cratedb.lttb.sql

/*
 * The MIT License

Copyright (c) 2013 by Sveinn Steinarsson

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

/*
Usage:

WITH 
	downsampleddata AS (
		SELECT lttb_with_text_array(
				array(
					SELECT CONCAT (ts,'***',reading) 
					FROM metrics
				)
				, 8) AS lttb
	),
	unnested AS (
		SELECT unnest(downsampleddata.lttb) AS lttb
		FROM downsampleddata
	),
	inarray AS (
		SELECT string_to_array(unnested.lttb, '***') AS lttb
		FROM unnested
	)
SELECT	inarray.lttb [1]::TIMESTAMP AS ts,
		inarray.lttb [2] AS reading
FROM inarray
ORDER BY 1;
*/

CREATE OR REPLACE FUNCTION lttb_with_text_array ("data" ARRAY(TEXT), threshold int)
RETURNS ARRAY(TEXT)
LANGUAGE JAVASCRIPT
AS
'   function lttb_with_text_array(data,threshold) {		
		
        var data_length = data.length;
        if (threshold >= data_length || threshold === 0) {
            return data; // Nothing to do
        }

        var sampled = [],
            sampled_index = 0;

        // Bucket size. Leave room for start and end data points
        var every = (data_length - 2) / (threshold - 2);

        var a = 0,  // Initially a is the first point in the triangle
            max_area_point,
            max_area,
            area,
            next_a;

        sampled[ sampled_index++ ] = data[ a ]; // Always add the first point

        for (var i = 0; i < threshold - 2; i++) {

            // Calculate point average for next bucket (containing c)
            var avg_x = 0,
                avg_y = 0,
                avg_range_start  = Math.floor( ( i + 1 ) * every ) + 1,
                avg_range_end    = Math.floor( ( i + 2 ) * every ) + 1;
            avg_range_end = avg_range_end < data_length ? avg_range_end : data_length;

            var avg_range_length = avg_range_end - avg_range_start;

            for ( ; avg_range_start<avg_range_end; avg_range_start++ ) {
              var datasplit_avg_range_start = data[avg_range_start].split("***");
			  avg_x += datasplit_avg_range_start[0] * 1; // * 1 enforces Number (value may be Date)
              avg_y += datasplit_avg_range_start[1] * 1;
            }
            avg_x /= avg_range_length;
            avg_y /= avg_range_length;

            // Get the range for this bucket
            var range_offs = Math.floor( (i + 0) * every ) + 1,
                range_to   = Math.floor( (i + 1) * every ) + 1;

            // Point a
            var datasplit_a = data[a].split("***");
			var point_a_x = datasplit_a[0] * 1, // enforce Number (value may be Date)
                point_a_y = datasplit_a[1] * 1;

            max_area = area = -1;

            for ( ; range_offs < range_to; range_offs++ ) {
                // Calculate triangle area over three buckets
                var datasplit_range_offs = data[range_offs].split("***");
				area = Math.abs( ( point_a_x - avg_x ) * ( datasplit_range_offs[1] - point_a_y ) -
                            ( point_a_x - datasplit_range_offs[0] ) * ( avg_y - point_a_y )
                          ) * 0.5;
                if ( area > max_area ) {
                    max_area = area;
                    max_area_point = [datasplit_range_offs[0],datasplit_range_offs[1]];
                    next_a = range_offs; // Next a is this b
                }
            }

            sampled[ sampled_index++ ] = max_area_point[0]+"***"+max_area_point[1]; // Pick this point from the bucket
            a = next_a; // This a is the next a (chosen b)
        }

        sampled[ sampled_index++ ] = data[ data_length - 1 ]; // Always add last

        return sampled;
    }
';