1 files changed, 110 insertions, 0 deletions
diff --git a/stacks/crdop/lib/hdlc_codec.py b/stacks/crdop/lib/hdlc_codec.py
new file mode 100644
index 0000000..942168b
--- /dev/null
+++ b/stacks/crdop/lib/hdlc_codec.py
@@ -0,0 +1,110 @@
+"""
+HDLC framing for AX.25 — flags, bit-stuffing, CRC-16-CCITT.
+"""
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+_ROOT = Path(__file__).resolve().parents[3]
+_DAEMON = _ROOT / "stacks" / "daemon"
+if str(_DAEMON) not in sys.path:
+    sys.path.insert(0, str(_DAEMON))
+
+from ax25_codec import ax25_crc  # noqa: E402
+
+HDLC_FLAG = 0x7E
+HDLC_ESCAPE = 0x7D
+HDLC_XOR = 0x20
+
+
+def bit_stuff(data: bytes) -> bytes:
+    out = bytearray()
+    ones = 0
+    for b in data:
+        for bit_i in range(7, -1, -1):
+            bit = (b >> bit_i) & 1
+            if bit:
+                ones += 1
+                out.append(1)
+                if ones == 5:
+                    out.append(0)
+                    ones = 0
+            else:
+                ones = 0
+                out.append(0)
+    # pack bits to bytes MSB-first
+    buf = bytearray()
+    acc = 0
+    n = 0
+    for bit in out:
+        acc = (acc << 1) | bit
+        n += 1
+        if n == 8:
+            buf.append(acc)
+            acc = 0
+            n = 0
+    if n:
+        buf.append(acc << (8 - n))
+    return bytes(buf)
+
+
+def bit_unstuff(data: bytes) -> bytes:
+    bits: list[int] = []
+    for b in data:
+        for bit_i in range(7, -1, -1):
+            bits.append((b >> bit_i) & 1)
+    out: list[int] = []
+    ones = 0
+    i = 0
+    while i < len(bits):
+        bit = bits[i]
+        if bit:
+            ones += 1
+            out.append(1)
+            if ones == 5 and i + 1 < len(bits) and bits[i + 1] == 0:
+                i += 1
+                ones = 0
+            elif ones == 6:
+                break
+        else:
+            ones = 0
+            out.append(0)
+        i += 1
+    buf = bytearray()
+    for j in range(0, len(out), 8):
+        chunk = out[j : j + 8]
+        if len(chunk) < 8:
+            break
+        val = 0
+        for bit in chunk:
+            val = (val << 1) | bit
+        buf.append(val)
+    return bytes(buf)
+
+
+def build_hdlc_frame(ax25_body: bytes) -> bytes:
+    """AX.25 body (no FCS) → on-air HDLC bit stream as bytes (pre tone encoding)."""
+    crc = ax25_crc(ax25_body)
+    payload = ax25_body + bytes((crc & 0xFF, crc >> 8))
+    stuffed = bit_stuff(payload)
+    return bytes([HDLC_FLAG]) + stuffed + bytes([HDLC_FLAG])
+
+
+def parse_hdlc_stream(raw_bits: bytes) -> list[bytes]:
+    """Extract AX.25 bodies (with FCS) from demodulated byte stream between flags."""
+    frames: list[bytes] = []
+    in_frame = False
+    cur = bytearray()
+    for b in raw_bits:
+        if b == HDLC_FLAG:
+            if in_frame and cur:
+                unstuffed = bit_unstuff(bytes(cur))
+                if len(unstuffed) >= 2:
+                    frames.append(unstuffed)
+            cur.clear()
+            in_frame = True
+            continue
+        if in_frame:
+            cur.append(b)
+    return frames