ENB: Two passes will simplify all reads and pastes

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Edward K. Ream

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Jul 31, 2023, 9:12:14 AM7/31/23

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This Engineering Notebook post suggests improvements to the code that reads outlines and pastes nodes.

These changes must wait until Leo 6.7.5. Recent changes automatically correct link errors in paste-retaining-clones. These changes suffice for Leo 6.7.4.

I propose the following two-pass strategy for code that reads outlines, including the code that handles paste-node and paste-retaining-clones:

A prepass will convert various file formats to a descriptor dict, similar to what p.archive returns. A postpass will create the outline from the descriptor dict.

The prepass

Leo's existing codebase contains four sets of duplicate code that handle reads and pastes in xml and json formats. The prepass will eliminate this code duplication.

The prepass will create a descriptor dict containing entries for each vnode without explicit structure data. In other words, outline structure will be implicit in the entries in the parents_dict and children_dict (see p.archive).

Pass one's most challenging task will be to create the parents_dict and children_dict entries in the descriptor dict. This task is tricky, just as in Leo's existing code.

Leo should support a new flat json format based on p.archive. This new format would make creating parents_dict and children_dict a trivial operation.

The descriptor dict will also contain a 'command' key, telling the postpass what command Leo is executing.

The postpass

The postpass will create vnodes from the descriptor dict. The postpass will have two phases.

Phase one will allocate all vnodes, assigning v.gnx. For read-outline and paste-node, this phase will recreate all vnodes. For paste-retaining-clones, this phase will create only missing vnodes.

Phase two will enforce the "pasted nodes rule" policy by overwriting everything: v.parents, p.children, v.h, v.b, and v.u.

Summary

A prepass will create a json-like dict descriptor dict containing parent/child links. Creating this dict will be a challenging task.

A postpass will allocate vnodes first and then overwrite everything, thus enforcing the pasted-nodes-rule requirement.

A new flat json format would represent parent/child links explicitly, thereby collapsing the complexity of the prepass.