world-builder — World Builder's Handbook reference implementation

Go library and CLI that generates Mongoose Traveller star systems per the procedures in the World Builder's Handbook (Geir Lanesskog, 2023). Given a seed, the tool produces a complete deterministic system — stars, planets, moons, belts, atmospheres, oceans, climate, geology, biology, habitability — rendered as the three IISS Survey forms (Class 0/I + Class II/III + Class IV-P) in Markdown, JSON, or a short profile string.

What it covers

WBH chapters 1-3 — pp.14-146:

• Stars (pp.14-35). Spectral types, luminosity classes, mass / diameter / temperature / luminosity, age, multi-star systems with companions, special objects (brown dwarfs, white dwarfs, etc. as primaries are stubbed; as companions they generate with type / mass / age).
• System Worlds and Orbits (pp.36-68). Habitable-zone central orbits, available orbits per stellar group, world counts, baseline orbit, system spread, anomalous slots, world placement, eccentricities.
• World Physical (pp.69-146). Per-body sizing, moons, designations, periods, HZ tagging, body physical (composition / density / gravity / mass), belt details, rotation / tilt / tidal lock / tidal effects, atmosphere / hydrographics / temperature (climate cluster), atmosphere taint typology, surface distribution, geology (residual seismic + tidal stress + tidal heating + tectonic plates), biology (biomass through compatibility), habitability rating, mainworld pick.
• IISS Survey Forms. Class 0/I (stellar census, p.35), Class II/III (per-body Objects table, pp.60-67), Class IV-P (mainworld detail, pp.138-146) with both PART P (planet/moon variants) and PART P.B (belt variant).

What's not here (out of scope): pp.147-234 (World Social Characteristics, Special Circumstances) and detailed special-object physics. The Special Circumstances chapter is explicitly deferred per project intent.

Quick start

# Build
go build ./cmd/world-builder

# Generate a full system as Markdown
go run ./cmd/world-builder -seed 42 -format markdown

# Short profile only
go run ./cmd/world-builder -seed 42 -format short
# → 2-1-9-8-0.7
# (G-P-T-N-S form per WBH p.58: gas-giants, belts, terrestrials, baseline, spread)

# Full system as JSON for downstream tooling
go run ./cmd/world-builder -seed 42 -format json

Sample of -format markdown output (truncated):

# A VII — System Survey

**Mainworld:** A VII

Short profile: `2-1-9-8-0.7`

Long profile: `A-7-T-G-T-T-G-T-T-T-0.7:B-0-T-T-P-T-0.7`

## Class 0/I — A VII

| Component | Class | Mass | Diameter | Temp (K) | Luminosity | HZCO |
| --------- | ----- | ---- | -------- | -------- | ---------- | ---- |
| A | A3 V | 1.780 | 1.914 | 8800 | 19.78 | 5.69 |
| B | A9 V | 1.380 | 1.830 | 7600 | 10.07 | 5.16 |

## Class II/III — A VII
...

Class IV-P renders only for the auto-picked mainworld (per WBH p.134's mainworld-priority chain: native sophonts → highest habitability → highest resource → first in iteration order), using whichever PART variant matches the mainworld's type (planet, moon, or belt).

Library usage

import (
    "fmt"

    "github.com/philoserf/world-builder/iiss"
    "github.com/philoserf/world-builder/worlds"
)

func main() {
    u, err := worlds.Generate(42)
    if err != nil {
        panic(err)
    }
    fmt.Print(iiss.MarkdownSystem(u.Detail.SystemForms))
}

Three layers:

• dice/ and roller/ — dice-notation parser and Roller interface with Seeded, Scripted, Fixed impls. A seed plus a procedure sequence fully determines a system; no package-level RNG anywhere.
• stars/ — WBH pp.14-35. stars.GenerateSystem(r, opts) returns a multi-star System.
• worlds/ — WBH pp.36-146. worlds.Generate(seed) returns a complete Universe (system + placement + per-body detail + system aggregations).
• iiss/ — IISS form structs and renderers (Markdown / JSON / PlainText). Boundary type is iiss.SystemForms, populated by worlds.BuildIISSForms and consumed by the renderers.

Lower-level entry points (stars.GenerateSystem, worlds.GenerateSystemPlacement, individual Apply* stages, per-procedure Roll* and Generate* functions) remain available for finer control — see docs/api-surface.md.

Architecture

The project went through a major re-layering in 2026. Pass-1 followed WBH pagination as architecture; pass-2 inverted that — the data dependency graph determines structure, worked-example fixtures determine the acceptance gate, the book is a citation system rather than an architecture.

Key design choices, all documented at docs/ root:

• Unified Body type. Moons are Body{Kind: BodyMoon, Parent: <planet>} walked by the same iterator as planets. The moon-path silent-zero anti-pattern that recurred four times in pass-1 is prevented at the type level.
• iiss/ package boundary. Renderers and form structs live in their own package; iiss/ does not import worlds/. iiss.SystemForms is the boundary aggregate.
• ApplyClimatePasses per-body solver. Folds partial geology (Residual + TSF + THF) into each rederive pass. Originally framed as a fixed-point solver; empirical investigation revealed the climate cluster is not a fixed point in the strict sense (RederiveAtmosphereHydrographics samples fresh dice per call), so the design settled on 2-pass-take-the-last behaviour, honestly named.
• Stage orchestrators: ApplyDetailFrontEnd, ApplyBodyPhysical, ApplyRotationTilt, ApplyClimate, ApplyTaintTypology, ApplyGeology, ApplyBiology, ApplyHabitability, AggregateSystem — each walks Body + Body.Children to drive per-procedure work.

The original pass-1 implementation is preserved on tag pass-1-final for archival.

Status

v1.0 shipped 2026-05-12 (tag v1.0, GitHub release). The 10 000-seed sweep produces 10 000 real, fully-formed systems with zero errors; seed determinism preserved. See docs/next-steps.md for the small set of post-v1.0 open items — all optional polish or explicitly-out-of-scope.

All gates currently green:

task        # check + test (modernizer + gofumpt + vet + golangci-lint + go test -race)

Coverage:

• Per-procedure value-exact tests for every WBH-narrated dice script (the book is the spec).
• Stage integration tests using Seeded shape-invariant assertions (per docs/history/spike-findings.md § Finding 2: full-pipeline gold scripts don't survive pipeline reorders).
• Façade end-to-end tests over 100 seeds.
• Property tests (8 invariants × 1000 seeds each).
• Misuse-path contract tests for every public function.
• Markdown regression baseline (5 seeds × full output) at iiss/testdata/seed_*.md — refresh via go test ./iiss/... -update.regression -run TestRegression.

Known limitations (out of pass-2 scope):

• Post-stellar primaries (white dwarf / neutron star / black hole as the primary star) hit stars.ErrPostStellarPrimaryUnsupported. About 3-4% of random seeds produce them.
• Peculiar primary dispatch, giant-companion-MAO gaps, missing class-IV table cells — same family. Skipped via isSpecialCircumstances(err) in property tests.
• WBH pp.147+ (World Social Characteristics, Special Circumstances chapter) — explicitly out of scope.

Documentation

• Evergreen design + reference docs (under docs/ root): design-intent.md (the why and the cuts), api-surface.md (every public signature), dependency-graph.md (every value, its inputs, fixed-point clusters), anti-patterns.md (don't-do-this catalog), harness.md (fixture catalog with status indicators), wbh-inconsistencies.md (six book-internal divergences with chosen interpretations), summary.md (one-page overview), next-steps.md (post-v1.0 open items).
• Historical artifacts (under docs/history/): pass-1 specs/plans/retrospective from the original implementation, plus the pass-2 rebuild's retrospective (lessons-learned.md, plan-clean-every-run.md, generator-error-catalog.md, spike-findings.md, allbodies-migration.md). Preserved for context; not authoritative. Pass-1 was buildable at tag pass-1-final; pass-2 design is now what's on main.

Development

task                        # default: check + test
task check                  # modernize + gofumpt + go vet + golangci-lint
task test                   # go test -race ./...
task fmt                    # gofumpt -l -w -extra .
task tidy                   # go mod tidy

# Single test:
go test ./worlds/ -run TestZed_ApplyStage5

# Run CLI:
go run ./cmd/world-builder -seed 42 -format markdown

task check runs go fix ./... first (modernizer pass) and fails on any diff — modernizer hints are mandatory. The gate is local; no CI.

Conventions: gofumpt formatting (gofumpt -l -w -extra .); typed Roller interface (no package-level RNG); WBH page citations in doc-comments; six book inconsistencies committed to specific interpretations in code (no runtime toggles for them, see docs/wbh-inconsistencies.md).

Source material

Every procedure, every table, and every formula references Mongoose Publishing's World Builder's Handbook (Geir Lanesskog, 2023) as the canonical authority. WBH page numbers appear in doc-comments next to the tables and procedures they encode — that's the project's traceability mechanism. To work with this repository locally and verify procedure fidelity against the source, place a copy of the handbook PDF at docs/World Builders Handbook.pdf (gitignored — copyright).

Where the book contradicts itself (worked example vs formula box, table A vs table B), this implementation surfaces the divergence in docs/wbh-inconsistencies.md rather than silently picking one. Each of the six known inconsistencies has a chosen interpretation with rationale and a test that asserts the chosen value.

License

MIT — see LICENSE. The code is licensed for reuse. The World Builder's Handbook itself is © Mongoose Publishing and is not redistributed here; users must obtain a copy of the handbook independently to verify procedure fidelity against the source.