# 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).