[gnc-datetime] add boost UK/US/ISO date parsers

Thus "30 Sep 2024" and "30 September 2024" are accepted.
But ICU's "30 Sept 2024" is rightfully rejected.

libgnucash/engine/gnc-datetime.cpp

@@ -77,13 +77,16 @@ static constexpr auto ticks_per_second = INT64_C(1000000);
 static constexpr auto ticks_per_second = INT64_C(1000000000);
 #endif
 
-/* Vector of date formats understood by gnucash and corresponding regex
- * to parse each from an external source
+/* Vector of date formats understood by gnucash and corresponding
+ * boost string->date or regex to parse each from an external source
  * Note: while the format names are using a "-" as separator, the
  * regexes will accept any of "-/.' " and will also work for dates
  * without separators.
  */
 const std::vector<GncDateFormat> GncDate::c_formats ({
+ GncDateFormat { N_("UK date"), boost::gregorian::from_uk_string },
+ GncDateFormat { N_("US date"), boost::gregorian::from_us_string },
+ GncDateFormat { N_("ISO date"), boost::gregorian::from_string },
  GncDateFormat {
  N_("y-m-d"),
  "(?:" // either y-m-d
@@ -617,6 +620,12 @@ GncDateImpl::GncDateImpl(const std::string str, const std::string fmt) :
  if (iter == GncDate::c_formats.cend())
  throw std::invalid_argument(N_("Unknown date format specifier passed as argument."));
 
+ if (iter->m_str_to_date)
+ {
+ m_greg = (*iter->m_str_to_date)(str);
+ return;
+ }
+
  boost::regex r(iter->m_re);
  boost::smatch what;
  if(!boost::regex_search(str, what, r)) // regex didn't find a match

libgnucash/engine/gnc-datetime.hpp

@@ -29,6 +29,10 @@
 #include <memory>
 #include <string>
 #include <vector>
+#include <functional>
+#include <optional>
+
+#include <boost/date_time/gregorian/gregorian.hpp>
 
 typedef struct
 {
@@ -172,6 +176,8 @@ class GncDateTime
  * GncDate::c_formats class variable and work with those.
  */
 
+using StringToDate = std::function<boost::gregorian::date(const std::string&)>;
+
 class GncDateFormat
 {
 public:
@@ -182,13 +188,16 @@ class GncDateFormat
  */
  GncDateFormat (const char* fmt, const char* re) :
  m_fmt(fmt), m_re(re) {}
+ GncDateFormat (const char* fmt, StringToDate str_to_date) :
+ m_fmt(fmt), m_str_to_date(str_to_date) {}
  /** A string representing the format. */
  const std::string m_fmt;
 private:
  /** Regular expression associated with the format string. This is to and
  * only be used internally by the gnc-datetime code.
  */
  const std::string m_re;
+ std::optional<StringToDate> m_str_to_date;
 
  friend class GncDateImpl;
 };