[fallintoplace]

What

• Make Rope::len() constant time by tracking finalized segment bytes.

Why

• Length checks currently scan every output segment.

Implementation

• Keep a running total for finalized segments and include the pending tail.
• Add Criterion cases for length checks, append-and-check, and write costs.

Benchmark

• Criterion, two runs: len() at 1/64/1,024 segments: 1.31/10.89/130.77 ns -> 0.92/0.94/0.92 ns.
• Append then check: 16 segments 320-353 ns -> 292 ns; 128 segments 2.81-2.89 us -> 1.77-1.81 us; 1,024 segments 79.8-86.7 us -> 12.3-13.2 us.
• Write checks (120 samples): small writes 404.6 -> 403.5 ns; shared segments 523.5 -> 528.7 ns (~1%).

[github-actions]

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