Improve ISO week-base-year formatter

include/fmt/chrono.h

@@ -1425,10 +1425,37 @@ template <typename FormatContext, typename OutputIt> struct tm_formatter {
     }
     return {q, r};
   }
+
+  // Algorithm:
+  // https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date
+  struct iso_weak {
+    int year;
+    int woy;
+  };
+  auto tm_iso_year_weaks(int year) -> int {
+    int p = (year + year / 4 - year / 100 + year / 400) % daysperweek;
+    int year_1 = year - 1;
+    int p_1 = (year_1 + year_1 / 4 - year_1 / 100 + year_1 / 400) % daysperweek;
+    return 52 + ((p == 4 || p_1 == 3) ? 1 : 0);
+  }
+  auto tm_iso_weak() -> iso_weak {
+    auto year = tm_year();
+    int w = (tm.tm_yday + 10 - (tm.tm_wday == 0 ? daysperweek : tm.tm_wday)) /
+            daysperweek;
+    if (w < 1) {
+      return {year - 1, tm_iso_year_weaks(year - 1)};
+    } else if (w > tm_iso_year_weaks(year)) {
+      return {year + 1, 1};
+    } else {
+      return {year, w};
+    }
+  }
+
   auto tm_hour12() const -> decltype(tm.tm_hour) {
     auto hour = tm.tm_hour % 12;
     return hour == 0 ? 12 : hour;
   }
+
   void write1(size_t value) { *out++ = detail::digits2(value)[1]; }
   void write2(size_t value) {
     out = std::copy_n(detail::digits2(value), 2, out);
@@ -1575,19 +1602,16 @@ template <typename FormatContext, typename OutputIt> struct tm_formatter {
   }
   void on_iso_week_of_year(numeric_system ns) {
     if (ns == numeric_system::standard) {
-      // TODO: Optimization
+      write2(detail::to_unsigned(tm_iso_weak().woy));
-      format_localized('V');
     } else {
       format_localized('V', 'O');
     }
   }
   void on_iso_week_based_year() {
-    // TODO: Optimization
+    out = detail::write<char_type>(out, tm_iso_weak().year);
-    format_localized('G');
   }
   void on_iso_week_based_year_last2() {
-    // TODO: Optimization
+    write2(detail::to_unsigned(tm_split_year(tm_iso_weak().year).lower));
-    format_localized('g');
   }
   void on_day_of_year() {
     auto yday = tm.tm_yday + 1;

test/chrono-test.cc

@@ -7,6 +7,8 @@
 
 #include "fmt/chrono.h"
 
+#include <time.h>
+
 #include <vector>
 
 #include "gtest-extra.h"  // EXPECT_THROW_MSG
@@ -57,6 +59,42 @@ TEST(chrono_test, format_tm) {
   EXPECT_EQ(fmt::format("{:%D}", tm), "04/25/16");
   EXPECT_EQ(fmt::format("{:%F}", tm), "2016-04-25");
   EXPECT_EQ(fmt::format("{:%T}", tm), "11:22:33");
+
+  // for week on the year
+  // https://www.cl.cam.ac.uk/~mgk25/iso-time.html
+  std::vector<std::string> str_tm_list = {
+      "1975-12-29",  // W01
+      "1977-01-02",  // W53
+      "1999-12-27",  // W52
+      "1999-12-31",  // W52
+      "2000-01-01",  // W52
+      "2000-01-02",  // W52
+      "2000-01-03",  // W1
+  };
+  std::vector<std::string> spec_list = {"%G", "%g", "%V"};
+  for (const auto& str_tm : str_tm_list) {
+    tm = std::tm();
+
+    // GCC 4 does not support std::get_time
+    // MSVC dows not support POSIX strptime
+#ifdef _WIN32
+    std::istringstream ss(str_tm);
+    ss >> std::get_time(&tm, "%Y-%m-%d");
+#else
+    strptime(str_tm.c_str(), "%Y-%m-%d", &tm);
+#endif
+    // Because std::get_time doesn't calculate tm_yday, tm_wday, etc.
+    tm.tm_isdst = 0;
+    auto t = std::mktime(&tm);
+    tm = *std::localtime(&t);
+
+    for (const auto& spec : spec_list) {
+      char output[256] = {};
+      std::strftime(output, sizeof(output), spec.c_str(), &tm);
+      auto fmt_spec = std::string("{:").append(spec).append("}");
+      EXPECT_EQ(output, fmt::format(fmt::runtime(fmt_spec), tm));
+    }
+  }
 }
 
 // MSVC: