/**

• callbag-scan

• ---

• Callbag operator that combines consecutive values from the same source.
• It's essentially like array .reduce, but delivers a new accumulated value
• for each value from the callbag source. Works on either pullable or
• listenable sources.
• npm install callbag-scan
• Example:

• const fromIter = require('callbag-from-iter');

• const iterate = require('callbag-iterate');

• const scan = require('callbag-scan');

• const iterSource = fromIter([1,2,3,4,5]);

• const scanned = scan((prev, x) => prev + x, 0)(iterSource);

• scanned(0, iterate(x => console.log(x))); // 1

•                                           // 3

•                                           // 6

•                                           // 10

•                                           // 15

*/

function scan(reducer, seed) { let hasAcc = arguments.length === 2; return source => (start, sink) => { if (start !== 0) return; let acc = seed; source(0, (t, d) => { if (t === 1) { acc = hasAcc ? reducer(acc, d) : ((hasAcc = true), d); sink(1, acc); } else sink(t, d); }); }; }

module.exports = scan;