multi-byte paths-doc

documentation/routing-paths.md

@@ -0,0 +1,66 @@
+# Routing Paths
+
+This page covers how MeshCore Open represents, selects, validates, and stores routing paths in the UI and data layer.
+
+## Path Routing
+
+MeshCore supports variable-length multi-byte routing paths so the app can scale from small meshes to very large node sets.
+
+### Path Encoding Format
+
+Each path byte packs both the hop count and the hash width:
+
+```
+Bit 7-6: Hash mode  (2 bits) → hash_bytes = 1 << mode
+Bit 5-0: Hop count   (6 bits) → max 63 hops
+```
+
+| Mode | Hash Bytes | Max Hops | Max Unique Nodes | Firmware |
+|------|-----------|----------|------------------|----------|
+| 0 | 1 | 63 | 256 | v1.13+ |
+| 1 | 2 | 63 | 65,536 | v1.14+ |
+| 2 | 3 | 63 | 16.7M | v1.14+ |
+| 3 | 4 | 63 | 4.3B | v1.14+ |
+
+### Path Byte Calculation
+
+```dart
+final pathLenRaw = frameData[index];
+final hopCount = pathLenRaw & 0x3F;
+final hashMode = (pathLenRaw >> 6) & 0x03;
+final hashBytes = 1 << hashMode;
+
+final pathByteLength = hopCount * hashBytes;
+final pathData = frameData.sublist(index + 1, index + 1 + pathByteLength);
+```
+
+### Device Capability Detection
+
+The app reads the device's reported capability byte to determine whether multi-byte paths are available:
+
+```dart
+if (firmwareBytes.length >= 82) {
+  final mode = (firmwareBytes[81] & 0xFF).clamp(0, 2);
+  pathHashByteWidth = mode + 1;
+} else {
+  pathHashByteWidth = 1;
+}
+```
+
+### Path Usage in Different Message Types
+
+- Direct messages extract the path from the decrypted payload to trace the sender route.
+- Channel messages decrypt a hop-by-hop routing chain from the payload.
+- Contact storage keeps the path length encoding in byte 32 and the raw path bytes in bytes 33-96.
+
+### Example Contact Path Parsing
+
+```dart
+// Mode 0 (1-byte): 3 hops = 3 bytes needed
+// [0x83, 0xA1, 0xB2, 0xC3] → hopCount=3, mode=0, path=[A1,B2,C3]
+
+// Mode 1 (2-byte): 3 hops = 6 bytes needed
+// [0xC3, 0xA1, 0xA2, 0xB1, 0xB2, 0xC1, 0xC2]
+// → hopCount=3, mode=1, path=[A1A2, B1B2, C1C2]
+```
+