#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
This module provides encoding and decoding functions for Hamming codes

(C) 2022 Louis Heredero  louis.heredero@edu.vs.ch
"""

class HammingError(Exception):
    pass

def encode(data, blocksize=7):
    result = []
    datasize = blocksize-blocksize.bit_length()
    data = list(map(int, data))
    if len(data) % datasize:
        raise HammingError(f"Length of data is not a multiple of {datasize}")
    
    nblocks = int(len(data)/datasize)
    
    for b in range(nblocks):
        for i in range(blocksize):
            # Power of 2
            if (i+1)&i == 0 or i == 0:
                result.append(0)
            
            else:
                result.append(data.pop(0))
        
        for i in range(blocksize.bit_length()):
            p = 1 << i
            c = sum([result[b*blocksize+j] for j in range(blocksize) if (j+1)&p])
            if c%2:
                result[b*blocksize+p-1] = 1
    
    return "".join(list(map(str, result)))

def decode(data, blocksize=7):
    result = []
    datasize = blocksize-blocksize.bit_length()
    data = list(map(int, data))
    if len(data) % blocksize:
        raise HammingError(f"Length of data is not a multiple of {blocksize}")
    
    nblocks = int(len(data)/blocksize)
    
    for b in range(nblocks):
        pos = 0
        for i in range(blocksize.bit_length()):
            p = 1 << i
            c = sum([data[b*blocksize+j] for j in range(blocksize) if (j+1)&p])
            if c%2:
                pos |= p
        
        if pos != 0:
            if pos > blocksize:
                raise HammingError("Too many errors")
                return
            
            data[b*blocksize+pos-1] = 1-data[b*blocksize+pos-1]
        
        for i in range(1, blocksize):
            if (i+1)&i != 0:
                result.append(data[b*blocksize+i])
    
    return "".join(list(map(str, result)))