# mnw-assumptions

MNW's business numbers as a typed, validated, substitutable source of truth.

Loads `assumptions.toml`, computes derived values (Stripe fee math, ARPU, break-even,
annual prices, per-creator marginal cost), checks the whole thing for internal
consistency, and substitutes `{{ dotted.path | filter }}` markers in markdown.

Sits on the generic substitution engine, [subst](../subst). Nothing here is reusable
outside MNW.

```rust
use mnw_assumptions::Assumptions;

let a = Assumptions::load("docs/business/assumptions.toml")?;
a.validate()?;
let resolved = a.substitute(&markdown)?;
let html = docengine::render_permissive(&resolved);
```

Canonical file: `MNW/server/docs/business/assumptions.toml`. The copy under
`tests/fixtures/` is a vendored snapshot so the crate's own tests don't reach across the
repo; `server/tests/assumptions.rs` is what tests the live one.

## What it produces

Three views of the same toml:

- A **typed mirror** (serde structs) for the fields validation and derived math need.
- A **derived registry** under the `derived.` prefix: `R_cap`, `ARPU_founding`,
  `ARPU_standard`, `stripe_fee_*`, `break_even_*`, `annual_<class>_<tier>`,
  `marginal_*`, the discount percentages.
- A **flat lookup table** of every key in the file, dotted-path keyed, which is what
  substitution reads. Unknown sections are walked into it too, so a doc author can add an
  ad-hoc key to the toml and use it in markdown without touching Rust.

## Validation

`validate()` collects every failure rather than stopping at the first, and returns them
as one `AssumptionsError::Validation(Vec<String>)`. The rules:

- `expenses.F_monthly` inside (100, 10000), a typo guard on the burn rate
- `tier_mix.assumed.*` sums to 1.0
- `reserve.surplus_split_{reserve,earnback}` sums to 1.0
- `reserve.rho_annual`, `reserve.rho_incident` inside (0, 1], and incident <= annual
- every founding tier price <= the matching standard price
- `cohort.cap_count > 0`, `cohort.cap_months > 0`
- each `tier_bytes.<k>` parses back to the display string in `tier_limits.<k>`

That last one is the shape to copy when a number has both a machine form and a
human-facing form. The server adds one more of its own in `tests/assumptions.rs`: the
`[synckit]` block must match the Rust constants in `src/synckit_billing.rs`, which stay
authoritative.

## Filters

Everything `subst` ships (`int`, `ceil`, `floor`, `round`, `money`, `percent`, `upper`,
`lower`), registered by default. Add MNW-specific ones with `with_filter(name, f)` before
substituting. `LookupValue` is a re-export of `subst::Value`.

## substitute_dir

```
substitute_dir <assumptions.toml> <docs_root> [--check]
```

Walks a markdown tree and substitutes in place. `--check` writes nothing and exits
non-zero if any file would change or any marker failed to resolve. Built for the
`_private/docs/` mirror, which cites business numbers but does not go through the boot-time
pipeline the public site-docs corpus does.

## License

MIT.