# partial strategy

## Purpose

The `partial` masking strategy preserves a configurable number of characters at
the start and end of an input string while replacing the middle portion with
asterisks.  This is the most commonly needed masking shape for fields like
API keys, account numbers, and reference codes where enough context must remain
for humans to identify a value in logs without exposing the sensitive body.

## Features

- Configurable `keepFirst` (default `0`) and `keepLast` (default `4`) character
  counts via `MaskRule`.
- Safe handling of short strings: when `keepFirst + keepLast >= length` the
  original value is returned unchanged rather than producing an empty or
  misleading result.
- No external dependencies — pure string slicing.

## Usage: Basic Example

```ts
import { maskPartial } from '@suaveplan/data-masking/strategies/partial';

// Keep last 4 characters (default keepLast = 4, keepFirst = 0)
const masked = maskPartial('sk_live_abc123xyz789', {
  fields: ['apiKey'],
  strategy: 'partial',
});
// masked → '****************789' (last 3 chars shown — "789" ends value)
```

## Usage: Advanced Example

```ts
import { maskPartial } from '@suaveplan/data-masking/strategies/partial';

// Keep first 2 and last 4 characters
const masked = maskPartial('4111111111111111', {
  fields: ['cardNumber'],
  strategy: 'partial',
  keepFirst: 2,
  keepLast: 4,
});
// masked → '41**********1111'

// Short string — no masking applied
const short = maskPartial('abc', {
  fields: ['code'],
  strategy: 'partial',
  keepFirst: 2,
  keepLast: 2,
});
// short → 'abc'  (keepFirst + keepLast >= length)
```

## API Reference

### `maskPartial(value, rule)`

```ts
function maskPartial(value: string, rule: MaskRule): string;
```

| Parameter        | Type       | Default | Description                              |
|------------------|------------|---------|------------------------------------------|
| `value`          | `string`   | —       | The original string.                     |
| `rule.keepFirst` | `number`   | `0`     | Characters to keep from the start.       |
| `rule.keepLast`  | `number`   | `4`     | Characters to keep from the end.         |

**Returns** the masked string, or the original value unchanged if it is too
short to mask.

**Does not throw.**

## Implementation Notes

- The mask character is always `"*"`.  A future extension could expose a
  `maskChar` option on `MaskRule` if needed.
- `keepFirst + keepLast >= len` guard prevents edge cases where `slice` would
  produce overlapping or inverted ranges, which can silently produce empty
  strings in JavaScript.
- Middle length is computed as `len - keepFirst - keepLast` to give a
  mask block that exactly fills the hidden region, preserving total length
  and allowing callers to spot-check that masking was applied.