# Copyright (c) 2017 Pieter Wuille
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# THE SOFTWARE.
"""Reference implementation for Bech32 and segwit addresses."""

CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"


def bech32_polymod( values ):
    """Internal function that computes the Bech32 checksum."""
    generator = [ 0x3B6A57B2, 0x26508E6D, 0x1EA119FA, 0x3D4233DD, 0x2A1462B3 ]
    chk = 1
    for value in values:
        top = chk >> 25
        chk = ( chk & 0x1FFFFFF ) << 5 ^ value
        for i in range( 5 ):
            chk ^= generator[ i ] if ( ( top >> i ) & 1 ) else 0
    return chk


def bech32_hrp_expand( hrp ):
    """Expand the HRP into values for checksum computation."""
    return [ ord( x ) >> 5 for x in hrp ] + [ 0
                                             ] + [ ord( x ) & 31 for x in hrp ]


def bech32_verify_checksum( hrp, data ):
    """Verify a checksum given HRP and converted data characters."""
    return bech32_polymod( bech32_hrp_expand( hrp ) + data ) == 1


def bech32_create_checksum( hrp, data ):
    """Compute the checksum values given HRP and data."""
    values = bech32_hrp_expand( hrp ) + data
    polymod = bech32_polymod( values + [ 0, 0, 0, 0, 0, 0 ] ) ^ 1
    return [ ( polymod >> 5 * ( 5 - i ) ) & 31 for i in range( 6 ) ]


def bech32_encode( hrp, data ):
    """Compute a Bech32 string given HRP and data values."""
    combined = data + bech32_create_checksum( hrp, data )
    return hrp + "1" + "".join( [ CHARSET[ d ] for d in combined ] )


def bech32_decode( bech ):
    """Validate a Bech32 string, and determine HRP and data."""
    if ( any( ord( x ) < 33 or ord( x ) > 126 for x in bech
             ) ) or ( bech.lower() != bech and bech.upper() != bech ):
        return ( None, None )
    bech = bech.lower()
    pos = bech.rfind( "1" )
    if pos < 1 or pos + 7 > len( bech ) or len( bech ) > 90:
        return ( None, None )
    if not all( x in CHARSET for x in bech[ pos + 1 : ] ):
        return ( None, None )
    hrp = bech[ : pos ]
    data = [ CHARSET.find( x ) for x in bech[ pos + 1 : ] ]
    if not bech32_verify_checksum( hrp, data ):
        return ( None, None )
    return ( hrp, data[ :-6 ] )


def convertbits( data, frombits, tobits, pad = True ):
    """General power-of-2 base conversion."""
    acc = 0
    bits = 0
    ret = []
    maxv = ( 1 << tobits ) - 1
    max_acc = ( 1 << ( frombits + tobits - 1 ) ) - 1
    for value in data:
        if value < 0 or ( value >> frombits ):
            return None
        acc = ( ( acc << frombits ) | value ) & max_acc
        bits += frombits
        while bits >= tobits:
            bits -= tobits
            ret.append( ( acc >> bits ) & maxv )
    if pad:
        if bits:
            ret.append( ( acc << ( tobits - bits ) ) & maxv )
    elif bits >= frombits or ( ( acc << ( tobits - bits ) ) & maxv ):
        return None
    return ret


def decode( hrp, addr ):
    """Decode a segwit address."""
    hrpgot, data = bech32_decode( addr )
    if hrpgot != hrp:
        return ( None, None )
    decoded = convertbits( data[ 1 : ], 5, 8, False )
    if decoded is None or len( decoded ) < 2 or len( decoded ) > 40:
        return ( None, None )
    if data[ 0 ] > 16:
        return ( None, None )
    if data[ 0 ] == 0 and len( decoded ) != 20 and len( decoded ) != 32:
        return ( None, None )
    return ( data[ 0 ], decoded )


def encode( hrp, witver, witprog ):
    """Encode a segwit address."""
    ret = bech32_encode( hrp, [ witver ] + convertbits( witprog, 8, 5 ) )
    if decode( hrp, ret ) == ( None, None ):
        return None
    return ret