callbag-filter

/**
* callbag-filter
* --------------
*
* Callbag operator that conditionally lets data pass through. Works on either
* pullable or listenable sources.
*
* npm install callbag-filter
*
* Example:
*
* const fromIter = require('callbag-from-iter');
* const iterate = require('callbag-iterate');
* const filter = require('callbag-filter');
*
* const source = filter(x => x % 2)(fromIter([1,2,3,4,5]));
*
* iterate(x => console.log(x))(source); // 1
* // 3
* // 5
*/

const filter = condition => source => (start, sink) => {
if (start !== 0) return;
let talkback;
source(0, (t, d) => {
if (t === 0) {
talkback = d;
sink(t, d);
} else if (t === 1) {
if (condition(d)) sink(t, d);
else talkback(1);
}
else sink(t, d);
});
};

module.exports = filter;