/*
 * Copyright (c) 2013-2016 Galois, Inc.
 * Distributed under the terms of the BSD3 license (see LICENSE file)
 */
module Base58 where

// Base 58 is a dependent format - the length of the encoded value depends on
// the value being encoded This does not play well with Cryptol, which expects
// a static type.   Thus we must consume the worst-case number of bytes and
// produce a length value which we return to the callee.

// Convert an N bit number (33 bytes for minilock) into a big endian base 58 representation (46 byte max for minilock).
base58enc : {n,m} (fin n, fin m, n >= 6 ) => [n] -> [m][8]
base58enc ns = [ x.1 | x <- reverse (take `{m} (drop `{1} enc))]
 where
  enc      = [(ns,zero)] # [ (n/58,charOf (n%58)) | (n,_) <- enc ]
  charOf x = charMap @ x

charMap : [58][8]
charMap = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"

base58dec : {n,m} (fin n, fin m, n >= 1, n >= width m, n >= 8) => [m][8] -> [n]
base58dec ms = sum [ (dec m) * 58 ^^ i | m <- reverse ms | i <- [0..m] ]
 where
  dec : [8] -> [n]
  dec x = zero # dec' x

  dec' : [8] -> [8]
  dec' x =
    if x >= 'm'
     then x - 'm' + 44
     else if x >= 'a'
            then x - 'a' + 33
            else if x >= 'P'
                  then x - 'P' + 22
                  else if x > 'I'
                        then x - 'J' + 17
                        else if x >= 'A'
                              then x - 'A' + 9
                              else x - '1'