Implement `compute_extended_matrix`

specs/_features/eip7594/das-core.md

@@ -17,7 +17,7 @@
     - [`DataColumnSidecar`](#datacolumnsidecar)
   - [Helper functions](#helper-functions)
     - [`get_custody_columns`](#get_custody_columns)
-    - [`compute_extended_data`](#compute_extended_data)
+    - [`compute_extended_matrix`](#compute_extended_matrix)
     - [`recover_matrix`](#recover_matrix)
     - [`get_data_column_sidecars`](#get_data_column_sidecars)
 - [Custody](#custody)
@@ -112,13 +112,20 @@ def get_custody_columns(node_id: NodeID, custody_subnet_count: uint64) -> Sequen
     ]
 ```
 
-#### `compute_extended_data`
+#### `compute_extended_matrix`
 
 ```python
-def compute_extended_data(data: Sequence[BLSFieldElement]) -> Sequence[BLSFieldElement]:
-    # TODO
-    # pylint: disable=unused-argument
-    ...
+def compute_extended_matrix(blobs: Sequence[Blob]) -> ExtendedMatrix:
+    """
+    Return the full ``ExtendedMatrix``.
+
+    This helper demonstrates the relationship between blobs and ``ExtendedMatrix``.
+    The data structure for storing cells is implementation-dependent.
+    """
+    extended_matrix = []
+    for blob in blobs:
+        extended_matrix.extend(compute_cells(blob))
+    return ExtendedMatrix(extended_matrix)
 ```
 
 #### `recover_matrix`
@@ -128,7 +135,7 @@ def recover_matrix(cells_dict: Dict[Tuple[BlobIndex, CellID], Cell], blob_count:
     """
     Return the recovered ``ExtendedMatrix``.
 
-    This helper demonstrate how to apply ``recover_polynomial``.
+    This helper demonstrates how to apply ``recover_polynomial``.
     The data structure for storing cells is implementation-dependent.
     """
     extended_matrix = []
@@ -208,7 +215,7 @@ A node runs a background peer discovery process, maintaining at least `TARGET_NU
 
 ## Extended data
 
-In this construction, we extend the blobs using a one-dimensional erasure coding extension. The matrix comprises maximum `MAX_BLOBS_PER_BLOCK` rows and fixed `NUMBER_OF_COLUMNS` columns, with each row containing a `Blob` and its corresponding extension.
+In this construction, we extend the blobs using a one-dimensional erasure coding extension. The matrix comprises maximum `MAX_BLOBS_PER_BLOCK` rows and fixed `NUMBER_OF_COLUMNS` columns, with each row containing a `Blob` and its corresponding extension. `compute_extended_matrix` demonstrates the relationship between blobs and custom type `ExtendedMatrix`.
 
 ## Column gossip
 

tests/core/pyspec/eth2spec/test/eip7594/unittests/das/test_das.py

@@ -9,6 +9,30 @@ from eth2spec.test.helpers.sharding import (
 )
 
 
+@with_eip7594_and_later
+@spec_test
+@single_phase
+def test_compute_extended_matrix(spec):
+    rng = random.Random(5566)
+
+    blob_count = 2
+    input_blobs = [get_sample_blob(spec, rng=rng) for _ in range(blob_count)]
+    extended_matrix = spec.compute_extended_matrix(input_blobs)
+    assert len(extended_matrix) == spec.CELLS_PER_BLOB * blob_count
+
+    rows = [extended_matrix[i:(i + spec.CELLS_PER_BLOB)] for i in range(0, len(extended_matrix), spec.CELLS_PER_BLOB)]
+    assert len(rows) == blob_count
+    assert len(rows[0]) == spec.CELLS_PER_BLOB
+
+    for blob_index, row in enumerate(rows):
+        extended_blob = []
+        for cell in row:
+            extended_blob.extend(cell)
+        blob_part = extended_blob[0:len(extended_blob) // 2]
+        blob = b''.join([spec.bls_field_to_bytes(x) for x in blob_part])
+        assert blob == input_blobs[blob_index]
+
+
 @with_eip7594_and_later
 @spec_test
 @single_phase