Find first missing positive: Difference between revisions

=={{header|JavaScript}}==

<lang javascript>(() => {

"use strict";

// ---------- FIRST MISSING NATURAL NUMBER -----------

// firstGap :: [Int] -> Int

const firstGap = xs =>

filterGen(

x => !xs.includes(x)

)(

enumFrom(1)

)

.next()

.value;

// ---------------------- TEST -----------------------

// main :: IO ()

const main = () => [

[1, 2, 0],

[3, 4, -1, 1],

[7, 8, 9, 11, 12]

]

.map(xs => `${show(xs)} -> ${firstGap(xs)}`)

.join("\n");

// --------------------- GENERIC ---------------------

// enumFrom :: Int -> [Int]

const enumFrom = function* (x) {

// A non-finite succession of

// integers, starting with n.

let v = x;

while (true) {

yield v;

v = 1 + v;

}

};

// filterGen :: (a -> Bool) -> Gen [a] -> Gen [a]

const filterGen = p =>

// A stream of values which are drawn

// from a generator, and satisfy p.

xs => {

const go = function* () {

let x = xs.next();

while (!x.done) {

const v = x.value;

if (p(v)) {

yield v;

}

x = xs.next();

}

};

return go(xs);

};

// show :: a -> String

const show = x => JSON.stringify(x);

// MAIN ---

return main();

})();</lang>

{{Out}}

<pre>[1,2,0] -> 3

[3,4,-1,1] -> 2

[7,8,9,11,12] -> 1</pre>