# @lpm.dev/nora-natio.record-diff

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> Detect added, changed and removed API records with deterministic SHA-256 snapshots, without persisting the original payloads.

- Version: 1.0.0
- Ecosystem: JavaScript
- Distribution: pool
- License: MIT

## Install

```bash
lpm install @lpm.dev/nora-natio.record-diff
```

## Agent quick reference

Detect added, changed, and removed records between API polls by comparing SHA-256 fingerprints of complete collections, without storing full payloads.

### Quick start

```js
import { diffRecords } from '@lpm.dev/nora-natio.record-diff';

const first = diffRecords(null, [{ id: 'a', price: 10 }, { id: 'b', price: 20 }]);
// first.added contains both records

const second = diffRecords(first.snapshot, [
  { price: 11, id: 'a' },
  { id: 'c', price: 30 },
]);
// second.added: [{ id: 'c', price: 30 }]
// second.changed: [{ price: 11, id: 'a' }]
// second.removed: ['b']
// second.unchanged: []
```

### Key exports

- `diffRecords` (function): Compare a previous snapshot (or null) with the current complete collection. Returns added, changed, removed, unchanged records and a new snapshot. — `(previous: Snapshot | null, records: T[], options?: Options<T>) => Diff<T>`
- `createSnapshot` (function): Create a baseline snapshot from records without computing a diff. Useful for initializing state. — `(records: T[], options?: Options<T>) => Snapshot`
- `fingerprint` (function): Compute the SHA-256 hex digest of a value's deterministic JSON encoding. — `(value: JsonValue) => string`
- `canonicalJson` (function): Produce a deterministic JSON string with sorted object keys. Used internally for fingerprinting. — `(value: JsonValue) => string`
- `Options` (type): Options for customizing record identity and fingerprint selection. — `{ key?: string | ((record: T) => string); select?: (record: T) => JsonValue; }`
- `Diff` (type): Result of a diff operation containing added, changed, removed, unchanged records and the new snapshot. — `{ added: T[]; changed: T[]; removed: string[]; unchanged: string[]; snapshot: Snapshot; }`
- `Snapshot` (type): Immutable snapshot structure with version, algorithm, and entries (id + hash). — `{ version: 1; algorithm: 'sha256'; entries: Array<{ id: string; hash: string }>; }`
- `JsonValue` (type): Allowed JSON value types: null, boolean, number, string, array, or plain object. — `null | boolean | number | string | JsonValue[] | { [key: string]: JsonValue }`

### First poll (no previous snapshot)

Use when you have no previous snapshot (first poll). All records are treated as added.

```js
const result = diffRecords(null, records);
// result.added contains all records
// save result.snapshot for next poll
```

### Subsequent poll with snapshot

Compare current records against a stored snapshot to detect additions, changes, and removals.

```js
const result = diffRecords(previousSnapshot, records);
// result.added, result.changed, result.removed, result.unchanged
// save result.snapshot for next poll
```

### Custom key and field selection

Use a custom ID field and ignore volatile fields (e.g., timestamps) by selecting only relevant properties for fingerprinting.

```js
const options = {
  key: (row) => String(row.productId),
  select: (row) => ({ price: row.price, stock: row.stock })
};
const result = diffRecords(prevSnapshot, records, options);
```

### Gotchas

- Values must be JSON-safe: non-finite numbers, undefined, BigInt, Date, sparse arrays, symbols, and nesting >100 levels throw TypeError. Convert before calling.
- Record IDs must be unique, non-empty strings. By default uses the 'id' field; use the key option for custom IDs.
- Array order counts as a change; object key order does not. Reordering items in an array will be detected as a change.
- Snapshots must be stored and passed back exactly. Reset the snapshot if you change key or select rules.
- No transactional delivery: commit the snapshot only after your processing succeeds to avoid data loss.

## README

# record-diff

Detect added, changed and removed records between API polls. Persist IDs and
SHA-256 fingerprints instead of full payloads. Zero dependencies, Node.js 18+,
ES modules and TypeScript declarations. No network calls, telemetry, filesystem
access, lifecycle hooks or background workers.

Useful for ETL jobs, product feeds, ticket sync, inventory polling and scheduled
automations that should only process new or changed records.

## Install

```sh
lpm install @lpm.dev/nora-natio.record-diff
```

## Quick start

```js
import { diffRecords } from '@lpm.dev/nora-natio.record-diff';

const first = diffRecords(null, [{ id: 'a', price: 10 }, { id: 'b', price: 20 }]);
// first.added contains both records

const second = diffRecords(first.snapshot, [
  { price: 11, id: 'a' },
  { id: 'c', price: 30 },
]);
// second.added: [{ id: 'c', price: 30 }]
// second.changed: [{ price: 11, id: 'a' }]
// second.removed: ['b']
// second.unchanged: []

// Save second.snapshot with JSON.stringify using your own storage.
// Commit it only after your processing succeeds; this library does not
// provide transactional delivery, retries, locking or exactly-once effects.
```

## Ignore volatile fields and use custom IDs

```js
const options = {
  key: row => String(row.productId),
  select: row => ({ price: row.price, stock: row.stock }),
};
const result = diffRecords(previousSnapshot, completeProductList, options);
```

Use the same key and select rules on every poll. Reset the snapshot when changing
those rules. Selection affects fingerprints, not returned records: `added` and
`changed` contain the original objects, without cloning or mutation.

## API

- `diffRecords(previous, records, options?)`: compare a snapshot (or `null` for
  first use) with the complete current collection. Returns `{ added, changed,
  removed, unchanged, snapshot }`.
- `createSnapshot(records, options?)`: establish a baseline without notifications.
- `fingerprint(value)`: SHA-256 hex digest of the deterministic JSON encoding.
- `canonicalJson(value)`: deterministic JSON encoding with sorted object keys.

Record IDs must be unique, non-empty strings. By default they come from `id`;
`key` may name another field or be a function. Results preserve current input
order; removed IDs preserve previous snapshot order. Record reordering and JSON
object key reordering do not count as changes. Array order does count.

Values must be JSON data: null, strings, booleans, finite numbers, dense arrays
and plain objects. Cycles, non-finite numbers, undefined, BigInt, Date instances,
accessors, sparse arrays, symbol keys and nesting beyond 100 levels are rejected
with `TypeError`. Convert such values explicitly before calling. This encoding
is intended for these snapshots; it is not an RFC 8785 implementation.

## Operational notes

- **Complete collections only.** Missing IDs are removals. Finish all API pages
  before comparing; do not replace good state with a failed or partial fetch.
- Snapshot format: `{ version: 1, algorithm: 'sha256', entries: [{ id, hash }] }`.
  Validate errors are thrown before a diff is returned. Keep state atomically
  and use one writer per feed, or supply your own transaction/lock.
- Snapshots retain IDs. Hashes are not encryption and can reveal low-entropy
  values by guessing. Apply your normal access controls; this is not an
  anonymization tool or a cryptographic signature scheme.
- Runtime and memory grow with the number and size of records. Bound your input
  sizes for large feeds. No external service or separate subscription is needed
  to run the code after you have obtained it.

## License and support

MIT. Commercial use, modification and redistribution are allowed under the
included license, without a seat limit; those rights do not expire with a
registry subscription. Registry access and any subscription billing are
managed separately by LPM.dev. No paid support or uptime service is included.
For reproducible issues, contact nora@natio.re without including private data.