Phase 8: ALT Landmark Preprocessing (Routing Backend v2) - Discussion Log

Audit trail only. Do not use as input to planning, research, or execution agents. Decisions captured in 08-CONTEXT.md — this log preserves the discussion.

Date: 2026-10-25 Phase: 08-alt-landmark-preprocessing-routing-backend-v2 Mode: discuss (default) Areas selected: all four presented — v1 graph compatibility, landmark budget × phone payload, optimality verification, limitations retirement

Prior-Decisions Carried Forward (not re-asked)

Phase 6 A1 ratification: Workers Paid (c1) rejected ($0 law); drop-classes ladder rejected on correctness; bidirectional A* (BH) kept — ALT builds on it
Measured-ladder law: budget gates print their measurement (BA9 precedent); p95 8 ms gate exists
Roadmap locks: build-time preprocessing, upload script unchanged, ALT admissible
Phase 11 dependency: same graph asset gets downloaded onto phones (payload awareness)

Questions & Answers

Area: v1 graph compatibility

Q	Options presented	Selected
How should old ARG1 binaries be treated once the v2 layout ships?	(A) ARG2-only fail-honest · (B) dual-version decode w/ plain-A* fallback · (C) decode both, route ALT-only	(A) ARG2-only, fail-honest

User first asked for a major/minor difference explanation between paths before deciding (second presentation).
Key rationale recorded: A swaps a small honest "unavailable" window for permanent code simplicity; B trades permanent complexity + silent degradation for zero unavailability. Fail-honest > silent, per repo law.

Area: landmark budget × phone payload

Q	Options presented	Selected
What is the landmark budget philosophy?	(a) lean ~16–32 · (b) generous ~48–64 · (c) measured constant	Scaling rule (approach locked; count node-proportional, measurement-justified)

Mid-area user asked: "what are landmarks here?" → explained ALT anchors = road-network nodes, not POIs; triangle-inequality lower bounds; destination-agnostic via snapping. Distinction preserved in CONTEXT.md.
Mid-area user asked: "if I pick generous, wouldn't that be more ideal when expanding to whole Malaysia?" → explained the reversal: per-landmark bytes grow with node count (~1.2 MB at KL scale → ~12–24 MB per landmark at country scale; 64-key markup ≈ 0.8–1.5 GB), ALT advantage narrows at scale (CH is that tier, deferred), and v1 scope is KL metro. This pushed "lean vs generous" into "scaling rule" and the count became future-proof.

Area: optimality verification

Q	Options presented	Selected
Q1 — what does the re-run gate assert?	equal seconds + ties allowed · bit-identical paths · identical steps + carve ties	Equal seconds, ties allowed
Q2 — does the sample get bigger?	extend to ~100 seeded cross-metro pairs for ALT verdict · keep 25 · full-corpus only	Extend sample for ALT gate

Note: this area was asked three times due to two session interruptions (server restart aborted the first two presentations); the third presentation received answers. No decision changed across presentations.

Area: limitations retirement

Q	Options presented	Selected
How does the c3 section retire?	retire with evidence · delete outright · superseded appendix	Retire with evidence

D-01 pre-collapsed the area's other half: no fallback-for-landmark-less-graphs documentation needed (old graphs are router-pending by design, not a documented fallback).

OpenCode's Discretion (explicitly left to downstream)

Landmark selection algorithm specifics, count formula constants, distance quantization
Preprocessing build-time budget; epsilon naming/value; ARG2 byte design; fixture version-bump mechanics; retirement wording

External Research

None required to resolve the gray areas — codebase + phase-6 ladder evidence sufficed. All library/algorithm research belongs to the researcher for this phase.

Deferred Ideas Noted

None captured (discussion stayed within phase scope; whole-Malaysia scale resolved as D-02's scaling rule rather than a deferral).

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