---
id: reconstruction-method
title: "Reconstruction Method — the loop, the no-stand-ins rule, decomp technique"
status: established
source_sections: "PROGRESS_LOG.md §10b, §5a (decompilation), §10c"
related_topics: [reconstruction-gotchas, decomp-reference, source-completeness]
key_terms: [decomp, oracle, bridge, BTL4OPT, WinTesla]
---

# Reconstruction Method

How the missing BT game logic is rebuilt from the binary. The governing rule: **no stand-ins.**
Full detail: `docs/PROGRESS_LOG.md §10b`; the systemic bug classes are [[reconstruction-gotchas]].

## The loop (per feature)
1. Read the RAW decomp `reference/decomp/all/part_*.c` for the relevant `FUN_xxxx`.
2. Map `FUN_`/`DAT_`/`this+0xNN` to engine symbols using: the BT headers + the WinTesla MUNGA
   source + `game/reconstructed/CLASSMAP.md` + **RP's parallel code** (VTV≈mech, WEAPSYS≈weapons).
3. Write the **real** reconstruction into `game/reconstructed/*.cpp`.
4. Build; run env-gated; read `btl4.log` (grep `[anim]/[drive]/[target]/[fire]/[damage]` markers).
5. cdb on any crash. `static_assert`-lock the layout against the binary's offsets. [T2]

## RULE: no stand-ins
The full game logic IS in the pseudocode (the binary ran the game); a "gap" is a reconstruction
stub not yet filled, not a hole in the original. **Never write stand-in/placeholder logic for an
apparent gap — read the decomp.** (User: "there are no gaps, just work to be done.") Bring-up
scaffolding (the `BT_AUTOFIRE`/`BT_AUTODRIVE`/`BT_GOTO` env harness; historically
explosion-for-beam and a player-gated drive, since replaced by the real reconstructions) is
clearly MARKED and meant to be REPLACED by the real reconstructed system, never to substitute
for reading the decomp. [T2]

## Decompilation stack (established)
- **Ghidra 12.1.2 + JDK21** are an EXTERNAL install (`tools/` holds only python utilities); the
  headless scripts live in `reference/ghidra_scripts/` (`ExportBTSource.java`, `DecompVSS.java`,
  `ExportAll.java`). `ExportBTSource.java` run headless on `BTL4OPT.EXE` → decompiled C in
  `reference/decomp/` (`bt_<src>.cpp.c` assert-anchored files; bulk pseudocode in
  `reference/decomp/all/part_*.c`). The pass is **assert-anchored** (few funcs/file — anchored
  on the asserts that carry source paths). [T2]
- For a `FUN_` the assert-anchored exporter skipped: **`tools/disas2.py <VA> [len]`** (capstone
  disassembly of `BTL4OPT.EXE` — recovers x87 math Ghidra drops, folds known calls + float
  constants). Or `DecompVSS.java` (headless address-list decompiler). [T2]
- **Resolving a `.data` fn-pointer** (a `PTR_LAB_xxxx` callback/vtable slot the decomp didn't
  export): PE-parse the DWORD at its VA, then capstone-disassemble the target. Used for the gait
  callbacks (`@0x4a6d8c`), the valve handler (`@0x4ae464`), gauge widget slots. [T2]
- **`.data` float constants** are the biggest "couldn't recover" bucket (tuning values); read them
  as the x87 80-bit `float10` the decomp uses, not 32-bit float (`scratchpad/rdtbyte.py`). [T2]

## Effort model (honest)
~0.5-1.5 hr/module → ~30-50 agent-hours + human review for ~40 modules, with a long tail (modules
with no surviving `.HPP` fragment, or split across decomp windows, cost more). Process upgrades:
per-class function lists (not address clusters), complete vtable rows, recover `.data` constants. [T3]

## Workflows (for scale)
For exhaustive multi-function decomp analysis, fan out a **read-only Workflow** (understanding
phase — one agent per function/class produces an offset map + findings, + an adversarial verify),
THEN implement hands-on so each change is visible + consistent. This is the §10c pattern; used for
the ground-model decode (10 agents), the alarm-unification (8), the gauge-widget decode (6). [T2]

## Key Relationships
- Bug classes: [[reconstruction-gotchas]] (check FIRST). Reference data: [[decomp-reference]].
- Why it's needed: [[source-completeness]].
