(*  Calculate Open Location Code (OLC co-ordinates).  Calculations from  *)
(*  https://en.wikipedia.org/wiki/Open_Location_Code  *)


southMostLat = -80;
northMostLat = 84 ;
EastWestSize = 180 ;
NorthSouthSize = 90 ;


convertLatLng(latitude_, longitude_) := Module(
    {OLCstr},
(*  error checking  *)
    If( Not(And(NumberQ(latitude), NumberQ(longitude) ) ) ,
        Return("Not a number"), );
    If( latitude < southMostLat ,
        Return("Latitude is too small"), );
    If( latitude > northMostLat ,
        Return("Latitude is too big"), );
    If( longitude < -EastWestSize ,
        Return("East longitude is too small"), );
    If( longitude >= EastWestSize ,
        Return("West longitude is too big"), );
    OLCstr = convertLatLngToOLC(latitude, longitude)
);


convertLatLngToOLC(latitude_, longitude_) := Module(
    {OLCdigits = "23456789CFGHJMPQRVWX",  OLCstr = "", j,
        OLCbase = 20, latIndex, lngIndex, tempA, tempB,
        plusPos = 5, plusBreak = "+", Yind, Xind
    },
    tempA = (latitude + NorthSouthSize) * OLCbase^3;
    tempB = (longitude + EastWestSize) * OLCbase^3;
    latIndex = IntegerDigits(Floor(tempA), OLCbase);
    lngIndex = IntegerDigits(Floor(tempB), OLCbase);
    For( j = 1, j <= 5, j++, 
        If( j == plusPos,
            OLCstr = StringJoin(OLCstr, plusBreak), );
        OLCstr = StringJoin(OLCstr, StringPart(OLCdigits, latIndex[[j]] + 1) );
        OLCstr = StringJoin(OLCstr, StringPart(OLCdigits, lngIndex[[j]] + 1) );
    );
    Yind = Floor((tempA - Floor(tempA)) * 5) * 4;
    Xind = Floor((tempB - Floor(tempB)) * 4);
    OLCstr = StringJoin(OLCstr, StringPart(OLCdigits, Yind + Xind + 1) )
);