#!/usr/bin/env python3
"""Decode a captured frame as SBUS: UART 100 kbit/s 8E2, 25-byte frame,
16 channels x 11 bits.

Usage: .venv/bin/python tools/decode.py captures/<name>.npz
"""
import sys

import numpy as np

BIT_US = 10.0

path = sys.argv[1]
d = np.load(path)
volts = d["volts"]
srate = float(d["srate"])
dt_us = 1e6 / srate

# idle level dominates the record; UART logic 1 == idle regardless of
# physical polarity (this line is standard inverted SBUS: idle low)
idle = np.median(volts)
p10, p90 = np.percentile(volts, [10, 90])
active = p10 if abs(p10 - idle) > abs(p90 - idle) else p90
thr = (idle + active) / 2
if active > idle:
    sig = (volts < thr).astype(np.int8)  # pulses up -> logic 0
else:
    sig = (volts > thr).astype(np.int8)

edges = np.flatnonzero(np.diff(sig)) + 1
starts = np.concatenate(([0], edges))
ends = np.concatenate((edges, [len(sig)]))
levels = sig[starts]
dur_us = (ends - starts) * dt_us

stream = []
for lv, du in zip(levels[1:-1], dur_us[1:-1]):
    stream.extend([int(lv)] * max(1, round(du / BIT_US)))
# trailing idle of last stop bits is trimmed by run cut; pad with idle-high
stream.extend([1] * 24)

# deframe 8E2: start=0, 8 data LSB-first, even parity, 2 stop=1
i = 0
frames = []
errors = []
while i + 12 <= len(stream):
    if stream[i] == 1:
        i += 1
        continue
    data = stream[i + 1 : i + 9]
    par = stream[i + 9]
    stops = stream[i + 10 : i + 12]
    byte = sum(b << k for k, b in enumerate(data))
    if par != (sum(data) & 1):
        errors.append((len(frames), "parity"))
    if stops != [1, 1]:
        errors.append((len(frames), f"stop={stops}"))
    frames.append(byte)
    i += 12

print(f"decoded {len(frames)} bytes, errors: {errors if errors else 'none'}")
print("hex:", " ".join(f"{b:02X}" for b in frames))

if len(frames) >= 25 and frames[0] == 0x0F:
    payload = frames[1:23]
    flags = frames[23]
    footer = frames[24]
    bits = 0
    for k, b in enumerate(payload):
        bits |= b << (8 * k)
    ch = [(bits >> (11 * n)) & 0x7FF for n in range(16)]
    print("\nSBUS frame OK" if footer == 0x00 else f"\nfooter unexpected: {footer:02X}")
    print("channels:", ch)
    print(f"flags: 0x{flags:02X} (bit0=ch17 bit1=ch18 bit2=frame_lost bit3=failsafe)")
else:
    print("not a valid SBUS frame (no 0x0F header)")