#include "base64.h"
#include <iostream>
#include <cctype>

std::string base64_decode(const std::string& encoded) {
    const std::string base64_chars = 
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz"
        "0123456789+/";
    
    size_t in_len = encoded.size();
    size_t i = 0;
    size_t in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];
    std::string decoded;
    
    while (in_ < in_len && encoded[in_] != '=' && 
           (std::isalnum(encoded[in_]) || encoded[in_] == '+' || encoded[in_] == '/')) {
        char_array_4[i++] = encoded[in_++];
        if (i == 4) {
            // Translate values in char_array_4 from base64 alphabet to indices
            for (i = 0; i < 4; i++) {
                char_array_4[i] = base64_chars.find(char_array_4[i]);
            }
            
            // Decode to original bytes
            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
            
            for (i = 0; i < 3; i++) {
                decoded += char_array_3[i];
            }
            i = 0;
        }
    }
    
    // Handle any remaining bytes
    if (i) {
        // Fill remaining positions with zeros
        for (size_t j = i; j < 4; j++) {
            char_array_4[j] = 0;
        }
        
        // Convert to indices
        for (size_t j = 0; j < 4; j++) {
            char_array_4[j] = base64_chars.find(char_array_4[j]);
        }
        
        // Decode remaining bytes
        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
        
        // Only add valid bytes based on how many characters we had
        for (size_t j = 0; j < i - 1; j++) {
            decoded += char_array_3[j];
        }
    }
    
    return decoded;
}