include/fast_float/decimal_to_binary.h

#ifndef FASTFLOAT_DECIMAL_TO_BINARY_H
#define FASTFLOAT_DECIMAL_TO_BINARY_H

#include "float_common.h"
#include "fast_table.h"
#include <cfloat>
#include <cinttypes>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <cstring>

namespace fast_float {

// This will compute or rather approximate w * 5**q and return a pair of 64-bit
// words approximating the result, with the "high" part corresponding to the
// most significant bits and the low part corresponding to the least significant
// bits.
//
template <int bit_precision>
fastfloat_really_inline FASTFLOAT_CONSTEXPR20 value128
compute_product_approximation(int64_t q, uint64_t w) {
  int const index = 2 * int(q - powers::smallest_power_of_five);
  // For small values of q, e.g., q in [0,27], the answer is always exact
  // because The line value128 firstproduct = full_multiplication(w,
  // power_of_five_128[index]); gives the exact answer.
  value128 firstproduct =
      full_multiplication(w, powers::power_of_five_128[index]);
  static_assert((bit_precision >= 0) && (bit_precision <= 64),
                " precision should  be in (0,64]");
  constexpr uint64_t precision_mask =
      (bit_precision < 64) ? (uint64_t(0xFFFFFFFFFFFFFFFF) >> bit_precision)
                           : uint64_t(0xFFFFFFFFFFFFFFFF);
  if ((firstproduct.high & precision_mask) ==
      precision_mask) { // could further guard with  (lower + w < lower)
    // regarding the second product, we only need secondproduct.high, but our
    // expectation is that the compiler will optimize this extra work away if
    // needed.
    value128 secondproduct =
        full_multiplication(w, powers::power_of_five_128[index + 1]);
    firstproduct.low += secondproduct.high;
    if (secondproduct.high > firstproduct.low) {
      firstproduct.high++;
    }
  }
  return firstproduct;
}

namespace detail {
/**
 * For q in (0,350), we have that
 *  f = (((152170 + 65536) * q ) >> 16);
 * is equal to
 *   floor(p) + q
 * where
 *   p = log(5**q)/log(2) = q * log(5)/log(2)
 *
 * For negative values of q in (-400,0), we have that
 *  f = (((152170 + 65536) * q ) >> 16);
 * is equal to
 *   -ceil(p) + q
 * where
 *   p = log(5**-q)/log(2) = -q * log(5)/log(2)
 */
constexpr fastfloat_really_inline int32_t power(int32_t q) noexcept {
  return (((152170 + 65536) * q) >> 16) + 63;
}
} // namespace detail

// create an adjusted mantissa, biased by the invalid power2
// for significant digits already multiplied by 10 ** q.
template <typename binary>
fastfloat_really_inline FASTFLOAT_CONSTEXPR14 adjusted_mantissa
compute_error_scaled(int64_t q, uint64_t w, int lz) noexcept {
  int hilz = int(w >> 63) ^ 1;
  adjusted_mantissa answer;
  answer.mantissa = w << hilz;
  int bias = binary::mantissa_explicit_bits() - binary::minimum_exponent();
  answer.power2 = int32_t(detail::power(int32_t(q)) + bias - hilz - lz - 62 +
                          invalid_am_bias);
  return answer;
}

// w * 10 ** q, without rounding the representation up.
// the power2 in the exponent will be adjusted by invalid_am_bias.
template <typename binary>
fastfloat_really_inline FASTFLOAT_CONSTEXPR20 adjusted_mantissa
compute_error(int64_t q, uint64_t w) noexcept {
  int lz = leading_zeroes(w);
  w <<= lz;
  value128 product =
      compute_product_approximation<binary::mantissa_explicit_bits() + 3>(q, w);
  return compute_error_scaled<binary>(q, product.high, lz);
}

// Computers w * 10 ** q.
// The returned value should be a valid number that simply needs to be
// packed. However, in some very rare cases, the computation will fail. In such
// cases, we return an adjusted_mantissa with a negative power of 2: the caller
// should recompute in such cases.
template <typename binary>
fastfloat_really_inline FASTFLOAT_CONSTEXPR20 adjusted_mantissa
compute_float(int64_t q, uint64_t w) noexcept {
  adjusted_mantissa answer;
  if ((w == 0) || (q < binary::smallest_power_of_ten())) {
    answer.power2 = 0;
    answer.mantissa = 0;
    // result should be zero
    return answer;
  }
  if (q > binary::largest_power_of_ten()) {
    // we want to get infinity:
    answer.power2 = binary::infinite_power();
    answer.mantissa = 0;
    return answer;
  }
  // At this point in time q is in [powers::smallest_power_of_five,
  // powers::largest_power_of_five].

  // We want the most significant bit of i to be 1. Shift if needed.
  int lz = leading_zeroes(w);
  w <<= lz;

  // The required precision is binary::mantissa_explicit_bits() + 3 because
  // 1. We need the implicit bit
  // 2. We need an extra bit for rounding purposes
  // 3. We might lose a bit due to the "upperbit" routine (result too small,
  // requiring a shift)

  value128 product =
      compute_product_approximation<binary::mantissa_explicit_bits() + 3>(q, w);
  // The computed 'product' is always sufficient.
  // Mathematical proof:
  // Noble Mushtak and Daniel Lemire, Fast Number Parsing Without Fallback (to
  // appear) See script/mushtak_lemire.py

  // The "compute_product_approximation" function can be slightly slower than a
  // branchless approach: value128 product = compute_product(q, w); but in
  // practice, we can win big with the compute_product_approximation if its
  // additional branch is easily predicted. Which is best is data specific.
  int upperbit = int(product.high >> 63);
  int shift = upperbit + 64 - binary::mantissa_explicit_bits() - 3;

  answer.mantissa = product.high >> shift;

  answer.power2 = int32_t(detail::power(int32_t(q)) + upperbit - lz -
                          binary::minimum_exponent());
  if (answer.power2 <= 0) { // we have a subnormal?
    // Here have that answer.power2 <= 0 so -answer.power2 >= 0
    if (-answer.power2 + 1 >=
        64) { // if we have more than 64 bits below the minimum exponent, you
              // have a zero for sure.
      answer.power2 = 0;
      answer.mantissa = 0;
      // result should be zero
      return answer;
    }
    // next line is safe because -answer.power2 + 1 < 64
    answer.mantissa >>= -answer.power2 + 1;
    // Thankfully, we can't have both "round-to-even" and subnormals because
    // "round-to-even" only occurs for powers close to 0 in the 32-bit and
    // and 64-bit case (with no more than 19 digits).
    answer.mantissa += (answer.mantissa & 1); // round up
    answer.mantissa >>= 1;
    // There is a weird scenario where we don't have a subnormal but just.
    // Suppose we start with 2.2250738585072013e-308, we end up
    // with 0x3fffffffffffff x 2^-1023-53 which is technically subnormal
    // whereas 0x40000000000000 x 2^-1023-53  is normal. Now, we need to round
    // up 0x3fffffffffffff x 2^-1023-53  and once we do, we are no longer
    // subnormal, but we can only know this after rounding.
    // So we only declare a subnormal if we are smaller than the threshold.
    answer.power2 =
        (answer.mantissa < (uint64_t(1) << binary::mantissa_explicit_bits()))
            ? 0
            : 1;
    return answer;
  }

  // usually, we round *up*, but if we fall right in between and and we have an
  // even basis, we need to round down
  // We are only concerned with the cases where 5**q fits in single 64-bit word.
  if ((product.low <= 1) && (q >= binary::min_exponent_round_to_even()) &&
      (q <= binary::max_exponent_round_to_even()) &&
      ((answer.mantissa & 3) == 1)) { // we may fall between two floats!
    // To be in-between two floats we need that in doing
    //   answer.mantissa = product.high >> (upperbit + 64 -
    //   binary::mantissa_explicit_bits() - 3);
    // ... we dropped out only zeroes. But if this happened, then we can go
    // back!!!
    if ((answer.mantissa << shift) == product.high) {
      answer.mantissa &= ~uint64_t(1); // flip it so that we do not round up
    }
  }

  answer.mantissa += (answer.mantissa & 1); // round up
  answer.mantissa >>= 1;
  if (answer.mantissa >= (uint64_t(2) << binary::mantissa_explicit_bits())) {
    answer.mantissa = (uint64_t(1) << binary::mantissa_explicit_bits());
    answer.power2++; // undo previous addition
  }

  answer.mantissa &= ~(uint64_t(1) << binary::mantissa_explicit_bits());
  if (answer.power2 >= binary::infinite_power()) { // infinity
    answer.power2 = binary::infinite_power();
    answer.mantissa = 0;
  }
  return answer;
}

} // namespace fast_float

#endif