# Rune master (`src/mods/runetrade.cpp`)

## Requests (2026-10-17)

1. Dropping a rune stack into the rune master takes one rune off the stack.
2. Buying: runes can always be exchanged directly for a chosen rune; the
   number of runes given lowers the price (decided: 4 runes 1,000, each
   missing rune x10: 3 runes 10,000, 2 runes 100,000, 1 rune 1,000,000). The
   rune to buy can always be chosen; Buy works only with at least one rune
   and enough gold. The last rune bought is remembered (decided: per hero,
   saved).

## The original window (`cUI_MasterLotto`, read)

| Offset | What |
|---|---|
| +0x154 + i * 0x114 | slot control i (0..3), hit test vf1c(x, y) |
| +0x24c + i * 0x114 | slot i's item descriptor {u16 kind 1, u16 class 0x1c, u32 id, ...} |
| +0x5a4 / +0x620 | arrows: next / previous rune (`doLottoSelect` mode 1 / 0) |
| +0x69c | the bought rune's control; +0x794 its descriptor, +0x798 id, +0x79c type |
| +0x7b4 | the chosen rune (item type) |
| +0x7c8 | Buy button |

- Prices: u32 table at `0xad2aac` by rune count; the game fills 2..4 from
  its settings (`0x5efc7b`); read by the render, the button state
  (`0x6d4390`, `0x6d3df0`) and the pay helper `0x6d3ce0(pay)` (gold at
  hero +0x3ee, paid with `cCreatureArts::AddGold` `0x57fc40`).
- Buy (`0x6d42a0`): 2 runes give a random rune, 3 a random one of the hero's
  class, 4 the chosen one. Buying needed 2 runes (`cmp dx, 2` at `0x6d3e20`,
  `0x6d4544`, `0x6d4a13`).
- `0x6d3d50` after slot changes: with 4 runes shows the arrows and keeps the
  choice (`doLottoSelect(hero type, 2)`, index in the global `0x17e81d0`),
  otherwise hides them and clears +0x7b4.

## Mod

- Owns the window's event hook (hud.cpp centres it via `hud::HandleCentred`).
- Prices: `kPrices` written into the table (on game start and with each
  window event); the three `cmp dx, 2` become 1.
- `0x6d3d50` replaced: arrows always shown (unless a bought rune waits), a
  rune always chosen; when none is chosen (window opened) the hero's last
  bought rune is chosen by stepping `doLottoSelect` forward to it.
- Buy press handled here (`CanBuy`): at least one rune, a chosen rune, gold
  (`0x6d3ce0(0)`) and room in the backpack (a stack of that rune with room,
  or a free cell). Buying pays (`0x6d3ce0(1)`), destroys the slot runes and
  puts the rune straight into the backpack (follow-up 2026-10-17: a bought
  rune taken from the window vanished; the window's waiting slot +0x794 /
  +0x798 is no longer used). The rune is remembered for the hero.
- The Buy button looks disabled whenever `CanBuy` fails: the render sets its
  colours each frame with `cUI_Button::SetColours` (`0x6d5d90`), hooked to
  hand it the disabled set (`0x9f0290`, text `0x9f028c`).
- Remembered rune: `"hero": {"lastRune": type}` in the hero's inventory JSON
  (`inventory_save::RegisterHeroData`); cleared when the session ends.
- Stack drops: see docs/item-stacks.md.