# checksum

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_(no submodules)_
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## Purpose

Provides SHA-256 checksum utilities for the migration package. The checksum system detects when a migration's source has been modified after it was applied to a database, preventing accidental schema drift from silent SQL edits. Using `node:crypto` directly avoids introducing a dependency on `@suaveplan/crypto-hash`, keeping the migration package's dependency graph minimal.

## Features

- `computeChecksum(content)` — returns the SHA-256 hex digest of any string.
- `checksumMigration(up)` — serialises a migration's `up` function to its source string and hashes it.

## Usage: Basic Example

```ts
import { computeChecksum } from "@suaveplan/migration";

const hash = computeChecksum("CREATE TABLE users (id TEXT PRIMARY KEY)");
// "a4c2f..." (64-char hex string)
```

## Usage: Advanced Example

```ts
import { checksumMigration } from "@suaveplan/migration";

async function validateChecksums(migrations, appliedRecords) {
  for (const migration of migrations) {
    const record = appliedRecords.find(r => r.id === migration.id);
    if (!record?.checksum) continue;
    const computed = checksumMigration(migration.up);
    if (computed !== record.checksum) {
      throw new Error(`Checksum mismatch for ${migration.id}`);
    }
  }
}
```

## API Reference

### `computeChecksum(content: string): string`

Returns the lowercase hex SHA-256 digest of `content`. Uses `node:crypto` synchronously.

### `checksumMigration(up: (ctx: unknown) => Promise<void>): string`

Calls `up.toString()` to obtain the function source and passes it to `computeChecksum`. The result is deterministic for a given function body.

## Implementation Notes

`Function.prototype.toString()` is mandated by the ECMAScript specification to return the original source text (or a representation of it). In practice, transpilers and minifiers may change function bodies; for best reliability, checksums should be computed from source-file content in a pre-commit hook rather than at runtime in production builds.