From a13326ea492c8f137632679cb7c1942dc254efc0 Mon Sep 17 00:00:00 2001
From: xanthospap <xanthos.papanikolaou@gmail.com>
Date: Wed, 3 Jan 2024 00:15:00 +0200
Subject: [PATCH] var

---
 src/cdatetime.hpp      |  3 +++
 src/datetime_tops.hpp  | 13 +++++++++++++
 src/datetime_write.hpp |  1 +
 src/dtdatetime.hpp     | 25 ++++++++++++++-----------
 4 files changed, 31 insertions(+), 11 deletions(-)

diff --git a/src/cdatetime.hpp b/src/cdatetime.hpp
index a8f0346..6b6c30c 100644
--- a/src/cdatetime.hpp
+++ b/src/cdatetime.hpp
@@ -37,6 +37,9 @@ constexpr const double MJD0_JD = 2400000.5e0;
 /** TT minus TAI in [sec] */
 constexpr const double TT_MINUS_TAI = 32.184e0;
 
+/** TT minus TAI in [nsec] */
+constexpr const long TT_MINUS_TAI_IN_NANOSEC = 32184 * 1'000'000L;
+
 } /* namespace dso */
 
 #endif
diff --git a/src/datetime_tops.hpp b/src/datetime_tops.hpp
index 8b3a4e7..dbb3922 100644
--- a/src/datetime_tops.hpp
+++ b/src/datetime_tops.hpp
@@ -415,6 +415,18 @@ template <typename Ssrc, typename Strg,
           typename = std::enable_if_t<Strg::is_of_sec_type>>
 #endif
 constexpr Strg cast_to(Ssrc s) noexcept {
+  /* underlying int type of the S with the highest resolution */
+  using SecIntType = std::conditional_t<
+      (Ssrc::template sec_factor<long>() > Strg::template sec_factor<long>()),
+      typename Ssrc::underlying_type, typename Strg::underlying_type>;
+
+  if constexpr (Strg::template sec_factor<long>() >=
+                Ssrc::template sec_factor<long>()) {
+    constexpr const SecIntType factor =
+        Strg::template sec_factor<SecIntType>() /
+        Ssrc::template sec_factor<SecIntType>();
+    return Strg(s.__member_ref__() * factor);
+  } else {
   // this is tricky! We must first compute the numerator and then the fraction.
   // why? check this out
   // seconds _s1 = cast_to<milliseconds, seconds>(milliseconds{2000L});
@@ -422,6 +434,7 @@ constexpr Strg cast_to(Ssrc s) noexcept {
   // (2000*1)/1000 = 2 which is correct
   const auto numerator = s.__member_ref__() * Strg::template sec_factor<long>();
   return Strg(numerator / Ssrc::template sec_factor<long>());
+  }
 }
 
 } /* namespace dso */
diff --git a/src/datetime_write.hpp b/src/datetime_write.hpp
index ca0839a..ba921cd 100644
--- a/src/datetime_write.hpp
+++ b/src/datetime_write.hpp
@@ -149,6 +149,7 @@ const char *to_char(const hms_time<S> &hms, char *buffer) {
  */
 template <YMDFormat FD, HMSFormat FT, typename S>
 const char *to_char(const datetime<S> &d, char *buffer) {
+  printf("\tNote that the date is: %ld %ld\n", d.imjd().as_underlying_type(), d.sec().as_underlying_type());
   /* write date to buffer */
   ymd_date ymd(d.as_ymd());
   if (SpitDate<FD>::spit(ymd, buffer) != SpitDate<FD>::numChars) {
diff --git a/src/dtdatetime.hpp b/src/dtdatetime.hpp
index ef6fcc1..712e385 100644
--- a/src/dtdatetime.hpp
+++ b/src/dtdatetime.hpp
@@ -402,6 +402,11 @@ class datetime {
    */
   constexpr S sec() const noexcept { return m_sec; }
 
+  /** Seconds in day as fractional days */
+  double fractional_days() const noexcept {
+    return dso::to_fractional_days<S>(m_sec);
+  }
+
   /** Operator '+' where the right-hand-side is an interval.
    * Note that the addition here is algebraic, i.e. the interval is added to
    * or subtracted from the instance, depending on its sign.
@@ -585,8 +590,8 @@ class datetime {
    * \f$ TT = TAI + ΔT \$ where \f$ ΔT = TT - TAI = 32.184 [sec] \f$
    */
   constexpr datetime<S> tai2tt() const noexcept {
-    constexpr const SecIntType dtat = static_cast<SecIntType>(
-        TT_MINUS_TAI * S::template sec_factor<double>());
+    constexpr const S dtat =
+        dso::cast_to<nanoseconds, S>(nanoseconds(TT_MINUS_TAI_IN_NANOSEC));
     return datetime(m_mjd, m_sec + dtat);
   }
 
@@ -606,19 +611,17 @@ class datetime {
    * The two time scales are connected by the formula:
    * \f$ TAI = GPSTime + 19 [sec] \f$
    */
-  constexpr datetime<S> tai2gps() const noexcept {
-    constexpr const SecIntType dt =
-        static_cast<SecIntType>(19 * S::template sec_factor<SecIntType>());
-    return datetime(m_mjd, m_sec - dt);
+  [[nodiscard]] constexpr datetime<S> tai2gps() const noexcept {
+    return datetime(m_mjd, m_sec - dso::cast_to<seconds, S>(seconds(19)));
   }
 
-  constexpr datetime<S> gps2tai() const noexcept {
-    constexpr const SecIntType dt =
-        static_cast<SecIntType>(19 * S::template sec_factor<SecIntType>());
-    return datetime(m_mjd, m_sec + dt);
+  [[nodiscard]] constexpr datetime<S> gps2tai() const noexcept {
+    return datetime(m_mjd, m_sec + dso::cast_to<seconds, S>(seconds(19)));
   }
 
-  constexpr datetime<S> gps2tt() const noexcept { return gps2tai().tai2tt(); }
+  [[nodiscard]] constexpr datetime<S> gps2tt() const noexcept {
+    return gps2tai().tai2tt();
+  }
 
 private:
   /** @brief Add any second type T where S is of higher resolution than T