#!/usr/bin/env bash

b64_encode() {
  local idx=0 numerics table_idxs table_idx encoded
  local table=( {A..Z} {a..z} {0..9} + / )

  for (( ; idx < ${#1}; idx+=3 )); do
    read -ra numerics < <(
      printf '%d %d %d\n' "'${1:idx:1}" "'${1:idx+1:1}" "'${1:idx+2:1}"
    )

    (( table_idxs[0] = numerics[0] >> 2 ))
    (( table_idxs[1] = (( (numerics[0] & 0x03) << 6) | (numerics[1] & 0xF0) >> 2) >> 2 ))
    (( table_idxs[2] = (( (numerics[1] & 0x0F) << 4) | (numerics[2] & 0xC0) >> 4) >> 2 ))
    (( table_idxs[3] = numerics[2] & 0x3F ))

    for table_idx in "${table_idxs[@]}"; do
      encoded+=${table[$table_idx]}
    done
  done

  if (( ${#1} % 3 == 1 )); then
    encoded=${encoded::-2}==
  elif (( ${#1} % 3 == 2 )); then
    encoded=${encoded::-1}=
  fi

  printf '%s\n' "$encoded"
}

b64_encode "$1"