dota_factory/docs/progression_design.md

# Progression & Balancing Design

Rules and principles that govern the production tree, progression pacing,
and balancing. This document contains **rules only** — concrete content
(item lists, recipes, unlock levels, stat numbers) lives in the config files
and `content_design.md`; those numbers must follow the rules stated here.

## Player-experience goals

What each phase of a run should feel like:

- **Early:** learning belts and ratios with forgiving chains. The building
  block economy is the main constraint; the player bootstraps a
  self-sustaining factory from the starting stock.
- **Mid:** deeper chains, the first real ratio puzzles, and the first
  meaningful drop decisions (which schematic, when to push).
- **Late:** combat feeds the factory — capital production requires salvage.
  Progress means extending and refactoring the existing factory, not
  rebuilding it. Strange ratios are deliberate optimization puzzles.

Overarching: an experienced player gains efficiency through **knowledge** —
layout foresight, understanding chains, exploiting shortcut recipes — never
through hidden mechanics. An inexperienced setup should not cost much more
than an experienced one; experience pays off in how easily the factory
adapts later (see Refactorability).

## Resource phases

- A run has exactly **four base inputs**:
  1. Two mined resources available from the start, minable on **every**
     asteroid tile.
  2. A third mined resource unlocked mid-game, minable **only on deposit
     patches** found in expansion territory (see Resource deposits).
  3. A fourth input unlocked late-game, obtainable **only** from
     reprocessing salvaged scrap.
- The fourth input is the core loop hook: capital ship production requires
  fighting (salvaging and reprocessing), not just mining.
- Every gating has a fictional reason (concrete fiction in
  `content_design.md`): the asteroid is a metal-rich body, so its bulk
  rock is minable anywhere; the mid resource sits in rare pockets; the
  late input is battle-forged — created only in the violence of ship
  destruction, which is why any wreck (including the player's own)
  yields it and no foundry can make it.
- The mid resource is **dual-gated**: schematics (knowledge, via drops)
  and territory (deposits, via expansions). Tuning must guarantee the
  deposit-bearing expansion is comfortably affordable by the time the
  first mid-tier schematics drop, or those drops are dead picks.
- There is no direct "resource unlock" mechanism. Miner recipes unlock
  **implicitly** (REQ-LOCK-IMPLICIT) when some unlocked schematic's material
  chain reaches that resource. Resource pacing is therefore controlled
  through the `unlock_at_station_level` values of ships, modules, and
  assembler recipe schematics — and the content must guarantee that the
  chains actually connect (a mid-game schematic must require an item whose
  chain reaches the mid resource, or it never unlocks).

### Resource deposits

- **Rule: freedom first, geography later.** The starting resources are
  minable everywhere, so the player has full layout freedom while
  learning. Later mined resources are bound to deposit patches — fixed
  geography as a layout puzzle, introduced once the player is competent.
- **Rule: deposits exist only in expansion territory.** Expansions buy
  space *and* access to resource tiers — the second leg of the growth
  curve (see Building block economy).
- **Rule: patch area is the throughput cap.** Deposits never deplete but
  are finite in area; the number of deposit tiles caps how many miners
  the chain supports. Buying deeper expansions raises the throughput
  ceiling of high-tier chains.
- **Rule: no empty expansions.** Deposit content per expansion is
  deterministic and config-defined; only the placement within the new
  columns is randomized. Buying an expansion never rolls "nothing".
- **Rule: mining is binary.** A miner whose footprint overlaps at least
  one deposit tile of a resource can select that resource's recipe; no
  partial-coverage rate scaling.
- Deposits arrive at the periphery (expansions add columns on the left),
  so each new chain starts in fresh space — supporting the
  refactorability property — and high-tier chains have the longest belt
  runs to the shipyards, escalating the logistics puzzle with tier.

## Production tree rules

### Structure

- **Each phase transition adds exactly one new base input chain.** A base
  input is a bottom-level resource entering the factory from outside — a
  mined resource or the scrap-only input. The early game starts with two
  ores as the baseline; the transition to mid adds one (the deposit-bound
  mid resource), the transition to late adds one (the scrap-only input).
  No transition ever introduces more than one unfamiliar bottom-level
  chain, so the factory grows in one direction at a time.
- **Intermediates are generic shared parts.** Keep the item count low —
  modules and hulls of a tier draw from a shared pool of that tier's and
  lower tiers' intermediates rather than each having bespoke inputs.
- **Thematic naming over thematic items.** Inputs should be plausible for
  what the recipe produces (crystals for lasers, heat sinks for bigger
  lasers). Achieve this through naming and chain membership, not by adding
  item types: rename a generic part, don't add a parallel one.

### Ratios

- **Ratio "niceness" degrades with tier.** The producer:consumer ratios
  needed for 100% throughput follow a curve:
  - Tier 1 (ore → basic material): trivially nice (e.g. 1:1 or 1:2
    miner:smelter).
  - Tier 2: slightly complex but still clean (e.g. 2:3).
  - Higher tiers: increasingly strange ratios, as deliberate optimization
    puzzles.
- Exceptions in both directions are allowed when there is a reason — a
  clean late chain as a breather, an odd early chain as a teaser — but the
  curve is the default.

### Shortcut recipes

- Some strange chains get a **shortcut recipe**: an explicitly unlockable
  assembler recipe schematic (`unlock_at_station_level ≥ 0`, drop-only per
  REQ-LOCK-EXPLICIT) that skips a step (e.g. t1 → t3 directly) and yields
  nice ratios for a chain whose base path is strange.
- **Not every strange chain gets a shortcut.** Some strangeness is
  permanent; the absence of a fix is a valid design choice.
- **Shortcuts drop only for known chains.** A shortcut recipe enters the
  drop pool only when both its input items and its output item are
  already unlocked (in addition to the station level check). The player
  is never offered a shortcut for a chain they have not built yet. The
  output-item half of this check already exists in
  REQ-DEF-SCHEMATIC-DROP; the input half is new (see Action items).
- **Shortcuts are pure rewards, never balance factors.** An item's threat
  value is the *maximum* across its producing recipes (REQ-THREAT-ITEM), so
  unlocking a cheaper recipe does not lower the item's threat accounting —
  the player gains real factory efficiency without their ships being
  valued cheaper and without enemy wave budgets shifting. Consequently:
  **balance every chain around its base (expensive) path**; the shortcut's
  savings define the size of the reward.

### Refactorability

- **Rule (the property):** unlocking the next tier or size of a thing must
  be a *local edit* of the existing production line — adding assemblers
  and belts, or replacing a machine or two in place — never a rebuild of
  the line.
- **What this buys the player:** foresight pays off in space, not blocks.
  An experienced player leaves a little slack in the middle of a line,
  knowing the next size or tier upgrade means tearing out one assembler
  and a few belts there and inserting the new step — plus maybe swapping
  a recipe or two elsewhere — while the rest of the line keeps running
  untouched.
- **Default technique:** the bigger version introduces one new intermediate
  that is produced from a subset of the smaller version's inputs (possibly
  plus one additional low-tier material), and otherwise reuses the smaller
  version's inputs. Existing lines keep running and feed the new
  intermediate's assemblers.
- The property is the rule; the technique is only the default. It may be
  broken where it fights thematic plausibility, as long as the property
  still holds.

## Cost archetypes

Every item has two cost knobs: **material quantity** and **cycle time**.
Both feed the threat value identically (threat = recursive
production-seconds, REQ-MOD-THREAT), so the split between them does not
change what an item is *worth* — it changes what kind of **factory
pressure** it creates:

- **Material-heavy, fast** (e.g. armor plates): simple items; stress belt
  throughput, splitter logistics, and miner/smelter counts.
- **Time-heavy, lean** (e.g. shield modules): technically complex items;
  few inputs — possibly higher-tier ones — but long cycles; stress
  assembler counts and parallelization.

**Rule:** each module family commits to a clear archetype, so factories
supporting different fleet doctrines feel structurally different to build.

## Threat model (balancing backbone)

- Threat cost = total recursive production-seconds (REQ-MOD-THREAT). One
  factory-second equals one threat; player output and enemy wave budgets
  are denominated in the same currency.
- **Rule: combat power per threat is roughly constant** across all ships,
  modules, and tiers. Higher tiers are better per *ship* and per *module
  slot*, not per invested factory-second — their advantage is
  concentration (fewer, bigger things; slot geometry per
  `content_design.md`) and qualitative capabilities, not a better exchange
  rate. Deviations from this rule are deliberate and documented.
- **Difficulty race:** the enemy threat rate (`threat_rate_formula`) is
  tuned against the factory output (threat/s) achievable by a competent
  player — slightly below it early, crossing above it eventually. The game
  is endless; enemy scaling must ultimately outpace any factory, and
  player skill shifts *when*, not *whether*.
- **All time scaling lives in the threat rate** — waves get bigger, ships
  of a given schematic never get individually stronger. There is no ship
  level dimension: stat formulas are plain values, and per-ship level
  scaling does not exist. Push
  scaling on enemy defence stations is the separate, player-triggered
  difficulty axis and keeps its level formulas.

## Unlock & drop pacing

- **Starting set rule:** the schematics unlocked at game start
  (`unlock_at_station_level = -1`) must be exactly enough to reach the
  first push unaided — a functioning block loop, small hulls, a basic
  weapon, and the salvage loop. Nothing more.
- The `unlock_at_station_level` ladder mirrors the resource phases:
  mid-tier hulls/modules/recipes at low station levels, capital content at
  higher levels. A schematic must not become available before the chains
  its materials need can be unlocked alongside it.
- **Schematics can require other schematics.** Beyond the station-level
  gate, a schematic (ship, module, or assembler recipe) may list
  prerequisite schematics (`unlock_requires`, REQ-LOCK-PREREQ) that must
  already be unlocked before it enters the drop pool — e.g. the medium
  laser requires the small laser; a future Mk2 requires its base version.
  Station level gates the earliest
  *when*; prerequisites gate the *order*, keeping drop offers coherent
  with what the player already owns.
- **No duplicate drops.** Ship and module schematics leave the drop pool
  once owned, exactly as assembler recipe schematics already do. There are
  no schematic level-ups; player power grows through unlock breadth and
  factory scale only, which keeps power-per-threat exact on both sides.
  The pool therefore shrinks over a run and late pushes increasingly offer
  artifacts — intended: the late game is a race for the win condition.
  Per-item progression may return later as Mk2 upgrade recipes (see Open
  tasks), never as free level-ups.
- **Artifacts trade power for progress.** Artifact options compete with
  schematic picks in the same choice dialog; the artifact chance must be
  tuned so that taking one is a real decision (giving up an unlock), not
  automatic in either direction.

## Scrap & reprocessing economy

- Scrap is the bridge from combat back into the factory, with two sinks:
  **smelting** (same basic materials as ore — the safe, boring option) and
  **reprocessing** (probabilistic higher intermediates, including the
  late-game input — the gamble that eventually becomes mandatory).
- The reprocessing output pool renormalizes over implicitly unlocked items
  (REQ-LOCK-REPROCESSING-POOL), so its output quality improves
  automatically as the run progresses. **Rule:** weights are authored for
  the *fully unlocked* pool state; early-game behavior falls out of
  renormalization for free and needs no separate staging.
- **Rule: ship scrap drops are derived, never authored.** A destroyed ship
  drops `threat cost × scrap_per_threat` (a `world.toml` key), with the
  threat cost computed from its actual hull plus installed modules
  (REQ-MOD-THREAT) — a kitted-out ship drops more scrap than a bare hull
  automatically. `ships.toml` carries no scrap value. Defence stations are
  the exception: they keep authored `scrap_drop_formula`s, because pushing
  rewards are tuned independently of ship production costs.
- Consequence: the threat value of scrap is the constant
  `1 / scrap_per_threat` (REQ-THREAT-SCRAP). The former min-`scrap_drop`
  schematic derivation and its potential circularity are gone.
- **Rule:** the late-game input's income rate meaningfully gates capital
  production — unlocking a capital hull must not mean spamming it; the
  input trickles in slowly enough that every capital ship is a noticeable
  investment. The tuning target is relative, not absolute: assume a
  reference player who destroys and salvages roughly the threat the game
  spawns ("fighting at parity"), and tune `scrap_per_threat`, the
  reprocessing weights, and capital material costs so that this player
  affords roughly N capital ships per boss cycle. An absolute income rate
  would be meaningless (income depends entirely on how much the player
  fights) and would not self-scale; per boss cycle, the target tracks the
  threat rate as it steps up.

## Building block economy

- Building blocks are the only global currency and the early game's
  central constraint. The early game is a bootstrap problem: convert the
  starting stock into a self-sustaining block loop before the first waves
  bite.
- **Rule:** the starting stock suffices for a minimal block loop plus the
  first shipyard — with a little slack for beginner mistakes, but not
  enough to skip the loop entirely.
- **Rule: the growth curve lives here.** A saturated building produces
  exactly 1 threat/s, so the player's output curve *is* their
  building-count curve — shaping growth over a run means shaping the
  block and space economy, there is nowhere else it can live. Intended
  shape: exponential bootstrap (block-limited) → ramp
  (expansion-limited) → asymptotic squeeze as expansion costs outrun
  income, racing the enemy threat rate throughout.
- **Rule: escalating expansion costs.** Expansion cost is a formula of
  the number of expansions already purchased, rising steeply enough that
  expansions eventually outrun any block income. The starting asteroid
  is deliberately small — filled within the first boss cycle or two, so
  the early exponential burst is a satisfying ramp, not a balance hole —
  and from then on the output curve is the expansion curve. Blocks keep
  a meaningful sink for the entire run, and "grow vs. army" stays a live
  decision at every moment.
- **Rule: designed doubling time.** Block production is a positive
  feedback loop (blocks buy assemblers, assemblers make blocks); its
  time constant is a designed quantity, never an accident of quantity
  choice. The block chain's depth and the per-building costs are tuned
  against a stated target of the form: "a factory spending X% of its
  capacity on blocks doubles in ~T minutes."
- **Rule: growth is limited by economy, never by waiting.** Construction
  times stay short; the serial build queue must not be used as a growth
  brake. Waiting for placed buildings to become operational — especially
  at the start of a run — is frustration, not gameplay. All growth
  limiting comes from block income and expansion pricing.
- Note: block income has a structural ceiling — blocks enter the stock
  through the HQ's single belt port, so income is capped at belt
  throughput regardless of assembler count. Per-building costs should be
  high enough that this cap can bind late-game (see the condensed-block
  idea under Open tasks).

## Numeric guardrails

Constraints that every recipe must respect, independent of tuning:

- **Belt throughput:** belt speed and per-tile capacity cap how fast a
  single belt can feed an input. A recipe whose per-cycle inputs cannot be
  sustained by one belt per input at 100% duty cycle is a *deliberate*
  design (forcing parallel belts/splitters as part of a high-tier puzzle)
  — never an accident of quantity choice.
- **Buffer burstiness:** input buffers hold 2× the per-cycle amount
  (REQ-MAT-INPUT-BUFFER), so large per-cycle quantities create bursty belt
  demand. Low tiers prefer small quantities with short cycles; big-batch
  recipes are reserved for high tiers where burstiness is part of the
  puzzle.
- **Cycle times scale with tier** monotonically — a higher-tier item never
  has a shorter total chain time than a lower-tier item of the same role.

## Open tasks / future work

- **Rework `recipes.toml`** once the rules in this document are fixed: the
  current tree feels too close to Factorio; apply the thematic-naming rule
  and the ratio curve to it (renames and quantity changes, not new items).
- **Balancing pass** (see placeholders in `content_design.md`): set the
  `unlock_at_station_level` ladder, real threat costs and
  `default_modules`, reprocessing weights, and the threat-rate formulas
  according to the rules above.
- **Condensed building blocks** — a drop-unlockable shortcut-style
  recipe that packs several blocks' worth of value into one belt item,
  relieving the HQ intake ceiling (see Building block economy) as a
  late-game reward. The ceiling is the puzzle, the drop is the fix —
  same philosophy as shortcut recipes.
- **Mk2 upgrade recipes** — the deferred design for per-item progression,
  to revisit once the config has stabilized. A duplicate-style drop
  unlocks a distinct `*_mk2` item whose recipe consumes the Mk1 item plus
  higher-tier parts. This preserves power-per-threat (the extra power is
  paid in real production-seconds, since threat is recursive), satisfies
  the refactorability rule (the Mk1 line keeps running and feeds one new
  assembler), and keeps balancing one-dimensional (no level variable
  anywhere). Enemy-side progression happens via `default_modules`
  variants per era instead of a level formula.

## Action items — changes beyond this document

Agreed changes that require edits to `requirements.md`, the code, and the
configs. Completed items are removed from this list (their outcomes live
in `requirements.md` and the git history). Still open:

1. **Fill unfillable schematic slots with artifacts.** With duplicates
   removed, the schematic drop pool can run dry — previously unreachable.
   Decision: every slot in the choice dialog that cannot be filled with a
   schematic because the eligible pool is exhausted is filled with an
   artifact option instead (in addition to any artifact option granted by
   the regular artifact roll). A push therefore always awards a full
   dialog. Update REQ-DEF-SCHEMATIC-DROP.
2. **Confirm wave scaling carries alone.** With per-ship level scaling
   removed, `threat_rate_formula` is the only time-scaling axis; verify
   the current `world.toml` values still produce the intended difficulty
   curve (likely a review, not a retune, since the config already ran
   with a flat ship level before the removal).
3. **Gate shortcut-recipe drops on their inputs.** Extend the assembler
   recipe schematic pool eligibility in REQ-DEF-SCHEMATIC-DROP: in
   addition to the existing station-level and output-item checks, all of
   the recipe's input item types must be implicitly unlocked as well.
4. **Expansion cost formula.** Replace the flat
   `world.toml [expansion].cost_building_blocks` with a formula of the
   number of expansions already purchased (x = expansions bought so
   far), so expansion costs can escalate per the block economy rules.
   Update REQ-EXP-COST and wherever the UI displays the expansion cost.
5. **Resource deposits.** Add a terrain deposit layer per the Resource
   deposits rules: deposit patches generated in expansion columns
   (deterministic content per expansion, randomized placement within the
   new columns), deposit rendering, and a miner condition (a resource
   recipe is selectable only if the miner's footprint overlaps at least
   one matching deposit tile). Touches REQ-BLD-MINER ("every asteroid
   tile is equivalent" no longer holds), REQ-GW-ASTEROID-EXPAND /
   REQ-EXP-*, `world.toml`, and `visuals.toml`.
6. **Scrap-consuming recipes as threat fallback only.** Amend
   REQ-THREAT-ITEM: recipes that take scrap as an input participate in
   an item's threat computation only if no scrap-free recipe (miner,
   smelter, or assembler) produces that item — mirroring the existing
   rule for the reprocessing path. Otherwise the scrap→ingot smelter
   recipe would inflate the basic materials' threat via the
   max-across-recipes rule, poisoning every downstream value.
7. **Per-unit item threat.** Amend REQ-THREAT-ITEM and
   `ThreatCostCalculator`: a recipe's threat is divided by its output
   amount, so item threat is production-seconds *per unit*. Currently a
   recipe producing 2 copper_wire per run assigns each wire the full
   run's threat, double-pricing multi-output items and everything
   downstream of them.
8. **Fixpoint resolution in ThreatCostCalculator.** Items downstream of
   reprocessing-only items (e.g. capital parts built from the scrap-only
   input) never resolve, because resolution stops after the reprocessing
   pass instead of iterating; their consumers silently drop the missing
   materials, so capital hull threat is currently underestimated (found
   by `tools/threat_report.py`, which implements the correct fixpoint).