add parent field to sexp_node for O(1) parent lookups - #7714
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The sexp_node struct used first/rest pointers but had no parent pointer, requiring O(n) linear scans of the entire Sexp_nodes array to find a node's parent. Add an int parent field (-1 for root/unlinked) and maintain it at all assignment sites: alloc_sexp, free_sexp, get_sexp parse loop, and save_branch. Refactor find_sexp_list(), find_parent_operator(), and is_sexp_top_level() to use the parent field directly; and add find_sexp_antecedent() and find_sexp_root(). Also add parent_node guards to several functions to allow them to gracefully fail on nodes with no parents. Also add documentation on how SEXP nodes and lists work, particularly the gotchas for top-level nodes. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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The sexp_node struct used first/rest pointers but had no parent pointer, requiring O(n) linear scans of the entire Sexp_nodes array to find a node's parent. Add an int parent field (-1 for root/unlinked) and maintain it at all assignment sites: alloc_sexp, free_sexp, get_sexp parse loop, and save_branch. Refactor find_sexp_list(), find_parent_operator(), and is_sexp_top_level() to use the parent field directly; and add find_sexp_antecedent() and find_sexp_root().
Also add parent_node guards to several functions to allow them to gracefully fail on nodes with no parents.
Also add documentation on how SEXP nodes and lists work, particularly the gotchas for top-level nodes.