import numpy as np

ALPHABET = (
    list('abcdefghijklmnopqrstuvwxyz') +
    list('ABCDEFGHIJKLMNOPQRSTUVWXYZ') +
    list('0123456789') +
    [' ', '.']
)

def get_hadamard(n):
    assert (n & (n - 1) == 0), "Hadamard order must be power of 2"
    H = np.array([[1]])
    while H.shape[0] < n:
        H = np.block([
            [ H,  H],
            [ H, -H]
        ])
    return H

def decode_signal(signal, alphabet=ALPHABET):
    code_length = 64
    n_chars = len(signal) // code_length
    H = get_hadamard(64)
    scale = 1 / np.sqrt(code_length)
    codebook = H * scale

    decoded = []
    for i in range(n_chars):
        y = signal[i*code_length : (i+1)*code_length]
        # The channel may have applied sqrt(10) gain to odds or evens
        # We don't know which, so try both options and pick best
        y_even = np.array(y)
        y_even[::2] /= np.sqrt(10)
        y_odd = np.array(y)
        y_odd[1::2] /= np.sqrt(10)
        # Try decoding both hypotheses
        scores_even = codebook @ y_even
        scores_odd = codebook @ y_odd
        idx_even = np.argmax(scores_even)
        idx_odd = np.argmax(scores_odd)
        score_even = np.max(scores_even)
        score_odd = np.max(scores_odd)
        idx_best = idx_even if score_even > score_odd else idx_odd
        decoded.append(alphabet[idx_best])
    return ''.join(decoded)

def main():
    import argparse
    parser = argparse.ArgumentParser(description="Receiver: decode y.txt to recovered message for PDC Project.")
    parser.add_argument('--input_file', type=str, required=True, help="Received y.txt from channel/server")
    parser.add_argument('--output_file', type=str, required=True, help="Text file for the decoded message")
    args = parser.parse_args()
    y = np.loadtxt(args.input_file)
    decoded = decode_signal(y)
    with open(args.output_file, 'w') as f:
        f.write(decoded)

if __name__ == '__main__':
    main()