# OpenSpec Change: enterprise-bitpacker

- **Change ID**: enterprise-bitpacker
- **Package**: `@suaveplan/bitpacker`
- **Category**: universal
- **Target path**: `universal/compression/bitpacker`
- **Status**: PROPOSED
- **Date**: 2026-04-24
- **Reference spec**: `/mnt/s/web/suaveplan/enterprise/bitpacker.md`

---

## Summary

Deliver `@suaveplan/bitpacker` on the genesis branch from scratch at enterprise quality. The package provides bit-level binary packing and unpacking: arbitrary-width unsigned/signed integers (1–53 bits), booleans, fixed-point numbers, bit-field schemas with named fields, and streaming read/write cursors. Designed for network protocols, game state serialisation, and compact binary formats. Pure TypeScript, zero deps beyond `@suaveplan/error`.

---

## Current-state audit (genesis)

The package does not exist on genesis yet — this is a fresh build. No prior source, no test results, no coverage data. The main-branch package had 91% coverage.

---

## Available workspace deps on genesis

| Package | Role |
|---|---|
| `@suaveplan/error` (`workspace:^`) | Base class for `BitpackerError`; `BITPACKER.*` codes |
| `@suaveplan/types` (`workspace:^`) | `LoggerLike` |
| `zod` (`catalog:forms`) | Zod schemas for schema definitions |
| `@opentelemetry/api` (`catalog:observability`) | Spans + counters |

Dev deps: `vitest`, `@vitest/coverage-istanbul`, `fast-check`, `mitata` (all `catalog:testing`).

---

## Public API

### BitWriter

```ts
class BitpackerError extends SuaveplanError

class BitWriter {
  constructor(initialCapacityBytes?: number)
  writeUint(value: number, bits: number): this   // 1–53 bits
  writeInt(value: number, bits: number): this    // 1–53 bits, two's complement
  writeBool(value: boolean): this
  writeFixed(value: number, intBits: number, fracBits: number): this
  writeBytes(bytes: Uint8Array): this
  align(toBytes?: number): this                  // pad to byte/word boundary
  toUint8Array(): Uint8Array
  get bitLength(): number
  get byteLength(): number
}
```

### BitReader

```ts
class BitReader {
  constructor(data: Uint8Array)
  readUint(bits: number): number
  readInt(bits: number): number
  readBool(): boolean
  readFixed(intBits: number, fracBits: number): number
  readBytes(byteCount: number): Uint8Array
  align(toBytes?: number): void
  get bitPosition(): number
  get bitsRemaining(): number
  peek(bits: number): number                     // read without advancing
  seek(bitPosition: number): void
}
```

### Schema-based packing

```ts
type FieldType = 'uint' | 'int' | 'bool' | 'fixed' | 'bytes'

interface FieldDef {
  name: string
  type: FieldType
  bits?: number               // for uint/int
  intBits?: number            // for fixed
  fracBits?: number           // for fixed
  byteCount?: number          // for bytes
}

interface BitSchema<T extends Record<string, unknown> = Record<string, unknown>> {
  fields: FieldDef[]
  pack(values: T): Uint8Array
  unpack(data: Uint8Array): T
  byteSize: number            // fixed size if computable; 0 = variable
}

function defineBitSchema<T extends Record<string, unknown>>(fields: FieldDef[]): BitSchema<T>
```

### Utilities

```ts
function packBits(values: number[], bitsPerValue: number): Uint8Array
function unpackBits(data: Uint8Array, count: number, bitsPerValue: number): number[]
function countBits(n: number): number              // position of highest set bit
function zigzagEncode(n: number): number           // signed → unsigned
function zigzagDecode(n: number): number           // unsigned → signed
```

### Error codes

```ts
type BitpackerErrorCode =
  | 'BITPACKER_OVERFLOW'
  | 'BITPACKER_UNDERFLOW'
  | 'BITPACKER_INVALID_BIT_WIDTH'
  | 'BITPACKER_VALUE_OUT_OF_RANGE'
  | 'BITPACKER_SCHEMA_INVALID'

const BITPACKER_ERROR_METADATA: Readonly<Record<BitpackerErrorCode, CodeMetadata>>
function registerBitpackerErrorCodes(): void
```

### Zod schemas

```ts
FieldDefSchema
```

---

## New additions for genesis

1. **`defineBitSchema`** — type-safe named-field schemas with compile-time inferred value types.
2. **Zigzag encoding** — `zigzagEncode`/`zigzagDecode` for efficient signed-integer variable-length encoding.
3. **`peek` and `seek`** on `BitReader` — non-destructive lookahead and random-access positioning.
4. **Alignment helpers** — `align()` pads writer or advances reader to next byte/word boundary for protocol framing.
5. **OTel instrumentation** — counter `bitpacker.pack_total`, counter `bitpacker.unpack_total`, histogram `bitpacker.pack_bytes`.
6. **Property-based tests** — pack/unpack round-trip for arbitrary bit widths and values within range, `zigzagEncode(zigzagDecode(n)) === n` for all signed 32-bit integers.
7. **Benchmark suite** — pack 1000 × 7-bit uints, schema pack/unpack 100-field struct, `packBits` on 10000 values.

---

## File layout

```text
universal/compression/bitpacker/
  package.json
  tsconfig.json
  vitest.config.ts
  src/
    index.ts
    error-codes.ts
    constants/
      index.ts
    types/
      common.ts
      common.md
    core/
      bit-writer/
        bit-writer.ts
        bit-writer.md
        bit-writer.types.ts
        bit-writer.test.ts
        index.ts
      bit-reader/
        bit-reader.ts
        bit-reader.md
        bit-reader.types.ts
        bit-reader.test.ts
        index.ts
      bit-schema/
        bit-schema.ts
        bit-schema.md
        bit-schema.types.ts
        bit-schema.test.ts
        index.ts
    utils/
      pack-bits.ts
      pack-bits.md
      pack-bits.test.ts
      zigzag.ts
      zigzag.md
      zigzag.test.ts
    __benchmarks__/
      bitpacker.bench.ts
  tests/
    integration.test.ts
    property.test.ts
```

---

## Impact

**Downstream**: `@suaveplan/packer`, network protocol packages, game state serialisation.
**Breaking changes**: none — fresh build.
**Prerequisites**: `@suaveplan/error`, `@suaveplan/types` published to Verdaccio.