Add back compute_cells function

specs/fulu/polynomial-commitments-sampling.md

@@ -39,6 +39,7 @@
     - [`coset_for_cell`](#coset_for_cell)
 - [Cells](#cells-1)
   - [Cell computation](#cell-computation)
+    - [`compute_cells`](#compute_cells)
     - [`compute_cells_and_kzg_proofs_polynomialcoeff`](#compute_cells_and_kzg_proofs_polynomialcoeff)
     - [`compute_cells_and_kzg_proofs`](#compute_cells_and_kzg_proofs)
   - [Cell verification](#cell-verification)
@@ -520,6 +521,28 @@ def coset_for_cell(cell_index: CellIndex) -> Coset:
 
 ### Cell computation
 
+#### `compute_cells`
+
+```python
+def compute_cells(blob: Blob) -> Vector[Cell, CELLS_PER_EXT_BLOB]:
+    """
+    Given a blob, extend it and return all the cells of the extended blob.
+
+    Public method.
+    """
+    assert len(blob) == BYTES_PER_BLOB
+
+    polynomial = blob_to_polynomial(blob)
+    polynomial_coeff = polynomial_eval_to_coeff(polynomial)
+
+    cells = []
+    for i in range(CELLS_PER_EXT_BLOB):
+        coset = coset_for_cell(CellIndex(i))
+        ys = CosetEvals([evaluate_polynomialcoeff(polynomial_coeff, z) for z in coset])
+        cells.append(coset_evals_to_cell(CosetEvals(ys)))
+    return cells
+```
+
 #### `compute_cells_and_kzg_proofs_polynomialcoeff`
 
 ```python

tests/formats/kzg_7594/compute_cells.md

@@ -0,0 +1,22 @@
+# Test format: Compute cells
+
+Compute the cells for a given `blob`.
+
+## Test case format
+
+The test data is declared in a `data.yaml` file:
+
+```yaml
+input:
+  blob: Blob -- the data blob
+output: List[Cell] -- the cells
+```
+
+- `Blob` is a 131072-byte hexadecimal string, prefixed with `0x`.
+- `Cell` is a 2048-byte hexadecimal string, prefixed with `0x`.
+
+All byte(s) fields are encoded as strings, hexadecimal encoding, prefixed with `0x`.
+
+## Condition
+
+The `compute_cells` handler should compute the cells (chunks of an extended blob) for `blob`, and the result should match the expected `output`. If the blob is invalid (e.g. incorrect length or one of the 32-byte blocks does not represent a BLS field element) it should error, i.e. the output should be `null`.

tests/generators/kzg_7594/main.py

@@ -27,6 +27,34 @@
 from eth2spec.utils import bls
 
 
+###############################################################################
+# Test cases for compute_cells
+###############################################################################
+
+def case_compute_cells():
+    # Valid cases
+    for blob in VALID_BLOBS:
+        cells = spec.compute_cells(blob)
+        identifier = make_id(blob)
+        yield f'compute_cells_case_valid_{identifier}', {
+            'input': {
+                'blob': encode_hex(blob),
+            },
+            'output': encode_hex_list(cells)
+        }
+
+    # Edge case: Invalid blobs
+    for blob in INVALID_BLOBS:
+        expect_exception(spec.compute_cells, blob)
+        identifier = make_id(blob)
+        yield f'compute_cells_invalid_blob_{identifier}', {
+            'input': {
+                'blob': encode_hex(blob)
+            },
+            'output': None
+        }
+
+
 ###############################################################################
 # Test cases for compute_cells_and_kzg_proofs
 ###############################################################################
@@ -565,6 +593,7 @@ def cases_fn() -> Iterable[gen_typing.TestCase]:
 if __name__ == "__main__":
     bls.use_arkworks()
     gen_runner.run_generator("kzg_7594", [
+        create_provider(FULU, 'compute_cells', case_compute_cells),
         create_provider(FULU, 'compute_cells_and_kzg_proofs', case_compute_cells_and_kzg_proofs),
         create_provider(FULU, 'verify_cell_kzg_proof_batch', case_verify_cell_kzg_proof_batch),
         create_provider(FULU, 'recover_cells_and_kzg_proofs', case_recover_cells_and_kzg_proofs),