Add timezone offset parsing and handle Clang compatibility for charconv (#6)

* Refactor parsing functions to return std::error_code and simplify error handling; update tests for ISO 8601 parsing.

* Refactor parsing functions to improve readability and maintainability; add free parsing functions for year, month, day, hour, minute, second, and fraction.

* Add timezone offset parsing and handle Clang compatibility for charconv

Author: mguludag

README.md

@@ -2,7 +2,7 @@
 
 parse date and times with {fmt} style into`std::chrono::time_point` {WIP}
 
-## [Usage](https://godbolt.org/z/nb3qdMYza)
+## [Usage](https://godbolt.org/z/PE3s1Y35d)
 
 ```C++
 #include "mgutility/chrono/parse.hpp"

include/mgutility/_common/definitions.hpp

@@ -82,4 +82,15 @@ SOFTWARE.
 #define MGUTILITY_CNSTEVL
 #endif
 
+/**
+ * @brief Detects if the compiler is Clang and its version is less than 7.
+ *
+ * If the compiler is Clang and its major version is less than 7,
+ * CLANG_NO_CHARCONV is defined. This is used to indicate that the
+ * <charconv> header is not fully supported in these versions of Clang.
+ */
+#if defined(__clang__) && __clang_major__ < 7
+#define CLANG_NO_CHARCONV
+#endif
+
 #endif // MGUTILITY_COMMON_DEFINITIONS_HPP

include/mgutility/chrono/parse.hpp

@@ -250,6 +250,48 @@ MGUTILITY_CNSTXPR auto parse_fraction(detail::tm &result, string_view date_str,
   return error;
 }
 
+MGUTILITY_CNSTXPR auto parse_timezone_offset(detail::tm &result, string_view date_str,
+                       uint32_t &next) -> std::errc {
+  std::errc error{};
+  if (--next < date_str.size() && date_str[next] == 'Z') {
+    handle_timezone(result, 0);
+    return error;
+  }
+
+  if (next >= date_str.size() || (date_str[next] != '+' && date_str[next] != '-')) {
+    return std::errc::invalid_argument;
+  }
+
+  char sign = date_str[next++];
+  int32_t hour = 0, minute = 0;
+
+  // Parse hour part (2 digits)
+  error = parse_integer(hour, date_str, 2, next);
+  if (error != std::errc{}) {
+    return error;
+  }
+
+  // Optional colon
+  if (next < date_str.size() && date_str[next] == ':') {
+    ++next;
+  }
+
+  // Parse minute part (2 digits)
+  error = parse_integer(minute, date_str, 2, next);
+  if (error != std::errc{}) {
+    return error;
+  }
+
+  int32_t offset = hour * 100 + minute;
+  error = check_range(offset, 0, 1200);
+  if (error != std::errc{}) {
+    return error;
+  }
+
+  handle_timezone(result, sign == '+' ? -offset : offset);
+  return error;
+}
+
 /**
  * @brief Parses a date and time string according to a specified format.
  *
@@ -367,55 +409,9 @@ MGUTILITY_CNSTXPR auto get_time(detail::tm &result, string_view format,
         }
         break;
       case 'z': { // Timezone offset (+/-HHMM)
-        if (*(date_str.begin() + next - 1) != 'Z') {
-          char sign{};
-          auto diff{0};
-          for (auto j{next - 1}; j < date_str.size(); ++j) {
-            if (date_str[j] == '-' || date_str[j] == '+') {
-              sign = date_str[j];
-              diff = j - next + 1;
-              break;
-            }
-          }
-          auto hour_offset_str =
-              string_view{date_str.data() + next + diff, date_str.size() - 1};
-          auto pos = hour_offset_str.find(':');
-          auto offset{0};
-          if (pos < 3 && pos > 0) {
-            next = 0;
-            int32_t hour_offset{0};
-            int32_t min_offset{0};
-            error = parse_integer(hour_offset, hour_offset_str, 2, next);
-            if (error != std::errc{}) {
-              return error;
-            }
-            error = parse_integer(min_offset, hour_offset_str, 2, next);
-            if (error != std::errc{}) {
-              return error;
-            }
-            offset = hour_offset * 100 + min_offset;
-          } else {
-            if (date_str.size() - next > 4 + diff) {
-              return std::errc::invalid_argument;
-            }
-            error = parse_integer(offset, date_str, date_str.size() - next,
-                                  next, diff);
-            if (error != std::errc{}) {
-              return error;
-            }
-          }
-          error = check_range(offset, 0, 1200);
-          if (error != std::errc{}) {
-            return error;
-          }
-          switch (sign) {
-          case '+':
-            handle_timezone(result, offset * -1);
-            break;
-          case '-':
-            handle_timezone(result, offset);
-            break;
-          }
+        error = parse_timezone_offset(result, date_str, next);
+        if (error != std::errc{}) {
+          return error;
         }
       } break;
       default:

include/mgutility/std/charconv.hpp

@@ -26,13 +26,13 @@ SOFTWARE.
 #include "mgutility/_common/definitions.hpp"
 #include <system_error>
 
-#if MGUTILITY_CPLUSPLUS >= 201703L
+#if MGUTILITY_CPLUSPLUS >= 201703L && !defined(CLANG_NO_CHARCONV)
 #include <charconv>
 #endif
 
 namespace mgutility {
 
-#if MGUTILITY_CPLUSPLUS < 201703L
+#if MGUTILITY_CPLUSPLUS < 201703L || defined(CLANG_NO_CHARCONV)
 
 /**
  * @brief Result structure for from_chars function.