#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# MIT License
# Copyright (c) 2018 zleffke
# Ported to GNU Radio 3.10+ by gr-mcp

"""
Biphase-L Decoder for Cospas-Sarsat 406 MHz beacons.

Biphase-L (Manchester) encoding represents:
- Bit 0: Low-to-High transition at bit center
- Bit 1: High-to-Low transition at bit center

This decoder extracts the data bit from each symbol pair.
"""

import numpy as np
from gnuradio import gr


class biphase_l_decode_bb(gr.decim_block):
    """
    Biphase-L Decoder.

    Input: Unpacked byte stream of Biphase-L encoded symbols (2 samples per bit).
    Output: Unpacked byte stream of decoded bits.

    Decimation factor: 2 (two input samples produce one output bit)
    """

    def __init__(self):
        gr.decim_block.__init__(
            self,
            name="biphase_l_decode_bb",
            in_sig=[np.int8],
            out_sig=[np.int8],
            decim=2,
        )

    def work(self, input_items, output_items):
        in0 = input_items[0]
        out = output_items[0]
        # Extract first sample of each symbol pair (the data bit value)
        out[:] = in0[::2]
        return len(output_items[0])