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venv/Lib/site-packages/django/db/migrations/optimizer.py

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+class MigrationOptimizer:
+    """
+    Power the optimization process, where you provide a list of Operations
+    and you are returned a list of equal or shorter length - operations
+    are merged into one if possible.
+
+    For example, a CreateModel and an AddField can be optimized into a
+    new CreateModel, and CreateModel and DeleteModel can be optimized into
+    nothing.
+    """
+
+    def optimize(self, operations, app_label):
+        """
+        Main optimization entry point. Pass in a list of Operation instances,
+        get out a new list of Operation instances.
+
+        Unfortunately, due to the scope of the optimization (two combinable
+        operations might be separated by several hundred others), this can't be
+        done as a peephole optimization with checks/output implemented on
+        the Operations themselves; instead, the optimizer looks at each
+        individual operation and scans forwards in the list to see if there
+        are any matches, stopping at boundaries - operations which can't
+        be optimized over (RunSQL, operations on the same field/model, etc.)
+
+        The inner loop is run until the starting list is the same as the result
+        list, and then the result is returned. This means that operation
+        optimization must be stable and always return an equal or shorter list.
+        """
+        # Internal tracking variable for test assertions about # of loops
+        if app_label is None:
+            raise TypeError('app_label must be a str.')
+        self._iterations = 0
+        while True:
+            result = self.optimize_inner(operations, app_label)
+            self._iterations += 1
+            if result == operations:
+                return result
+            operations = result
+
+    def optimize_inner(self, operations, app_label):
+        """Inner optimization loop."""
+        new_operations = []
+        for i, operation in enumerate(operations):
+            right = True  # Should we reduce on the right or on the left.
+            # Compare it to each operation after it
+            for j, other in enumerate(operations[i + 1:]):
+                result = operation.reduce(other, app_label)
+                if isinstance(result, list):
+                    in_between = operations[i + 1:i + j + 1]
+                    if right:
+                        new_operations.extend(in_between)
+                        new_operations.extend(result)
+                    elif all(op.reduce(other, app_label) is True for op in in_between):
+                        # Perform a left reduction if all of the in-between
+                        # operations can optimize through other.
+                        new_operations.extend(result)
+                        new_operations.extend(in_between)
+                    else:
+                        # Otherwise keep trying.
+                        new_operations.append(operation)
+                        break
+                    new_operations.extend(operations[i + j + 2:])
+                    return new_operations
+                elif not result:
+                    # Can't perform a right reduction.
+                    right = False
+            else:
+                new_operations.append(operation)
+        return new_operations