BUG: track nan values before filling them GH6945

seperate real na and filled na

Author: peterpanmj

pandas/_libs/algos_rank_helper.pxi.in

@@ -44,17 +44,21 @@ def rank_1d_{{dtype}}(object in_arr, ties_method='average', ascending=True,
 
         {{if dtype == 'object'}}
         ndarray sorted_data, values
+        ndarray[np.uint8_t, cast=True] sorted_namask
+        {{elif dtype != 'uint64'}}
+        ndarray[{{ctype}}] sorted_data, values
+        ndarray[np.uint8_t, cast=True] sorted_namask
         {{else}}
         ndarray[{{ctype}}] sorted_data, values
         {{endif}}
-
+        
         ndarray[float64_t] ranks
         ndarray[int64_t] argsorted
 
         {{if dtype == 'uint64'}}
         {{ctype}} val
         {{else}}
-        {{ctype}} val, nan_value
+        {{ctype}} val, nan_value, isnan
         {{endif}}
 
         float64_t sum_ranks = 0
@@ -121,18 +125,23 @@ def rank_1d_{{dtype}}(object in_arr, ties_method='average', ascending=True,
 
     if not ascending:
         _as = _as[::-1]
-
+    
     sorted_data = values.take(_as)
+    # need to distinguish between pos/neg nan and real nan when keep_na is true
+    {{if dtype != 'uint64'}}
+    sorted_namask = mask.take(_as)
+    sorted_namask = sorted_namask.astype(np.bool)
+    {{endif}}
     argsorted = _as.astype('i8')
 
     {{if dtype == 'object'}}
     for i in range(n):
         sum_ranks += i + 1
         dups += 1
-
+        isnan = sorted_namask[i]
         val = util.get_value_at(sorted_data, i)
 
-        if (val is nan_value) and keep_na:
+        if isnan and keep_na:
             ranks[argsorted[i]] = nan
             continue
 
@@ -168,7 +177,8 @@ def rank_1d_{{dtype}}(object in_arr, ties_method='average', ascending=True,
             val = sorted_data[i]
 
             {{if dtype != 'uint64'}}
-            if (val == nan_value) and keep_na:
+            isnan = sorted_namask[i]
+            if  isnan and keep_na:
                 ranks[argsorted[i]] = nan
                 continue
             {{endif}}