From 3cc5e7b57b0ca9bd5143d746022cb69349023a68 Mon Sep 17 00:00:00 2001
From: Munyoki Kilyungi
Date: Mon, 6 Feb 2023 15:05:56 +0300
Subject: Get the N-th matrix given N, an index value

* gn3/db/matrix.py (get_nth_matrix): New function.

Signed-off-by: Munyoki Kilyungi <me@bonfacemunyoki.com>
---
 gn3/db/matrix.py | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

(limited to 'gn3/db/matrix.py')

diff --git a/gn3/db/matrix.py b/gn3/db/matrix.py
index 71e7ca0..0fc4bf4 100644
--- a/gn3/db/matrix.py
+++ b/gn3/db/matrix.py
@@ -26,6 +26,31 @@ def get_total_versions(db_path: str) -> int:
             return 0
         return int(len(versions_hash) / BLOB_HASH_DIGEST)
 
+
+def get_nth_matrix(index: int, db_path: str) -> Optional[Matrix]:
+    """Get the NTH matrix from the DB_PATH.  The most recent matrix is 0."""
+    env = lmdb.open(db_path)
+    with env.begin(write=False) as txn:
+        versions_hash = txn.get(b"versions")
+        if (index * 32) + 32 > len(versions_hash):
+            return None
+        _hash, nrows = versions_hash[index * 32: (index * 32) + 32], 0
+        row_pointers = txn.get(_hash + b":row-pointers")
+        if _hash:
+            (nrows,) = struct.unpack("<Q", txn.get(_hash + b":nrows"))
+        if row_pointers:
+            return Matrix(
+                data=[
+                    json.loads(txn.get(row_pointers[i: i + 32]).decode())
+                    for i in range(0, nrows * 32, 32)
+                ],
+                metadata=json.loads(
+                    txn.get(_hash + b":metadata").rstrip(b"\x00").decode()
+                ),
+            )
+        return None
+
+
 def get_current_matrix(db_path: str) -> Optional[Matrix]:
     """Get the most recent matrix from DB_PATH.  This is functionally
     equivalent to get_nth_matrix(0, db_path)"""
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