switch to avr-libc 1.8.1 time library

[z180-stamp.git] / time / daylight_seconds.c

diff --git a/time/daylight_seconds.c b/time/daylight_seconds.c
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+++ b/time/daylight_seconds.c
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+/*
+ * (C)2012 Michael Duane Rice All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer. Redistributions in binary
+ * form must reproduce the above copyright notice, this list of conditions
+ * and the following disclaimer in the documentation and/or other materials
+ * provided with the distribution. Neither the name of the copyright holders
+ * nor the names of contributors may be used to endorse or promote products
+ * derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/* $Id: daylight_seconds.c 2369 2013-04-28 14:19:35Z swfltek $ */
+
+/*
+    Determine the amount of time the sun is above the horizon. At high latitudes, around the
+    solstices, this can be zero or greater than ONE_DAY.
+
+*/
+
+#include <time.h>
+#include <math.h>
+
+extern long     __latitude;
+
+long
+daylight_seconds(const time_t * timer)
+{
+    double          l, d;
+    long            n;
+
+    /* convert latitude to radians */
+    l = __latitude / 206264.806;
+
+    d = -solar_declination(timer);
+
+    /* partial 'Sunrise Equation' */
+    d = tan(l) * tan(d);
+
+    /* magnitude of d may exceed 1.0 at near solstices */
+    if (d > 1.0)
+        d = 1.0;
+
+    if (d < -1.0)
+        d = -1.0;
+
+    /* derive hour angle */
+    d = acos(d);
+
+    /* but for atmospheric refraction, this would be d /= M_PI */
+    d /= 3.112505;
+
+    n = ONE_DAY * d;
+
+    return n;
+}