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openspec/archive/2026-04-24-enterprise-bitpacker/proposal.md

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+# 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.