base62.nim

import tables
import private/resultSet as rs

# Alphabet is provided to factory functions that return the encoder and decoder
# as a tuple of functions.

const
  base: string = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

type 
  DecoderFunc* = proc(input: string): int
  EncoderFunc* = proc(input: int):    string
  FunctionPair* = tuple[encoder: EncoderFunc, decoder: DecoderFunc]

proc generateEncoder(alphabet: string = base): EncoderFunc =
  let encode: EncoderFunc = (proc(input: int): string =
      let
        baseNum: int = len(alphabet)
      var
        remainder:  int = input mod baseNum
        integerDiv: int = input div baseNum
        encoded:    string = "" & alphabet[remainder]

      while (integerDiv != 0):
        remainder  = integerDiv mod baseNum
        integerDiv = integerDiv div baseNum
        encoded    = alphabet[remainder] & encoded

      result = encoded)
  result = encode

proc generateDecoder(alphabet: string = base): DecoderFunc =
  let PositionMap = rs.toCharMap(alphabet)
  # Should return either decoded result or -1 for error
  let decode: DecoderFunc = (proc(input: string): int =
    if not rs.isValidResult(input,alphabet):
      return -1
    var 
        decoded: int = PositionMap[input[0]]
    let
      input_limit: int = len(input)
      baseNum:     int = len(alphabet)
    # For each letter of this, want to get position in input
    for i in 1..<input_limit:
      decoded = baseNum * decoded + PositionMap[input[i]]
    result = decoded)
  result = decode
  
proc GenerateFunctionPair*(alphabet: string = base): FunctionPair =
  return (generateEncoder(alphabet),generateDecoder(alphabet))

when isMainModule:
  let
    (encode,decode) = GenerateFunctionPair()
    test_encoded:   string = "5e3T6"
    test_decoded:   int    = 77233220
    encoded_output: string = encode(test_decoded)
    decoded_output: int    = decode(test_encoded)

    
  echo "ENCODING: Expected " & test_encoded & " and got: " & encoded_output
  echo("DECODING: Expected " & $test_decoded & " and got: " & $decoded_output)
  doAssert encoded_output == test_encoded
  doAssert decoded_output == test_decoded