patch: split fp_to_string.h into fptostring.h and fptostring.cpp

src/fptostring.cpp

@@ -0,0 +1,207 @@
+// SPDX-FileCopyrightText: 2024 Simon Gene Gottlieb
+// SPDX-License-Identifier: MIT
+
+#include "contrib/dragonbox.h"
+
+#include <array>
+#include <cassert>
+#include <cmath>
+#include <limits>
+#include <sstream>
+#include <tuple>
+
+namespace YAML {
+namespace detail {
+namespace fp_formatting {
+
+/**
+ * Converts a integer into its ASCII digits.
+ *
+ * @param begin/end - a buffer, must be at least 20bytes long
+ * @param value   - input value
+ * @param width   - minimum number of digits, fill with '0' to the left. Must be equal or smaller than the buffer size.
+ * @return      - number of digits filled into the buffer.
+ *
+ * Example:
+ * std::array<char, 20> buffer;
+ * auto ct = ConvertToChars(buffer.begin(), buffer.end(), 23, 3);
+ * assert(ct = 3);
+ * assert(buffer[0] == '0');
+ * assert(buffer[1] == '2');
+ * assert(buffer[2] == '3');
+ */
+int ConvertToChars(char* begin, char* end, size_t value, int width=1) {
+  assert(width >= 1);
+  assert(end >= begin);     // end must be after begin
+  assert(end-begin >= width); // Buffer must be large enough
+  assert(end-begin >= 20);  // 2^64 has 20digits, so at least 20 digits must be available
+
+  // count number of digits, and fill digits array accordingly
+  int digits_ct{};
+  while (value > 0) {
+    char c = value % 10 + '0';
+    value = value / 10;
+    digits_ct += 1;
+    *(end-digits_ct) = c;
+  }
+  while(digits_ct < width) {
+    assert(digits_ct < 64);
+    digits_ct += 1;
+    *(end-digits_ct) = '0';
+  }
+  // move data to the front of the array
+  std::memmove(begin, end-digits_ct, digits_ct);
+  return digits_ct;
+}
+
+/**
+ * Converts a float or double to a string.
+ *
+ * converts a value 'v' to a string. Uses dragonbox for formatting.
+ */
+template <typename T>
+std::string FpToString(T v, int precision = 0) {
+//  assert(precision > 0);
+  // hardcoded constant, at which exponent should switch to a scientific notation
+  int const lowerExponentThreshold = -5;
+  int const upperExponentThreshold =  (precision==0)?6:precision;
+  if (precision == 0) {
+    precision = 6;
+  }
+
+  // dragonbox/to_decimal does not handle value 0, inf, NaN
+  if (v == 0 || std::isinf(v) || std::isnan(v)) {
+    std::stringstream ss;
+    ss << v;
+    return ss.str();
+  }
+
+  auto r = jkj::dragonbox::to_decimal(v);
+
+  auto digits  = std::array<char, 20>{}; // max digits of size_t is 20.
+  auto digits_ct = ConvertToChars(digits.data(), digits.data() + digits.size(), r.significand);
+
+  // check if requested precision is lower than
+  // required digits for exact representation
+  if (digits_ct > precision) {
+    auto diff = digits_ct - precision;
+    r.exponent += diff;
+    digits_ct = precision;
+
+    // round numbers if required
+    if (digits[digits_ct] >= '5') {
+      int i{digits_ct-1};
+      digits[i] += 1;
+      while (digits[i] == '9'+1) {
+        digits_ct -= 1;
+        r.exponent += 1;
+        if (i > 0) {
+          digits[i-1] += 1;
+          i -= 1;
+        } else {
+          digits_ct = 1;
+          digits[0] = '1';
+          break;
+        }
+      }
+    }
+  }
+
+  std::array<char, 28> output_buffer; // max digits of size_t plus sign, a dot and 2 letters for 'e+' or 'e-' and 4 letters for the exponent
+  auto output_ptr = &output_buffer[0];
+
+  // print '-' symbol for negative numbers
+  if (r.is_negative) {
+    *(output_ptr++) = '-';
+  }
+
+  // exponent if only a single non-zero digit is before the decimal point
+  int const exponent = r.exponent + digits_ct - 1;
+
+  // case 1: scientific notation
+  if (exponent >= upperExponentThreshold || exponent <= lowerExponentThreshold) {
+    // print first digit
+    *(output_ptr++) = digits[0];
+
+    // print digits after decimal point
+    if (digits_ct > 1) {
+      *(output_ptr++) = '.';
+      // print significant numbers after decimal point
+      for (int i{1}; i < digits_ct; ++i) {
+         *(output_ptr++) = digits[i];
+      }
+    }
+    *(output_ptr++) = 'e';
+    *(output_ptr++) = (exponent>=0)?'+':'-';
+    auto exp_digits = std::array<char, 20>{};
+    auto exp_digits_ct = ConvertToChars(exp_digits.data(), exp_digits.data() + exp_digits.size(), std::abs(exponent), /*.precision=*/ 2);
+    for (int i{0}; i < exp_digits_ct; ++i) {
+      *(output_ptr++) = exp_digits[i];
+    }
+
+  // case 2: default notation
+  } else {
+    auto const digits_end   = digits.begin() + digits_ct;
+    auto digits_iter    = digits.begin();
+
+    // print digits before point
+    int const before_decimal_digits = digits_ct + r.exponent;
+    if (before_decimal_digits > 0) {
+      // print digits before point
+      for (int i{0}; i < std::min(before_decimal_digits, digits_ct); ++i) {
+        *(output_ptr++) = *(digits_iter++);
+      }
+      // print trailing zeros before point
+      for (int i{0}; i < before_decimal_digits - digits_ct; ++i) {
+        *(output_ptr++) = '0';
+      }
+
+    // print 0 before point if none where printed before
+    } else {
+      *(output_ptr++) = '0';
+    }
+
+    if (digits_iter != digits_end) {
+      *(output_ptr++) = '.';
+      // print 0 afer decimal point, to fill until first digits
+      int const after_decimal_zeros = -digits_ct - r.exponent;
+      for (int i{0}; i < after_decimal_zeros; ++i) {
+         *(output_ptr++) = '0';
+      }
+
+      // print significant numbers after decimal point
+      for (;digits_iter < digits_end; ++digits_iter) {
+         *(output_ptr++) = *digits_iter;
+      }
+    }
+  }
+  *output_ptr = '\0';
+  return std::string{&output_buffer[0], output_ptr};
+}
+
+}
+}
+
+std::string FpToString(float v, size_t precision) {
+  return detail::fp_formatting::FpToString(v, precision);
+}
+
+std::string FpToString(double v, size_t precision) {
+  return detail::fp_formatting::FpToString(v, precision);
+}
+
+/**
+ * dragonbox only works for floats/doubles not long double
+ */
+std::string FpToString(long double v, size_t precision) {
+  std::stringstream ss;
+  ss.imbue(std::locale("C"));
+  if (precision == 0) {
+     precision = std::numeric_limits<long double>::max_digits10;
+  }
+  ss.precision(precision);
+  ss << v;
+  return ss.str();
+}
+
+}