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Choose non-colliding boundaries for nested multiparts: ancestor prefix · case 03

A nested boundary such as b_10 extends the outer boundary b_1, so the outer delimiter matches inside the inner one.

Member previewVariant 3 · 3 implementations · 6 checks per implementation

Case contract

Multipart nodes have "parts", leaves have "text". Nodes are visited depth-first (pre-order) and a single counter produces candidates base_1, base_2, ... A candidate is rejected when it occurs in any text anywhere below the node, when it is a prefix of an ancestor boundary or an ancestor boundary is a prefix of it, and a candidate longer than 70 characters makes that node's boundary None (its subtree gets none). Result: boundaries in visiting order.

Why this case matters

A boundary that appears inside a part, or that extends an outer boundary, splits the message at the wrong place.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
counter passes ten	`["q3_1", "q3_10"]`	`["q3_1", "q3_20"]`	Failed

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