Make ACTZipper::reset return to an off-trie root instead of a real ancestor - #72
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imlvts wants to merge 1 commit into
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Make ACTZipper::reset return to an off-trie root instead of a real ancestor#72imlvts wants to merge 1 commit into
imlvts wants to merge 1 commit into
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A zipper can be *rooted* at a path that does not exist: `fork_read_zipper`
at an off-trie focus makes one, and so does `read_zipper_at_path` with
such a path. `reset` put the path back but then cleared `invalid`
unconditionally, so the zipper came back believing it was rooted on the
trie, at whatever real node the stack still pointed at, its deepest
existing ancestor. It then answered `val()` with that ancestor's value:
trie: root=38, [1,0,2]=22, [1,1]=72
fork at [1,1,3] (does not exist): val None -> after reset(): Some(72)
fork at [9] (does not exist): val None -> after reset(): Some(38)
The zipper knew where its root was (`origin_depth`, `origin_node_depth`)
but not how far off the trie it was, so `reset` had nothing to restore.
`origin_invalid` records that, `with_root_here` captures it when a fork
takes the focus as its root, and `reset` restores it.
Regression test: `act_zipper_reset_returns_to_an_off_trie_root`, forks at
three off-trie paths and a zipper created at one. Fails before this
change.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BZmoASqM5FUuzvJeJaYQjR
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A zipper can be rooted at a path that does not exist:
fork_read_zipperat an off-trie focus makes one, and so doesread_zipper_at_pathwith such a path.resetput the path back but then clearedinvalidunconditionally, so the zipper came back believing it was rooted on the trie, at whatever real node the stack still pointed at, its deepest existing ancestor. It then answeredval()with that ancestor's value:The zipper knew where its root was (
origin_depth,origin_node_depth) but not how far off the trie it was, soresethad nothing to restore.origin_invalidrecords that,with_root_herecaptures it when a fork takes the focus as its root, andresetrestores it.Regression test:
act_zipper_reset_returns_to_an_off_trie_root, forks at three off-trie paths and a zipper created at one. Fails before this change.