diff --git a/src/j1939_decoder.py b/src/j1939_decoder.py
new file mode 100644
index 0000000..e49788c
--- /dev/null
+++ b/src/j1939_decoder.py
@@ -0,0 +1,14 @@
+import argparse
+import polars as pl
+
+def get_j1939_mask() -> pl.Expr:
+    id_int = pl.col("ID").str.to_integer(base=16).cast(pl.UInt32)
+    return (
+        (id_int > 0x7FF) &
+        ((id_int & 0x02000000) == 0) &
+        (pl.col("DLC") <= 8)
+    )
+
+
+
+if __name__ == "__main__":