74 lines
3.2 KiB
Markdown
74 lines
3.2 KiB
Markdown
# Architecture
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## The one big idea: engine-first, everything is a command
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`ledger-core` owns all accounting knowledge. Every state change is a method
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on `Ledger`:
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- `add_account` — extend the chart of accounts
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- `create_entry` / `add_line` — build a draft journal entry
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- `post_entry` — validate and apply (one-way)
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- `reverse_entry` — correct via reversal (one-way)
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Each command validates *before* it applies, and each failure is a typed
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`LedgerError` variant. There is no other mutation path: struct fields are
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private, and constructors are `pub(crate)` where it matters. This is what
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makes the ledger auditable and what will make the future oracle harness
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trustworthy — the harness only needs to drive the command surface.
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Reports (`trial_balance`, `balance_sheet`, `profit_and_loss`, `snapshot`)
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are free functions: pure, total, and deterministic. Same ledger in, same
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report out, every time, on every platform. That determinism is the entire
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basis of `docs/oracle.md`.
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## Current crate layout
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```
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crates/ledger-core/
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src/
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lib.rs # crate docs, re-exports, design rules
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money.rs # Currency + Money (Decimal, checked arithmetic)
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account.rs # AccountType, Side, Account, ChartOfAccounts
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journal.rs # JournalLine, JournalEntry, validate()
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ledger.rs # Ledger: the command surface, append-only log
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reports.rs # trial_balance, profit_and_loss, balance_sheet, snapshot
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tests/
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invariants.rs # accounting-law tests + randomized property test
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golden/ # hand-verified expected report snapshots
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```
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## Dependency rules
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`ledger-core` may depend only on:
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- `rust_decimal` — exact decimal arithmetic
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- `thiserror` — the error type
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- `chrono` — dates (`NaiveDate`; no wall-clock reads in the core)
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- `serde` / `serde_json` — the oracle interchange format
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New dependencies need justification in the commit message. In particular:
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no I/O, no networking, no async runtime, no logging framework in the core.
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The core is a pure domain library and stays that way.
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## Future crates (do not create until the roadmap says so)
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| Crate | Purpose | Depends on |
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| `ledger-store` | Journal (hash-chained JSONL) + SQLite projection, blob store | `ledger-core` |
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| `ledger-import` | QuickBooks CSV read-only import (bank transactions, chart of accounts) | `ledger-core` |
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| `ledger-export` | Format renderers: CSV, XLSX, IIF journal entries (QuickBooks Desktop) | `ledger-core` |
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| `ledger-oracle` | Diff harness: our engine vs GnuCash on fixture books | `ledger-core`, `ledger-import` |
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| `books-app` | egui UI (native + WASM), command-driven | `ledger-core`, `ledger-store` |
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Dependency direction is always inward toward `ledger-core`. Nothing in the
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core may depend on a future crate. UI concerns (formatting, locale, widgets)
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never leak into the core; the core speaks `Decimal` and ISO codes.
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## Why Rust
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Exact decimal semantics, exhaustive pattern matching on the five account
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types, and a type system that makes invalid states (mutable posted entries,
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float money) unrepresentable. The storytold pipeline this project is modeled
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on is Rust + egui for the same reasons; when the UI crate arrives it will be
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one codebase compiling to native and WASM.
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