Taking a float and adding hundreds of zeros, you may get a truncated value that is just one over the desired value (bug fix) (#1388)

* Found a bug where if take some float and add many zeros, you may get a truncated value that is just one over the desired value.

src/from_chars.cpp

@@ -98,7 +98,20 @@ decimal parse_decimal(const char *&p) noexcept {
     }
     answer.decimal_point = int32_t(first_after_period - p);
   }
-
+  if(answer.num_digits > 0) {
+    const char *preverse = p - 1;
+    int32_t trailing_zeros = 0;
+    while ((*preverse == '0') || (*preverse == '.')) {
+      if(*preverse == '0') { trailing_zeros++; };
+      --preverse;
+    }
+    answer.decimal_point += int32_t(answer.num_digits);
+    answer.num_digits -= uint32_t(trailing_zeros);
+  }
+  if(answer.num_digits > max_digits ) {
+    answer.num_digits = max_digits;
+    answer.truncated = true;
+  }
   if (('e' == *p) || ('E' == *p)) {
     ++p;
     bool neg_exp = false;
@@ -118,11 +131,6 @@ decimal parse_decimal(const char *&p) noexcept {
     }
     answer.decimal_point += (neg_exp ? -exp_number : exp_number);
   }
-  answer.decimal_point += answer.num_digits;
-  if(answer.num_digits > max_digits ) {
-    answer.num_digits = max_digits;
-    answer.truncated = true;
-  }
   return answer;
 }
 

tests/basictests.cpp

@@ -280,6 +280,12 @@ namespace number_tests {
     std::cout << std::dec;
     return true;
   }
+  bool truncated_borderline() {
+    std::cout << __func__ << std::endl;
+    std::string round_to_even = "9007199254740993.0";
+    for(size_t i = 0; i < 1000; i++) { round_to_even += "0"; }
+    return basic_test_64bit(round_to_even,9007199254740992);
+  }
 
   bool specific_tests() {
     std::cout << __func__ << std::endl;
@@ -299,7 +305,8 @@ namespace number_tests {
   }
 
   bool run() {
-    return specific_tests() &&
+    return truncated_borderline() &&
+           specific_tests() &&
            ground_truth() &&
            small_integers() &&
            powers_of_two() &&

tests/random_string_number_tests.cpp

@@ -119,9 +119,9 @@ bool check_float(double result, const char *buf) {
     return false;
   }
   if (expected != result) {
-    fprintf(stderr, "parsed %.128e from \n", result);
-    fprintf(stderr, "       %.32s whereas strtod gives\n", buf);
-    fprintf(stderr, "       %.128e,", expected);
+    std::cerr << std::hexfloat << " parsed " << result << " from "
+              << buf << " whereas strtod gives " << expected << std::endl;
+    std::cerr << std::defaultfloat;
     return false;
   }
   return true;