# FPS/1 — Functional Primitive Surface

Status: Approved
Owner: Architecture
Version: 1.0.0
SoT: Yes
Last Updated: 2026-02-21
Tags: [primitives, determinism, bytes]

**Document ID:** `FPS/1`
**Layer:** L1 — Deterministic functional primitives over payload bytes

**Depends on (normative):**

* `ASL/1-CORE` — `Artifact`, `Reference`, hash identity
* `HASH/ASL1` — hash family registry
* `ADR-006` — deterministic error semantics
* `ADR-015` — CAS rejection governance (alignment)

**Informative references:**

* `SRS` §12 — Functional Primitive Surface summary

---

## 0. Conventions

The key words **MUST**, **MUST NOT**, **REQUIRED**, **SHOULD**, and **MAY** are
to be interpreted as in RFC 2119.

FPS/1 defines **pure, deterministic** operations over canonical byte payloads.
The primitives are functional: identical inputs MUST produce identical outputs.

---

## 1. CID Derivation

All primitives produce exactly one output payload and one CID.

The CID for a payload is derived as:

```
cid = algo_id || H("CAS:OBJ\0" || payload)
```

* `algo_id` is an `ASL/1-CORE` hash identifier.
* For v1, `algo_id` defaults to `HASH/ASL1` SHA-256.
* The domain separator **MUST** be exactly `"CAS:OBJ\0"`.

---

## 2. Primitives (Normative)

### 2.1 put

```
put(payload_bytes) -> CID
```

Canonical write; identity only. Storage durability is out of scope here.

Errors: `ERR_IO_FAILURE`, `ERR_NORMALIZATION`

### 2.2 get

```
get(CID) -> payload_bytes
```

Fetch canonical bytes from a conformant store.

Errors: `ERR_CID_NOT_FOUND`

### 2.3 slice

```
slice(CID, offset, length) -> CID
```

Extract a contiguous byte range.

Errors: `ERR_SLICE_RANGE`

### 2.4 concatenate

```
concatenate([CID1, ..., CIDn]) -> CID
```

Join payloads in order.

Errors: `ERR_EMPTY_INPUTS`

### 2.5 reverse

```
reverse(CID, level) -> CID
```

Reverse payload order at a specified granularity:

* `bit` — reverse all bits in the payload
* `byte` — reverse byte order
* `word` — reverse 2-byte units
* `long` — reverse 4-byte units

Errors: `ERR_REV_ALIGNMENT`, `ERR_INVALID_LEVEL`

### 2.6 splice

```
splice(CID_a, offset, CID_b) -> CID
```

Insert payload `b` into `a` at `offset`.

Errors: `ERR_SPLICE_RANGE`

---

## 3. Determinism & Closure

* Identical inputs MUST produce identical outputs.
* Inputs are immutable.
* Outputs MUST be valid inputs to any FPS/1 primitive.

---

## 4. Conformance Vectors

The canonical vectors live in:

```
/amduat/tests/fixtures/conformance/fps_1/
```

Vectors are required for each primitive and include output bytes and CID.