs.np_datetime cimport NPY_DATETIMEUNIT
 
 
 cdef str npy_unit_to_abbrev(NPY_DATETIMEUNIT unit)
 cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) nogil
+cdef int64_t periods_per_day(NPY_DATETIMEUNIT reso=*)
 
 cdef dict attrname_to_abbrevs
 

pandas/_libs/tslibs/dtypes.pyx

@@ -307,6 +307,36 @@ cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) nogil:
         return NPY_DATETIMEUNIT.NPY_FR_D
 
 
+cdef int64_t periods_per_day(NPY_DATETIMEUNIT reso=NPY_DATETIMEUNIT.NPY_FR_ns):
+    """
+    How many of the given time units fit into a single day?
+    """
+    cdef:
+        int64_t day_units
+
+    if reso == NPY_DATETIMEUNIT.NPY_FR_ps:
+        # pico is the smallest unit for which we don't overflow, so
+        #  we exclude fempto and atto
+        day_units = 24 * 3600 * 1_000_000_000_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ns:
+        day_units = 24 * 3600 * 1_000_000_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
+        day_units = 24 * 3600 * 1_000_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
+        day_units = 24 * 3600 * 1_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
+        day_units = 24 * 3600
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
+        day_units = 24 * 60
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_h:
+        day_units = 24
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_D:
+        day_units = 1
+    else:
+        raise NotImplementedError(reso)
+    return day_units
+
+
 cdef dict _reso_str_map = {
     Resolution.RESO_NS.value: "nanosecond",
     Resolution.RESO_US.value: "microsecond",

pandas/_libs/tslibs/vectorized.pyx

@@ -21,7 +21,9 @@ cnp.import_array()
 from .conversion cimport normalize_i8_stamp
 
 from .dtypes import Resolution
+
 from .ccalendar cimport DAY_NANOS
+from .dtypes cimport c_Resolution
 from .nattype cimport (
     NPY_NAT,
     c_NaT as NaT,
@@ -168,27 +170,19 @@ def ints_to_pydatetime(
 
 # -------------------------------------------------------------------------
 
-cdef:
-    int RESO_US = Resolution.RESO_US.value
-    int RESO_MS = Resolution.RESO_MS.value
-    int RESO_SEC = Resolution.RESO_SEC.value
-    int RESO_MIN = Resolution.RESO_MIN.value
-    int RESO_HR = Resolution.RESO_HR.value
-    int RESO_DAY = Resolution.RESO_DAY.value
-
 
-cdef inline int _reso_stamp(npy_datetimestruct *dts):
+cdef inline c_Resolution _reso_stamp(npy_datetimestruct *dts):
     if dts.us != 0:
         if dts.us % 1000 == 0:
-            return RESO_MS
-        return RESO_US
+            return c_Resolution.RESO_MS
+        return c_Resolution.RESO_US
     elif dts.sec != 0:
-        return RESO_SEC
+        return c_Resolution.RESO_SEC
     elif dts.min != 0:
-        return RESO_MIN
+        return c_Resolution.RESO_MIN
     elif dts.hour != 0:
-        return RESO_HR
-    return RESO_DAY
+        return c_Resolution.RESO_HR
+    return c_Resolution.RESO_DAY
 
 
 @cython.wraparound(False)
@@ -205,7 +199,7 @@ def get_resolution(const int64_t[:] stamps, tzinfo tz=None) -> Resolution:
         str typ
 
         npy_datetimestruct dts
-        int reso = RESO_DAY, curr_reso
+        c_Resolution reso = c_Resolution.RESO_DAY, curr_reso
 
     if is_utc(tz) or tz is None:
         use_utc = True