From 0598a6d315ecadc5dd58cd8f5dde7c603c3f493c Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Mon, 30 Mar 2020 20:47:38 +0300
Subject: Add daytime() function

---
 libbutl/timestamp.cxx      | 28 ++++++++++++++++++++++++++++
 libbutl/timestamp.mxx      | 10 ++++++++++
 tests/timestamp/driver.cxx | 12 ++++++++++++
 3 files changed, 50 insertions(+)

diff --git a/libbutl/timestamp.cxx b/libbutl/timestamp.cxx
index 296e070..0242758 100644
--- a/libbutl/timestamp.cxx
+++ b/libbutl/timestamp.cxx
@@ -662,4 +662,32 @@ namespace butl
     return system_clock::from_time_t (time) +
       chrono::duration_cast<duration> (t.second);
   }
+
+  duration
+  daytime (timestamp t)
+  {
+    // Convert the time from Epoch to the local time.
+    //
+    time_t time (system_clock::to_time_t (t));
+
+    std::tm tm;
+    if (details::localtime (&time, &tm) == nullptr)
+      throw_generic_error (errno);
+
+    // Roll the time back to the latest midnight.
+    //
+    tm.tm_hour = 0;
+    tm.tm_min = 0;
+    tm.tm_sec = 0;
+
+    // Convert the local midnight to time from Epoch.
+    //
+    time = mktime (&tm);
+    if (time == -1)
+      throw_generic_error (errno);
+
+    // Note: preserves the accuracy of the original time.
+    //
+    return t - system_clock::from_time_t (time);
+  }
 }
diff --git a/libbutl/timestamp.mxx b/libbutl/timestamp.mxx
index f4a06f9..9525ec0 100644
--- a/libbutl/timestamp.mxx
+++ b/libbutl/timestamp.mxx
@@ -194,4 +194,14 @@ LIBBUTL_MODEXPORT namespace butl
                const char* format,
                bool local,
                const char** end = nullptr);
+
+  // Rebase a time point from UNIX epoch to midnight in the local time zone
+  // (so the returned duration is always less than 24 hours).
+  //
+  // Specifically, convert the time point from Epoch to the local time and
+  // return the time elapsed since midnight. Throw std::system_error on
+  // underlying time conversion function failures.
+  //
+  LIBBUTL_SYMEXPORT duration
+  daytime (timestamp);
 }
diff --git a/tests/timestamp/driver.cxx b/tests/timestamp/driver.cxx
index 1746c2e..11e1821 100644
--- a/tests/timestamp/driver.cxx
+++ b/tests/timestamp/driver.cxx
@@ -225,4 +225,16 @@ main ()
 
   assert (parse (
     "Apr  8 19:31:10 2016", "%b %d %H:%M:%S %Y", "Apr 08 19:31:10 2016"));
+
+  {
+    timestamp t (from_string ("Apr 8 19:31:10 2016",
+                              "%b %d %H:%M:%S %Y",
+                              true /* local */));
+
+    timestamp mt (from_string ("Apr 8 00:00:00 2016",
+                               "%b %d %H:%M:%S %Y",
+                               true /* local */));
+
+    assert (daytime (t) == t - mt);
+  }
 }
-- 
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