---
title: "Marching fleet shooter: API"
package: "@phaser-ae/marching-fleet"
tier: Pro
version: 3.0.0-1
released: 2026-10-19
updated: 2026-10-20
description: "The rules of a fixed-screen rack shooter, with a marching fleet that speeds up as it thins, typed alien shots from targeting tables, a mystery saucer and eroding shields."
url: https://phaser.io/packs/marching-fleet/api
site: Phaser - https://phaser.io
---

# Marching fleet shooter: API

`@phaser-ae/marching-fleet` — Fixed-screen rack shooting: a rack that marches one invader a frame, typed shots from targeting tables, a saucer and eroding shields, as in Space Invaders.

## The pack

### `marchingFleet`

```ts
function marchingFleet(): Pack<{ marchingFleet: MarchingFleetApi }>
```

Install the renderer-independent marching-fleet rules (Space Invaders). Its shields are `destructible-cover` masks.

## Helpers

### `createInvaders`

```ts
function createInvaders(config: InvadersConfig)
```

Create an independent marching-fleet game. No renderer or kernel clock is required.

### `invadersPreset`

```ts
function invadersPreset(o: InvadersPresetOptions = {}): InvadersConfig
```

Space Invaders (Taito, 1978) as data. Values from the ROM disassembly (computerarcheology.com):
224x256 screen; 11x5 rack of squid (30), crab (20), octopus (10) on 16-pixel centres; one invader moved
per frame, 2 px (the last invader 3 px right, 2 left), 8 px drop at an edge; rounds start lower from
the AlienStartTable; one player shot at 4 px/frame that needs the button released; rolling (aimed at
the column over the cannon), plunger and squiggly shots from the ColFireTable, each stepping 4 px
every third frame (5 px with 8 or fewer invaders), reloading by score; the saucer every 1536 frames
with 8+ invaders, its score from the 15-entry wrap of SaucerScrTab; four 22x16 shields eroded by the
ROM's explosion stencils; 3 cannons and one extra at 1500.

## Classes

### `Invaders`

```ts
class Invaders
```

Owns a marching fleet game: rack, cursor march, shot types, saucer, eroding cover, lives and rounds.

- `readonly config: InvadersConfig` — The validated definition (a private copy).
- `readonly cover: DestructibleCover` — The shields, a destructible-cover instance owned by this game.
- `constructor(config: InvadersConfig)`
- `get state(): InvadersState` — A read-only copy of the whole state.
- `box(inv: InvaderState)` — Live invaders' hit boxes, for renderers and tests.
- `reloadSteps(score = this.s.score)` — Current reload threshold (steps) for the score.
- `saucerValue()` — Score the saucer would award if hit now.
- `tick(seconds: number, input: InvadersInput = {})` — Advance by real seconds (fixed frames inside); returns the state.
- `step(input: InvadersInput = {})` — Advance exactly one simulation frame.
- `drain()` — Take the events since the last drain.
- `snapshot()` — Serializable snapshot of the whole game.
- `restore(snap: ReturnType<Invaders["snapshot"]>)` — Restore a snapshot taken from the same configuration.
- `reset(round = 0)` — Start a new game with the same configuration, optionally at a later (0-based) round.

## Types

### `InvaderKind`

```ts
interface InvaderKind
```

One invader type: hitbox (centred in its fleet cell) and score.

- `id: string` — Kind id used by fleet rows.
- `score: number` — Points for shooting one.
- `width: number` — Hit-box width.
- `height: number` — Hit-box height.

### `InvadersConfig`

```ts
interface InvadersConfig
```

Complete marching-fleet definition. Every original value is a preset in {@link invadersPreset}.

- `width: number` — Playfield width in model pixels.
- `height: number` — Playfield height in model pixels.
- `hz: number` — Simulation frames per second.
- `seed?: number` — Seed for random-column shots.
- `kinds: InvaderKind[]` — Invader kinds.
- `fleet: { columns: number; /** Kind id per row, TOP row first. */ rows: string[]; /** Cell size; invaders' hitboxes are centred in their cell. */ cellWidth: number; cellHeight: number; /** Distance between column / row origins. */ spacingX: number; spacingY: number; /** Left edge of column 0 at the start of a round. */ left: number; /** Top of the BOTTOM row per round; the last entry repeats from \`repeatFrom\`. */ startY: number[]; repeatFrom?: number; }` — Rack layout and the start height of each round.
- `march: { /** Invaders moved per frame (original 1: one invader per frame). */ movesPerFrame: number; stepX: number; /** Horizontal steps of the very last invader, right and left (original 3 and 2). */ lastStepRight: number; lastStepLeft: number; /** Drop applied in the sweep after the rack touches an edge. */ stepDown: number; /** Playfield edges that reverse the rack. */ minX: number; maxX: number; /** Frames the rack freezes while a shot invader explodes. Original: 16. */ killPause: number; /** Invaders erase any cover pixels they overlap (original behaviour). */ eraseCover: boolean; }` — Cursor march rules.
- `invasionY: number` — An invader's bottom reaching this line lands the invasion: game over regardless of lives.
- `player: { /** Top of the cannon. */ y: number; width: number; height: number; /** Pixels per frame. */ speed: number; minX: number; maxX: number; startX: number; lives: number; /** Extra cannons awarded at these scores (once each). Original: 1500. */ extraLives: number[]; /** Frames of the death explosion, during which the rack and its fire freeze. */ deathFrames: number; /** Frames before the new cannon appears after the explosion. */ respawnFrames: number; }` — Cannon rules.
- `playerShot: { /** Pixels per frame. Original: 4. */ speed: number; /** Simultaneous player shots. Original: 1. */ maxActive: number; /** Fire must be released between shots (original). */ requireRelease: boolean; /** The shot explodes here (top of the playfield). */ ceiling: number; width: number; height: number; /** Player shots can destroy alien shots they touch (original). */ hitsAlienShots: boolean; /** Frames the shot's explosion lingers before another shot may fire. */ explodeFrames: number; }` — Player shot rules.
- `alienShots: { types: InvaderShotType[]; /** Each shot type takes a turn every \`cadence\` frames, offset by its index. Original: 3. */ cadence: number; /** Pixels per step, and the faster step once \`fastAtOrBelow\` invaders remain. Original: 4, 5 and 8. */ step: number; fastStep: number; fastAtOrBelow: number; /** * Reload: a type may fire only when every other in-flight shot has taken MORE than this many steps. * Looked up by score: \`rates[i]\` applies while score < \`scores[i]\`, the last rate beyond. * Original: scores 200/1000/2000/3000, rates 48/16/11/8/7. */ reload: { scores: number[]; rates: number[] }; /** Frames at the start of each life/round before aliens may fire. */ initialDelay: number; /** Shots are lost below this line (the ground). */ floor: number; }` — Alien shot types, cadence, speed and reload.
- `saucer: SaucerRules | null` — Mystery ship rules, or null for none.
- `cover: CoverDefinition[]` — Damageable shields (destructible-cover definitions).
- `splats: { player: string[] | null; alien: string[] | null; inset: number }` — Explosion stencils (null = destroy the single contacted cell), centred `inset` pixels into the cover.
- `roundDelay: number` — Frames between clearing a round and the next rack.
- `restoreCover: boolean` — Cover restored for each new round (original).

### `InvadersEvent`

```ts
type InvadersEvent = | { kind: "player-fire"; x: number; y: number } | { kind: "invader-killed"; id: string; invader: string; score: number; x: number; y: number } | { kind: "saucer-start"; dir: number } | { kind: "saucer-killed"; score: number; x: number; y: number } | { kind: "saucer-gone" } | { kind: "shot-exploded"; team: "player" | "alien"; x: number; y: number; on: "cover" | "ceiling" | "floor" | "shot" } | { kind: "player-hit"; x: number; y: number } | { kind: "extra-life"; lives: number } | { kind: "march"; sweep: number } | { kind: "round-clear"; round: number } | { kind: "round-start"; round: number } | { kind: "game-over"; reason: "lives" | "invasion" }
```

Ordered notifications for presentation: drain() them each frame.

### `InvaderShot`

```ts
interface InvaderShot
```

One alien shot in flight.

- `id: number` — Stable id for drawing.
- `type: string` — Its shot type id.
- `x: number` — Centre x.
- `y: number` — Top y.
- `steps: number` — Steps taken (reload bookkeeping and animation).
- `active: boolean` — In flight.

### `InvaderShotSelect`

```ts
type InvaderShotSelect = /** The lowest live invader in the column over the player's centre (the "rolling" shot). */ | { kind: "player-column" } /** The lowest live invader in the next column of a looping 1-based column table. */ | { kind: "column-table"; table: number[]; start?: number } /** The lowest live invader in a seeded random column (not in the original; a variant policy). */ | { kind: "random" }
```

How an alien shot type picks the invader that drops it.

### `InvaderShotType`

```ts
interface InvaderShotType
```

One alien shot type. At most one shot of each type is in flight, so N types cap alien fire at N shots.

- `id: string` — Shot type id (also its drawing key).
- `select: InvaderShotSelect` — Which invader drops it.
- `disableAtOrBelow?: number` — Do not fire this type while this many invaders or fewer remain (the plunger stops at 1).
- `sharesSaucerSlot?: boolean` — This type's turn is shared with the saucer, which waits for it to land (the squiggly shot).
- `width?: number` — Hit-box width (default 3).
- `height?: number` — Hit-box height (default 7).

### `InvadersInput`

```ts
interface InvadersInput
```

One frame of player input.

- `move?: number` — -1 left, 0 still, +1 right.
- `fire?: boolean` — Fire held.

### `InvadersPresetOptions`

```ts
interface InvadersPresetOptions
```

Options for {@link invadersPreset}: the classic values unless overridden.

- `columns?: number` — Invaders per row (11 = classic).
- `rows?: string[]` — Kind ids per row, top first. Default: squid, crab, crab, octopus, octopus.
- `lives?: number` — Starting cannons (3 = classic).
- `bunkers?: number` — Shields (4 = classic, at the original positions).
- `saucer?: boolean` — Include the mystery ship (default true).
- `movesPerFrame?: number` — Invaders moved per frame (1 = original; raise for a faster, smoother march).
- `playerShots?: number` — Simultaneous player shots (1 = original).
- `shotTypes?: ("rolling" | "plunger" | "squiggly")[]` — Which original shot types to use.
- `extraLives?: number[]` — Scores awarding an extra cannon ([1500] = classic).
- `seed?: number` — Seed for random-column shots.

### `InvadersState`

```ts
interface InvadersState
```

Serializable game state; read it through `state`.

- `frame: number` — Frames since the game began.
- `round: number` — Rack number, from 0.
- `score: number` — Score.
- `lives: number` — Cannons left, including the one in play.
- `extraAwarded: number` — Extra cannons awarded so far.
- `status: InvadersStatus` — Game status.
- `gameOverReason: "" | "lives" | "invasion"` — Why the game ended.
- `statusFrames: number` — Frames left in the current status.
- `player: { x: number; alive: boolean; visible: boolean }` — The cannon: left x, alive and visible.
- `invaders: InvaderState[]` — Every invader in march order (bottom row first).
- `alive: number` — Invaders left.
- `cursor: number` — Cursor into the bottom-up march order; direction +1 right, -1 left.
- `dir: number` — Rack direction: +1 right, -1 left.
- `sweepDown: number` — Drop applied during the current sweep.
- `bumped: boolean` — A bump was detected during this sweep: next sweep drops and reverses.
- `killPause: number` — Frames the rack stays frozen for an explosion.
- `sweeps: number` — Total completed march sweeps (presentation: the four-note fleet sound).
- `shots: InvaderShot[]` — Alien shots in flight.
- `playerShots: PlayerShotState[]` — Player shots in flight.
- `shotsFired: number` — Player shots fired this game (drives the saucer table and direction).
- `fireHeld: boolean` — Fire was held on the last frame (release rule).
- `shotLock: number` — Frames before another player shot may fire.
- `fireDelay: number` — Frames before aliens may fire.
- `tablePointers: Record<string, number>` — Column-table position per shot type.
- `saucer: SaucerState` — Mystery ship.
- `nextId: number` — Next shot id.
- `random: number[]` — The kernel generator's state (four words from `createRng`), advanced in place, so a snapshot carries the stream.

### `InvadersStatus`

```ts
type InvadersStatus = | "playing" | "player-dying" | "round-clear" | "game-over"
```

Playing, the cannon exploding, between racks, or over.

### `InvaderState`

```ts
interface InvaderState
```

One invader.

- `id: string` — Stable id `r<row>c<col>`.
- `kind: string` — Kind id.
- `col: number` — Fleet column.
- `row: number` — Fleet row, 0 at the top.
- `x: number` — Top-left of the invader's cell.
- `y: number` — Top of its cell.
- `alive: boolean` — Not yet shot.
- `frame: number` — Two-frame march animation; flips each time this invader moves.

### `MarchingFleetApi`

```ts
type MarchingFleetApi = typeof invaders
```

`game.marchingFleet`: the marching-fleet rules owner and its preset.

### `PlayerShotState`

```ts
interface PlayerShotState
```

One player shot in flight.

- `id: number` — Stable id for drawing.
- `x: number` — Centre x.
- `y: number` — Top y.

### `SaucerRules`

```ts
interface SaucerRules
```

The mystery ship.

- `interval: number` — Frames between saucer launches. Original: 0x600 = 1536 (25.6 s).
- `minInvaders: number` — No saucer when fewer invaders remain. Original: 8.
- `y: number` — Top of the saucer's 16x8 box.
- `width: number` — Saucer width.
- `height: number` — Saucer height.
- `step: number` — Pixels per step; the saucer steps on its shared shot turn. Original: 2.
- `scores: number[]` — Score table indexed by player shots fired, wrapping after `wrap` entries.
- `wrap: number` — Table entries used before the index wraps (original: 15 of 16, a ROM quirk).
- `directionByShots: boolean` — Odd shot count → enter from the left, even → from the right (original). Otherwise alternate.
- `timerBelowY?: number` — The saucer timer only runs while some live invader's top is below this y (original: the rack has dropped since round 1's start).
- `scoreFrames: number` — Frames the score is shown after a hit (presentation hint and saucer-slot lockout).

### `SaucerState`

```ts
interface SaucerState
```

Mystery ship state.

- `active: boolean` — Crossing now.
- `x: number` — Left edge.
- `dir: number` — +1 moving right, -1 left.
- `timer: number` — Frames since the last saucer left or was hit.
- `shown: number` — Score shown after a hit, counting down `showFrames`.
- `showFrames: number` — Frames the shown score lasts.
- `showX: number` — Where the score is shown.

---

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