REF: de-duplicate tslibs.fields (pandas-dev#38950)

Author: jbrockmendel

pandas/_libs/tslibs/fields.pyx

@@ -174,6 +174,18 @@ def get_date_name_field(const int64_t[:] dtindex, str field, object locale=None)
     return out
 
 
+cdef inline bint _is_on_month(int month, int compare_month, int modby) nogil:
+    """
+    Analogous to DateOffset.is_on_offset checking for the month part of a date.
+    """
+    if modby == 1:
+        return True
+    elif modby == 3:
+        return (month - compare_month) % 3 == 0
+    else:
+        return month == compare_month
+
+
 @cython.wraparound(False)
 @cython.boundscheck(False)
 def get_start_end_field(const int64_t[:] dtindex, str field,
@@ -191,6 +203,7 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
         int start_month = 1
         ndarray[int8_t] out
         npy_datetimestruct dts
+        int compare_month, modby
 
     out = np.zeros(count, dtype='int8')
 
@@ -215,102 +228,15 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
         end_month = 12
         start_month = 1
 
-    if field == 'is_month_start':
-        if is_business:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if dts.day == get_firstbday(dts.year, dts.month):
-                    out[i] = 1
-
-        else:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if dts.day == 1:
-                    out[i] = 1
-
-    elif field == 'is_month_end':
-        if is_business:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if dts.day == get_lastbday(dts.year, dts.month):
-                    out[i] = 1
-
-        else:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if dts.day == get_days_in_month(dts.year, dts.month):
-                    out[i] = 1
-
-    elif field == 'is_quarter_start':
-        if is_business:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if ((dts.month - start_month) % 3 == 0) and (
-                        dts.day == get_firstbday(dts.year, dts.month)):
-                    out[i] = 1
-
-        else:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if ((dts.month - start_month) % 3 == 0) and dts.day == 1:
-                    out[i] = 1
-
-    elif field == 'is_quarter_end':
-        if is_business:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if ((dts.month - end_month) % 3 == 0) and (
-                        dts.day == get_lastbday(dts.year, dts.month)):
-                    out[i] = 1
-
-        else:
-            for i in range(count):
-                if dtindex[i] == NPY_NAT:
-                    out[i] = 0
-                    continue
-
-                dt64_to_dtstruct(dtindex[i], &dts)
-
-                if ((dts.month - end_month) % 3 == 0) and (
-                        dts.day == get_days_in_month(dts.year, dts.month)):
-                    out[i] = 1
+    compare_month = start_month if "start" in field else end_month
+    if "month" in field:
+        modby = 1
+    elif "quarter" in field:
+        modby = 3
+    else:
+        modby = 12
 
-    elif field == 'is_year_start':
+    if field in ["is_month_start", "is_quarter_start", "is_year_start"]:
         if is_business:
             for i in range(count):
                 if dtindex[i] == NPY_NAT:
@@ -319,7 +245,7 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
 
                 dt64_to_dtstruct(dtindex[i], &dts)
 
-                if (dts.month == start_month) and (
+                if _is_on_month(dts.month, compare_month, modby) and (
                         dts.day == get_firstbday(dts.year, dts.month)):
                     out[i] = 1
 
@@ -331,10 +257,10 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
 
                 dt64_to_dtstruct(dtindex[i], &dts)
 
-                if (dts.month == start_month) and dts.day == 1:
+                if _is_on_month(dts.month, compare_month, modby) and dts.day == 1:
                     out[i] = 1
 
-    elif field == 'is_year_end':
+    elif field in ["is_month_end", "is_quarter_end", "is_year_end"]:
         if is_business:
             for i in range(count):
                 if dtindex[i] == NPY_NAT:
@@ -343,7 +269,7 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
 
                 dt64_to_dtstruct(dtindex[i], &dts)
 
-                if (dts.month == end_month) and (
+                if _is_on_month(dts.month, compare_month, modby) and (
                         dts.day == get_lastbday(dts.year, dts.month)):
                     out[i] = 1
 
@@ -355,7 +281,7 @@ def get_start_end_field(const int64_t[:] dtindex, str field,
 
                 dt64_to_dtstruct(dtindex[i], &dts)
 
-                if (dts.month == end_month) and (
+                if _is_on_month(dts.month, compare_month, modby) and (
                         dts.day == get_days_in_month(dts.year, dts.month)):
                     out[i] = 1