From 4f472497807bab3c1f18922ef85324aea69a36e8 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Sat, 18 Feb 2023 17:13:59 -0800
Subject: [PATCH] REF: de-duplicate precision_from_unit

---
 pandas/_libs/tslibs/conversion.pyx  | 49 +++++++----------------------
 pandas/_libs/tslibs/np_datetime.pyx |  2 ++
 2 files changed, 13 insertions(+), 38 deletions(-)

diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index 03a53b1b451e9..18ea6bbb0a257 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -37,6 +37,7 @@ 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,
@@ -83,9 +84,9 @@ 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
@@ -106,10 +107,6 @@ 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,
@@ -148,8 +145,8 @@ 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
@@ -162,38 +159,14 @@ 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)
 
-    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}")
+    if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+        reso = NPY_FR_ns
+
+    m = get_conversion_factor(reso, out_reso)
+
     p = <int>log10(m)  # number of digits in 'm' minus 1
     return m, p
 
diff --git a/pandas/_libs/tslibs/np_datetime.pyx b/pandas/_libs/tslibs/np_datetime.pyx
index aa3411385595b..d9aac87384952 100644
--- a/pandas/_libs/tslibs/np_datetime.pyx
+++ b/pandas/_libs/tslibs/np_datetime.pyx
@@ -571,6 +571,8 @@ 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(