# The Arsenal develop economy, and what the save's blob indexes

**2026-08-11.** The save file's 54-byte blob was shown to be per-item development
state by [a runtime differential](structures/savegame-format.md#the-develop-differential-one-weapon-three-fields) —
one weapon bought, two entries moved, `2 → 4` for the item and `0 → 2` for the
successor. That named the *values*. This note names the *index space*, and in
passing recovers the whole develop economy from the disc.

## Where the Arsenal gets its data

`dat/GP_HANGAR_ARSENAL.pak` carries one screen-config record per language whose
fields point at the rest:

```
PATH  = dat\GP_HANGAR_ARSENAL.pak+eng\
WEAPON = weapon.tbl          STRINGS = strings.tbl
WEAPON_CATEGORIES = 8        Detail_Window_Known / Detail_Window_Unknown
ConditionToDevelop = 757,228   WeaponDesc = 734,502   Range = 891,73 …
```

`WEAPON_CATEGORIES = 8` is the eight tabs the screen cycles with LB/RB (GUN,
BEAM, LASER, MULTIPURPOSE MISSILE, ANTI-SHIP MISSILE, BOMB/ROCKET, CANNON,
SPECIAL). The `…= x,y` fields are screen positions for that screen's labels —
`ConditionToDevelop` is *where the conditions text is drawn*, not a condition.

### `eng\weapon.tbl` — the item table (IDXD, schema `0x03c244b5`, 66 records)

Its string pool opens with a stage list, then the literal key `WEAPONS`
followed by the item ids **in table order**:

```
No_Equipment, Machiene_Cannon_MG1, Machiene_Cannon_MG2H, Machiene_Cannon_MG3,
Machiene_Cannon_MG5H, Broad_Sword_SG1, Swing_Sword_SG2H, Twin_Sword_SG3II,
Frail_GP37, Stiletto_BG1, Dagger_BG2, Rapier_BG4H, Pilum_BP, … , Wep_83,
Wep_84, Wep_85, NullWeapon_Arm1, NullWeapon_Arm2, NullWeapon_Arm3
```

The ids are grouped by the eight categories in the same order the screen shows
them, which is why `No_Equipment … Wep_83` is exactly **54** entries — the blob's
length.

Each item then has a record: `Size, Weight, Range, Power, Speed, Stage,
MissionObjective, Dependency, Points`. For the **first** record all of it reads
straight out, and it matches the running game exactly:

| field | disc | in-game DATA SHEET for Light Machine Gun MG I |
|---|---|---|
| Range | `D` | `Range Class D` |
| Power | `E` | `Damage Class E` |
| Speed | `-` | `Speed Class –` |
| Weight | `0.3` | `Weight Class Light` |
| Points | `4000` | `4000 P` on the develop row |

**Caveat, and it is a real one:** IDXD string pools store each distinct string
*once*, so from the second record onward every value that repeats an earlier
string is absent from the token stream. Record 2 (MG2H) shows only its unique
values — `0.4` and `3000` — and record 6 shows no number at all because its cost
had already been seen. So `weapon.tbl`'s per-item fields cannot be read from the
pool alone; they need the binary node/index region, which is still undecoded
(see the module docs in `idxd.rs`). The prose table below is the practical
substitute.

### `eng\strings.tbl` — the economy in prose (IXUD, UTF-16BE)

The same pak's string table holds, per item: the internal id, the display name,
the description, and a **"Conditions to obtain …"** block that spells out the
gate and the price:

> Conditions to obtain Light Swivel Machine Gun MG2H — Completed development of
> the Light Machine Gun MG1 — Must spend 3000 points to develop

All 60 of them are extracted to
[`captures/arsenal-develop-conditions.csv`](captures/arsenal-develop-conditions.csv)
with the stage gate, the cost and the prerequisite split out. The shape of the
economy:

- gates are **stage completion** (`Complete Stage 1 … 15`), **a predecessor
  item** (`Completed development of the …`), or **an ace kill** (`Shoot down the
  ace pilot in Stage 11/13/14`);
- costs run 3 000 → 350 000 points, and **twenty items have no cost at all** —
  they are granted the moment their gate opens (`Tomahawk Alpha Rail Gun —
  Complete Stage 1`), which is why they read as `Developed` in a list the player
  never spent points in;
- two items are anomalies worth flagging: `Adhesive Mine B2A` says "Must spend
  points to develop" with **no number**, and ten late entries (`Ballista GSH`,
  `Designator LH`, `Smoke Marker Launcher`, `Bumble UV`, `Ballista 24 Rocket
  Launcher`, `Thrush 220AM`, `Divider L3GP`, `Mace GP25H`, `Thor Gun System`,
  `Spitfire BX`) have a conditions *header* and no conditions — ❔ cut content or
  non-player weapons.

## What the blob indexes: the first 32 entries, confirmed

Predicting the save state from the conditions and comparing against the actual
blob is a strong test, because it is made **before** looking at the blob. At the
captured state (Stage 1 complete, Stage 02 at standby, 4101 P) the conditions say
exactly six items are developable — Light Machine Gun MG1, Broad Sword SG1,
Dagger BG2, Pilum BP, Hound SMH, Dart 23 Rocket — and the blob's **six** `2`s sit
at WEAPONS-order indices **1, 5, 10, 12, 27, 31**, which are those six items, in
order. Nothing had to be fitted.

| blob | value | item (WEAPONS order) | confirmed by |
|---|---|---|---|
| 0 | 4 | `No_Equipment` | always available |
| 1 | 2→4 | `Machiene_Cannon_MG1` | the differential: bought for 4000 P |
| 2 | 0→2 | `Machiene_Cannon_MG2H` | the differential: announced as newly developable |
| 5 | 2 | `Broad_Sword_SG1` | `BROAD SWORD SG1 5000P`, **below the fold** in the GUN list |
| 9 | 4 | `Stiletto_BG1` | `Developed`; "Initially Mounted"; mounted as NOSE WEAPON |
| 10 | 2 | `Dagger_BG2` | `DAGGER BG2 5000P` |
| 12 | 2 | `Pilum_BP` | `PILUM BP 6000P` |
| 21 | 4 | `Falcon_9AM` | `Developed`; "Initially Mounted"; mounted as MAIN WEAPON 1 |
| 22 | 4 | `Buzzard_10AM` | `Developed`; free at Stage 1 |
| 26 | 4 | `Terrier_SMH` | `Developed`; free at Stage 1 |
| 27 | 2 | `Hound_SMH` | `HOUND SMH 10000P` |
| 31 | 2 | `Dirt_23_Rocket` | `DART 23 ROCKET 3500P` ("Dart" is the localised "Dirt") |

Twelve concordances, no contradiction, over indices 0–31.

## What the blob does **not** index: the tail is unexplained

Past index 31 the same alignment fails, and it fails against the screen, not
against a theory:

- `Tomahawk_a_Rail_Gun` is WEAPONS index **38** and the CANNON list shows it
  `Developed` (it is free once Stage 1 is complete) — but **blob[38] = 0**.
- The blob's remaining `4`s are at **33, 39, 45, 46, 47**. In WEAPONS order those
  are `EMP_Mine_B8E`, `Sling_75KG`, `Shield_Doubler`, `Booster`,
  `FireControlSystem` — and **every one of them is shown as not developed**:
  BOMB/ROCKET lists only `DART 23 ROCKET 3500P`, CANNON only the Tomahawk, and
  **the SPECIAL tab is entirely empty** (all rows dashed). Their conditions agree
  — Shield Doubler needs Stage 12, Booster an ace kill in Stage 11, FCS Stage 6.

So five `4`s in the tail correspond to nothing the Arsenal reports as owned, and
one genuinely-owned item reads `0`. A single `+1` shift does not repair it
either: `blob[39] = 4` would put the Tomahawk right, but then `blob[33] = 4`
lands on `Arrow_27_Rocket` (Stage 2 + 5000 P, unreachable here) and 45/46/47
still land on locked SPECIAL items.

### Tested actively, with a hand-written save

Waiting for a mission payout was not necessary. The save container's derived
fields turned out to be reproducible (see
[`savegame_edit.py`](../../tools/re-capture/savegame_edit.py) — the whole
container re-wraps **byte-identically**), so the blob could be *written* and the
result read off the screen. A probe save was built in our own throwaway slot 03
with the blob zeroed except `0, 9, 21, 33, 38, 45 = 4` and `39, 46 = 2`, and it
**loaded**.

What it showed:

- **The controls rendered.** `9` → BEAM shows `STILETTO BG1 Developed`; `21` →
  MULTIPURPOSE MISSILE shows `FALCON 9AM Developed`. So a hand-written `4` does
  reach the screen, and indices 9 and 21 are those items — now by experiment,
  not by correlation.
- **The tail probes rendered nothing.** `33` should have made a BOMB/ROCKET row
  `Developed` (row 3 of that list, on screen, not below the fold) — it stayed
  dashed. `45` should have made a SPECIAL row `Developed` — the SPECIAL tab
  stayed entirely empty. And `38` should have been the Tomahawk — instead the
  CANNON list showed `TOMAHAWK ALPHA RAIL GUN — 0 P`, i.e. *not owned*.
- **Clearing the real save's tail `4`s cost the Tomahawk its status.** In the
  unedited save the Tomahawk reads `Developed` while `blob[38] = 0`; the probe
  cleared `{22, 26, 39, 46, 47}` and the Tomahawk stopped being owned — so its
  flag is one of the cleared entries, with **39** the positional candidate. A
  uniform `+1` shift past index 32 would explain that, but it fails elsewhere:
  under it the real save's `45/46/47` are Chaff Flare Dispencer / Shield Doubler
  / Booster, and the SPECIAL tab is empty in that same save.

So the tail is not a shifted copy of the list either. **Status: ❔** — the value
alphabet and indices 0–31 are settled; indices ≥32 are not, and must not go into
the reimplementation. The next probe is cheap now that saves can be written: set
one tail index at a time and read which row changes.

### Two behaviours the probe exposed

- **`2` is derived, not stored.** On load the title recomputes which items are
  developable from its own conditions and *announces the difference* — zeroing
  Light Machine Gun MG1 and Broad Sword SG1 produced "You can now develop Broad
  Sword …" and both came back priced in the list. Only the owned (`4`) entries
  are authoritative state; writing `2` is pointless, and writing `0` over an
  item whose conditions are met is undone at load.
- **The free items are bought for zero, not granted.** `TOMAHAWK ALPHA RAIL GUN
  — 0 P` is what an unowned no-cost item looks like. That is the real reason a
  save where the player never spent points still shows items as `Developed`
  (this note previously said "granted" — the mechanism is a 0 P purchase).

## Evidence

- [`captures/arsenal-develop-conditions.csv`](captures/arsenal-develop-conditions.csv) — all 60 conditions blocks.
- [`captures/arsenal-categories-all.png`](captures/arsenal-categories-all.png) — the eight category tabs.
- [`captures/arsenal-gun-below-fold.png`](captures/arsenal-gun-below-fold.png) — Broad Sword SG1, the entry that is not visible without scrolling.
- [`captures/arsenal-probe-save-result.png`](captures/arsenal-probe-save-result.png) — the hand-written save's result: controls at 9/21 render `Developed`, the tail probes render nothing, and the Tomahawk shows `0 P`.
- [`captures/arsenal-gun-list-predevelop.png`](captures/arsenal-gun-list-predevelop.png) · [`captures/arsenal-mg1-developed.png`](captures/arsenal-mg1-developed.png) — the differential itself.