# ENC-ASL-LOG

### Encoding Specification for ASL Append-Only Log

---

## 1. Purpose

This document defines the **exact encoding** of the ASL append-only log.

It translates **ASL/LOG/1** semantics into a deterministic **bytes-on-disk** format.

It does **not** define log semantics (see `ASL/LOG/1`).

---

## 2. Encoding Principles

1. **Little-endian** integers
2. **Packed structures** (no compiler padding)
3. **Forward-compatible** versioning via header fields
4. **Deterministic serialization**: identical log content -> identical bytes
5. **Hash-chained integrity** as defined by ASL/LOG/1

---

## 3. Log File Layout

```
+----------------+
| LogHeader      |
+----------------+
| LogRecord[]    |
+----------------+
```

* **LogHeader**: fixed-size, mandatory, begins file
* **LogRecord[]**: append-only entries, variable number

---

## 4. LogHeader

```c
#pragma pack(push,1)
typedef struct {
    uint64_t magic;       // "ASLLOG01"
    uint32_t version;     // Encoding version (1)
    uint32_t header_size; // Total header bytes including this struct
    uint64_t flags;       // Reserved, must be zero for v1
} LogHeader;
#pragma pack(pop)
```

Notes:

* `magic` is ASCII bytes: `0x41 0x53 0x4c 0x4c 0x4f 0x47 0x30 0x31`
* `version` allows forward compatibility

---

## 5. LogRecord Envelope

Each record is encoded as:

```c
#pragma pack(push,1)
typedef struct {
    uint64_t logseq;       // Monotonic sequence number
    uint32_t record_type;  // Record type tag
    uint32_t payload_len;  // Payload byte length
    uint8_t  payload[payload_len];
    uint8_t  record_hash[32]; // Hash-chained integrity (SHA-256)
} LogRecord;
#pragma pack(pop)
```

Hash chain rule (normative):

```
record_hash = H(prev_record_hash || logseq || record_type || payload_len || payload)
```

* `prev_record_hash` is the previous record's `record_hash`
* For the first record, `prev_record_hash` is 32 bytes of zero
* `H` is SHA-256 for v1

Readers MUST skip unknown `record_type` values using `payload_len` and MUST
continue replay without failure.

---

## 6. Record Type IDs (v1)

These type IDs bind the ASL/LOG/1 semantics to bytes-on-disk:

| Type ID | Record Type        |
| ------- | ------------------ |
| 0x01    | SEGMENT_SEAL       |
| 0x10    | TOMBSTONE          |
| 0x11    | TOMBSTONE_LIFT     |
| 0x20    | SNAPSHOT_ANCHOR    |
| 0x30    | ARTIFACT_PUBLISH   |
| 0x31    | ARTIFACT_UNPUBLISH |

---

## 6.1 Payload Schemas (v1)

All payloads are little-endian and packed. Variable-length fields are encoded
inline and accounted for by `payload_len`.

### 6.1.1 ArtifactRef

```c
#pragma pack(push,1)
typedef struct {
    uint32_t hash_id;     // Hash algorithm identifier
    uint16_t digest_len;  // Digest length in bytes
    uint16_t reserved0;   // Must be 0
    uint8_t  digest[digest_len];
} ArtifactRef;
#pragma pack(pop)
```

Notes:

* `digest_len` MUST be > 0.
* If StoreConfig fixes the hash, `digest_len` MUST match that hash's length.

### 6.1.2 SEGMENT_SEAL (Type 0x01)

```c
#pragma pack(push,1)
typedef struct {
    uint64_t segment_id;      // Store-local segment identifier
    uint8_t  segment_hash[32]; // SHA-256 over the segment file bytes
} SegmentSealPayload;
#pragma pack(pop)
```

### 6.1.3 TOMBSTONE (Type 0x10)

```c
#pragma pack(push,1)
typedef struct {
    ArtifactRef artifact;
    uint32_t scope;        // Opaque to ASL/LOG/1
    uint32_t reason_code;  // Opaque to ASL/LOG/1
} TombstonePayload;
#pragma pack(pop)
```

### 6.1.4 TOMBSTONE_LIFT (Type 0x11)

```c
#pragma pack(push,1)
typedef struct {
    ArtifactRef artifact;
    uint64_t tombstone_logseq; // logseq of the tombstone being lifted
} TombstoneLiftPayload;
#pragma pack(pop)
```

### 6.1.5 SNAPSHOT_ANCHOR (Type 0x20)

```c
#pragma pack(push,1)
typedef struct {
    uint64_t snapshot_id;
    uint8_t  root_hash[32]; // Hash of snapshot-visible state
} SnapshotAnchorPayload;
#pragma pack(pop)
```

### 6.1.6 ARTIFACT_PUBLISH (Type 0x30)

```c
#pragma pack(push,1)
typedef struct {
    ArtifactRef artifact;
} ArtifactPublishPayload;
#pragma pack(pop)
```

### 6.1.7 ARTIFACT_UNPUBLISH (Type 0x31)

```c
#pragma pack(push,1)
typedef struct {
    ArtifactRef artifact;
} ArtifactUnpublishPayload;
#pragma pack(pop)
```

---

## 7. Versioning Rules

* `version = 1` for this specification.
* New record types MAY be added without bumping the version.
* Layout changes to `LogHeader` or `LogRecord` require a new version.

---

## 8. Relationship to Other Layers

| Layer            | Responsibility                                   |
| ---------------- | ------------------------------------------------ |
| ASL/LOG/1        | Semantic log behavior and replay rules           |
| ASL-STORE-INDEX  | Store lifecycle and snapshot/log contracts       |
| ENC-ASL-LOG      | Exact byte layout for log encoding (this doc)    |
| ENC-ASL-CORE-INDEX | Exact byte layout for index segments          |