/*

* BigInt Encoding/Decoding

* (C) 1999-2010,2012,2019,2021 Jack Lloyd

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include <botan/bigint.h>

#include <botan/internal/divide.h>

#include <botan/hex.h>

namespace Botan {

std::string BigInt::to_dec_string() const

   {

   // Use the largest power of 10 that fits in a word

#if (BOTAN_MP_WORD_BITS == 64)

   const word conversion_radix = 10000000000000000000U;

   const word radix_digits = 19;

#else

   const word conversion_radix = 1000000000U;

   const word radix_digits = 9;

#endif

   // (over-)estimate of the number of digits needed; log2(10) ~ 3.3219

   const size_t digit_estimate = static_cast<size_t>(1 + (this->bits() / 3.32));

   // (over-)estimate of db such that conversion_radix^db > *this

   const size_t digit_blocks = (digit_estimate + radix_digits - 1) / radix_digits;

   BigInt value = *this;

   value.set_sign(Positive);

   // Extract groups of digits into words

   std::vector<word> digit_groups(digit_blocks);

   for(size_t i = 0; i != digit_blocks; ++i)

      {

      word remainder = 0;

      ct_divide_word(value, conversion_radix, value, remainder);

      digit_groups[i] = remainder;

      }

   BOTAN_ASSERT_NOMSG(value.is_zero());

   // Extract digits from the groups

   std::vector<uint8_t> digits(digit_blocks * radix_digits);

   for(size_t i = 0; i != digit_blocks; ++i)

      {

      word remainder = digit_groups[i];

      for(size_t j = 0; j != radix_digits; ++j)

         {

         // Compiler should convert div/mod by 10 into mul by magic constant

         const word digit = remainder % 10;

         remainder /= 10;

         digits[radix_digits*i + j] = static_cast<uint8_t>(digit);

         }

      }

   // remove leading zeros

   while(!digits.empty() && digits.back() == 0)

      {

      digits.pop_back();

      }

   BOTAN_ASSERT_NOMSG(digit_estimate >= digits.size());

   // Reverse the digits to big-endian and format to text

   std::string s;

   s.reserve(1 + digits.size());

   if(is_negative())

      s += "-";

   // Reverse and convert to textual digits

   for(auto i = digits.rbegin(); i != digits.rend(); ++i)

      {

      s.push_back(*i + '0'); // assumes ASCII

      }

   if(s.empty())

      s += "0";

   return s;

   }

std::string BigInt::to_hex_string() const

   {

   std::vector<uint8_t> bits = BigInt::encode(*this);

   if(bits.empty())

      bits.push_back(0);

   std::string hrep;

   if(is_negative())

      hrep += "-";

   hrep += "0x";

   hrep += hex_encode(bits);

   return hrep;

   }

/*

* Encode a BigInt, with leading 0s if needed

*/

secure_vector<uint8_t> BigInt::encode_1363(const BigInt& n, size_t bytes)

   {

   if(n.bytes() > bytes)

      throw Encoding_Error("encode_1363: n is too large to encode properly");

   secure_vector<uint8_t> output(bytes);

   n.binary_encode(output.data(), output.size());

   return output;

   }

//static

void BigInt::encode_1363(uint8_t output[], size_t bytes, const BigInt& n)

   {

   if(n.bytes() > bytes)

      throw Encoding_Error("encode_1363: n is too large to encode properly");

   n.binary_encode(output, bytes);

   }

/*

* Encode two BigInt, with leading 0s if needed, and concatenate

*/

secure_vector<uint8_t> BigInt::encode_fixed_length_int_pair(const BigInt& n1, const BigInt& n2, size_t bytes)

   {

   if(n1.is_negative() || n2.is_negative())

      throw Encoding_Error("encode_fixed_length_int_pair: values must be positive");

   if(n1.bytes() > bytes || n2.bytes() > bytes)

      throw Encoding_Error("encode_fixed_length_int_pair: values too large to encode properly");

   secure_vector<uint8_t> output(2 * bytes);

   n1.binary_encode(output.data()        , bytes);

   n2.binary_encode(output.data() + bytes, bytes);

   return output;

   }

/*

* Decode a BigInt

*/

BigInt BigInt::decode(const uint8_t buf[], size_t length, Base base)

   {

   BigInt r;

   if(base == Binary)

      {

      r.binary_decode(buf, length);

      }

   else if(base == Hexadecimal)

      {

      secure_vector<uint8_t> binary;

      if(length % 2)

         {

         // Handle lack of leading 0

         const char buf0_with_leading_0[2] =

            { '0', static_cast<char>(buf[0]) };

         binary = hex_decode_locked(buf0_with_leading_0, 2);

         binary += hex_decode_locked(cast_uint8_ptr_to_char(&buf[1]),

                                     length - 1,

                                     false);

         }

      else

         binary = hex_decode_locked(cast_uint8_ptr_to_char(buf),

                                    length, false);

      r.binary_decode(binary.data(), binary.size());

      }

   else if(base == Decimal)

      {

      // This could be made faster using the same trick as to_dec_string

      for(size_t i = 0; i != length; ++i)

         {

         const char c = buf[i];

         if(c < '0' || c > '9')

            throw Invalid_Argument("BigInt::decode: invalid decimal char");

         const uint8_t x = c - '0';

         BOTAN_ASSERT_NOMSG(x < 10);

         r *= 10;

         r += x;

         }

      }

   else

      throw Invalid_Argument("Unknown BigInt decoding method");

   return r;

   }

}