# CORE-001A Decimal Unit Value Object Architecture

## Scope

This document defines the first implementation slice for:

- **CORE-001** — Explicit Unit/Value Classes for Domain Quantities
- **CORE-001A** — Decimal Unit Value Object Foundation

The goal is to introduce a small, strict, reusable domain layer that prevents silent unit confusion across portfolio, hedge, and backtesting code.

This slice is intentionally limited to:

- enums / typed constants for currency and weight units
- immutable Decimal-based value objects
- explicit conversion methods
- explicitly allowed arithmetic operators
- fail-closed defaults for invalid or ambiguous arithmetic

This slice should **not** yet migrate every page or calculation path. That belongs to:

- **CORE-001B** — overview and hedge migration
- **CORE-001C** — backtests and event-comparison migration
- **CORE-001D** — persistence / API / integration cleanup

---

## Design goals

1. **Eliminate unit ambiguity by construction.** Values must carry unit metadata.
2. **Use Decimal for bookkeeping accuracy.** No binary floating point in core domain value objects.
3. **Fail closed by default.** Arithmetic only works when explicitly defined and unit-safe.
4. **Keep the first slice small.** Add primitives first, then migrate consumers incrementally.
5. **Make edge conversions explicit.** Float-heavy libraries remain at the boundaries only.

---

## Core design decisions

### 1. Canonical numeric type: `Decimal`

All domain value objects introduced in this slice should use Python `Decimal` as the canonical numeric representation.

Implementation guidance:

- never construct `Decimal` directly from `float`
- construct from:
  - `str`
  - `int`
  - existing `Decimal`
- introduce a small helper such as `to_decimal(value: Decimal | int | str) -> Decimal`
- if a float enters from an external provider, convert it at the edge using a deliberate helper, e.g. `decimal_from_float(value: float) -> Decimal`

### 2. Immutable value objects

Use frozen dataclasses for predictable behavior and easy testing.

Recommended style:

- `@dataclass(frozen=True, slots=True)`
- validation in `__post_init__`
- methods return new values rather than mutating in place

### 3. Unit metadata is mandatory

A raw numeric value without unit/currency metadata must not be considered a domain quantity.

Examples:

- `Money(amount=Decimal("1000"), currency=BaseCurrency.USD)`
- `GoldQuantity(amount=Decimal("220"), unit=WeightUnit.OUNCE_TROY)`
- `PricePerWeight(amount=Decimal("4400"), currency=BaseCurrency.USD, per_unit=WeightUnit.OUNCE_TROY)`

### 4. Unsupported operators should fail

Do not make these classes behave like plain numbers.

Examples of operations that should fail unless explicitly defined:

- `Money + GoldQuantity`
- `GoldQuantity + PricePerWeight`
- `Money * Money`
- `PricePerWeight + Money`
- adding values with different currencies without explicit conversion support

### 5. Explicit conversions only

Unit changes must be requested directly.

Examples:

- `gold.to_unit(WeightUnit.GRAM)`
- `price.to_unit(WeightUnit.KILOGRAM)`
- `money.assert_currency(BaseCurrency.USD)`

---

## Proposed module layout

Recommended first location:

- `app/domain/units.py`

Optional supporting files if it grows:

- `app/domain/__init__.py`
- `app/domain/decimal_utils.py`
- `app/domain/exceptions.py`

Reasoning:

- this is core domain logic, not page logic
- it should be usable from models, services, calculations, and backtesting
- avoid burying it inside `app/models/` if the types are broader than persistence models

---

## Proposed enums

## `BaseCurrency`

```python
from enum import StrEnum

class BaseCurrency(StrEnum):
    USD = "USD"
    EUR = "EUR"
    CHF = "CHF"
```

Notes:

- omit `Invalid`; prefer validation failure over sentinel invalid values
- add currencies only when needed

## `WeightUnit`

```python
from enum import StrEnum

class WeightUnit(StrEnum):
    GRAM = "g"
    KILOGRAM = "kg"
    OUNCE_TROY = "ozt"
```

Notes:

- use **troy ounce**, not generic `oz`, because gold math should be explicit
- naming should be domain-precise to avoid ounce ambiguity

---

## Proposed conversion constants

Use Decimal constants, not floats.

```python
GRAMS_PER_KILOGRAM = Decimal("1000")
GRAMS_PER_TROY_OUNCE = Decimal("31.1034768")
```

Recommended helper:

```python
def weight_unit_factor(unit: WeightUnit) -> Decimal:
    ...
```

Interpretation:

- factor returns grams per given unit
- conversions can normalize through grams

Example:

```python
def convert_weight(amount: Decimal, from_unit: WeightUnit, to_unit: WeightUnit) -> Decimal:
    grams = amount * weight_unit_factor(from_unit)
    return grams / weight_unit_factor(to_unit)
```

---

## Proposed value objects

## `Money`

```python
@dataclass(frozen=True, slots=True)
class Money:
    amount: Decimal
    currency: BaseCurrency
```

### Allowed operations

- `Money + Money -> Money` if same currency
- `Money - Money -> Money` if same currency
- `Money * Decimal -> Money`
- `Money / Decimal -> Money`
- unary negation
- equality on same currency and amount

### Must fail

- addition/subtraction across different currencies
- multiplication by `Money`
- addition to non-money quantities
- division by `Money` unless a future ratio type is added explicitly

### Recommended methods

- `zero(currency: BaseCurrency) -> Money`
- `assert_currency(currency: BaseCurrency) -> Money`
- `quantize_cents() -> Money` (optional, for display/persistence edges only)

## `Weight`

Use a neutral weight-bearing quantity rather than a `Gold` type for the foundation layer.

```python
@dataclass(frozen=True, slots=True)
class Weight:
    amount: Decimal
    unit: WeightUnit
```

### Allowed operations

- `Weight + Weight -> Weight` after explicit normalization or same-unit conversion inside method
- `Weight - Weight -> Weight`
- `Weight * Decimal -> Weight`
- `Weight / Decimal -> Weight`
- `to_unit(unit: WeightUnit) -> Weight`

### Must fail

- implicit multiplication with `Weight`
- addition to `Money`
- comparison without normalization if not explicitly handled

## `PricePerWeight`

```python
@dataclass(frozen=True, slots=True)
class PricePerWeight:
    amount: Decimal
    currency: BaseCurrency
    per_unit: WeightUnit
```

Interpretation:

- `4400 USD / ozt`
- `141.46 USD / g`

### Allowed operations

- `PricePerWeight.to_unit(unit: WeightUnit) -> PricePerWeight`
- `Weight * PricePerWeight -> Money`
- `PricePerWeight * Weight -> Money`
- `PricePerWeight * Decimal -> PricePerWeight` (optional; acceptable if useful)

### Must fail

- adding prices of different currencies without explicit conversion
- adding `PricePerWeight` to `Money`
- multiplying `PricePerWeight * PricePerWeight`

## `AssetQuantity`

Backtesting currently uses neutral `underlying_units`, while live pages also use physical-gold semantics. For the foundation layer, keep this explicit.

Two viable approaches:

### Option A: only introduce `Weight` in CORE-001A

Pros:

- simpler first slice
- cleanly solves current gold/spot confusion first

Cons:

- historical `underlying_units` remain primitive a bit longer

### Option B: also introduce a neutral counted-asset quantity

```python
@dataclass(frozen=True, slots=True)
class AssetQuantity:
    amount: Decimal
    symbol: str
```

Recommendation:

- **Option A for CORE-001A**
- defer `AssetQuantity` to CORE-001C where historical scenario boundaries are migrated

---

## Explicit operator design

The main rule is:

> only define operators that are unquestionably unit-safe and domain-obvious.

Recommended first operator set:

```python
Weight * PricePerWeight -> Money
PricePerWeight * Weight -> Money
Money + Money -> Money
Money - Money -> Money
Weight + Weight -> Weight
Weight - Weight -> Weight
```

Example pseudocode:

```python
def __mul__(self, other: object) -> Money:
    if isinstance(other, PricePerWeight):
        adjusted_price = other.to_unit(self.unit)
        return Money(
            amount=self.amount * adjusted_price.amount,
            currency=adjusted_price.currency,
        )
    return NotImplemented
```

Important note:

- returning `NotImplemented` is correct for unsupported operator pairs
- if the reverse operation is also unsupported, Python will raise a `TypeError`
- that is the desired fail-closed behavior

---

## Validation rules

Recommended invariants:

### Money

- currency required
- amount finite Decimal

### Weight

- unit required
- amount finite Decimal
- negative values allowed only if the domain needs them; otherwise reject at construction

Recommendation:

- allow negative values in the primitive type
- enforce business positivity at higher-level models where appropriate

### PricePerWeight

- currency required
- per-unit required
- amount must be non-negative for current use cases

---

## Serialization guidance

For CORE-001A, serialization can remain explicit and boring.

Recommended shape:

```python
{
  "amount": "4400.00",
  "currency": "USD",
  "per_unit": "ozt"
}
```

Guidelines:

- serialize `Decimal` as string, not float
- keep enum serialization stable and human-readable
- avoid hidden coercion in JSON helpers

Persistence migration itself belongs primarily to **CORE-001D**, but the foundational classes should be serialization-friendly.

---

## Interop boundaries

Several existing libraries/services are float-heavy:

- yfinance payloads
- chart libraries
- some existing calculations in `app/core/`
- option pricing inputs/outputs

CORE-001A should establish a clear policy:

### Inside the core domain

- use Decimal-bearing unit-safe types

### At external edges

- accept floats only in adapters
- immediately convert to Decimal-bearing domain types
- convert back to floats only when required by third-party/chart APIs

Recommended helper names:

- `decimal_from_provider_float(...)`
- `money_from_float_usd(...)`
- `price_per_ounce_usd_from_float(...)`
- `to_chart_float(...)`

The helper names should make the boundary obvious.

---

## Suggested implementation order

### Step 1: add foundational helpers

- `to_decimal(...)`
- `decimal_from_float(...)`
- weight conversion constants and helpers

### Step 2: add enums

- `BaseCurrency`
- `WeightUnit`

### Step 3: add frozen value objects

- `Money`
- `Weight`
- `PricePerWeight`

### Step 4: add unit tests first

Test at least:

- `Weight(Decimal("1"), OUNCE_TROY).to_unit(GRAM)`
- `PricePerWeight(USD/ozt).to_unit(GRAM)`
- `Weight * PricePerWeight -> Money`
- `Money + Money` same currency succeeds
- `Money + Money` different currency fails
- invalid operator combinations raise `TypeError`
- decimal construction helpers reject unsafe ambiguous input patterns if desired

### Step 5: add one thin usage seam

Before full migration, wire one small non-invasive helper or calculation path to prove ergonomics.

Recommendation:

- introduce a helper used by the overview quote fallback path or a standalone calculation test first
- keep page-wide migration for CORE-001B

---

## Proposed first test file

- `tests/test_units.py`

Recommended test groups:

1. Decimal normalization helpers
2. Weight conversions
3. Price-per-weight conversions
4. Unit-safe multiplication
5. Currency mismatch failures
6. Unsupported operator failures
7. Serialization shape helpers if implemented in this slice

---

## Migration notes for later slices

## CORE-001B

Migrate:

- overview spot resolution / fallback
- margin-call math
- hedge starting-position math
- hedge scenario contribution math where unit-bearing values are mixed

## CORE-001C

Migrate:

- workspace `gold_value -> historical underlying_units` conversion
- backtest scenario portfolio construction
- event comparison scenario materialization
- explicit distinction between:
  - physical gold weight
  - USD notional collateral value
  - historical underlying units

## CORE-001D

Clean up:

- persistence schemas
- API serialization
- cache payloads
- third-party provider adapters

---

## Non-goals for CORE-001A

Do not attempt all of this in the first slice:

- full currency FX conversion
- replacing every float in the app
- redesigning all existing Pydantic models at once
- full options contract quantity modeling
- chart formatting overhaul
- database migration complexity

---

## Recommendation

Implement `CORE-001A` as a small, strict, test-first domain package introducing:

- `BaseCurrency`
- `WeightUnit`
- `Money`
- `Weight`
- `PricePerWeight`

all backed by `Decimal`, immutable, with explicit conversions and only a tiny allowed operator surface.

That creates the foundation needed to safely migrate the visible calculation paths in `CORE-001B` and the historical scenario paths in `CORE-001C` without repeating the unit-confusion bugs already discovered in overview and backtesting.