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0632eba6c3
Fixes #906. ## What Part of #904 (net-cost policy completion tracking). Follows up #856 / #857 with the corrected gain term raised in [this #856 comment](https://github.com/chopratejas/headroom/issues/856#issuecomment-4679706939) — prerequisite for P2 (pipeline consumption), which would otherwise wire in a formula that is always-pro-mutation by exactly `P_alive·(w−r)·ΔT`. ## Why the corrected form is right With a live cache, the ΔT tokens a mutation removes are **already cache-written** — keeping them costs only reads (`ΔT·r·R`), so a mutation cannot avoid a fresh write of them. Blending alive (`ΔT·r·R − (w−r)·S`) and dead (`ΔT·(w + r·(R−1))`, no suffix penalty) cases over `P_alive`: ``` gain = ΔT·(w + r·(R−1)) − P_alive·(w−r)·(S + ΔT) ``` Three independent confirmations: 1. **Direct cost check** (w=1.25, r=0.1, warm, ΔT=50K, S=10K, R=2): keeping costs 60K·0.1·2 = 12,000 in reads; mutating costs 10K·1.25 (suffix rewrite, the first of the R touches) + 10K·0.1 (remaining read) = 13,500 — mutation loses 1,500, matching the corrected gain of −1,500. The old form said +56,000. 2. **The issue's own anchors**: corrected break-even is exactly `R = 11.5·S/ΔT` → 2K/50K = 287.5 (~290, as the issue says) and 50K/10K = 2.3 — the spec text's anchor numbers can only be derived from the corrected penalty. The implemented form gave 276 and *negative*. 3. **Internal consistency**: `break_even_reads` already shipped with the ~11.5·S/ΔT shape; this PR reconciles `net_mutation_gain` with it (and drops break_even's stray −1 term). ## Behavior changes (formula is still dead code — nothing consumes it yet) - 50K-shave/10K-suffix/R=3 golden: +61,000 → **+3,500** (tight win, consistent with 2.3-read break-even). - 2K-shave/50K-suffix/R=10 golden: −53,200 → **−55,500**. - S=0 boundary: an edit of already-cached content with no suffix is profitable whenever ≥1 read remains (`gain = ΔT·r·R`), and exactly 0 at R=0 warm. Not-yet-cached (live-zone) content should bypass the formula — now documented on both implementations. Rust + Python goldens updated in lockstep: 13 Rust + 19 Python tests green. ## Next (separate PRs) - **P2**: flag-gated consumption (`HEADROOM_NET_COST_POLICY=1`) with decision telemetry. - **P3**: batch deep edits (reclaim threshold), idle-timer compaction near TTL lapse. Co-authored-by: integration-check <integration@local> |
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