PERF: restore kh_get before kh_put in hashtable operations (pandas-dev#65026)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jbrockmendel

pandas/_libs/hashtable_class_helper.pxi.in

@@ -757,10 +757,11 @@ cdef class {{name}}HashTable(HashTable):
                         append_data_uint8(rmd, 1)
                         continue
 
-                k = kh_put_{{dtype}}(self.table, val, &ret)
+                k = kh_get_{{dtype}}(self.table, val)
 
-                if ret != 0:
+                if k == self.table.n_buckets:
                     # k hasn't been seen yet
+                    k = kh_put_{{dtype}}(self.table, val, &ret)
 
                     if needs_resize(ud.size, ud.capacity):
                         with gil:
@@ -898,8 +899,12 @@ cdef class {{name}}HashTable(HashTable):
                     labels[i] = -1
                     continue
 
-                k = kh_put_{{dtype}}(self.table, val, &ret)
-                if ret != 0:
+                k = kh_get_{{dtype}}(self.table, val)
+                if k != self.table.n_buckets:
+                    idx = self.table.vals[k]
+                    labels[i] = idx
+                else:
+                    k = kh_put_{{dtype}}(self.table, val, &ret)
                     self.table.vals[k] = count
 
                     if needs_resize(ud.size, ud.capacity):
@@ -908,9 +913,6 @@ cdef class {{name}}HashTable(HashTable):
                     append_data_{{dtype}}(ud, val)
                     labels[i] = count
                     count += 1
-                else:
-                    idx = self.table.vals[k]
-                    labels[i] = idx
 
         arr_uniques = uniques.to_array()
 
@@ -1219,9 +1221,10 @@ cdef class StringHashTable(HashTable):
                     continue
 
                 v = vecs[i]
-                k = kh_put_str(self.table, v, &ret)
-                if ret != 0:
+                k = kh_get_str(self.table, v)
+                if k == self.table.n_buckets:
                     # k hasn't been seen yet
+                    k = kh_put_str(self.table, v, &ret)
                     uindexer[count] = i
                     if return_inverse:
                         self.table.vals[k] = count
@@ -1491,9 +1494,10 @@ cdef class PyObjectHashTable(HashTable):
                 labels[i] = na_sentinel
                 continue
 
-            k = kh_put_pymap(self.table, <PyObject*>val, &ret)
-            if ret != 0:
+            k = kh_get_pymap(self.table, <PyObject*>val)
+            if k == self.table.n_buckets:
                 # k hasn't been seen yet
+                k = kh_put_pymap(self.table, <PyObject*>val, &ret)
                 uniques.append(val)
                 if return_inverse:
                     self.table.vals[k] = count

pandas/_libs/hashtable_func_helper.pxi.in

@@ -66,12 +66,13 @@ cdef value_count_{{dtype}}(const {{dtype}}_t[:] values, bint dropna, const uint8
     for i in range(n):
         val = values[i]
         if not dropna or not checknull(val):
-            k = kh_put_{{ttype}}(table, {{to_c_type}}val, &ret)
-            if ret != 0:
+            k = kh_get_{{ttype}}(table, {{to_c_type}}val)
+            if k != table.n_buckets:
+                table.vals[k] += 1
+            else:
+                k = kh_put_{{ttype}}(table, {{to_c_type}}val, &ret)
                 table.vals[k] = 1
                 result_keys.append(val)
-            else:
-                table.vals[k] += 1
     {{else}}
     kh_resize_{{ttype}}(table, n)
 
@@ -89,12 +90,13 @@ cdef value_count_{{dtype}}(const {{dtype}}_t[:] values, bint dropna, const uint8
             if uses_mask and isna_entry:
                 na_counter += 1
             else:
-                k = kh_put_{{ttype}}(table, val, &ret)
-                if ret != 0:
+                k = kh_get_{{ttype}}(table, val)
+                if k != table.n_buckets:
+                    table.vals[k] += 1
+                else:
+                    k = kh_put_{{ttype}}(table, val, &ret)
                     table.vals[k] = 1
                     result_keys.append(val)
-                else:
-                    table.vals[k] += 1
     {{endif}}
 
     # collect counts in the order corresponding to result_keys:
@@ -191,13 +193,14 @@ cdef duplicated_{{dtype}}(const {{dtype}}_t[:] values, object keep='first', cons
 
                 else:
                     value = {{to_c_type}}(values[i])
-                    k = kh_put_{{ttype}}(table, value, &ret)
-                    if ret != 0:
-                        table.vals[k] = i
-                        out[i] = 0
-                    else:
+                    k = kh_get_{{ttype}}(table, value)
+                    if k != table.n_buckets:
                         out[table.vals[k]] = 1
                         out[i] = 1
+                    else:
+                        k = kh_put_{{ttype}}(table, value, &ret)
+                        table.vals[k] = i
+                        out[i] = 0
 
     kh_destroy_{{ttype}}(table)
     return out