path: root/libc/src/stdio/printf_core/float_dec_converter.h

//===-- Decimal Float Converter for printf ----------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_FLOAT_DEC_CONVERTER_H
#define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_FLOAT_DEC_CONVERTER_H

#include "src/__support/CPP/string_view.h"
#include "src/__support/FPUtil/FPBits.h"
#include "src/__support/FPUtil/rounding_mode.h"
#include "src/__support/float_to_string.h"
#include "src/__support/integer_to_string.h"
#include "src/__support/libc_assert.h"
#include "src/stdio/printf_core/converter_utils.h"
#include "src/stdio/printf_core/core_structs.h"
#include "src/stdio/printf_core/float_inf_nan_converter.h"
#include "src/stdio/printf_core/writer.h"

#include <inttypes.h>
#include <stddef.h>

namespace LIBC_NAMESPACE {
namespace printf_core {

using StorageType = fputil::FPBits<long double>::StorageType;
using DecimalString = IntegerToString<intmax_t>;
using ExponentString =
    IntegerToString<intmax_t, radix::Dec::WithWidth<2>::WithSign>;

// Returns true if value is divisible by 2^p.
template <typename T>
LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_integral_v<T>, bool>
multiple_of_power_of_2(T value, uint32_t p) {
  return (value & ((T(1) << p) - 1)) == 0;
}

constexpr size_t BLOCK_SIZE = 9;
constexpr uint32_t MAX_BLOCK = 999999999;

// constexpr size_t BLOCK_SIZE = 18;
// constexpr uint32_t MAX_BLOCK = 999999999999999999;
constexpr char DECIMAL_POINT = '.';

// This is used to represent which direction the number should be rounded.
enum class RoundDirection { Up, Down, Even };

LIBC_INLINE RoundDirection get_round_direction(int last_digit, bool truncated,
                                               fputil::Sign sign) {
  switch (fputil::quick_get_round()) {
  case FE_TONEAREST:
    // Round to nearest, if it's exactly halfway then round to even.
    if (last_digit != 5) {
      return last_digit > 5 ? RoundDirection::Up : RoundDirection::Down;
    } else {
      return !truncated ? RoundDirection::Even : RoundDirection::Up;
    }
  case FE_DOWNWARD:
    if (sign.is_neg() && (truncated || last_digit > 0)) {
      return RoundDirection::Up;
    } else {
      return RoundDirection::Down;
    }
  case FE_UPWARD:
    if (sign.is_pos() && (truncated || last_digit > 0)) {
      return RoundDirection::Up;
    } else {
      return RoundDirection::Down;
    }
    return sign.is_neg() ? RoundDirection::Down : RoundDirection::Up;
  case FE_TOWARDZERO:
    return RoundDirection::Down;
  default:
    return RoundDirection::Down;
  }
}

template <typename T>
LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_integral_v<T>, bool>
zero_after_digits(int32_t base_2_exp, int32_t digits_after_point, T mantissa,
                  const int32_t mant_width) {
  const int32_t required_twos = -base_2_exp - digits_after_point - 1;
  // Add 8 to mant width since this is a loose bound.
  const bool has_trailing_zeros =
      required_twos <= 0 ||
      (required_twos < (mant_width + 8) &&
       multiple_of_power_of_2(mantissa, static_cast<uint32_t>(required_twos)));
  return has_trailing_zeros;
}

class PaddingWriter {
  bool left_justified = false;
  bool leading_zeroes = false;
  char sign_char = 0;
  size_t min_width = 0;

public:
  LIBC_INLINE PaddingWriter() {}
  LIBC_INLINE PaddingWriter(const FormatSection &to_conv, char init_sign_char)
      : left_justified((to_conv.flags & FormatFlags::LEFT_JUSTIFIED) > 0),
        leading_zeroes((to_conv.flags & FormatFlags::LEADING_ZEROES) > 0),
        sign_char(init_sign_char),
        min_width(to_conv.min_width > 0 ? to_conv.min_width : 0) {}

  LIBC_INLINE int write_left_padding(Writer *writer, size_t total_digits) {
    // The pattern is (spaces) (sign) (zeroes), but only one of spaces and
    // zeroes can be written, and only if the padding amount is positive.
    int padding_amount =
        static_cast<int>(min_width - total_digits - (sign_char > 0 ? 1 : 0));
    if (left_justified || padding_amount < 0) {
      if (sign_char > 0) {
        RET_IF_RESULT_NEGATIVE(writer->write(sign_char));
      }
      return 0;
    }
    if (!leading_zeroes) {
      RET_IF_RESULT_NEGATIVE(writer->write(' ', padding_amount));
    }
    if (sign_char > 0) {
      RET_IF_RESULT_NEGATIVE(writer->write(sign_char));
    }
    if (leading_zeroes) {
      RET_IF_RESULT_NEGATIVE(writer->write('0', padding_amount));
    }
    return 0;
  }

  LIBC_INLINE int write_right_padding(Writer *writer, size_t total_digits) {
    // If and only if the conversion is left justified, there may be trailing
    // spaces.
    int padding_amount =
        static_cast<int>(min_width - total_digits - (sign_char > 0 ? 1 : 0));
    if (left_justified && padding_amount > 0) {
      RET_IF_RESULT_NEGATIVE(writer->write(' ', padding_amount));
    }
    return 0;
  }
};

/*
  We only need to round a given segment if all of the segments below it are
  the max (or this is the last segment). This means that we don't have to
  write those initially, we can just keep the most recent non-maximal
  segment and a counter of the number of maximal segments. When we reach a
  non-maximal segment, we write the stored segment as well as as many 9s as
  are necessary. Alternately, if we reach the end and have to round up, then
  we round the stored segment, and write zeroes following it. If this
  crosses the decimal point, then we have to shift it one space to the
  right.
  This FloatWriter class does the buffering and counting, and writes to the
  output when necessary.
*/
class FloatWriter {
  char block_buffer[BLOCK_SIZE]; // The buffer that holds a block.
  size_t buffered_digits = 0;    // The number of digits held in the buffer.
  bool has_written = false;      // True once any digits have been output.
  size_t max_block_count = 0; // The # of blocks of all 9s currently buffered.
  size_t total_digits = 0;    // The number of digits that will be output.
  size_t digits_before_decimal = 0; // The # of digits to write before the '.'
  size_t total_digits_written = 0;  // The # of digits that have been output.
  bool has_decimal_point;           // True if the number has a decimal point.
  Writer *writer;                   // Writes to the final output.
  PaddingWriter padding_writer; // Handles prefixes/padding, uses total_digits.

  LIBC_INLINE int flush_buffer(bool round_up_max_blocks = false) {
    const char MAX_BLOCK_DIGIT = (round_up_max_blocks ? '0' : '9');

    // Write the most recent buffered block, and mark has_written
    if (!has_written) {
      has_written = true;
      RET_IF_RESULT_NEGATIVE(
          padding_writer.write_left_padding(writer, total_digits));
    }

    // if the decimal point is the next character, or is in the range covered
    // by the buffered block, write the appropriate digits and the decimal
    // point.
    if (total_digits_written < digits_before_decimal &&
        total_digits_written + buffered_digits >= digits_before_decimal &&
        has_decimal_point) {
      size_t digits_to_write = digits_before_decimal - total_digits_written;
      if (digits_to_write > 0) {
        // Write the digits before the decimal point.
        RET_IF_RESULT_NEGATIVE(writer->write({block_buffer, digits_to_write}));
      }
      RET_IF_RESULT_NEGATIVE(writer->write(DECIMAL_POINT));
      if (buffered_digits - digits_to_write > 0) {
        // Write the digits after the decimal point.
        RET_IF_RESULT_NEGATIVE(
            writer->write({block_buffer + digits_to_write,
                           (buffered_digits - digits_to_write)}));
      }
      // add 1 for the decimal point
      total_digits_written += buffered_digits + 1;
      // Mark the buffer as empty.
      buffered_digits = 0;
    }

    // Clear the buffered digits.
    if (buffered_digits > 0) {
      RET_IF_RESULT_NEGATIVE(writer->write({block_buffer, buffered_digits}));
      total_digits_written += buffered_digits;
      buffered_digits = 0;
    }

    // if the decimal point is the next character, or is in the range covered
    // by the max blocks, write the appropriate digits and the decimal point.
    if (total_digits_written < digits_before_decimal &&
        total_digits_written + BLOCK_SIZE * max_block_count >=
            digits_before_decimal &&
        has_decimal_point) {
      size_t digits_to_write = digits_before_decimal - total_digits_written;
      if (digits_to_write > 0) {
        RET_IF_RESULT_NEGATIVE(writer->write(MAX_BLOCK_DIGIT, digits_to_write));
      }
      RET_IF_RESULT_NEGATIVE(writer->write(DECIMAL_POINT));
      if ((BLOCK_SIZE * max_block_count) - digits_to_write > 0) {
        RET_IF_RESULT_NEGATIVE(writer->write(
            MAX_BLOCK_DIGIT, (BLOCK_SIZE * max_block_count) - digits_to_write));
      }
      // add 1 for the decimal point
      total_digits_written += BLOCK_SIZE * max_block_count + 1;
      // clear the buffer of max blocks
      max_block_count = 0;
    }

    // Clear the buffer of max blocks
    if (max_block_count > 0) {
      RET_IF_RESULT_NEGATIVE(
          writer->write(MAX_BLOCK_DIGIT, max_block_count * BLOCK_SIZE));
      total_digits_written += max_block_count * BLOCK_SIZE;
      max_block_count = 0;
    }
    return 0;
  }

  // -exponent will never overflow because all long double types we support
  // have at most 15 bits of mantissa and the C standard defines an int as
  // being at least 16 bits.
  static_assert(fputil::FPBits<long double>::EXP_LEN < (sizeof(int) * 8));

public:
  LIBC_INLINE FloatWriter(Writer *init_writer, bool init_has_decimal_point,
                          const PaddingWriter &init_padding_writer)
      : has_decimal_point(init_has_decimal_point), writer(init_writer),
        padding_writer(init_padding_writer) {}

  LIBC_INLINE void init(size_t init_total_digits,
                        size_t init_digits_before_decimal) {
    total_digits = init_total_digits;
    digits_before_decimal = init_digits_before_decimal;
  }

  LIBC_INLINE void write_first_block(BlockInt block, bool exp_format = false) {
    const DecimalString buf(block);
    const cpp::string_view int_to_str = buf.view();
    size_t digits_buffered = int_to_str.size();
    // Block Buffer is guaranteed to not overflow since block cannot have more
    // than BLOCK_SIZE digits.
    // TODO: Replace with memcpy
    for (size_t count = 0; count < digits_buffered; ++count) {
      block_buffer[count] = int_to_str[count];
    }
    buffered_digits = digits_buffered;

    // In the exponent format (%e) we know how many digits will be written even
    // before calculating any blocks, whereas the decimal format (%f) has to
    // write all of the blocks that would come before the decimal place.
    if (!exp_format) {
      total_digits += digits_buffered;
      digits_before_decimal += digits_buffered;
    }
  }

  LIBC_INLINE int write_middle_block(BlockInt block) {
    if (block == MAX_BLOCK) { // Buffer max blocks in case of rounding
      ++max_block_count;
    } else { // If a non-max block has been found
      RET_IF_RESULT_NEGATIVE(flush_buffer());

      // Now buffer the current block. We add 1 + MAX_BLOCK to force the
      // leading zeroes, and drop the leading one. This is probably inefficient,
      // but it works. See https://xkcd.com/2021/
      const DecimalString buf(block + (MAX_BLOCK + 1));
      const cpp::string_view int_to_str = buf.view();
      // TODO: Replace with memcpy
      for (size_t count = 0; count < BLOCK_SIZE; ++count) {
        block_buffer[count] = int_to_str[count + 1];
      }

      buffered_digits = BLOCK_SIZE;
    }
    return 0;
  }

  LIBC_INLINE int write_last_block(BlockInt block, size_t block_digits,
                                   RoundDirection round, int exponent = 0,
                                   char exp_char = '\0') {
    bool has_exp = (exp_char != '\0');

    char end_buff[BLOCK_SIZE];

    {
      const DecimalString buf(block + (MAX_BLOCK + 1));
      const cpp::string_view int_to_str = buf.view();

      // copy the last block_digits characters into the start of end_buff.
      // TODO: Replace with memcpy
      for (size_t count = 0; count < block_digits; ++count) {
        end_buff[count] = int_to_str[count + 1 + (BLOCK_SIZE - block_digits)];
      }
    }

    char low_digit = '0';
    if (block_digits > 0) {
      low_digit = end_buff[block_digits - 1];
    } else if (max_block_count > 0) {
      low_digit = '9';
    } else if (buffered_digits > 0) {
      low_digit = block_buffer[buffered_digits - 1];
    }

    bool round_up_max_blocks = false;

    // Round up
    if (round == RoundDirection::Up ||
        (round == RoundDirection::Even && low_digit % 2 != 0)) {
      bool has_carry = true;
      round_up_max_blocks = true; // if we're rounding up, we might need to
                                  // round up the max blocks that are buffered.

      // handle the low block that we're adding
      for (int count = static_cast<int>(block_digits) - 1;
           count >= 0 && has_carry; --count) {
        if (end_buff[count] == '9') {
          end_buff[count] = '0';
        } else {
          end_buff[count] += 1;
          has_carry = false;
          round_up_max_blocks = false; // If the low block isn't all nines, then
                                       // the max blocks aren't rounded up.
        }
      }
      // handle the high block that's buffered
      for (int count = static_cast<int>(buffered_digits) - 1;
           count >= 0 && has_carry; --count) {
        if (block_buffer[count] == '9') {
          block_buffer[count] = '0';
        } else {
          block_buffer[count] += 1;
          has_carry = false;
        }
      }

      // has_carry should only be true here if every previous digit is 9, which
      // implies that the number has never been written.
      if (has_carry /* && !has_written */) {
        if (has_exp) { // This is in %e style
          // Since this is exponential notation, we don't write any more digits
          // but we do increment the exponent.
          ++exponent;

          const ExponentString buf(exponent);
          const cpp::string_view int_to_str = buf.view();

          // TODO: also change this to calculate the width of the number more
          // efficiently.
          size_t exponent_width = int_to_str.size();
          size_t number_digits =
              buffered_digits + (max_block_count * BLOCK_SIZE) + block_digits;

          // Here we have to recalculate the total number of digits since the
          // exponent's width may have changed. We're only adding 1 to exponent
          // width since exp_str appends the sign.
          total_digits =
              (has_decimal_point ? 1 : 0) + number_digits + 1 + exponent_width;

          // Normally write_left_padding is called by flush_buffer but since
          // we're rounding up all of the digits, the ones in the buffer are
          // wrong and can't be flushed.
          RET_IF_RESULT_NEGATIVE(
              padding_writer.write_left_padding(writer, total_digits));
          // Now we know we need to print a leading 1, the decimal point, and
          // then zeroes after it.
          RET_IF_RESULT_NEGATIVE(writer->write('1'));
          // digits_before_decimal - 1 to account for the leading '1'
          if (has_decimal_point) {
            RET_IF_RESULT_NEGATIVE(writer->write(DECIMAL_POINT));
            // This is just the length of the number, not including the decimal
            // point, or exponent.

            if (number_digits > 1) {
              RET_IF_RESULT_NEGATIVE(writer->write('0', number_digits - 1));
            }
          }
          RET_IF_RESULT_NEGATIVE(writer->write(exp_char));
          RET_IF_RESULT_NEGATIVE(writer->write(int_to_str));

          total_digits_written = total_digits;
          return WRITE_OK;
        } else { // This is in %f style
          ++total_digits;
          ++digits_before_decimal;
          // Normally write_left_padding is called by flush_buffer but since
          // we're rounding up all of the digits, the ones in the buffer are
          // wrong and can't be flushed.
          RET_IF_RESULT_NEGATIVE(
              padding_writer.write_left_padding(writer, total_digits));
          // Now we know we need to print a leading 1, zeroes up to the decimal
          // point, the decimal point, and then finally digits after it.
          RET_IF_RESULT_NEGATIVE(writer->write('1'));
          // digits_before_decimal - 1 to account for the leading '1'
          RET_IF_RESULT_NEGATIVE(writer->write('0', digits_before_decimal - 1));
          if (has_decimal_point) {
            RET_IF_RESULT_NEGATIVE(writer->write(DECIMAL_POINT));
            // add one to digits_before_decimal to account for the decimal point
            // itself.
            if (total_digits > digits_before_decimal + 1) {
              RET_IF_RESULT_NEGATIVE(writer->write(
                  '0', total_digits - (digits_before_decimal + 1)));
            }
          }
          total_digits_written = total_digits;
          return WRITE_OK;
        }
      }
    }
    // Either we intend to round down, or the rounding up is complete. Flush the
    // buffers.

    RET_IF_RESULT_NEGATIVE(flush_buffer(round_up_max_blocks));

    // And then write the final block. It's written via the buffer so that if
    // this is also the first block, the decimal point will be placed correctly.

    // TODO: Replace with memcpy
    for (size_t count = 0; count < block_digits; ++count) {
      block_buffer[count] = end_buff[count];
    }
    buffered_digits = block_digits;
    RET_IF_RESULT_NEGATIVE(flush_buffer());

    if (has_exp) {
      RET_IF_RESULT_NEGATIVE(writer->write(exp_char));
      const ExponentString buf(exponent);
      RET_IF_RESULT_NEGATIVE(writer->write(buf.view()));
    }
    total_digits_written = total_digits;

    return WRITE_OK;
  }

  LIBC_INLINE int write_zeroes(uint32_t num_zeroes) {
    RET_IF_RESULT_NEGATIVE(flush_buffer());
    RET_IF_RESULT_NEGATIVE(writer->write('0', num_zeroes));
    return 0;
  }

  LIBC_INLINE int right_pad() {
    return padding_writer.write_right_padding(writer, total_digits);
  }
};

// This implementation is based on the Ryu Printf algorithm by Ulf Adams:
// Ulf Adams. 2019. Ryū revisited: printf floating point conversion.
// Proc. ACM Program. Lang. 3, OOPSLA, Article 169 (October 2019), 23 pages.
// https://doi.org/10.1145/3360595
template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
LIBC_INLINE int convert_float_decimal_typed(Writer *writer,
                                            const FormatSection &to_conv,
                                            fputil::FPBits<T> float_bits) {
  // signed because later we use -FRACTION_LEN
  constexpr int32_t FRACTION_LEN = fputil::FPBits<T>::FRACTION_LEN;
  int exponent = float_bits.get_explicit_exponent();

  char sign_char = 0;

  if (float_bits.is_neg())
    sign_char = '-';
  else if ((to_conv.flags & FormatFlags::FORCE_SIGN) == FormatFlags::FORCE_SIGN)
    sign_char = '+'; // FORCE_SIGN has precedence over SPACE_PREFIX
  else if ((to_conv.flags & FormatFlags::SPACE_PREFIX) ==
           FormatFlags::SPACE_PREFIX)
    sign_char = ' ';

  // If to_conv doesn't specify a precision, the precision defaults to 6.
  const unsigned int precision = to_conv.precision < 0 ? 6 : to_conv.precision;
  bool has_decimal_point =
      (precision > 0) || ((to_conv.flags & FormatFlags::ALTERNATE_FORM) != 0);

  // nonzero is false until a nonzero digit is found. It is used to determine if
  // leading zeroes should be printed, since before the first digit they are
  // ignored.
  bool nonzero = false;

  PaddingWriter padding_writer(to_conv, sign_char);
  FloatWriter float_writer(writer, has_decimal_point, padding_writer);
  FloatToString<T> float_converter(float_bits.get_val());

  const size_t positive_blocks = float_converter.get_positive_blocks();

  if (positive_blocks >= 0) {
    // This loop iterates through the number a block at a time until it finds a
    // block that is not zero or it hits the decimal point. This is because all
    // zero blocks before the first nonzero digit or the decimal point are
    // ignored (no leading zeroes, at least at this stage).
    int32_t i = static_cast<int32_t>(positive_blocks) - 1;
    for (; i >= 0; --i) {
      BlockInt digits = float_converter.get_positive_block(i);
      if (nonzero) {
        RET_IF_RESULT_NEGATIVE(float_writer.write_middle_block(digits));
      } else if (digits != 0) {
        size_t blocks_before_decimal = i;
        float_writer.init((blocks_before_decimal * BLOCK_SIZE) +
                              (has_decimal_point ? 1 : 0) + precision,
                          blocks_before_decimal * BLOCK_SIZE);
        float_writer.write_first_block(digits);

        nonzero = true;
      }
    }
  }

  // if we haven't yet found a valid digit, buffer a zero.
  if (!nonzero) {
    float_writer.init((has_decimal_point ? 1 : 0) + precision, 0);
    float_writer.write_first_block(0);
  }

  if (exponent < FRACTION_LEN) {
    const uint32_t blocks = (precision / static_cast<uint32_t>(BLOCK_SIZE)) + 1;
    uint32_t i = 0;
    // if all the blocks we should write are zero
    if (blocks <= float_converter.zero_blocks_after_point()) {
      i = blocks; // just write zeroes up to precision
      RET_IF_RESULT_NEGATIVE(float_writer.write_zeroes(precision));
    } else if (i < float_converter.zero_blocks_after_point()) {
      // else if there are some blocks that are zeroes
      i = static_cast<uint32_t>(float_converter.zero_blocks_after_point());
      // write those blocks as zeroes.
      RET_IF_RESULT_NEGATIVE(float_writer.write_zeroes(9 * i));
    }
    // for each unwritten block
    for (; i < blocks; ++i) {
      if (float_converter.is_lowest_block(i)) {
        const uint32_t fill = precision - 9 * i;
        RET_IF_RESULT_NEGATIVE(float_writer.write_zeroes(fill));
        break;
      }
      BlockInt digits = float_converter.get_negative_block(i);
      if (i < blocks - 1) {
        RET_IF_RESULT_NEGATIVE(float_writer.write_middle_block(digits));
      } else {

        const uint32_t maximum =
            static_cast<uint32_t>(precision - BLOCK_SIZE * i);
        uint32_t last_digit = 0;
        for (uint32_t k = 0; k < BLOCK_SIZE - maximum; ++k) {
          last_digit = digits % 10;
          digits /= 10;
        }
        RoundDirection round;
        const bool truncated = !zero_after_digits(
            exponent - FRACTION_LEN, precision,
            float_bits.get_explicit_mantissa(), FRACTION_LEN);
        round = get_round_direction(last_digit, truncated, float_bits.sign());

        RET_IF_RESULT_NEGATIVE(
            float_writer.write_last_block(digits, maximum, round));
        break;
      }
    }
  } else {
    RET_IF_RESULT_NEGATIVE(float_writer.write_zeroes(precision));
  }
  RET_IF_RESULT_NEGATIVE(float_writer.right_pad());
  return WRITE_OK;
}

template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
LIBC_INLINE int convert_float_dec_exp_typed(Writer *writer,
                                            const FormatSection &to_conv,
                                            fputil::FPBits<T> float_bits) {
  // signed because later we use -FRACTION_LEN
  constexpr int32_t FRACTION_LEN = fputil::FPBits<T>::FRACTION_LEN;
  int exponent = float_bits.get_explicit_exponent();
  StorageType mantissa = float_bits.get_explicit_mantissa();

  const char a = (to_conv.conv_name & 32) | 'A';

  char sign_char = 0;

  if (float_bits.is_neg())
    sign_char = '-';
  else if ((to_conv.flags & FormatFlags::FORCE_SIGN) == FormatFlags::FORCE_SIGN)
    sign_char = '+'; // FORCE_SIGN has precedence over SPACE_PREFIX
  else if ((to_conv.flags & FormatFlags::SPACE_PREFIX) ==
           FormatFlags::SPACE_PREFIX)
    sign_char = ' ';

  // If to_conv doesn't specify a precision, the precision defaults to 6.
  const unsigned int precision = to_conv.precision < 0 ? 6 : to_conv.precision;
  bool has_decimal_point =
      (precision > 0) || ((to_conv.flags & FormatFlags::ALTERNATE_FORM) != 0);

  PaddingWriter padding_writer(to_conv, sign_char);
  FloatWriter float_writer(writer, has_decimal_point, padding_writer);
  FloatToString<T> float_converter(float_bits.get_val());

  size_t digits_written = 0;
  int final_exponent = 0;

  // Here we would subtract 1 to account for the fact that block 0 counts as a
  // positive block, but the loop below accounts for this by starting with
  // subtracting 1 from cur_block.
  int cur_block;

  if (exponent < 0) {
    cur_block = -static_cast<int>(float_converter.zero_blocks_after_point());
  } else {
    cur_block = static_cast<int>(float_converter.get_positive_blocks());
  }

  BlockInt digits = 0;

  // If the mantissa is 0, then the number is 0, meaning that looping until a
  // non-zero block is found will loop forever. The first block is just 0.
  if (mantissa != 0) {
    // This loop finds the first block.
    while (digits == 0) {
      --cur_block;
      digits = float_converter.get_block(cur_block);
    }
  } else {
    cur_block = 0;
  }

  const size_t block_width = IntegerToString<intmax_t>(digits).size();

  final_exponent = static_cast<int>(cur_block * BLOCK_SIZE) +
                   static_cast<int>(block_width - 1);
  int positive_exponent = final_exponent < 0 ? -final_exponent : final_exponent;

  size_t exponent_width = IntegerToString<intmax_t>(positive_exponent).size();

  // Calculate the total number of digits in the number.
  // 1 - the digit before the decimal point
  // 1 - the decimal point (optional)
  // precision - the number of digits after the decimal point
  // 1 - the 'e' at the start of the exponent
  // 1 - the sign at the start of the exponent
  // max(2, exp width) - the digits of the exponent, min 2.

  float_writer.init(1 + (has_decimal_point ? 1 : 0) + precision + 2 +
                        (exponent_width < 2 ? 2 : exponent_width),
                    1);

  // If this block is not the last block
  if (block_width <= precision + 1) {
    float_writer.write_first_block(digits, true);
    digits_written += block_width;
    --cur_block;
  }

  // For each middle block.
  for (; digits_written + BLOCK_SIZE < precision + 1; --cur_block) {
    digits = float_converter.get_block(cur_block);

    RET_IF_RESULT_NEGATIVE(float_writer.write_middle_block(digits));
    digits_written += BLOCK_SIZE;
  }

  digits = float_converter.get_block(cur_block);

  size_t last_block_size = BLOCK_SIZE;

  // if the last block is also the first block, then ignore leading zeroes.
  if (digits_written == 0) {
    last_block_size = IntegerToString<intmax_t>(digits).size();
  }

  // This tracks if the number is truncated, that meaning that the digits after
  // last_digit are non-zero.
  bool truncated = false;

  // This is the last block.
  const size_t maximum = precision + 1 - digits_written;
  uint32_t last_digit = 0;
  for (uint32_t k = 0; k < last_block_size - maximum; ++k) {
    if (last_digit > 0)
      truncated = true;

    last_digit = digits % 10;
    digits /= 10;
  }

  // If the last block we read doesn't have the digit after the end of what
  // we'll print, then we need to read the next block to get that digit.
  if (maximum == last_block_size) {
    --cur_block;
    BlockInt extra_block = float_converter.get_block(cur_block);
    last_digit = extra_block / ((MAX_BLOCK / 10) + 1);
    if (extra_block % ((MAX_BLOCK / 10) + 1) > 0) {
      truncated = true;
    }
  }

  RoundDirection round;

  // If we've already seen a truncated digit, then we don't need to check any
  // more.
  if (!truncated) {
    // Check the blocks above the decimal point
    if (cur_block >= 0) {
      // Check every block until the decimal point for non-zero digits.
      for (int cur_extra_block = cur_block - 1; cur_extra_block >= 0;
           --cur_extra_block) {
        BlockInt extra_block = float_converter.get_block(cur_extra_block);
        if (extra_block > 0) {
          truncated = true;
          break;
        }
      }
    }
    // If it's still not truncated and there are digits below the decimal point
    if (!truncated && exponent - FRACTION_LEN < 0) {
      // Use the formula from %f.
      truncated = !zero_after_digits(
          exponent - FRACTION_LEN, precision - final_exponent,
          float_bits.get_explicit_mantissa(), FRACTION_LEN);
    }
  }
  round = get_round_direction(last_digit, truncated, float_bits.sign());

  RET_IF_RESULT_NEGATIVE(float_writer.write_last_block(
      digits, maximum, round, final_exponent, a + 'E' - 'A'));

  RET_IF_RESULT_NEGATIVE(float_writer.right_pad());
  return WRITE_OK;
}

template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
LIBC_INLINE int convert_float_dec_auto_typed(Writer *writer,
                                             const FormatSection &to_conv,
                                             fputil::FPBits<T> float_bits) {
  // signed because later we use -FRACTION_LEN
  constexpr int32_t FRACTION_LEN = fputil::FPBits<T>::FRACTION_LEN;
  int exponent = float_bits.get_explicit_exponent();
  StorageType mantissa = float_bits.get_explicit_mantissa();

  // From the standard: Let P (init_precision) equal the precision if nonzero, 6
  // if the precision is omitted, or 1 if the precision is zero.
  const unsigned int init_precision = to_conv.precision <= 0
                                          ? (to_conv.precision == 0 ? 1 : 6)
                                          : to_conv.precision;

  //  Then, if a conversion with style E would have an exponent of X
  //  (base_10_exp):
  int base_10_exp = 0;
  // If P > X >= -4 the conversion is with style F and precision P - (X + 1).
  // Otherwise, the conversion is with style E and precision P - 1.

  // For calculating the base 10 exponent, we need to process the number as if
  // it has style E, so here we calculate the precision we'll use in that case.
  const unsigned int exp_precision = init_precision - 1;

  FloatToString<T> float_converter(float_bits.get_val());

  // Here we would subtract 1 to account for the fact that block 0 counts as a
  // positive block, but the loop below accounts for this by starting with
  // subtracting 1 from cur_block.
  int cur_block;

  if (exponent < 0) {
    cur_block = -static_cast<int>(float_converter.zero_blocks_after_point());
  } else {
    cur_block = static_cast<int>(float_converter.get_positive_blocks());
  }

  BlockInt digits = 0;

  // If the mantissa is 0, then the number is 0, meaning that looping until a
  // non-zero block is found will loop forever.
  if (mantissa != 0) {
    // This loop finds the first non-zero block.
    while (digits == 0) {
      --cur_block;
      digits = float_converter.get_block(cur_block);
    }
  } else {
    // In the case of 0.0, then it's always decimal format. If we don't have alt
    // form then the trailing zeroes are trimmed to make "0", else the precision
    // is 1 less than specified by the user.
    FormatSection new_conv = to_conv;
    if ((to_conv.flags & FormatFlags::ALTERNATE_FORM) != 0) {
      // This is a style F conversion, making the precision P - 1 - X, but since
      // this is for the number 0, X (the base 10 exponent) is always 0.
      new_conv.precision = init_precision - 1;
    } else {
      new_conv.precision = 0;
    }
    return convert_float_decimal_typed<T>(writer, new_conv, float_bits);
  }

  const size_t block_width = IntegerToString<intmax_t>(digits).size();

  size_t digits_checked = 0;
  // TODO: look into unifying trailing_zeroes and trailing_nines. The number can
  // end in a nine or a zero, but not both.
  size_t trailing_zeroes = 0;
  size_t trailing_nines = 0;

  base_10_exp = static_cast<int>(cur_block * BLOCK_SIZE) +
                static_cast<int>(block_width - 1);

  // If the first block is not also the last block
  if (block_width <= exp_precision + 1) {
    const DecimalString buf(digits);
    const cpp::string_view int_to_str = buf.view();

    for (size_t i = 0; i < block_width; ++i) {
      if (int_to_str[i] == '9') {
        ++trailing_nines;
        trailing_zeroes = 0;
      } else if (int_to_str[i] == '0') {
        ++trailing_zeroes;
        trailing_nines = 0;
      } else {
        trailing_nines = 0;
        trailing_zeroes = 0;
      }
    }
    digits_checked += block_width;
    --cur_block;
  }

  // Handle middle blocks
  for (; digits_checked + BLOCK_SIZE < exp_precision + 1; --cur_block) {
    digits = float_converter.get_block(cur_block);
    digits_checked += BLOCK_SIZE;
    if (digits == MAX_BLOCK) {
      trailing_nines += 9;
      trailing_zeroes = 0;
    } else if (digits == 0) {
      trailing_zeroes += 9;
      trailing_nines = 0;
    } else {
      // The block is neither all nines nor all zeroes, so we need to figure out
      // what it ends with.
      trailing_nines = 0;
      trailing_zeroes = 0;
      BlockInt copy_of_digits = digits;
      BlockInt cur_last_digit = copy_of_digits % 10;
      // We only care if it ends in nines or zeroes.
      while (copy_of_digits > 0 &&
             (cur_last_digit == 9 || cur_last_digit == 0)) {
        // If the next digit is not the same as the previous one, then there are
        // no more contiguous trailing digits.
        if (copy_of_digits % 10 != cur_last_digit) {
          break;
        }
        if (cur_last_digit == 9) {
          ++trailing_nines;
        } else if (cur_last_digit == 0) {
          ++trailing_zeroes;
        } else {
          break;
        }
        copy_of_digits /= 10;
      }
    }
  }

  // Handle the last block

  digits = float_converter.get_block(cur_block);

  size_t last_block_size = BLOCK_SIZE;

  const DecimalString buf(digits);
  const cpp::string_view int_to_str = buf.view();

  size_t implicit_leading_zeroes = BLOCK_SIZE - int_to_str.size();

  // if the last block is also the first block, then ignore leading zeroes.
  if (digits_checked == 0) {
    last_block_size = int_to_str.size();
    implicit_leading_zeroes = 0;
  }

  unsigned int digits_requested =
      (exp_precision + 1) - static_cast<unsigned int>(digits_checked);

  int digits_to_check =
      digits_requested - static_cast<int>(implicit_leading_zeroes);
  if (digits_to_check < 0) {
    digits_to_check = 0;
  }

  // If the block is not the maximum size, that means it has leading
  // zeroes, and zeroes are not nines.
  if (implicit_leading_zeroes > 0) {
    trailing_nines = 0;
  }

  // But leading zeroes are zeroes (that could be trailing). We take the
  // minimum of the leading zeroes and digits requested because if there are
  // more requested digits than leading zeroes we shouldn't count those.
  trailing_zeroes +=
      (implicit_leading_zeroes > digits_requested ? digits_requested
                                                  : implicit_leading_zeroes);

  // Check the upper digits of this block.
  for (int i = 0; i < digits_to_check; ++i) {
    if (int_to_str[i] == '9') {
      ++trailing_nines;
      trailing_zeroes = 0;
    } else if (int_to_str[i] == '0') {
      ++trailing_zeroes;
      trailing_nines = 0;
    } else {
      trailing_nines = 0;
      trailing_zeroes = 0;
    }
  }

  bool truncated = false;

  // Find the digit after the lowest digit that we'll actually print to
  // determine the rounding.
  const uint32_t maximum =
      exp_precision + 1 - static_cast<uint32_t>(digits_checked);
  uint32_t last_digit = 0;
  for (uint32_t k = 0; k < last_block_size - maximum; ++k) {
    if (last_digit > 0)
      truncated = true;

    last_digit = digits % 10;
    digits /= 10;
  }

  // If the last block we read doesn't have the digit after the end of what
  // we'll print, then we need to read the next block to get that digit.
  if (maximum == last_block_size) {
    --cur_block;
    BlockInt extra_block = float_converter.get_block(cur_block);
    last_digit = extra_block / ((MAX_BLOCK / 10) + 1);

    if (extra_block % ((MAX_BLOCK / 10) + 1) > 0)
      truncated = true;
  }

  // TODO: unify this code across the three float conversions.
  RoundDirection round;

  // If we've already seen a truncated digit, then we don't need to check any
  // more.
  if (!truncated) {
    // Check the blocks above the decimal point
    if (cur_block >= 0) {
      // Check every block until the decimal point for non-zero digits.
      for (int cur_extra_block = cur_block - 1; cur_extra_block >= 0;
           --cur_extra_block) {
        BlockInt extra_block = float_converter.get_block(cur_extra_block);
        if (extra_block > 0) {
          truncated = true;
          break;
        }
      }
    }
    // If it's still not truncated and there are digits below the decimal point
    if (!truncated && exponent - FRACTION_LEN < 0) {
      // Use the formula from %f.
      truncated = !zero_after_digits(
          exponent - FRACTION_LEN, exp_precision - base_10_exp,
          float_bits.get_explicit_mantissa(), FRACTION_LEN);
    }
  }

  round = get_round_direction(last_digit, truncated, float_bits.sign());

  bool round_up;
  if (round == RoundDirection::Up) {
    round_up = true;
  } else if (round == RoundDirection::Down) {
    round_up = false;
  } else {
    // RoundDirection is even, so check the lowest digit that will be printed.
    uint32_t low_digit;

    // maximum is the number of digits that will remain in digits after getting
    // last_digit. If it's greater than zero, we can just check the lowest digit
    // in digits.
    if (maximum > 0) {
      low_digit = digits % 10;
    } else {
      // Else if there are trailing nines, then the low digit is a nine, same
      // with zeroes.
      if (trailing_nines > 0) {
        low_digit = 9;
      } else if (trailing_zeroes > 0) {
        low_digit = 0;
      } else {
        // If there are no trailing zeroes or nines, then the round direction
        // doesn't actually matter here. Since this conversion passes off the
        // value to another one for final conversion, rounding only matters to
        // determine if the exponent is higher than expected (with an all nine
        // number) or to determine the trailing zeroes to trim. In this case
        // low_digit is set to 0, but it could be set to any number.

        low_digit = 0;
      }
    }
    round_up = (low_digit % 2) != 0;
  }

  digits_checked += digits_requested;
  LIBC_ASSERT(digits_checked == init_precision);
  // At this point we should have checked all the digits requested by the
  // precision. We may increment this number 1 more if we round up all of the
  // digits, but at this point in the code digits_checked should always equal
  // init_precision.

  if (round_up) {
    // If all the digits that would be printed are nines, then rounding up means
    // that the base 10 exponent is one higher and all those nines turn to
    // zeroes (e.g. 999 -> 1000).
    if (trailing_nines == init_precision) {
      ++base_10_exp;
      trailing_zeroes = digits_checked;
      ++digits_checked;
    } else {
      // If there are trailing nines, they turn into trailing zeroes when
      // they're rounded up.
      if (trailing_nines > 0) {
        trailing_zeroes += trailing_nines;
      } else if (trailing_zeroes > 0) {
        // If there are trailing zeroes, then the last digit will be rounded up
        // to a 1 so they aren't trailing anymore.
        trailing_zeroes = 0;
      }
    }
  }

  // if P > X >= -4, the conversion is with style f (or F) and precision equals
  //  P - (X + 1).
  if (static_cast<int>(init_precision) > base_10_exp && base_10_exp >= -4) {
    FormatSection new_conv = to_conv;
    const int conv_precision = init_precision - (base_10_exp + 1);

    if ((to_conv.flags & FormatFlags::ALTERNATE_FORM) != 0) {
      new_conv.precision = conv_precision;
    } else {
      // If alt form isn't set, then we need to determine the number of trailing
      // zeroes and set the precision such that they are removed.

      /*
      Here's a diagram of an example:

      printf("%.15g", 22.25);

                            +--- init_precision = 15
                            |
                            +-------------------+
                            |                   |
                            |  ++--- trimmed_precision = 2
                            |  ||               |
                            22.250000000000000000
                            ||   |              |
                            ++   +--------------+
                             |   |
       base_10_exp + 1 = 2 --+   +--- trailing_zeroes = 11
      */
      int trimmed_precision = static_cast<int>(
          digits_checked - (base_10_exp + 1) - trailing_zeroes);
      if (trimmed_precision < 0) {
        trimmed_precision = 0;
      }
      new_conv.precision = (trimmed_precision > conv_precision)
                               ? conv_precision
                               : trimmed_precision;
    }

    return convert_float_decimal_typed<T>(writer, new_conv, float_bits);
  } else {
    // otherwise, the conversion is with style e (or E) and precision equals
    // P - 1
    const int conv_precision = init_precision - 1;
    FormatSection new_conv = to_conv;
    if ((to_conv.flags & FormatFlags::ALTERNATE_FORM) != 0) {
      new_conv.precision = conv_precision;
    } else {
      // If alt form isn't set, then we need to determine the number of trailing
      // zeroes and set the precision such that they are removed.
      int trimmed_precision =
          static_cast<int>(digits_checked - 1 - trailing_zeroes);
      if (trimmed_precision < 0) {
        trimmed_precision = 0;
      }
      new_conv.precision = (trimmed_precision > conv_precision)
                               ? conv_precision
                               : trimmed_precision;
    }
    return convert_float_dec_exp_typed<T>(writer, new_conv, float_bits);
  }
}

// TODO: unify the float converters to remove the duplicated checks for inf/nan.
LIBC_INLINE int convert_float_decimal(Writer *writer,
                                      const FormatSection &to_conv) {
  if (to_conv.length_modifier == LengthModifier::L) {
    fputil::FPBits<long double>::StorageType float_raw = to_conv.conv_val_raw;
    fputil::FPBits<long double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_decimal_typed<long double>(writer, to_conv,
                                                      float_bits);
    }
  } else {
    fputil::FPBits<double>::StorageType float_raw =
        static_cast<fputil::FPBits<double>::StorageType>(to_conv.conv_val_raw);
    fputil::FPBits<double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_decimal_typed<double>(writer, to_conv, float_bits);
    }
  }

  return convert_inf_nan(writer, to_conv);
}

LIBC_INLINE int convert_float_dec_exp(Writer *writer,
                                      const FormatSection &to_conv) {
  if (to_conv.length_modifier == LengthModifier::L) {
    fputil::FPBits<long double>::StorageType float_raw = to_conv.conv_val_raw;
    fputil::FPBits<long double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_dec_exp_typed<long double>(writer, to_conv,
                                                      float_bits);
    }
  } else {
    fputil::FPBits<double>::StorageType float_raw =
        static_cast<fputil::FPBits<double>::StorageType>(to_conv.conv_val_raw);
    fputil::FPBits<double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_dec_exp_typed<double>(writer, to_conv, float_bits);
    }
  }

  return convert_inf_nan(writer, to_conv);
}

LIBC_INLINE int convert_float_dec_auto(Writer *writer,
                                       const FormatSection &to_conv) {
  if (to_conv.length_modifier == LengthModifier::L) {
    fputil::FPBits<long double>::StorageType float_raw = to_conv.conv_val_raw;
    fputil::FPBits<long double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_dec_auto_typed<long double>(writer, to_conv,
                                                       float_bits);
    }
  } else {
    fputil::FPBits<double>::StorageType float_raw =
        static_cast<fputil::FPBits<double>::StorageType>(to_conv.conv_val_raw);
    fputil::FPBits<double> float_bits(float_raw);
    if (!float_bits.is_inf_or_nan()) {
      return convert_float_dec_auto_typed<double>(writer, to_conv, float_bits);
    }
  }

  return convert_inf_nan(writer, to_conv);
}

} // namespace printf_core
} // namespace LIBC_NAMESPACE

#endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_FLOAT_DEC_CONVERTER_H