Optimize jsonpatch.make_patch for lists. Fixes #24

This optimization tries to work only with really different lists
subsequences and trying to preserve as much as possible common ones.
This reduces overall of generated operations since now it's possible
to simple insertions. Also, it converts pairs of remove-add operations
into single replace (if target path is the same) or move (if target
value is the same).

Author: kxepal

jsonpatch.py

@@ -38,6 +38,7 @@
 import copy
 import functools
 import inspect
+import itertools
 import json
 import sys
 
@@ -53,9 +54,6 @@
 # pylint: disable=E0611,W0404
 if sys.version_info >= (3, 0):
     basestring = (bytes, str)  # pylint: disable=C0103,W0622
-    from itertools import zip_longest
-else:
-    from itertools import izip_longest as zip_longest
 
 
 class JsonPatchException(Exception):
@@ -282,15 +280,15 @@ def compare_values(path, value, other):
             if value == other:
                 return
             if isinstance(value, dict) and isinstance(other, dict):
-                for operation in compare_dict(path, value, other):
+                for operation in compare_dicts(path, value, other):
                     yield operation
             elif isinstance(value, list) and isinstance(other, list):
-                for operation in compare_list(path, value, other):
+                for operation in compare_lists(path, value, other):
                     yield operation
             else:
                 yield {'op': 'replace', 'path': '/'.join(path), 'value': other}
 
-        def compare_dict(path, src, dst):
+        def compare_dicts(path, src, dst):
             for key in src:
                 if key not in dst:
                     yield {'op': 'remove', 'path': '/'.join(path + [key])}
@@ -304,23 +302,10 @@ def compare_dict(path, src, dst):
                            'path': '/'.join(path + [key]),
                            'value': dst[key]}
 
-        def compare_list(path, src, dst):
-            lsrc, ldst = len(src), len(dst)
-            for idx in range(min(lsrc, ldst)):
-                current = path + [str(idx)]
-                for operation in compare_values(current, src[idx], dst[idx]):
-                    yield operation
-            if lsrc < ldst:
-                for idx in range(lsrc, ldst):
-                    current = path + [str(idx)]
-                    yield {'op': 'add',
-                           'path': '/'.join(current),
-                           'value': dst[idx]}
-            elif lsrc > ldst:
-                for idx in reversed(range(ldst, lsrc)):
-                    yield {'op': 'remove', 'path': '/'.join(path + [str(idx)])}
+        def compare_lists(path, src, dst):
+            return _compare_lists(path, src, dst)
 
-        return cls(list(compare_dict([''], src, dst)))
+        return cls(list(compare_dicts([''], src, dst)))
 
     def to_string(self):
         """Returns patch set as JSON string."""
@@ -527,3 +512,230 @@ def apply(self, obj):
         }).apply(obj)
 
         return obj
+
+
+def _compare_lists(path, src, dst):
+    """Compares two lists objects and return JSON patch about."""
+    return _optimize(_compare(path, src, dst, *_split_by_common_seq(src, dst)))
+
+
+def _longest_common_subseq(src, dst):
+    """Returns pair of ranges of longest common subsequence for the `src`
+    and `dst` lists.
+
+    >>> src = [1, 2, 3, 4]
+    >>> dst = [0, 1, 2, 3, 5]
+    >>> # The longest common subsequence for these lists is [1, 2, 3]
+    ... # which is located at (0, 3) index range for src list and (1, 4) for
+    ... # dst one. Tuple of these ranges we should get back.
+    ... assert ((0, 3), (1, 4)) == _longest_common_subseq(src, dst)
+    """
+    lsrc, ldst = len(src), len(dst)
+    drange = list(range(ldst))
+    matrix = [[0] * ldst for _ in range(lsrc)]
+    z = 0  # length of the longest subsequence
+    range_src, range_dst = None, None
+    for i, j in itertools.product(range(lsrc), drange):
+        if src[i] == dst[j]:
+            if i == 0 or j == 0:
+                matrix[i][j] = 1
+            else:
+                matrix[i][j] = matrix[i-1][j-1] + 1
+            if matrix[i][j] > z:
+                z = matrix[i][j]
+            if matrix[i][j] == z:
+                range_src = (i-z+1, i+1)
+                range_dst = (j-z+1, j+1)
+        else:
+            matrix[i][j] = 0
+    return range_src, range_dst
+
+
+def _split_by_common_seq(src, dst, bx=(0, -1), by=(0, -1)):
+    """Recursively splits the `dst` list onto two parts: left and right.
+    The left part contains differences on left from common subsequence,
+    same as the right part by for other side.
+
+    To easily understand the process let's take two lists: [0, 1, 2, 3] as
+    `src` and [1, 2, 4, 5] for `dst`. If we've tried to generate the binary tree
+    where nodes are common subsequence for both lists, leaves on the left
+    side are subsequence for `src` list and leaves on the right one for `dst`,
+    our tree would looks like::
+
+        [1, 2]
+       /     \
+    [0]       []
+             /  \
+          [3]   [4, 5]
+
+    This function generate the similar structure as flat tree, but without
+    nodes with common subsequences - since we're don't need them - only with
+    left and right leaves::
+
+        []
+       / \
+    [0]  []
+        / \
+     [3]  [4, 5]
+
+    The `bx` is the absolute range for currently processed subsequence of
+    `src` list.  The `by` means the same, but for the `dst` list.
+    """
+    # Prevent useless comparisons in future
+    bx = bx if bx[0] != bx[1] else None
+    by = by if by[0] != by[1] else None
+
+    if not src:
+        return [None, by]
+    elif not dst:
+        return [bx, None]
+
+    # note that these ranges are relative for processed sublists
+    x, y = _longest_common_subseq(src, dst)
+
+    if x is None or y is None:  # no more any common subsequence
+        return [bx, by]
+
+    return [_split_by_common_seq(src[:x[0]], dst[:y[0]],
+                                 (bx[0], bx[0] + x[0]),
+                                 (by[0], by[0] + y[0])),
+            _split_by_common_seq(src[x[1]:], dst[y[1]:],
+                                 (bx[0] + x[1], bx[0] + len(src)),
+                                 (bx[0] + y[1], bx[0] + len(dst)))]
+
+
+def _compare(path, src, dst, left, right):
+    """Same as :func:`_compare_with_shift` but strips emitted `shift` value."""
+    for op, _ in _compare_with_shift(path, src, dst, left, right, 0):
+        yield op
+
+
+def _compare_with_shift(path, src, dst, left, right, shift):
+    """Recursively compares differences from `left` and `right` sides
+    from common subsequences.
+
+    The `shift` parameter is used to store index shift which caused
+    by ``add`` and ``remove`` operations.
+
+    Yields JSON patch operations and list index shift.
+    """
+    if isinstance(left, list):
+        for item, shift in _compare_with_shift(path, src, dst, *left,
+                                               shift=shift):
+            yield item, shift
+    elif left is not None:
+        for item, shift in _compare_left(path, src, left, shift):
+            yield item, shift
+
+    if isinstance(right, list):
+        for item, shift in _compare_with_shift(path, src, dst, *right,
+                                               shift=shift):
+            yield item, shift
+    elif right is not None:
+        for item, shift in _compare_right(path, dst, right, shift):
+            yield item, shift
+
+
+def _compare_left(path, src, left, shift):
+    """Yields JSON patch ``remove`` operations for elements that are only
+    exists in the `src` list."""
+    start, end = left
+    if end == -1:
+        end = len(src)
+    # we need to `remove` elements from list tail to not deal with index shift
+    for idx in reversed(range(start + shift, end + shift)):
+        current = path + [str(idx)]
+        yield (
+            {'op': 'remove',
+             # yes, there should be any value field, but we'll use it
+             # to apply `move` optimization a bit later and will remove
+             # it in _optimize function.
+             'value': src[idx - shift],
+             'path': '/'.join(current)},
+            shift - 1
+        )
+        shift -= 1
+
+
+def _compare_right(path, dst, right, shift):
+    """Yields JSON patch ``add`` operations for elements that are only
+    exists in the `dst` list"""
+    start, end = right
+    if end == -1:
+        end = len(dst)
+    for idx in range(start, end):
+        current = path + [str(idx)]
+        yield (
+            {'op': 'add', 'path': '/'.join(current), 'value': dst[idx]},
+            shift + 1
+        )
+        shift += 1
+
+
+def _optimize(operations):
+    """Optimizes operations which was produced by lists comparison.
+
+    Actually it does two kinds of optimizations:
+
+    1. Seeks pair of ``remove`` and ``add`` operations against the same path
+       and replaces them with ``replace`` operation.
+    2. Seeks pair of ``remove`` and ``add`` operations for the same value
+       and replaces them with ``move`` operation.
+    """
+    result = []
+    ops_by_path = {}
+    ops_by_value = {}
+    add_remove = set(['add', 'remove'])
+    for item in operations:
+        # could we apply "move" optimization for dict values?
+        hashable_value = not isinstance(item['value'], (dict, list))
+        if item['path'] in ops_by_path:
+            _optimize_using_replace(ops_by_path[item['path']], item)
+            continue
+        if hashable_value and item['value'] in ops_by_value:
+            prev_item = ops_by_value[item['value']]
+            # ensure that we processing pair of add-remove ops
+            if set([item['op'], prev_item['op']]) == add_remove:
+                _optimize_using_move(prev_item, item)
+                ops_by_value.pop(item['value'])
+                continue
+        result.append(item)
+        ops_by_path[item['path']] = item
+        if hashable_value:
+            ops_by_value[item['value']] = item
+
+    # cleanup
+    ops_by_path.clear()
+    ops_by_value.clear()
+    for item in result:
+        if item['op'] == 'remove':
+            item.pop('value')  # strip our hack
+        yield item
+
+
+def _optimize_using_replace(prev, cur):
+    """Optimises JSON patch by using ``replace`` operation instead of
+    ``remove`` and ``add`` against the same path."""
+    prev['op'] = 'replace'
+    if cur['op'] == 'add':
+        prev['value'] = cur['value']
+
+
+def _optimize_using_move(prev_item, item):
+    """Optimises JSON patch by using ``move`` operation instead of
+    ``remove` and ``add`` against the different paths but for the same value."""
+    prev_item['op'] = 'move'
+    move_from, move_to = [
+        (item['path'], prev_item['path']),
+        (prev_item['path'], item['path']),
+    ][item['op'] == 'add']
+    if item['op'] == 'add':  # first was remove then add
+        prev_item['from'] = move_from
+        prev_item['path'] = move_to
+    else:  # first was add then remove
+        head, move_from = move_from.rsplit('/', 1)
+        # since add operation was first it incremented
+        # overall index shift value. we have to fix this
+        move_from = int(move_from) - 1
+        prev_item['from'] = head + '/%d' % move_from
+        prev_item['path'] = move_to

tests.py

@@ -297,6 +297,43 @@ def test_add_nested(self):
                    }
         self.assertEqual(expected, res)
 
+    def test_should_just_add_new_item_not_rebuild_all_list(self):
+        src = {'foo': [1, 2, 3]}
+        dst = {'foo': [3, 1, 2, 3]}
+        patch = list(jsonpatch.make_patch(src, dst))
+        self.assertEqual(len(patch), 1)
+        self.assertEqual(patch[0]['op'], 'add')
+        res = jsonpatch.apply_patch(src, patch)
+        self.assertEqual(res, dst)
+
+    def test_use_replace_instead_of_remove_add(self):
+        src = {'foo': [1, 2, 3]}
+        dst = {'foo': [3, 2, 3]}
+        patch = list(jsonpatch.make_patch(src, dst))
+        self.assertEqual(len(patch), 1)
+        self.assertEqual(patch[0]['op'], 'replace')
+        res = jsonpatch.apply_patch(src, patch)
+        self.assertEqual(res, dst)
+
+    def test_use_move_instead_of_remove_add(self):
+        src = {'foo': [4, 1, 2, 3]}
+        dst = {'foo': [1, 2, 3, 4]}
+        patch = list(jsonpatch.make_patch(src, dst))
+        self.assertEqual(len(patch), 1)
+        self.assertEqual(patch[0]['op'], 'move')
+        res = jsonpatch.apply_patch(src, patch)
+        self.assertEqual(res, dst)
+
+    def test_use_move_instead_of_add_remove(self):
+        src = {'foo': [1, 2, 3]}
+        dst = {'foo': [3, 1, 2]}
+        patch = list(jsonpatch.make_patch(src, dst))
+        self.assertEqual(len(patch), 1)
+        self.assertEqual(patch[0]['op'], 'move')
+        res = jsonpatch.apply_patch(src, patch)
+        self.assertEqual(res, dst)
+
+
 
 class InvalidInputTests(unittest.TestCase):