# ASL-CORE Patch: Block Semantics

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## 1. Purpose

Introduce **blocks** into the ASL-CORE semantic model to provide a minimal foundation for:

* Artifact → block → bytes mapping
* Immutability guarantees
* Snapshot-safe references

The patch is **minimal**, leaving lifecycle, sealing, retention, and GC to ASL-STORE-INDEX.

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## 2. New Core Concepts

### 2.1 Block

* **Definition:** A block is an **atomic, immutable storage unit** containing a sequence of bytes.
* **Identifier:** `BlockID` — opaque and unique across the system.
* **Properties:**

  1. Contents are immutable once created (semantic guarantee).
  2. Blocks can be referenced by one or more artifacts.
  3. Blocks are existential; their layout, size, and packing are **implementation concerns**.
* **Notation:** `(BlockID, offset, length)` denotes a **byte slice** within a block.

  * Offset and length must refer to bytes **inside the block**.
  * Semantic operations may reference these slices but **cannot modify them**.

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### 2.2 Artifact and Block Relationship

* Each **Artifact** in ASL-CORE can be fully or partially contained in one or more blocks.
* Semantic mapping:

```
ArtifactKey → {ArtifactLocation1, ArtifactLocation2, ...}
```

Where each `ArtifactLocation` is:

```
ArtifactLocation = (BlockID, offset, length)
```

* Guarantees:

  1. **Determinism:** Given the same ArtifactKey, the locations are always the same at the same snapshot.
  2. **Immutability:** The bytes addressed by ArtifactLocation never change.
  3. **Snapshot consistency:** If an artifact is referenced by a snapshot, the bytes remain valid for the lifetime of that snapshot.

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### 2.3 Block Visibility and Referencing

* **Blocks themselves** are not directly visible in ASL-CORE; they exist as **supporting storage units for artifacts**.
* **ArtifactKey references** to blocks must only point to **immutable bytes**.
* Operations on artifacts **cannot modify or relocate block bytes**; only new artifacts can be created referencing blocks.

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### 2.4 Minimal Invariants

1. **Block Immutability:** Once a block exists semantically, its bytes never change.
2. **Artifact Integrity:** ArtifactKey → ArtifactLocation mapping is stable and deterministic.
3. **Existence:** A referenced BlockID must exist in the system at the semantic level (physical existence is store-level concern).
4. **Slice Validity:** `(offset, length)` is always within the bounds of the block.

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### 2.5 Non-Goals of ASL-CORE Regarding Blocks

* **Sealing mechanics** – handled by ASL-STORE-INDEX
* **Retention / GC** – handled by ASL-STORE-INDEX
* **Snapshot pinning or lifecycle events** – store concern
* **Size class, packing, or performance optimizations** – store concern

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## 3. Summary

This patch introduces:

* **BlockID**: opaque, unique identifier
* **Blocks**: immutable byte sequences, existential atoms
* **ArtifactLocation**: `(BlockID, offset, length)` primitive to map artifacts to blocks

It **does not define storage, GC, or lifecycle mechanics**, leaving these to ASL-STORE-INDEX.

**Semantic impact:**

* Artifacts can now reference storage in a stable, deterministic, and immutable way.
* Higher layers (index, store, encoding) can build on blocks for deterministic persistence, snapshot safety, and replay without modifying ASL-CORE semantics.