ENH: Added 'direction' parameter to merge_asof() (#14887)

doc/source/whatsnew/v0.20.0.txt

@@ -115,6 +115,7 @@ Other enhancements
 - ``pandas.io.json.json_normalize()`` gained the option ``errors='ignore'|'raise'``; the default is ``errors='raise'`` which is backward compatible. (:issue:`14583`)
 
 - ``.select_dtypes()`` now allows the string 'datetimetz' to generically select datetimes with tz (:issue:`14910`)
+- ``pd.merge_asof()`` gained the option ``direction='backward'|'forward'|'nearest'`` (:issue:`14887`)
 
 
 .. _whatsnew_0200.api_breaking:

pandas/src/joins_func_helper.pxi.in

@@ -28,7 +28,8 @@ from hashtable cimport *
 {{for on_dtype in on_dtypes}}
 
 
-def asof_join_{{on_dtype}}_by_{{by_dtype}}(ndarray[{{on_dtype}}] left_values,
+def asof_join_backward_{{on_dtype}}_by_{{by_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
         ndarray[{{on_dtype}}] right_values,
         ndarray[{{by_dtype}}] left_by_values,
         ndarray[{{by_dtype}}] right_by_values,
@@ -40,6 +41,7 @@ def asof_join_{{on_dtype}}_by_{{by_dtype}}(ndarray[{{on_dtype}}] left_values,
         ndarray[int64_t] left_indexer, right_indexer
         bint has_tolerance = 0
         {{on_dtype}} tolerance_
+        {{on_dtype}} diff
         {{table_type}} hash_table
         {{by_dtype}} by_value
 
@@ -62,7 +64,7 @@ def asof_join_{{on_dtype}}_by_{{by_dtype}}(ndarray[{{on_dtype}}] left_values,
         if right_pos < 0:
             right_pos = 0
 
-        # find last position in right whose value is less than left's value
+        # find last position in right whose value is less than left's
         if allow_exact_matches:
             while right_pos < right_size and\
                 right_values[right_pos] <= left_values[left_pos]:
@@ -90,6 +92,119 @@ def asof_join_{{on_dtype}}_by_{{by_dtype}}(ndarray[{{on_dtype}}] left_values,
 
     return left_indexer, right_indexer
 
+
+def asof_join_forward_{{on_dtype}}_by_{{by_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
+        ndarray[{{on_dtype}}] right_values,
+        ndarray[{{by_dtype}}] left_by_values,
+        ndarray[{{by_dtype}}] right_by_values,
+        bint allow_exact_matches=1,
+        tolerance=None):
+
+    cdef:
+        Py_ssize_t left_pos, right_pos, left_size, right_size, found_right_pos
+        ndarray[int64_t] left_indexer, right_indexer
+        bint has_tolerance = 0
+        {{on_dtype}} tolerance_
+        {{on_dtype}} diff
+        {{table_type}} hash_table
+        {{by_dtype}} by_value
+
+    # if we are using tolerance, set our objects
+    if tolerance is not None:
+        has_tolerance = 1
+        tolerance_ = tolerance
+
+    left_size = len(left_values)
+    right_size = len(right_values)
+
+    left_indexer = np.empty(left_size, dtype=np.int64)
+    right_indexer = np.empty(left_size, dtype=np.int64)
+
+    hash_table = {{table_type}}(right_size)
+
+    right_pos = right_size - 1
+    for left_pos in range(left_size - 1, -1, -1):
+        # restart right_pos if it went over in a previous iteration
+        if right_pos == right_size:
+            right_pos = right_size - 1
+
+        # find first position in right whose value is greater than left's
+        if allow_exact_matches:
+            while right_pos >= 0 and\
+                right_values[right_pos] >= left_values[left_pos]:
+                hash_table.set_item(right_by_values[right_pos], right_pos)
+                right_pos -= 1
+        else:
+            while right_pos >= 0 and\
+                right_values[right_pos] > left_values[left_pos]:
+                hash_table.set_item(right_by_values[right_pos], right_pos)
+                right_pos -= 1
+        right_pos += 1
+
+        # save positions as the desired index
+        by_value = left_by_values[left_pos]
+        found_right_pos = hash_table.get_item(by_value)\
+                          if by_value in hash_table else -1
+        left_indexer[left_pos] = left_pos
+        right_indexer[left_pos] = found_right_pos
+
+        # if needed, verify that tolerance is met
+        if has_tolerance and found_right_pos != -1:
+            diff = right_values[found_right_pos] - left_values[left_pos]
+            if diff > tolerance_:
+                right_indexer[left_pos] = -1
+
+    return left_indexer, right_indexer
+
+
+def asof_join_nearest_{{on_dtype}}_by_{{by_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
+        ndarray[{{on_dtype}}] right_values,
+        ndarray[{{by_dtype}}] left_by_values,
+        ndarray[{{by_dtype}}] right_by_values,
+        bint allow_exact_matches=1,
+        tolerance=None):
+
+    cdef:
+        Py_ssize_t left_size, right_size, i
+        ndarray[int64_t] left_indexer, right_indexer, bli, bri, fli, fri
+        {{on_dtype}} bdiff, fdiff
+
+    left_size = len(left_values)
+    right_size = len(right_values)
+
+    left_indexer = np.empty(left_size, dtype=np.int64)
+    right_indexer = np.empty(left_size, dtype=np.int64)
+
+    # search both forward and backward
+    bli, bri =\
+        asof_join_backward_{{on_dtype}}_by_{{by_dtype}}(left_values,
+                                                        right_values,
+                                                        left_by_values,
+                                                        right_by_values,
+                                                        allow_exact_matches,
+                                                        tolerance)
+    fli, fri =\
+        asof_join_forward_{{on_dtype}}_by_{{by_dtype}}(left_values,
+                                                       right_values,
+                                                       left_by_values,
+                                                       right_by_values,
+                                                       allow_exact_matches,
+                                                       tolerance)
+
+    for i in range(len(bri)):
+        # choose timestamp from right with smaller difference
+        if bri[i] != -1 and fri[i] != -1:
+            bdiff = left_values[bli[i]] - right_values[bri[i]]
+            fdiff = right_values[fri[i]] - left_values[fli[i]]
+            right_indexer[i] = bri[i] if bdiff <= fdiff else fri[i]
+        else:
+            right_indexer[i] = bri[i] if bri[i] != -1 else fri[i]
+        left_indexer[i] = bli[i]
+
+    return left_indexer, right_indexer
+
 {{endfor}}
 {{endfor}}
 
@@ -110,7 +225,8 @@ dtypes = ['uint8_t', 'uint16_t', 'uint32_t', 'uint64_t',
 {{for on_dtype in dtypes}}
 
 
-def asof_join_{{on_dtype}}(ndarray[{{on_dtype}}] left_values,
+def asof_join_backward_{{on_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
         ndarray[{{on_dtype}}] right_values,
         bint allow_exact_matches=1,
         tolerance=None):
@@ -120,6 +236,7 @@ def asof_join_{{on_dtype}}(ndarray[{{on_dtype}}] left_values,
         ndarray[int64_t] left_indexer, right_indexer
         bint has_tolerance = 0
         {{on_dtype}} tolerance_
+        {{on_dtype}} diff
 
     # if we are using tolerance, set our objects
     if tolerance is not None:
@@ -138,7 +255,7 @@ def asof_join_{{on_dtype}}(ndarray[{{on_dtype}}] left_values,
         if right_pos < 0:
             right_pos = 0
 
-        # find last position in right whose value is less than left's value
+        # find last position in right whose value is less than left's
         if allow_exact_matches:
             while right_pos < right_size and\
                 right_values[right_pos] <= left_values[left_pos]:
@@ -161,5 +278,96 @@ def asof_join_{{on_dtype}}(ndarray[{{on_dtype}}] left_values,
 
     return left_indexer, right_indexer
 
+
+def asof_join_forward_{{on_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
+        ndarray[{{on_dtype}}] right_values,
+        bint allow_exact_matches=1,
+        tolerance=None):
+
+    cdef:
+        Py_ssize_t left_pos, right_pos, left_size, right_size
+        ndarray[int64_t] left_indexer, right_indexer
+        bint has_tolerance = 0
+        {{on_dtype}} tolerance_
+        {{on_dtype}} diff
+
+    # if we are using tolerance, set our objects
+    if tolerance is not None:
+        has_tolerance = 1
+        tolerance_ = tolerance
+
+    left_size = len(left_values)
+    right_size = len(right_values)
+
+    left_indexer = np.empty(left_size, dtype=np.int64)
+    right_indexer = np.empty(left_size, dtype=np.int64)
+
+    right_pos = right_size - 1
+    for left_pos in range(left_size - 1, -1, -1):
+        # restart right_pos if it went over in a previous iteration
+        if right_pos == right_size:
+            right_pos = right_size - 1
+
+        # find first position in right whose value is greater than left's
+        if allow_exact_matches:
+            while right_pos >= 0 and\
+                right_values[right_pos] >= left_values[left_pos]:
+                right_pos -= 1
+        else:
+            while right_pos >= 0 and\
+                right_values[right_pos] > left_values[left_pos]:
+                right_pos -= 1
+        right_pos += 1
+
+        # save positions as the desired index
+        left_indexer[left_pos] = left_pos
+        right_indexer[left_pos] = right_pos\
+                                  if right_pos != right_size else -1
+
+        # if needed, verify that tolerance is met
+        if has_tolerance and right_pos != right_size:
+            diff = right_values[right_pos] - left_values[left_pos]
+            if diff > tolerance_:
+                right_indexer[left_pos] = -1
+
+    return left_indexer, right_indexer
+
+
+def asof_join_nearest_{{on_dtype}}(
+        ndarray[{{on_dtype}}] left_values,
+        ndarray[{{on_dtype}}] right_values,
+        bint allow_exact_matches=1,
+        tolerance=None):
+
+    cdef:
+        Py_ssize_t left_size, right_size, i
+        ndarray[int64_t] left_indexer, right_indexer, bli, bri, fli, fri
+        {{on_dtype}} bdiff, fdiff
+
+    left_size = len(left_values)
+    right_size = len(right_values)
+
+    left_indexer = np.empty(left_size, dtype=np.int64)
+    right_indexer = np.empty(left_size, dtype=np.int64)
+
+    # search both forward and backward
+    bli, bri = asof_join_backward_{{on_dtype}}(left_values, right_values,
+                                               allow_exact_matches, tolerance)
+    fli, fri = asof_join_forward_{{on_dtype}}(left_values, right_values,
+                                              allow_exact_matches, tolerance)
+
+    for i in range(len(bri)):
+        # choose timestamp from right with smaller difference
+        if bri[i] != -1 and fri[i] != -1:
+            bdiff = left_values[bli[i]] - right_values[bri[i]]
+            fdiff = right_values[fri[i]] - left_values[fli[i]]
+            right_indexer[i] = bri[i] if bdiff <= fdiff else fri[i]
+        else:
+            right_indexer[i] = bri[i] if bri[i] != -1 else fri[i]
+        left_indexer[i] = bli[i]
+
+    return left_indexer, right_indexer
+
 {{endfor}}