// After editing this file, run "go generate" in the parent directory.

// Copyright 2020 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// ---------------- String Conversions

// wuffs_base__parse_number__foo_digits entries are 0x00 for invalid digits,
// and (0x80 | v) for valid digits, where v is the 4 bit value.

static const uint8_t wuffs_base__parse_number__decimal_digits[256] = {
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x00 ..= 0x07.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x08 ..= 0x0F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x10 ..= 0x17.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x18 ..= 0x1F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x20 ..= 0x27.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x28 ..= 0x2F.
    0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,  // 0x30 ..= 0x37. '0'-'7'.
    0x88, 0x89, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x38 ..= 0x3F. '8'-'9'.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x40 ..= 0x47.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x48 ..= 0x4F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x50 ..= 0x57.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x58 ..= 0x5F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x60 ..= 0x67.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x68 ..= 0x6F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x70 ..= 0x77.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x78 ..= 0x7F.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x80 ..= 0x87.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x88 ..= 0x8F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x90 ..= 0x97.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x98 ..= 0x9F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xA0 ..= 0xA7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xA8 ..= 0xAF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xB0 ..= 0xB7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xB8 ..= 0xBF.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xC0 ..= 0xC7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xC8 ..= 0xCF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xD0 ..= 0xD7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xD8 ..= 0xDF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xE0 ..= 0xE7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xE8 ..= 0xEF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xF0 ..= 0xF7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xF8 ..= 0xFF.
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
};

static const uint8_t wuffs_base__parse_number__hexadecimal_digits[256] = {
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x00 ..= 0x07.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x08 ..= 0x0F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x10 ..= 0x17.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x18 ..= 0x1F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x20 ..= 0x27.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x28 ..= 0x2F.
    0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,  // 0x30 ..= 0x37. '0'-'7'.
    0x88, 0x89, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x38 ..= 0x3F. '8'-'9'.

    0x00, 0x8A, 0x8B, 0x8C, 0x8D, 0x8E, 0x8F, 0x00,  // 0x40 ..= 0x47. 'A'-'F'.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x48 ..= 0x4F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x50 ..= 0x57.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x58 ..= 0x5F.
    0x00, 0x8A, 0x8B, 0x8C, 0x8D, 0x8E, 0x8F, 0x00,  // 0x60 ..= 0x67. 'a'-'f'.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x68 ..= 0x6F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x70 ..= 0x77.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x78 ..= 0x7F.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x80 ..= 0x87.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x88 ..= 0x8F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x90 ..= 0x97.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x98 ..= 0x9F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xA0 ..= 0xA7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xA8 ..= 0xAF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xB0 ..= 0xB7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xB8 ..= 0xBF.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xC0 ..= 0xC7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xC8 ..= 0xCF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xD0 ..= 0xD7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xD8 ..= 0xDF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xE0 ..= 0xE7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xE8 ..= 0xEF.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xF0 ..= 0xF7.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0xF8 ..= 0xFF.
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
};

// --------

wuffs_base__result_i64  //
wuffs_base__parse_number_i64(wuffs_base__slice_u8 s) {
  uint8_t* p = s.ptr;
  uint8_t* q = s.ptr + s.len;

  for (; (p < q) && (*p == '_'); p++) {
  }

  bool negative = false;
  if (p >= q) {
    goto fail_bad_argument;
  } else if (*p == '-') {
    p++;
    negative = true;
  } else if (*p == '+') {
    p++;
  }

  do {
    wuffs_base__result_u64 r = wuffs_base__parse_number_u64(
        wuffs_base__make_slice_u8(p, (size_t)(q - p)));
    if (r.status.repr != NULL) {
      wuffs_base__result_i64 ret;
      ret.status.repr = r.status.repr;
      ret.value = 0;
      return ret;
    } else if (negative) {
      if (r.value > 0x8000000000000000) {
        goto fail_out_of_bounds;
      }
      wuffs_base__result_i64 ret;
      ret.status.repr = NULL;
      ret.value = -(int64_t)(r.value);
      return ret;
    } else if (r.value > 0x7FFFFFFFFFFFFFFF) {
      goto fail_out_of_bounds;
    } else {
      wuffs_base__result_i64 ret;
      ret.status.repr = NULL;
      ret.value = +(int64_t)(r.value);
      return ret;
    }
  } while (0);

fail_bad_argument:
  do {
    wuffs_base__result_i64 ret;
    ret.status.repr = wuffs_base__error__bad_argument;
    ret.value = 0;
    return ret;
  } while (0);

fail_out_of_bounds:
  do {
    wuffs_base__result_i64 ret;
    ret.status.repr = wuffs_base__error__out_of_bounds;
    ret.value = 0;
    return ret;
  } while (0);
}

wuffs_base__result_u64  //
wuffs_base__parse_number_u64(wuffs_base__slice_u8 s) {
  uint8_t* p = s.ptr;
  uint8_t* q = s.ptr + s.len;

  for (; (p < q) && (*p == '_'); p++) {
  }

  if (p >= q) {
    goto fail_bad_argument;

  } else if (*p == '0') {
    p++;
    if (p >= q) {
      goto ok_zero;
    }
    if (*p == '_') {
      p++;
      for (; p < q; p++) {
        if (*p != '_') {
          goto fail_bad_argument;
        }
      }
      goto ok_zero;
    }

    if ((*p == 'x') || (*p == 'X')) {
      p++;
      for (; (p < q) && (*p == '_'); p++) {
      }
      if (p < q) {
        goto hexadecimal;
      }

    } else if ((*p == 'd') || (*p == 'D')) {
      p++;
      for (; (p < q) && (*p == '_'); p++) {
      }
      if (p < q) {
        goto decimal;
      }
    }

    goto fail_bad_argument;
  }

decimal:
  do {
    uint64_t v = wuffs_base__parse_number__decimal_digits[*p++];
    if (v == 0) {
      goto fail_bad_argument;
    }
    v &= 0x0F;

    // UINT64_MAX is 18446744073709551615, which is ((10 * max10) + max1).
    const uint64_t max10 = 1844674407370955161;
    const uint8_t max1 = 5;

    for (; p < q; p++) {
      if (*p == '_') {
        continue;
      }
      uint8_t digit = wuffs_base__parse_number__decimal_digits[*p];
      if (digit == 0) {
        goto fail_bad_argument;
      }
      digit &= 0x0F;
      if ((v > max10) || ((v == max10) && (digit > max1))) {
        goto fail_out_of_bounds;
      }
      v = (10 * v) + ((uint64_t)(digit));
    }

    wuffs_base__result_u64 ret;
    ret.status.repr = NULL;
    ret.value = v;
    return ret;
  } while (0);

hexadecimal:
  do {
    uint64_t v = wuffs_base__parse_number__hexadecimal_digits[*p++];
    if (v == 0) {
      goto fail_bad_argument;
    }
    v &= 0x0F;

    for (; p < q; p++) {
      if (*p == '_') {
        continue;
      }
      uint8_t digit = wuffs_base__parse_number__hexadecimal_digits[*p];
      if (digit == 0) {
        goto fail_bad_argument;
      }
      digit &= 0x0F;
      if ((v >> 60) != 0) {
        goto fail_out_of_bounds;
      }
      v = (v << 4) | ((uint64_t)(digit));
    }

    wuffs_base__result_u64 ret;
    ret.status.repr = NULL;
    ret.value = v;
    return ret;
  } while (0);

ok_zero:
  do {
    wuffs_base__result_u64 ret;
    ret.status.repr = NULL;
    ret.value = 0;
    return ret;
  } while (0);

fail_bad_argument:
  do {
    wuffs_base__result_u64 ret;
    ret.status.repr = wuffs_base__error__bad_argument;
    ret.value = 0;
    return ret;
  } while (0);

fail_out_of_bounds:
  do {
    wuffs_base__result_u64 ret;
    ret.status.repr = wuffs_base__error__out_of_bounds;
    ret.value = 0;
    return ret;
  } while (0);
}

  // ---------------- IEEE 754 Floating Point

#define WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE 1023
#define WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION 500

// WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL is the largest N
// such that ((10 << N) < (1 << 64)).
#define WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL 60

// wuffs_base__private_implementation__high_prec_dec (abbreviated as HPD) is a
// fixed precision floating point decimal number, augmented with ±infinity
// values, but it cannot represent NaN (Not a Number).
//
// An HPD isn't for general purpose arithmetic, only for conversions to and
// from IEEE 754 double-precision floating point, where the largest and
// smallest positive, finite values are approximately 1.8e+308 and 4.9e-324.
// HPD exponents above +1023 mean infinity, below -1023 mean zero. The ±1023
// bounds are further away from zero than ±(324 + 500), where 500 and 1023 is
// WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION and
// WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE.
//
// digits[.. num_digits] are the number's digits in big-endian order. The
// uint8_t values are in the range [0 ..= 9], not ['0' ..= '9'], where e.g. '7'
// is the ASCII value 0x37.
//
// decimal_point is the index (within digits) of the decimal point. It may be
// negative or be larger than num_digits, in which case the explicit digits are
// padded with implicit zeroes.
//
// For example, if num_digits is 3 and digits is "\x07\x08\x09":
//   - A decimal_point of -2 means ".00789"
//   - A decimal_point of -1 means ".0789"
//   - A decimal_point of -0 means ".789"
//   - A decimal_point of +1 means "7.89"
//   - A decimal_point of +2 means "78.9"
//   - A decimal_point of +3 means "789."
//   - A decimal_point of +4 means "7890."
//   - A decimal_point of +5 means "78900."
//
// As above, a decimal_point higher than +1023 means that the overall value is
// infinity, lower than -1023 means zero.
//
// negative is a sign bit. An HPD can distinguish positive and negative zero.
//
// truncated is whether there are more than
// WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION digits, and at
// least one of those extra digits are non-zero. The existence of long-tail
// digits can affect rounding.
//
// The "all fields are zero" value is valid, and represents the number +0.
typedef struct {
  uint32_t num_digits;
  int32_t decimal_point;
  bool negative;
  bool truncated;
  uint8_t digits[WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION];
} wuffs_base__private_implementation__high_prec_dec;

// wuffs_base__private_implementation__high_prec_dec__trim trims trailing
// zeroes from the h->digits[.. h->num_digits] slice. They have no benefit,
// since we explicitly track h->decimal_point.
//
// Preconditions:
//  - h is non-NULL.
static inline void  //
wuffs_base__private_implementation__high_prec_dec__trim(
    wuffs_base__private_implementation__high_prec_dec* h) {
  while ((h->num_digits > 0) && (h->digits[h->num_digits - 1] == 0)) {
    h->num_digits--;
  }
}

static wuffs_base__status  //
wuffs_base__private_implementation__high_prec_dec__parse(
    wuffs_base__private_implementation__high_prec_dec* h,
    wuffs_base__slice_u8 s) {
  if (!h) {
    return wuffs_base__make_status(wuffs_base__error__bad_receiver);
  }
  h->num_digits = 0;
  h->decimal_point = 0;
  h->negative = false;
  h->truncated = false;

  uint8_t* p = s.ptr;
  uint8_t* q = s.ptr + s.len;

  for (; (p < q) && (*p == '_'); p++) {
  }
  if (p >= q) {
    return wuffs_base__make_status(wuffs_base__error__bad_argument);
  }

  // Parse sign.
  do {
    if (*p == '+') {
      p++;
    } else if (*p == '-') {
      h->negative = true;
      p++;
    } else {
      break;
    }
    for (; (p < q) && (*p == '_'); p++) {
    }
  } while (0);

  // Parse digits.
  uint32_t nd = 0;
  int32_t dp = 0;
  bool saw_digits = false;
  bool saw_non_zero_digits = false;
  bool saw_dot = false;
  for (; p < q; p++) {
    if (*p == '_') {
      // No-op.

    } else if ((*p == '.') || (*p == ',')) {
      // As per https://en.wikipedia.org/wiki/Decimal_separator, both '.' or
      // ',' are commonly used. We just parse either, regardless of LOCALE.
      if (saw_dot) {
        return wuffs_base__make_status(wuffs_base__error__bad_argument);
      }
      saw_dot = true;
      dp = (int32_t)nd;

    } else if ('0' == *p) {
      if (!saw_dot && !saw_non_zero_digits && saw_digits) {
        // We don't allow unnecessary leading zeroes: "000123" or "0644".
        return wuffs_base__make_status(wuffs_base__error__bad_argument);
      }
      saw_digits = true;
      if (nd == 0) {
        // Track leading zeroes implicitly.
        dp--;
      } else if (nd <
                 WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
        h->digits[nd++] = 0;
      } else {
        // Long-tail zeroes are ignored.
      }

    } else if (('0' < *p) && (*p <= '9')) {
      if (!saw_dot && !saw_non_zero_digits && saw_digits) {
        // We don't allow unnecessary leading zeroes: "000123" or "0644".
        return wuffs_base__make_status(wuffs_base__error__bad_argument);
      }
      saw_digits = true;
      saw_non_zero_digits = true;
      if (nd < WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
        h->digits[nd++] = (uint8_t)(*p - '0');
      } else {
        // Long-tail non-zeroes set the truncated bit.
        h->truncated = true;
      }

    } else {
      break;
    }
  }

  if (!saw_digits) {
    return wuffs_base__make_status(wuffs_base__error__bad_argument);
  }
  if (!saw_dot) {
    dp = (int32_t)nd;
  }

  // Parse exponent.
  if ((p < q) && ((*p == 'E') || (*p == 'e'))) {
    p++;
    for (; (p < q) && (*p == '_'); p++) {
    }
    if (p >= q) {
      return wuffs_base__make_status(wuffs_base__error__bad_argument);
    }

    int32_t exp_sign = +1;
    if (*p == '+') {
      p++;
    } else if (*p == '-') {
      exp_sign = -1;
      p++;
    }

    int32_t exp = 0;
    const int32_t exp_large =
        WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE +
        WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION;
    bool saw_exp_digits = false;
    for (; p < q; p++) {
      if (*p == '_') {
        // No-op.
      } else if (('0' <= *p) && (*p <= '9')) {
        saw_exp_digits = true;
        if (exp < exp_large) {
          exp = (10 * exp) + ((int32_t)(*p - '0'));
        }
      } else {
        break;
      }
    }
    if (!saw_exp_digits) {
      return wuffs_base__make_status(wuffs_base__error__bad_argument);
    }
    dp += exp_sign * exp;
  }

  // Finish.
  if (p != q) {
    return wuffs_base__make_status(wuffs_base__error__bad_argument);
  }
  h->num_digits = nd;
  if (nd == 0) {
    h->decimal_point = 0;
  } else if (dp <
             -WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE) {
    h->decimal_point =
        -WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE - 1;
  } else if (dp >
             +WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE) {
    h->decimal_point =
        +WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE + 1;
  } else {
    h->decimal_point = dp;
  }
  wuffs_base__private_implementation__high_prec_dec__trim(h);
  return wuffs_base__make_status(NULL);
}

// --------

// The etc__hpd_left_shift and etc__powers_of_5 tables were printed by
// script/print-hpd-left-shift.go. That script has an optional -comments flag,
// whose output is not copied here, which prints further detail.
//
// These tables are used in
// wuffs_base__private_implementation__high_prec_dec__lshift_num_new_digits.

// wuffs_base__private_implementation__hpd_left_shift[i] encodes the number of
// new digits created after multiplying a positive integer by (1 << i): the
// additional length in the decimal representation. For example, shifting "234"
// by 3 (equivalent to multiplying by 8) will produce "1872". Going from a
// 3-length string to a 4-length string means that 1 new digit was added (and
// existing digits may have changed).
//
// Shifting by i can add either N or N-1 new digits, depending on whether the
// original positive integer compares >= or < to the i'th power of 5 (as 10
// equals 2 * 5). Comparison is lexicographic, not numerical.
//
// For example, shifting by 4 (i.e. multiplying by 16) can add 1 or 2 new
// digits, depending on a lexicographic comparison to (5 ** 4), i.e. "625":
//  - ("1"      << 4) is "16",       which adds 1 new digit.
//  - ("5678"   << 4) is "90848",    which adds 1 new digit.
//  - ("624"    << 4) is "9984",     which adds 1 new digit.
//  - ("62498"  << 4) is "999968",   which adds 1 new digit.
//  - ("625"    << 4) is "10000",    which adds 2 new digits.
//  - ("625001" << 4) is "10000016", which adds 2 new digits.
//  - ("7008"   << 4) is "112128",   which adds 2 new digits.
//  - ("99"     << 4) is "1584",     which adds 2 new digits.
//
// Thus, when i is 4, N is 2 and (5 ** i) is "625". This etc__hpd_left_shift
// array encodes this as:
//  - etc__hpd_left_shift[4] is 0x1006 = (2 << 11) | 0x0006.
//  - etc__hpd_left_shift[5] is 0x1009 = (? << 11) | 0x0009.
// where the ? isn't relevant for i == 4.
//
// The high 5 bits of etc__hpd_left_shift[i] is N, the higher of the two
// possible number of new digits. The low 11 bits are an offset into the
// etc__powers_of_5 array (of length 0x051C, so offsets fit in 11 bits). When i
// is 4, its offset and the next one is 6 and 9, and etc__powers_of_5[6 .. 9]
// is the string "\x06\x02\x05", so the relevant power of 5 is "625".
//
// Thanks to Ken Thompson for the original idea.
static const uint16_t wuffs_base__private_implementation__hpd_left_shift[65] = {
    0x0000, 0x0800, 0x0801, 0x0803, 0x1006, 0x1009, 0x100D, 0x1812, 0x1817,
    0x181D, 0x2024, 0x202B, 0x2033, 0x203C, 0x2846, 0x2850, 0x285B, 0x3067,
    0x3073, 0x3080, 0x388E, 0x389C, 0x38AB, 0x38BB, 0x40CC, 0x40DD, 0x40EF,
    0x4902, 0x4915, 0x4929, 0x513E, 0x5153, 0x5169, 0x5180, 0x5998, 0x59B0,
    0x59C9, 0x61E3, 0x61FD, 0x6218, 0x6A34, 0x6A50, 0x6A6D, 0x6A8B, 0x72AA,
    0x72C9, 0x72E9, 0x7B0A, 0x7B2B, 0x7B4D, 0x8370, 0x8393, 0x83B7, 0x83DC,
    0x8C02, 0x8C28, 0x8C4F, 0x9477, 0x949F, 0x94C8, 0x9CF2, 0x051C, 0x051C,
    0x051C, 0x051C,
};

// wuffs_base__private_implementation__powers_of_5 contains the powers of 5,
// concatenated together: "5", "25", "125", "625", "3125", etc.
static const uint8_t wuffs_base__private_implementation__powers_of_5[0x051C] = {
    5, 2, 5, 1, 2, 5, 6, 2, 5, 3, 1, 2, 5, 1, 5, 6, 2, 5, 7, 8, 1, 2, 5, 3, 9,
    0, 6, 2, 5, 1, 9, 5, 3, 1, 2, 5, 9, 7, 6, 5, 6, 2, 5, 4, 8, 8, 2, 8, 1, 2,
    5, 2, 4, 4, 1, 4, 0, 6, 2, 5, 1, 2, 2, 0, 7, 0, 3, 1, 2, 5, 6, 1, 0, 3, 5,
    1, 5, 6, 2, 5, 3, 0, 5, 1, 7, 5, 7, 8, 1, 2, 5, 1, 5, 2, 5, 8, 7, 8, 9, 0,
    6, 2, 5, 7, 6, 2, 9, 3, 9, 4, 5, 3, 1, 2, 5, 3, 8, 1, 4, 6, 9, 7, 2, 6, 5,
    6, 2, 5, 1, 9, 0, 7, 3, 4, 8, 6, 3, 2, 8, 1, 2, 5, 9, 5, 3, 6, 7, 4, 3, 1,
    6, 4, 0, 6, 2, 5, 4, 7, 6, 8, 3, 7, 1, 5, 8, 2, 0, 3, 1, 2, 5, 2, 3, 8, 4,
    1, 8, 5, 7, 9, 1, 0, 1, 5, 6, 2, 5, 1, 1, 9, 2, 0, 9, 2, 8, 9, 5, 5, 0, 7,
    8, 1, 2, 5, 5, 9, 6, 0, 4, 6, 4, 4, 7, 7, 5, 3, 9, 0, 6, 2, 5, 2, 9, 8, 0,
    2, 3, 2, 2, 3, 8, 7, 6, 9, 5, 3, 1, 2, 5, 1, 4, 9, 0, 1, 1, 6, 1, 1, 9, 3,
    8, 4, 7, 6, 5, 6, 2, 5, 7, 4, 5, 0, 5, 8, 0, 5, 9, 6, 9, 2, 3, 8, 2, 8, 1,
    2, 5, 3, 7, 2, 5, 2, 9, 0, 2, 9, 8, 4, 6, 1, 9, 1, 4, 0, 6, 2, 5, 1, 8, 6,
    2, 6, 4, 5, 1, 4, 9, 2, 3, 0, 9, 5, 7, 0, 3, 1, 2, 5, 9, 3, 1, 3, 2, 2, 5,
    7, 4, 6, 1, 5, 4, 7, 8, 5, 1, 5, 6, 2, 5, 4, 6, 5, 6, 6, 1, 2, 8, 7, 3, 0,
    7, 7, 3, 9, 2, 5, 7, 8, 1, 2, 5, 2, 3, 2, 8, 3, 0, 6, 4, 3, 6, 5, 3, 8, 6,
    9, 6, 2, 8, 9, 0, 6, 2, 5, 1, 1, 6, 4, 1, 5, 3, 2, 1, 8, 2, 6, 9, 3, 4, 8,
    1, 4, 4, 5, 3, 1, 2, 5, 5, 8, 2, 0, 7, 6, 6, 0, 9, 1, 3, 4, 6, 7, 4, 0, 7,
    2, 2, 6, 5, 6, 2, 5, 2, 9, 1, 0, 3, 8, 3, 0, 4, 5, 6, 7, 3, 3, 7, 0, 3, 6,
    1, 3, 2, 8, 1, 2, 5, 1, 4, 5, 5, 1, 9, 1, 5, 2, 2, 8, 3, 6, 6, 8, 5, 1, 8,
    0, 6, 6, 4, 0, 6, 2, 5, 7, 2, 7, 5, 9, 5, 7, 6, 1, 4, 1, 8, 3, 4, 2, 5, 9,
    0, 3, 3, 2, 0, 3, 1, 2, 5, 3, 6, 3, 7, 9, 7, 8, 8, 0, 7, 0, 9, 1, 7, 1, 2,
    9, 5, 1, 6, 6, 0, 1, 5, 6, 2, 5, 1, 8, 1, 8, 9, 8, 9, 4, 0, 3, 5, 4, 5, 8,
    5, 6, 4, 7, 5, 8, 3, 0, 0, 7, 8, 1, 2, 5, 9, 0, 9, 4, 9, 4, 7, 0, 1, 7, 7,
    2, 9, 2, 8, 2, 3, 7, 9, 1, 5, 0, 3, 9, 0, 6, 2, 5, 4, 5, 4, 7, 4, 7, 3, 5,
    0, 8, 8, 6, 4, 6, 4, 1, 1, 8, 9, 5, 7, 5, 1, 9, 5, 3, 1, 2, 5, 2, 2, 7, 3,
    7, 3, 6, 7, 5, 4, 4, 3, 2, 3, 2, 0, 5, 9, 4, 7, 8, 7, 5, 9, 7, 6, 5, 6, 2,
    5, 1, 1, 3, 6, 8, 6, 8, 3, 7, 7, 2, 1, 6, 1, 6, 0, 2, 9, 7, 3, 9, 3, 7, 9,
    8, 8, 2, 8, 1, 2, 5, 5, 6, 8, 4, 3, 4, 1, 8, 8, 6, 0, 8, 0, 8, 0, 1, 4, 8,
    6, 9, 6, 8, 9, 9, 4, 1, 4, 0, 6, 2, 5, 2, 8, 4, 2, 1, 7, 0, 9, 4, 3, 0, 4,
    0, 4, 0, 0, 7, 4, 3, 4, 8, 4, 4, 9, 7, 0, 7, 0, 3, 1, 2, 5, 1, 4, 2, 1, 0,
    8, 5, 4, 7, 1, 5, 2, 0, 2, 0, 0, 3, 7, 1, 7, 4, 2, 2, 4, 8, 5, 3, 5, 1, 5,
    6, 2, 5, 7, 1, 0, 5, 4, 2, 7, 3, 5, 7, 6, 0, 1, 0, 0, 1, 8, 5, 8, 7, 1, 1,
    2, 4, 2, 6, 7, 5, 7, 8, 1, 2, 5, 3, 5, 5, 2, 7, 1, 3, 6, 7, 8, 8, 0, 0, 5,
    0, 0, 9, 2, 9, 3, 5, 5, 6, 2, 1, 3, 3, 7, 8, 9, 0, 6, 2, 5, 1, 7, 7, 6, 3,
    5, 6, 8, 3, 9, 4, 0, 0, 2, 5, 0, 4, 6, 4, 6, 7, 7, 8, 1, 0, 6, 6, 8, 9, 4,
    5, 3, 1, 2, 5, 8, 8, 8, 1, 7, 8, 4, 1, 9, 7, 0, 0, 1, 2, 5, 2, 3, 2, 3, 3,
    8, 9, 0, 5, 3, 3, 4, 4, 7, 2, 6, 5, 6, 2, 5, 4, 4, 4, 0, 8, 9, 2, 0, 9, 8,
    5, 0, 0, 6, 2, 6, 1, 6, 1, 6, 9, 4, 5, 2, 6, 6, 7, 2, 3, 6, 3, 2, 8, 1, 2,
    5, 2, 2, 2, 0, 4, 4, 6, 0, 4, 9, 2, 5, 0, 3, 1, 3, 0, 8, 0, 8, 4, 7, 2, 6,
    3, 3, 3, 6, 1, 8, 1, 6, 4, 0, 6, 2, 5, 1, 1, 1, 0, 2, 2, 3, 0, 2, 4, 6, 2,
    5, 1, 5, 6, 5, 4, 0, 4, 2, 3, 6, 3, 1, 6, 6, 8, 0, 9, 0, 8, 2, 0, 3, 1, 2,
    5, 5, 5, 5, 1, 1, 1, 5, 1, 2, 3, 1, 2, 5, 7, 8, 2, 7, 0, 2, 1, 1, 8, 1, 5,
    8, 3, 4, 0, 4, 5, 4, 1, 0, 1, 5, 6, 2, 5, 2, 7, 7, 5, 5, 5, 7, 5, 6, 1, 5,
    6, 2, 8, 9, 1, 3, 5, 1, 0, 5, 9, 0, 7, 9, 1, 7, 0, 2, 2, 7, 0, 5, 0, 7, 8,
    1, 2, 5, 1, 3, 8, 7, 7, 7, 8, 7, 8, 0, 7, 8, 1, 4, 4, 5, 6, 7, 5, 5, 2, 9,
    5, 3, 9, 5, 8, 5, 1, 1, 3, 5, 2, 5, 3, 9, 0, 6, 2, 5, 6, 9, 3, 8, 8, 9, 3,
    9, 0, 3, 9, 0, 7, 2, 2, 8, 3, 7, 7, 6, 4, 7, 6, 9, 7, 9, 2, 5, 5, 6, 7, 6,
    2, 6, 9, 5, 3, 1, 2, 5, 3, 4, 6, 9, 4, 4, 6, 9, 5, 1, 9, 5, 3, 6, 1, 4, 1,
    8, 8, 8, 2, 3, 8, 4, 8, 9, 6, 2, 7, 8, 3, 8, 1, 3, 4, 7, 6, 5, 6, 2, 5, 1,
    7, 3, 4, 7, 2, 3, 4, 7, 5, 9, 7, 6, 8, 0, 7, 0, 9, 4, 4, 1, 1, 9, 2, 4, 4,
    8, 1, 3, 9, 1, 9, 0, 6, 7, 3, 8, 2, 8, 1, 2, 5, 8, 6, 7, 3, 6, 1, 7, 3, 7,
    9, 8, 8, 4, 0, 3, 5, 4, 7, 2, 0, 5, 9, 6, 2, 2, 4, 0, 6, 9, 5, 9, 5, 3, 3,
    6, 9, 1, 4, 0, 6, 2, 5,
};

// wuffs_base__private_implementation__high_prec_dec__lshift_num_new_digits
// returns the number of additional decimal digits when left-shifting by shift.
//
// See below for preconditions.
static uint32_t  //
wuffs_base__private_implementation__high_prec_dec__lshift_num_new_digits(
    wuffs_base__private_implementation__high_prec_dec* h,
    uint32_t shift) {
  // Masking with 0x3F should be unnecessary (assuming the preconditions) but
  // it's cheap and ensures that we don't overflow the
  // wuffs_base__private_implementation__hpd_left_shift array.
  shift &= 63;

  uint32_t x_a = wuffs_base__private_implementation__hpd_left_shift[shift];
  uint32_t x_b = wuffs_base__private_implementation__hpd_left_shift[shift + 1];
  uint32_t num_new_digits = x_a >> 11;
  uint32_t pow5_a = 0x7FF & x_a;
  uint32_t pow5_b = 0x7FF & x_b;

  const uint8_t* pow5 =
      &wuffs_base__private_implementation__powers_of_5[pow5_a];
  uint32_t i = 0;
  uint32_t n = pow5_b - pow5_a;
  for (; i < n; i++) {
    if (i >= h->num_digits) {
      return num_new_digits - 1;
    } else if (h->digits[i] == pow5[i]) {
      continue;
    } else if (h->digits[i] < pow5[i]) {
      return num_new_digits - 1;
    } else {
      return num_new_digits;
    }
  }
  return num_new_digits;
}

// --------

// wuffs_base__private_implementation__high_prec_dec__rounded_integer returns
// the integral (non-fractional) part of h, provided that it is 18 or fewer
// decimal digits. For 19 or more digits, it returns UINT64_MAX. Note that:
//   - (1 << 53) is    9007199254740992, which has 16 decimal digits.
//   - (1 << 56) is   72057594037927936, which has 17 decimal digits.
//   - (1 << 59) is  576460752303423488, which has 18 decimal digits.
//   - (1 << 63) is 9223372036854775808, which has 19 decimal digits.
// and that IEEE 754 double precision has 52 mantissa bits.
//
// That integral part is rounded-to-even: rounding 7.5 or 8.5 both give 8.
//
// h's negative bit is ignored: rounding -8.6 returns 9.
//
// See below for preconditions.
static uint64_t  //
wuffs_base__private_implementation__high_prec_dec__rounded_integer(
    wuffs_base__private_implementation__high_prec_dec* h) {
  if ((h->num_digits == 0) || (h->decimal_point < 0)) {
    return 0;
  } else if (h->decimal_point > 18) {
    return UINT64_MAX;
  }

  uint32_t dp = (uint32_t)(h->decimal_point);
  uint64_t n = 0;
  uint32_t i = 0;
  for (; i < dp; i++) {
    n = (10 * n) + ((i < h->num_digits) ? h->digits[i] : 0);
  }

  bool round_up = false;
  if (dp < h->num_digits) {
    round_up = h->digits[dp] >= 5;
    if ((h->digits[dp] == 5) && (dp + 1 == h->num_digits)) {
      // We are exactly halfway. If we're truncated, round up, otherwise round
      // to even.
      round_up = h->truncated ||  //
                 ((dp > 0) && (1 & h->digits[dp - 1]));
    }
  }
  if (round_up) {
    n++;
  }

  return n;
}

// wuffs_base__private_implementation__high_prec_dec__small_xshift shifts h's
// number (where 'x' is 'l' or 'r' for left or right) by a small shift value.
//
// Preconditions:
//  - h is non-NULL.
//  - h->decimal_point is "not extreme".
//  - shift is non-zero.
//  - shift is "a small shift".
//
// "Not extreme" means within
// ±WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE.
//
// "A small shift" means not more than
// WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL.
//
// wuffs_base__private_implementation__high_prec_dec__rounded_integer and
// wuffs_base__private_implementation__high_prec_dec__lshift_num_new_digits
// have the same preconditions.

static void  //
wuffs_base__private_implementation__high_prec_dec__small_lshift(
    wuffs_base__private_implementation__high_prec_dec* h,
    uint32_t shift) {
  if (h->num_digits == 0) {
    return;
  }
  uint32_t num_new_digits =
      wuffs_base__private_implementation__high_prec_dec__lshift_num_new_digits(
          h, shift);
  uint32_t rx = h->num_digits - 1;                   // Read  index.
  uint32_t wx = h->num_digits - 1 + num_new_digits;  // Write index.
  uint64_t n = 0;

  // Repeat: pick up a digit, put down a digit, right to left.
  while (((int32_t)rx) >= 0) {
    n += ((uint64_t)(h->digits[rx])) << shift;
    uint64_t quo = n / 10;
    uint64_t rem = n - (10 * quo);
    if (wx < WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
      h->digits[wx] = (uint8_t)rem;
    } else if (rem > 0) {
      h->truncated = true;
    }
    n = quo;
    wx--;
    rx--;
  }

  // Put down leading digits, right to left.
  while (n > 0) {
    uint64_t quo = n / 10;
    uint64_t rem = n - (10 * quo);
    if (wx < WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
      h->digits[wx] = (uint8_t)rem;
    } else if (rem > 0) {
      h->truncated = true;
    }
    n = quo;
    wx--;
  }

  // Finish.
  h->num_digits += num_new_digits;
  if (h->num_digits >
      WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
    h->num_digits = WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION;
  }
  h->decimal_point += (int32_t)num_new_digits;
  wuffs_base__private_implementation__high_prec_dec__trim(h);
}

static void  //
wuffs_base__private_implementation__high_prec_dec__small_rshift(
    wuffs_base__private_implementation__high_prec_dec* h,
    uint32_t shift) {
  uint32_t rx = 0;  // Read  index.
  uint32_t wx = 0;  // Write index.
  uint64_t n = 0;

  // Pick up enough leading digits to cover the first shift.
  while ((n >> shift) == 0) {
    if (rx < h->num_digits) {
      // Read a digit.
      n = (10 * n) + h->digits[rx++];
    } else if (n == 0) {
      // h's number used to be zero and remains zero.
      return;
    } else {
      // Read sufficient implicit trailing zeroes.
      while ((n >> shift) == 0) {
        n = 10 * n;
        rx++;
      }
      break;
    }
  }
  h->decimal_point -= ((int32_t)(rx - 1));
  if (h->decimal_point <
      -WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE) {
    // After the shift, h's number is effectively zero.
    h->num_digits = 0;
    h->decimal_point = 0;
    h->negative = false;
    h->truncated = false;
    return;
  }

  // Repeat: pick up a digit, put down a digit, left to right.
  uint64_t mask = (((uint64_t)(1)) << shift) - 1;
  while (rx < h->num_digits) {
    uint8_t new_digit = ((uint8_t)(n >> shift));
    n = (10 * (n & mask)) + h->digits[rx++];
    h->digits[wx++] = new_digit;
  }

  // Put down trailing digits, left to right.
  while (n > 0) {
    uint8_t new_digit = ((uint8_t)(n >> shift));
    n = 10 * (n & mask);
    if (wx < WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DIGITS_PRECISION) {
      h->digits[wx++] = new_digit;
    } else if (new_digit > 0) {
      h->truncated = true;
    }
  }

  // Finish.
  h->num_digits = wx;
  wuffs_base__private_implementation__high_prec_dec__trim(h);
}

// --------

wuffs_base__result_f64  //
wuffs_base__parse_number_f64_special(wuffs_base__slice_u8 s,
                                     const char* fallback_status_repr) {
  do {
    uint8_t* p = s.ptr;
    uint8_t* q = s.ptr + s.len;

    for (; (p < q) && (*p == '_'); p++) {
    }
    if (p >= q) {
      goto fallback;
    }

    // Parse sign.
    bool negative = false;
    do {
      if (*p == '+') {
        p++;
      } else if (*p == '-') {
        negative = true;
        p++;
      } else {
        break;
      }
      for (; (p < q) && (*p == '_'); p++) {
      }
    } while (0);
    if (p >= q) {
      goto fallback;
    }

    bool nan = false;
    switch (p[0]) {
      case 'I':
      case 'i':
        if (((q - p) < 3) ||                     //
            ((p[1] != 'N') && (p[1] != 'n')) ||  //
            ((p[2] != 'F') && (p[2] != 'f'))) {
          goto fallback;
        }
        p += 3;

        if ((p >= q) || (*p == '_')) {
          break;
        } else if (((q - p) < 5) ||                     //
                   ((p[0] != 'I') && (p[0] != 'i')) ||  //
                   ((p[1] != 'N') && (p[1] != 'n')) ||  //
                   ((p[2] != 'I') && (p[2] != 'i')) ||  //
                   ((p[3] != 'T') && (p[3] != 't')) ||  //
                   ((p[4] != 'Y') && (p[4] != 'y'))) {
          goto fallback;
        }
        p += 5;

        if ((p >= q) || (*p == '_')) {
          break;
        }
        goto fallback;

      case 'N':
      case 'n':
        if (((q - p) < 3) ||                     //
            ((p[1] != 'A') && (p[1] != 'a')) ||  //
            ((p[2] != 'N') && (p[2] != 'n'))) {
          goto fallback;
        }
        p += 3;

        if ((p >= q) || (*p == '_')) {
          nan = true;
          break;
        }
        goto fallback;

      default:
        goto fallback;
    }

    // Finish.
    for (; (p < q) && (*p == '_'); p++) {
    }
    if (p != q) {
      goto fallback;
    }
    wuffs_base__result_f64 ret;
    ret.status.repr = NULL;
    ret.value = wuffs_base__ieee_754_bit_representation__to_f64(
        (nan ? 0x7FFFFFFFFFFFFFFF : 0x7FF0000000000000) |
        (negative ? 0x8000000000000000 : 0));
    return ret;
  } while (0);

fallback:
  do {
    wuffs_base__result_f64 ret;
    ret.status.repr = fallback_status_repr;
    ret.value = 0;
    return ret;
  } while (0);
}

wuffs_base__result_f64  //
wuffs_base__parse_number_f64(wuffs_base__slice_u8 s) {
  wuffs_base__private_implementation__high_prec_dec h;

  do {
    // powers converts decimal powers of 10 to binary powers of 2. For example,
    // (10000 >> 13) is 1. It stops before the elements exceed 60, also known
    // as WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL.
    static const uint32_t num_powers = 19;
    static const uint8_t powers[19] = {
        0,  3,  6,  9,  13, 16, 19, 23, 26, 29,  //
        33, 36, 39, 43, 46, 49, 53, 56, 59,      //
    };

    wuffs_base__status status =
        wuffs_base__private_implementation__high_prec_dec__parse(&h, s);
    if (status.repr) {
      return wuffs_base__parse_number_f64_special(s, status.repr);
    }

    // Handle zero and obvious extremes. The largest and smallest positive
    // finite f64 values are approximately 1.8e+308 and 4.9e-324.
    if ((h.num_digits == 0) || (h.decimal_point < -326)) {
      goto zero;
    } else if (h.decimal_point > 310) {
      goto infinity;
    }

    // Scale by powers of 2 until we're in the range [½ .. 1], which gives us
    // our exponent (in base-2). First we shift right, possibly a little too
    // far, ending with a value certainly below 1 and possibly below ½...
    const int32_t bias = -1023;
    int32_t exp2 = 0;
    while (h.decimal_point > 0) {
      uint32_t n = (uint32_t)(+h.decimal_point);
      uint32_t shift =
          (n < num_powers)
              ? powers[n]
              : WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL;

      wuffs_base__private_implementation__high_prec_dec__small_rshift(&h,
                                                                      shift);
      if (h.decimal_point <
          -WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE) {
        goto zero;
      }
      exp2 += (int32_t)shift;
    }
    // ...then we shift left, putting us in [½ .. 1].
    while (h.decimal_point <= 0) {
      uint32_t shift;
      if (h.decimal_point == 0) {
        if (h.digits[0] >= 5) {
          break;
        }
        shift = (h.digits[0] <= 2) ? 2 : 1;
      } else {
        uint32_t n = (uint32_t)(-h.decimal_point);
        shift = (n < num_powers)
                    ? powers[n]
                    : WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL;
      }

      wuffs_base__private_implementation__high_prec_dec__small_lshift(&h,
                                                                      shift);
      if (h.decimal_point >
          +WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__DECIMAL_POINT__RANGE) {
        goto infinity;
      }
      exp2 -= (int32_t)shift;
    }

    // We're in the range [½ .. 1] but f64 uses [1 .. 2].
    exp2--;

    // The minimum normal exponent is (bias + 1).
    while ((bias + 1) > exp2) {
      uint32_t n = (uint32_t)((bias + 1) - exp2);
      if (n > WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL) {
        n = WUFFS_BASE__PRIVATE_IMPLEMENTATION__HPD__SHIFT__MAX_INCL;
      }
      wuffs_base__private_implementation__high_prec_dec__small_rshift(&h, n);
      exp2 += (int32_t)n;
    }

    // Check for overflow.
    if ((exp2 - bias) >= 0x07FF) {  // (1 << 11) - 1.
      goto infinity;
    }

    // Extract 53 bits for the mantissa (in base-2).
    wuffs_base__private_implementation__high_prec_dec__small_lshift(&h, 53);
    uint64_t man2 =
        wuffs_base__private_implementation__high_prec_dec__rounded_integer(&h);

    // Rounding might have added one bit. If so, shift and re-check overflow.
    if ((man2 >> 53) != 0) {
      man2 >>= 1;
      exp2++;
      if ((exp2 - bias) >= 0x07FF) {  // (1 << 11) - 1.
        goto infinity;
      }
    }

    // Handle subnormal numbers.
    if ((man2 >> 52) == 0) {
      exp2 = bias;
    }

    // Pack the bits and return.
    uint64_t exp2_bits = (uint64_t)((exp2 - bias) & 0x07FF);  // (1 << 11) - 1.
    uint64_t bits = (man2 & 0x000FFFFFFFFFFFFF) |             // (1 << 52) - 1.
                    (exp2_bits << 52) |                       //
                    (h.negative ? 0x8000000000000000 : 0);    // (1 << 63).

    wuffs_base__result_f64 ret;
    ret.status.repr = NULL;
    ret.value = wuffs_base__ieee_754_bit_representation__to_f64(bits);
    return ret;
  } while (0);

zero:
  do {
    uint64_t bits = h.negative ? 0x8000000000000000 : 0;

    wuffs_base__result_f64 ret;
    ret.status.repr = NULL;
    ret.value = wuffs_base__ieee_754_bit_representation__to_f64(bits);
    return ret;
  } while (0);

infinity:
  do {
    uint64_t bits = h.negative ? 0xFFF0000000000000 : 0x7FF0000000000000;

    wuffs_base__result_f64 ret;
    ret.status.repr = NULL;
    ret.value = wuffs_base__ieee_754_bit_representation__to_f64(bits);
    return ret;
  } while (0);
}

// ---------------- Hexadecimal

size_t  //
wuffs_base__hexadecimal__decode2(wuffs_base__slice_u8 dst,
                                 wuffs_base__slice_u8 src) {
  size_t src_len2 = src.len / 2;
  size_t len = dst.len < src_len2 ? dst.len : src_len2;
  uint8_t* d = dst.ptr;
  uint8_t* s = src.ptr;
  size_t n = len;

  while (n--) {
    *d = (uint8_t)((wuffs_base__parse_number__hexadecimal_digits[s[0]] << 4) |
                   (wuffs_base__parse_number__hexadecimal_digits[s[1]] & 0x0F));
    d += 1;
    s += 2;
  }

  return len;
}

size_t  //
wuffs_base__hexadecimal__decode4(wuffs_base__slice_u8 dst,
                                 wuffs_base__slice_u8 src) {
  size_t src_len4 = src.len / 4;
  size_t len = dst.len < src_len4 ? dst.len : src_len4;
  uint8_t* d = dst.ptr;
  uint8_t* s = src.ptr;
  size_t n = len;

  while (n--) {
    *d = (uint8_t)((wuffs_base__parse_number__hexadecimal_digits[s[2]] << 4) |
                   (wuffs_base__parse_number__hexadecimal_digits[s[3]] & 0x0F));
    d += 1;
    s += 4;
  }

  return len;
}

// ---------------- Unicode and UTF-8

size_t  //
wuffs_base__utf_8__encode(wuffs_base__slice_u8 dst, uint32_t code_point) {
  if (code_point <= 0x7F) {
    if (dst.len >= 1) {
      dst.ptr[0] = (uint8_t)(code_point);
      return 1;
    }

  } else if (code_point <= 0x07FF) {
    if (dst.len >= 2) {
      dst.ptr[0] = (uint8_t)(0xC0 | ((code_point >> 6)));
      dst.ptr[1] = (uint8_t)(0x80 | ((code_point >> 0) & 0x3F));
      return 2;
    }

  } else if (code_point <= 0xFFFF) {
    if ((dst.len >= 3) && ((code_point < 0xD800) || (0xDFFF < code_point))) {
      dst.ptr[0] = (uint8_t)(0xE0 | ((code_point >> 12)));
      dst.ptr[1] = (uint8_t)(0x80 | ((code_point >> 6) & 0x3F));
      dst.ptr[2] = (uint8_t)(0x80 | ((code_point >> 0) & 0x3F));
      return 3;
    }

  } else if (code_point <= 0x10FFFF) {
    if (dst.len >= 4) {
      dst.ptr[0] = (uint8_t)(0xF0 | ((code_point >> 18)));
      dst.ptr[1] = (uint8_t)(0x80 | ((code_point >> 12) & 0x3F));
      dst.ptr[2] = (uint8_t)(0x80 | ((code_point >> 6) & 0x3F));
      dst.ptr[3] = (uint8_t)(0x80 | ((code_point >> 0) & 0x3F));
      return 4;
    }
  }

  return 0;
}

// wuffs_base__utf_8__byte_length_minus_1 is the byte length (minus 1) of a
// UTF-8 encoded code point, based on the encoding's initial byte.
//  - 0x00 is 1-byte UTF-8 (ASCII).
//  - 0x01 is the start of 2-byte UTF-8.
//  - 0x02 is the start of 3-byte UTF-8.
//  - 0x03 is the start of 4-byte UTF-8.
//  - 0x40 is a UTF-8 tail byte.
//  - 0x80 is invalid UTF-8.
//
// RFC 3629 (UTF-8) gives this grammar for valid UTF-8:
//    UTF8-1      = %x00-7F
//    UTF8-2      = %xC2-DF UTF8-tail
//    UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
//                  %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
//    UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
//                  %xF4 %x80-8F 2( UTF8-tail )
//    UTF8-tail   = %x80-BF
static const uint8_t wuffs_base__utf_8__byte_length_minus_1[256] = {
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x00 ..= 0x07.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x08 ..= 0x0F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x10 ..= 0x17.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x18 ..= 0x1F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x20 ..= 0x27.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x28 ..= 0x2F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x30 ..= 0x37.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x38 ..= 0x3F.

    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x40 ..= 0x47.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x48 ..= 0x4F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x50 ..= 0x57.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x58 ..= 0x5F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x60 ..= 0x67.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x68 ..= 0x6F.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x70 ..= 0x77.
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  // 0x78 ..= 0x7F.

    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x80 ..= 0x87.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x88 ..= 0x8F.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x90 ..= 0x97.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x98 ..= 0x9F.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0xA0 ..= 0xA7.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0xA8 ..= 0xAF.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0xB0 ..= 0xB7.
    0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0xB8 ..= 0xBF.

    0x80, 0x80, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,  // 0xC0 ..= 0xC7.
    0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,  // 0xC8 ..= 0xCF.
    0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,  // 0xD0 ..= 0xD7.
    0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01,  // 0xD8 ..= 0xDF.
    0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02,  // 0xE0 ..= 0xE7.
    0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02,  // 0xE8 ..= 0xEF.
    0x03, 0x03, 0x03, 0x03, 0x03, 0x80, 0x80, 0x80,  // 0xF0 ..= 0xF7.
    0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80,  // 0xF8 ..= 0xFF.
    // 0     1     2     3     4     5     6     7
    // 8     9     A     B     C     D     E     F
};

wuffs_base__utf_8__next__output  //
wuffs_base__utf_8__next(wuffs_base__slice_u8 s) {
  if (s.len == 0) {
    return wuffs_base__make_utf_8__next__output(0, 0);
  }
  uint32_t c = s.ptr[0];
  switch (wuffs_base__utf_8__byte_length_minus_1[c & 0xFF]) {
    case 0:
      return wuffs_base__make_utf_8__next__output(c, 1);

    case 1:
      if (s.len < 2) {
        break;
      }
      c = wuffs_base__load_u16le__no_bounds_check(s.ptr);
      if ((c & 0xC000) != 0x8000) {
        break;
      }
      c = (0x0007C0 & (c << 6)) | (0x00003F & (c >> 8));
      return wuffs_base__make_utf_8__next__output(c, 2);

    case 2:
      if (s.len < 3) {
        break;
      }
      c = wuffs_base__load_u24le__no_bounds_check(s.ptr);
      if ((c & 0xC0C000) != 0x808000) {
        break;
      }
      c = (0x00F000 & (c << 12)) | (0x000FC0 & (c >> 2)) |
          (0x00003F & (c >> 16));
      if ((c <= 0x07FF) || ((0xD800 <= c) && (c <= 0xDFFF))) {
        break;
      }
      return wuffs_base__make_utf_8__next__output(c, 3);

    case 3:
      if (s.len < 4) {
        break;
      }
      c = wuffs_base__load_u32le__no_bounds_check(s.ptr);
      if ((c & 0xC0C0C000) != 0x80808000) {
        break;
      }
      c = (0x1C0000 & (c << 18)) | (0x03F000 & (c << 4)) |
          (0x000FC0 & (c >> 10)) | (0x00003F & (c >> 24));
      if ((c <= 0xFFFF) || (0x110000 <= c)) {
        break;
      }
      return wuffs_base__make_utf_8__next__output(c, 4);
  }

  return wuffs_base__make_utf_8__next__output(
      WUFFS_BASE__UNICODE_REPLACEMENT_CHARACTER, 1);
}

size_t  //
wuffs_base__utf_8__longest_valid_prefix(wuffs_base__slice_u8 s) {
  // TODO: possibly optimize the all-ASCII case (4 or 8 bytes at a time).
  //
  // TODO: possibly optimize this by manually inlining the
  // wuffs_base__utf_8__next calls.
  size_t original_len = s.len;
  while (s.len > 0) {
    wuffs_base__utf_8__next__output o = wuffs_base__utf_8__next(s);
    if ((o.code_point > 0x7F) && (o.byte_length == 1)) {
      break;
    }
    s.ptr += o.byte_length;
    s.len -= o.byte_length;
  }
  return original_len - s.len;
}

size_t  //
wuffs_base__ascii__longest_valid_prefix(wuffs_base__slice_u8 s) {
  // TODO: possibly optimize this by checking 4 or 8 bytes at a time.
  uint8_t* original_ptr = s.ptr;
  uint8_t* p = s.ptr;
  uint8_t* q = s.ptr + s.len;
  for (; (p != q) && ((*p & 0x80) == 0); p++) {
  }
  return (size_t)(p - original_ptr);
}