aa5f688407
mbtiles files often have two indexes on the map table. This doesn't impact our ability to do an optimized query against it for ZXYStream.
58 lines
2.0 KiB
JavaScript
58 lines
2.0 KiB
JavaScript
var stream = require('stream');
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var util = require('util');
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module.exports = ZXYStream;
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util.inherits(ZXYStream, stream.Readable);
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// Readable stream of line-delimited z/x/y coordinates
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// contained within the MBTiles `tiles` table/view.
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//
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// The `batch` option exists to allow tests to check that
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// multiple calls to `_read` are handled properly. IRL the
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// default offset of 1000 should be reasonably efficient
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// and not worth messing with.
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function ZXYStream(source, options) {
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if (!source) throw new TypeError('MBTiles source required');
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options = options || {};
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if (options.batch !== undefined && typeof options.batch !== 'number')
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throw new TypeError('options.batch must be a positive integer');
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this.source = source;
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this.batch = options.batch || 1000;
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this.offset = 0;
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stream.Readable.call(this);
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}
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ZXYStream.prototype._read = function() {
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var stream = this;
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// Check for the existence of a map table that is indexed.
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if (!stream.table) {
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return this.source._db.get("select count(1) as count from sqlite_master where type = 'index' and tbl_name = 'map';", function(err, row) {
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if (err) return stream.emit('error', err);
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stream.table = row.count >= 1 ? 'map' : 'tiles';
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return stream._read();
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});
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}
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this.source._db.all('SELECT zoom_level AS z, tile_column AS x, tile_row AS y FROM ' + this.table + ' LIMIT ' + this.batch + ' OFFSET ' + this.offset, function(err, rows) {
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if (err && err.code === 'SQLITE_ERROR' && /no such table/.test(err.message)) return stream.push(null);
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if (err) return stream.emit('error', err);
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if (!rows.length) return stream.push(null);
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stream.offset += stream.batch;
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var chunk = '';
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for (var i = 0; i < rows.length; i++) chunk += toLine(rows[i]);
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stream.push(chunk);
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});
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};
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function toLine(row) {
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// Flip Y coordinate because MBTiles files are TMS.
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var y = row.y = (1 << row.z) - 1 - row.y;
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return row.z + '/' + row.x + '/' + y + '\n';
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}
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