DZone Snippets: Mrclay's Code Snippets

GPS distance and initial bearing between points (MySQL)

mrclay (Steve Clay) — Wed, 16 Jan 2008 21:14:20 GMT

Assume you have a table of locations with Latitude and Longitude for each one. In my case the table is "station" and the primary key is "LocID".

First we create a view to help with the 3D geometry (6378 = Earth's radius in km):



CREATE VIEW gpsGlb AS

    SELECT 

        LocID

        ,6378 * COS(RADIANS(Latitude)) * COS(RADIANS(Longitude)) AS x

        ,6378 * COS(RADIANS(Latitude)) * SIN(RADIANS(Longitude)) AS y

        ,6378 * SIN(RADIANS(Latitude)) AS z

    FROM station;

Here I query for distances to all my locations that are NOT LocID = 405 (rounded miles in my case):



SELECT 

    LocID

    ,ROUND((2 * 6378 * ASIN(d / 2 / 6378)) * 0.621371192) AS dist_mi

FROM

    (SELECT

        SQRT(dx * dx + dy * dy + dz * dz) AS d

        ,LocID

     FROM

        (SELECT

            p1.x - p2.x AS dx

            ,p1.y - p2.y AS dy

            ,p1.z - p2.z AS dz

            ,p2.LocID

        FROM gpsGlb p1

        JOIN gpsGlb p2 ON (p1.LocID = 405 AND p2.LocID != 405)

       ) t1

    ) t2

ORDER BY dist_mi

Here I get the initial bearing to the locations. The "boxed" calculation will come in handy later.



SELECT

    LocID

    ,(360 + DEGREES(ATAN2(y, x))) % 360 AS initBearing_deg

    ,ROUND(((360 + DEGREES(ATAN2(y, x))) % 360) / 22.5) * 22.5 

     AS initBearingBoxed_deg

FROM

    (SELECT

        SIN(RADIANS(s2.Longitude - s1.Longitude)) * COS(RADIANS(s2.Latitude)) 

        AS y

        ,COS(RADIANS(s1.Latitude)) * SIN(RADIANS(s2.Latitude))

            - SIN(RADIANS(s1.Latitude)) * COS(RADIANS(s2.Latitude))

               * COS(RADIANS(s2.Longitude - s1.Longitude)) 

        AS x

        ,s2.LocID

    FROM station s1

    JOIN station s2 ON (s1.LocID = 405 AND s2.LocID != 405)

    ) q1

Here's the combined query plus boxed degrees converted to 'NNE', etc. I've also added a limit for the distance in the qq1 subquery.



SELECT

    qq2.LocID

    ,dist_mi

    ,CASE initBearingBoxed_deg

        WHEN 22.5 THEN 'NNE'   WHEN 45 THEN 'NE'

        WHEN 67.5 THEN 'ENE'   WHEN 90 THEN 'E'

        WHEN 112.5 THEN 'ESE'  WHEN 135 THEN 'SE'

        WHEN 157.5 THEN 'SSE'  WHEN 180 THEN 'S'

        WHEN 202.5 THEN 'SSW'  WHEN 225 THEN 'SW'

        WHEN 247.5 THEN 'WSW'  WHEN 270 THEN 'W'

        WHEN 292.5 THEN 'WNW'  WHEN 315 THEN 'NW'

        WHEN 337.5 THEN 'NNW'  ELSE 'N'

     END AS bearing

FROM (

    SELECT 

        LocID

        ,ROUND((2 * 6378 * ASIN(d / 2 / 6378)) * 0.621371192) AS dist_mi

    FROM

        (SELECT

            SQRT(dx * dx + dy * dy + dz * dz) AS d

            ,LocID

         FROM

            (SELECT

                p1.x - p2.x AS dx

                ,p1.y - p2.y AS dy

                ,p1.z - p2.z AS dz

                ,p2.LocID

            FROM gpsGlb p1

            JOIN gpsGlb p2 ON (p1.LocID = 405 AND p2.LocID != 405)

           ) t1

        ) t2

    ) qq1

JOIN (

    SELECT

        LocID

        ,(360 + DEGREES(ATAN2(y, x))) % 360 AS initBearing_deg

        ,(360 + ROUND((DEGREES(ATAN2(y, x))) / 22.5) * 22.5) % 360 

         AS initBearingBoxed_deg

    FROM

        (SELECT

            SIN(RADIANS(s2.Longitude - s1.Longitude)) * COS(RADIANS(s2.Latitude)) 

             AS y

            ,COS(RADIANS(s1.Latitude)) * SIN(RADIANS(s2.Latitude))

                - SIN(RADIANS(s1.Latitude)) * COS(RADIANS(s2.Latitude))

                   * COS(RADIANS(s2.Longitude - s1.Longitude)) 

             AS x

            ,s2.LocID

        FROM station s1

        JOIN station s2 ON (s1.LocID = 405 AND s2.LocID != 405)

        ) q1

    ) qq2 ON (qq1.LocID = qq2.LocID

              AND qq1.dist_mi <= 60)

ORDER BY dist_mi

Detect Daylight Saving Time on a MySQL server in the America/New_York timezone.

mrclay (Steve Clay) — Tue, 15 Jan 2008 19:21:42 GMT



SELECT -5 - CAST(REPLACE(TIMEDIFF(NOW(), UTC_TIMESTAMP()), ':00:00', '') AS SIGNED) AS isDst

To test in other timezones, replace -5 with your local standard time GMT offset.

RecordSet to tab-separated values

mrclay (Steve Clay) — Wed, 07 Mar 2007 21:51:49 GMT



Function TSV(rs)

	Dim field

	For Each field In rs.Fields

		TSV = TSV & field.Name & VBTab

	Next

	TSV = Left(TSV, Len(TSV) - 1) & vbCr & rs.GetString()

End Function

The rows are separated by vbCr ("\r" in most languages).
The first row is the field names.

Unix time

mrclay (Steve Clay) — Wed, 07 Mar 2007 21:43:08 GMT



Function UnixTime(gmtHrsOffset)

	UnixTime = DateDiff("s", "1/1/1970 00:00:00", Now()) - (3600 * gmtHrsOffset)

End Function



Repsonse.Write(UnixTime(-5)) 'E.S.T.

Adding in the GMT offset allowed this to match PHP's time() function on a separate server.

Style a collection of elements quickly

mrclay (Steve Clay) — Sun, 26 Mar 2006 07:24:30 GMT

Quickly set multiple style properties to single elements or arrays/nodeLists of elements.



function setStyles(el, css) {

	var l = el.length;

	if (typeof l == 'undefined') {el = [el]; l = 1;}

	for (var i=0; i		for (var s in css) el[i].style[s] = css[s];

	}

}

Usage:



window.onload = function() {

	// single element

	setStyles(document.body, {background:'yellow'});

	// nodeList

	setStyles(document.getElementsByTagName('div'), {

		background: '#def',

		color: 'red'

	});

	// array

	var a = [document.body, document.getElementsByTagName('div')[0]];

	setStyles(a, {border: '1px black dotted'});

};

Empty a DOM node

mrclay (Steve Clay) — Fri, 24 Mar 2006 21:09:53 GMT



while (node.hasChildNodes()) {node.removeChild(node.firstChild);}

Faster looping over DOMCollections

mrclay (Steve Clay) — Fri, 24 Mar 2006 20:59:35 GMT



var i=0, el;

while (el = document.getElementsByTagName('div').item(i++)) {

    // use el here

}

Calling document.getElementsByTagName('div').item() directly saves the browser from having to return a whole DOMCollection. Using the while loop prevents the browser from ever having to calculate a DOMCollection's length, and gives us a syntactically nicer reference to the current node.

Source: David "liorean" Andersson. Here's an e-mail excerpt:

> DOM implementations are optimised for single element access, and DOMCollections [like document.getElementsByTagName()] are particularly badly optimised since they are dynamic, which means they need to be either polled at regular intervals, event controlled or recollected at each access. So, instead of caching the DOMCollection, you can make the script never have to create a DOMCollection at all. A perfomance win that will be considerable, and that can actually be felt if you're doing DHTML. ... [calling item() directly, the browser] never has to create a dynamic collection, it just searches for the element with the given order, and discards the created list. Browsers do optimise for accessing collection members this way.