Implement some formatting options in Grisu

include/fmt/format-inl.h

@@ -364,12 +364,14 @@ FMT_FUNC fp get_cached_power(int min_exponent, int &pow10_exponent) {
   return fp(data::POW10_SIGNIFICANDS[index], data::POW10_EXPONENTS[index]);
 }
 
-// Writes the exponent exp in the form "[-]d{1,3}" to buffer.
+// Writes the exponent exp in the form "[+-]d{1,3}" to buffer.
 FMT_FUNC char *write_exponent(char *buffer, int exp) {
   FMT_ASSERT(-1000 < exp && exp < 1000, "exponent out of range");
   if (exp < 0) {
     *buffer++ = '-';
     exp = -exp;
+  } else {
+    *buffer++ = '+';
   }
   if (exp >= 100) {
     *buffer++ = '0' + static_cast<char>(exp / 100);
@@ -446,25 +448,56 @@ FMT_FUNC void grisu2_gen_digits(
 
 // Prettifies the output of the Grisu2 algorithm.
 // The number is given as v = buffer * 10^exp.
-FMT_FUNC void grisu2_prettify(char *buffer, size_t &size, int exp) {
+FMT_FUNC void grisu2_prettify(char *buffer, size_t &size, int exp, char type,
+                              int precision, bool print_decimal_point) {
+  int int_size = static_cast<int>(size);
   // 10^(full_exp - 1) <= v <= 10^full_exp.
-  int full_exp = static_cast<int>(size) + exp;
-  // Insert a decimal point after the first digit and add an exponent.
-  std::memmove(buffer + 2, buffer + 1, size - 1);
-  buffer[1] = '.';
-  char *p = buffer + size + 1;
-  *p++ = 'e';
-  size = to_unsigned(write_exponent(p, full_exp - 1) - buffer);
+  int full_exp = int_size + exp;
+  if (int_size <= full_exp && full_exp <= 21) {
+    // 1234e7 -> 12340000000
+    char *p = std::uninitialized_fill_n(
+          buffer + int_size, full_exp - int_size, '0');
+    if (print_decimal_point && int_size < precision) {
+      *p++ = '.';
+      p = std::uninitialized_fill_n(p, precision - int_size, '0');
+    }
+    size = to_unsigned(p - buffer);
+  } else if (0 < full_exp && full_exp <= 21) {
+    // 1234e-2 -> 12.34
+    size_t fractional_size = int_size - full_exp;
+    std::memmove(buffer + full_exp + 1, buffer + full_exp, fractional_size);
+    buffer[full_exp] = '.';
+    if (type == 'f' && fractional_size < precision) {
+      size_t num_zeros = precision - fractional_size;
+      std::uninitialized_fill_n(buffer + size + 1, num_zeros, '0');
+      size += num_zeros;
+    }
+    ++size;
+  } else if (-6 < full_exp && full_exp <= 0) {
+    // 1234e-6 -> 0.001234
+    int offset = 2 - full_exp;
+    std::memmove(buffer + offset, buffer, size);
+    buffer[0] = '0';
+    buffer[1] = '.';
+    std::uninitialized_fill_n(buffer + 2, -full_exp, '0');
+    size = to_unsigned(int_size + offset);
+  } else {
+    // Insert a decimal point after the first digit and add an exponent.
+    std::memmove(buffer + 2, buffer + 1, size - 1);
+    buffer[1] = '.';
+    char *p = buffer + size + 1;
+    *p++ = 'e';
+    size = to_unsigned(write_exponent(p, full_exp - 1) - buffer);
+  }
 }
 
-// Formats value using Grisu2 algorithm. Grisu2 doesn't give any guarantees on
-// the shortness of the result.
-FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size) {
+FMT_FUNC void grisu2_format_positive(double value, char *buffer, size_t &size,
+                                     int &dec_exp) {
+  FMT_ASSERT(value > 0, "value is nonpositive");
   fp fp_value(value);
   fp lower, upper;  // w^- and w^+ in the Grisu paper.
   fp_value.compute_boundaries(lower, upper);
   // Find a cached power of 10 close to 1 / upper.
-  int dec_exp = 0;  // K in Grisu.
   const int min_exp = -60;  // alpha in Grisu.
   auto dec_pow = get_cached_power(  // \tilde{c}_{-k} in Grisu.
       min_exp - (upper.e + fp::significand_size), dec_exp);
@@ -477,7 +510,27 @@ FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size) {
   --scaled_upper.f;  // \tilde{M}^+ - 1 ulp -> M^+_{\downarrow}.
   uint64_t delta = scaled_upper.f - scaled_lower.f;
   grisu2_gen_digits(scaled_value, scaled_upper, delta, buffer, size, dec_exp);
-  grisu2_prettify(buffer, size, dec_exp);
+}
+
+// Formats value using Grisu2 algorithm. Grisu2 doesn't give any guarantees on
+// the shortness of the result.
+FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size, char type,
+                            int precision, bool print_decimal_point) {
+  int dec_exp = 0;  // K in Grisu.
+  if (value != 0) {
+    grisu2_format_positive(value, buffer, size, dec_exp);
+  } else {
+    *buffer = '0';
+    size = 1;
+  }
+  if (precision < 0)
+    precision = 6;
+  if (size > precision) {
+    // TODO: round instead of truncating
+    dec_exp += size - precision;
+    size = precision;
+  }
+  grisu2_prettify(buffer, size, dec_exp, type, precision, print_decimal_point);
 }
 }  // namespace internal
 

include/fmt/format.h

@@ -367,7 +367,8 @@ FMT_API fp get_cached_power(int min_exponent, int &pow10_exponent);
 
 // Formats value using Grisu2 algorithm:
 // https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
-FMT_API void grisu2_format(double value, char *buffer, size_t &size);
+FMT_API void grisu2_format(double value, char *buffer, size_t &size, char type,
+                           int precision, bool print_decimal_point);
 
 template <typename Allocator>
 typename Allocator::value_type *allocate(Allocator& alloc, std::size_t n) {
@@ -2949,12 +2950,12 @@ void basic_writer<Range>::write_double(T value, const format_specs &spec) {
   basic_memory_buffer<char_type> buffer;
   if (internal::const_check(FMT_USE_GRISU && sizeof(T) <= sizeof(double) &&
       std::numeric_limits<double>::is_iec559)) {
-    // The max size = 10 (hi) + 20 (lo) + 5 (exp).
-    enum { BUF_SIZE = 35 };
-    char buf[BUF_SIZE];
+    char buf[100]; // TODO: correct buffer size
     size_t size = 0;
-    internal::grisu2_format(static_cast<double>(value), buf, size);
-    FMT_ASSERT(size <= BUF_SIZE, "buffer overflow");
+    internal::grisu2_format(
+          static_cast<double>(value), buf, size, static_cast<char>(spec.type()),
+          spec.precision(), spec.flag(HASH_FLAG));
+    FMT_ASSERT(size <= 100, "buffer overflow");
     buffer.append(buf, buf + size); // TODO: avoid extra copy
   } else {
     format_specs normalized_spec(spec);