Revert "REF: de-duplicate precision_from_unit" (#51584)

Revert "REF: de-duplicate precision_from_unit (#51483)"

This reverts commit d08d451.

Author: phofl

pandas/_libs/tslibs/conversion.pyx

@@ -37,7 +37,6 @@ from pandas._libs.tslibs.np_datetime cimport (
     NPY_FR_us,
     check_dts_bounds,
     convert_reso,
-    get_conversion_factor,
     get_datetime64_unit,
     get_datetime64_value,
     get_implementation_bounds,
@@ -84,9 +83,9 @@ TD64NS_DTYPE = np.dtype("m8[ns]")
 # Unit Conversion Helpers
 
 cdef int64_t cast_from_unit(
-    object ts,
-    str unit,
-    NPY_DATETIMEUNIT out_reso=NPY_FR_ns
+        object ts,
+        str unit,
+        NPY_DATETIMEUNIT out_reso=NPY_FR_ns
 ) except? -1:
     """
     Return a casting of the unit represented to nanoseconds
@@ -107,6 +106,10 @@ cdef int64_t cast_from_unit(
 
     m, p = precision_from_unit(unit, out_reso)
 
+    # just give me the unit back
+    if ts is None:
+        return m
+
     if unit in ["Y", "M"]:
         if is_float_object(ts) and not ts.is_integer():
             # GH#47267 it is clear that 2 "M" corresponds to 1970-02-01,
@@ -145,8 +148,8 @@ cdef int64_t cast_from_unit(
 
 
 cpdef inline (int64_t, int) precision_from_unit(
-    str unit,
-    NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
+        str unit,
+        NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
 ):
     """
     Return a casting of the unit represented to nanoseconds + the precision
@@ -159,14 +162,38 @@ cpdef inline (int64_t, int) precision_from_unit(
     """
     cdef:
         int64_t m
+        int64_t multiplier
         int p
         NPY_DATETIMEUNIT reso = abbrev_to_npy_unit(unit)
 
-    if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
-        reso = NPY_FR_ns
-
-    m = get_conversion_factor(reso, out_reso)
-
+    multiplier = periods_per_second(out_reso)
+
+    if reso == NPY_DATETIMEUNIT.NPY_FR_Y:
+        # each 400 years we have 97 leap years, for an average of 97/400=.2425
+        #  extra days each year. We get 31556952 by writing
+        #  3600*24*365.2425=31556952
+        m = multiplier * 31556952
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_M:
+        # 2629746 comes from dividing the "Y" case by 12.
+        m = multiplier * 2629746
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_W:
+        m = multiplier * 3600 * 24 * 7
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_D:
+        m = multiplier * 3600 * 24
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_h:
+        m = multiplier * 3600
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
+        m = multiplier * 60
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
+        m = multiplier
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
+        m = multiplier // 1_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
+        m = multiplier // 1_000_000
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ns or reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+        m = multiplier // 1_000_000_000
+    else:
+        raise ValueError(f"cannot cast unit {unit}")
     p = <int>log10(m)  # number of digits in 'm' minus 1
     return m, p
 

pandas/_libs/tslibs/np_datetime.pyx

@@ -571,8 +571,6 @@ cdef int64_t get_conversion_factor(
         return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_fs, to_unit)
     elif from_unit == NPY_DATETIMEUNIT.NPY_FR_fs:
         return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_as, to_unit)
-    else:
-        raise ValueError("Converting from M or Y units is not supported.")
 
 
 cdef int64_t convert_reso(