Add Lua script to check if we should have daylight or not

diff --git a/home/suntime.lua b/home/suntime.lua
new file mode 100755 (executable)
index 0000000..3dd53f6
--- /dev/null
+++ b/home/suntime.lua
@@ -0,0 +1,171 @@
+#!/usr/bin/lua
+--[[
+    Utility to let you figure out if we should've daylight now.
+    Usage: ./suntime.lua [debug]
+    Exit code is either 0, we have daylight - or 1, no daylight.
+    Adjust lon, lat, tolerance for your location and the required
+    tolerance in seconds.
+]]
+
+-- input values
+lon,lat = "3.1234","51.1234" 
+tolerance = 4800
+dateTable = os.date("*t")
+curEpoch = os.time(dateTable)
+year = dateTable["year"]
+month = dateTable["month"]
+day = dateTable["day"]
+
+function force_range(v, max)
+    if( v < 0 ) then
+        return (v + max)
+    elseif( v >= max) then
+        return (v - max)
+    end
+    return v
+end
+
+function daysInMonth(year, month)
+    month2days = {"31", "28", "31", "30", "31", "30", "31", "31", "30", "31", "30", "31"}
+
+    -- special case February http://www.henk-reints.nl/cal/gregcal.htm
+    if(month == 2) then
+        -- no century change and can be devided by 4
+        if(((year % 100) ~= 0) and ((year % 4) == 0)) then
+            month2days[2] = 29
+        -- change of a century and can be devided by 400
+        elseif(((year % 100) == 0) and ((year % 400) == 0)) then
+            month2days[2] = 29
+        end
+    end
+    return month2days[month]
+end
+
+--[[ Ported from https://pypi.org/project/suntime/
+     which is based on https://stackoverflow.com/questions/19615350/calculate-sunrise-and-sunset-times-for-a-given-gps-coordinate-within-postgresql
+     which took the algorithm from https://web.archive.org/web/20161022214335/https://williams.best.vwh.net/sunrise_sunset_algorithm.htm
+
+     year: number
+     month: number
+     day: number
+     lon: string, longitude of your location
+     lat: string, latitude of your location
+     getRiseTime: bool, true -> sunrise time, false -> sunset time
+
+     return value: time in Unix epoch
+     ]]
+function calc_sun_time(year, month, day, lon, lat, getRiseTime)
+    -- constants we need
+    TO_RAD = (math.pi/180)
+    zenith = 90.8
+
+    -- 1. first calculate the day of the year
+    N1 = math.floor(275 * month / 9)
+    N2 = math.floor((month + 9 ) / 12)
+    N3 = ( 1 + math.floor((year - 4 * math.floor(year / 4) + 2) / 3))
+    N = N1 - (N2 * N3) + day - 30
+
+    -- 2. convert the longitude to hour value and calculate an approximate time
+    lonHour=lon/15
+
+    if(getRiseTime) then
+        t = N + ((6 - lonHour) / 24)
+    else
+        t = N + ((18 - lonHour) / 24)
+    end
+
+    -- 3. calculate the Sun's mean anomaly
+    M = (0.9856 * t) - 3.289
+
+    -- 4. calculate the Sun's true longitude
+    L = M + (1.916 * math.sin(TO_RAD*M)) + (0.020 * math.sin(TO_RAD * 2 * M)) + 282.634
+    L = force_range(L, 360 ) -- NOTE: L adjusted into the range [0,360)
+
+    -- 5a. calculate the Sun's right ascension
+    RA = (1/TO_RAD) * math.atan(0.91764 * math.tan(TO_RAD*L))
+    RA = force_range(RA, 360 ) -- NOTE: RA adjusted into the range [0,360)
+
+    -- 5b. right ascension value needs to be in the same quadrant as L
+    Lquadrant  = (math.floor( L/90)) * 90
+    RAquadrant = (math.floor(RA/90)) * 90
+    RA = RA + (Lquadrant - RAquadrant)
+
+    -- 5c. right ascension value needs to be converted into hours
+    RA = RA / 15
+
+    -- 6. calculate the Sun's declination
+    sinDec = 0.39782 * math.sin(TO_RAD * L)
+    cosDec = math.cos(math.asin(sinDec))
+
+    -- 7a. calculate the Sun's local hour angle
+    cosH = (math.cos(TO_RAD * zenith) - (sinDec * math.sin(TO_RAD * lat))) / (cosDec * math.cos(TO_RAD * lat))
+    if(cosH > 1) then
+        -- return None -- The sun never rises on this location (on the specified date)
+        print("Sun will never rise here and now.")
+        os.exit()
+    elseif(cosH < -1) then
+        -- return None -- The sun never sets on this location (on the specified date)
+        print("Sun will never rise here and now.")
+        os.exit()
+    end
+
+    -- 7b. finish calculating H and convert into hours
+    if(getRiseTime) then
+        H = 360 - (1/TO_RAD) * math.acos(cosH)
+    else
+        H = (1/TO_RAD) * math.acos(cosH)
+    end
+    H = H / 15
+
+    -- 8. calculate local mean time of rising/setting
+    T = H + RA - (0.06571 * t) - 6.622
+
+    -- 9. adjust back to UTC
+    UT = T - lonHour
+    UT = force_range(UT, 24)   -- UTC time in decimal format (e.g. 23.23)
+
+    -- 10. Return
+    hr = force_range(math.floor(UT), 24)
+    min = tonumber(string.format("%.0f", (UT - math.floor(UT))*60))
+    if(min == 60) then
+        hr = (hr + 1)
+        min = 0
+    end
+
+    -- 10. check corner case https://github.com/SatAgro/suntime/issues/1 and issue 8
+    if(hr == 24) then
+        hr = 0
+        day = (day + 1)
+        
+        if(day > daysInMonth(year, month)) then
+            day = 1
+            month = (month + 1)
+
+            if(month > 12) then
+                month = 1
+                year = (year + 1)
+            end
+        end
+    end
+
+    return os.time{year=year, month=month, day=day, hour=hr, min=math.floor(min)}
+end
+
+-- calculate sunrise and sunset time adjusted by tolerance seconds
+sunrise = (calc_sun_time(year, month, day, lon, lat, true) + tolerance)
+sunset = (calc_sun_time(year, month, day, lon, lat, false) - tolerance)
+
+-- debugging aide
+if(arg[1] == "debug") then
+    print("Expected sunrise for today in UTC seconds adjusted +", tolerance)
+    print(os.date("%c", sunrise))
+    print("Expected sunset for today in UTC seconds adjusted -", tolerance)
+    print(os.date("%c", sunset))
+end
+
+-- return 0 in case we should've daylight, otherwise return 1
+if((curEpoch >= sunrise) and (curEpoch <= sunset)) then
+    os.exit(0)
+else
+    os.exit(1)
+end
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