Initial commit

src/meshbot/messages.py

@@ -0,0 +1,21 @@
+"""Helpers for shaping outgoing mesh messages."""
+
+from __future__ import annotations
+
+
+def trim_to_bytes(text: str, max_bytes: int) -> str:
+    """Return ``text`` truncated so its UTF-8 encoding is at most ``max_bytes`` bytes.
+
+    Backs off if the cut lands inside a multi-byte UTF-8 sequence so we never emit
+    invalid UTF-8 to the radio.
+    """
+    if max_bytes <= 0:
+        return ""
+    encoded = text.encode("utf-8")
+    if len(encoded) <= max_bytes:
+        return text
+    cut = encoded[:max_bytes]
+    # Continuation bytes start with bits 10xxxxxx; rewind past them.
+    while cut and (cut[-1] & 0xC0) == 0x80:
+        cut = cut[:-1]
+    return cut.decode("utf-8", errors="ignore")