Implement get_supported_reso (#48956)

Author: jbrockmendel

pandas/_libs/tslibs/dtypes.pxd

@@ -8,6 +8,7 @@ cdef NPY_DATETIMEUNIT abbrev_to_npy_unit(str abbrev)
 cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) nogil
 cpdef int64_t periods_per_day(NPY_DATETIMEUNIT reso=*) except? -1
 cpdef int64_t periods_per_second(NPY_DATETIMEUNIT reso) except? -1
+cpdef NPY_DATETIMEUNIT get_supported_reso(NPY_DATETIMEUNIT reso)
 
 cdef dict attrname_to_abbrevs
 

pandas/_libs/tslibs/dtypes.pyi

@@ -9,6 +9,7 @@ def periods_per_day(reso: int) -> int: ...
 def periods_per_second(reso: int) -> int: ...
 def is_supported_unit(reso: int) -> bool: ...
 def npy_unit_to_abbrev(reso: int) -> str: ...
+def get_supported_reso(reso: int) -> int: ...
 
 class PeriodDtypeBase:
     _dtype_code: int  # PeriodDtypeCode

pandas/_libs/tslibs/dtypes.pyx

@@ -278,6 +278,19 @@ class NpyDatetimeUnit(Enum):
     NPY_FR_GENERIC = NPY_DATETIMEUNIT.NPY_FR_GENERIC
 
 
+cpdef NPY_DATETIMEUNIT get_supported_reso(NPY_DATETIMEUNIT reso):
+    # If we have an unsupported reso, return the nearest supported reso.
+    if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+        # TODO: or raise ValueError? trying this gives unraisable errors, but
+        #  "except? -1" breaks at compile-time for unknown reasons
+        return NPY_DATETIMEUNIT.NPY_FR_ns
+    if reso < NPY_DATETIMEUNIT.NPY_FR_s:
+        return NPY_DATETIMEUNIT.NPY_FR_s
+    elif reso > NPY_DATETIMEUNIT.NPY_FR_ns:
+        return NPY_DATETIMEUNIT.NPY_FR_ns
+    return reso
+
+
 def is_supported_unit(NPY_DATETIMEUNIT reso):
     return (
         reso == NPY_DATETIMEUNIT.NPY_FR_ns