freeCodeCamp/curriculum/challenges/german/22-rosetta-code/rosetta-code-challenges/hash-join.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5956795bc9e2c415eb244de1	Hash-Join	1	302284	hash-join

--description--

An inner join is an operation that combines two data tables into one table, based on matching column values. The simplest way of implementing this operation is the nested loop join algorithm, but a more scalable alternative is the hash join algorithm.

Der "Hash Join"-Algorithmus besteht aus zwei Schritten:

1. Hash phase: Create a multimap from one of the two tables, mapping from each join column value to all the rows that contain it.
• The multimap must support hash-based lookup which scales better than a simple linear search, because that's the whole point of this algorithm.
• Idealerweise sollte die Multimap für die kleinere Tabelle erstellt werden, um die Erstellungszeit und die Speichergröße zu minimieren.
2. Join-Phase: Scanne die andere Tabelle und finde übereinstimmende Zeilen in der zuvor erstellten Multimap.

Als Pseudocode könnte der Algorithmus wie folgt ausgedrückt werden:

let A = the first input table (or ideally, the larger one)
let B = the second input table (or ideally, the smaller one)
let jA = the join column ID of table A
let jB = the join column ID of table B
let MB = a multimap for mapping from single values to multiple rows of table B (starts out empty)
let C = the output table (starts out empty)
for each row b in table B:
  place b in multimap MB under key b(jB)
for each row a in table A:
  for each row b in multimap MB under key a(jA):
    let c = the concatenation of row a and row b
    place row c in table C

--instructions--

Implementiere den "hash join"-Algorithmus als Funktion und zeige, dass er den unten aufgeführten Testfall besteht. Die Funktion sollte zwei Anordnungen mit Objekten akzeptieren und eine Anordnung mit kombinierten Objekten zurückgeben.

Eingabe

A = Age Name 27 Jonah 18 Alan 28 Glory 18 Popeye 28 Alan B = Character Nemesis Jonah Whales Jonah Spiders Alan Ghosts Alan Zombies Glory Buffy jA = Name (i.e. column 1) jB = Character (i.e. column 0)

Output

A_age	A_name	B_character	B_nemesis
27	Jonah	Jonah	Whales
27	Jonah	Jonah	Spiders
18	Alan	Alan	Ghosts
18	Alan	Alan	Zombies
28	Glory	Glory	Buffy
28	Alan	Alan	Ghosts
28	Alan	Alan	Zombies

Die Reihenfolge der Zeilen in der Ausgabetabelle ist nicht von Bedeutung.

--hints--

hashJoin sollte eine Funktion sein.

assert(typeof hashJoin === 'function');

hashJoin([{ age: 27, name: "Jonah" }, { age: 18, name: "Alan" }, { age: 28, name: "Glory" }, { age: 18, name: "Popeye" }, { age: 28, name: "Alan" }], [{ character: "Jonah", nemesis: "Whales" }, { character: "Jonah", nemesis: "Spiders" }, { character: "Alan", nemesis: "Ghosts" }, { character:"Alan", nemesis: "Zombies" }, { character: "Glory", nemesis: "Buffy" }, { character: "Bob", nemesis: "foo" }]) sollte [{"A_age": 27,"A_name": "Jonah", "B_character": "Jonah", "B_nemesis": "Whales"}, {"A_age": 27,"A_name": "Jonah", "B_character": "Jonah", "B_nemesis": "Spiders"}, {"A_age": 18,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Ghosts"}, {"A_age": 18,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Zombies"}, {"A_age": 28,"A_name": "Glory", "B_character": "Glory", "B_nemesis": "Buffy"}, {"A_age": 28,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Ghosts"}, {"A_age": 28,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Zombies"}] zurückgeben

assert.deepEqual(hashJoin(hash1, hash2), res);

--seed--

--after-user-code--

const hash1 = [
    { age: 27, name: 'Jonah' },
    { age: 18, name: 'Alan' },
    { age: 28, name: 'Glory' },
    { age: 18, name: 'Popeye' },
    { age: 28, name: 'Alan' }
];

const hash2 = [
    { character: 'Jonah', nemesis: 'Whales' },
    { character: 'Jonah', nemesis: 'Spiders' },
    { character: 'Alan', nemesis: 'Ghosts' },
    { character: 'Alan', nemesis: 'Zombies' },
    { character: 'Glory', nemesis: 'Buffy' },
    { character: 'Bob', nemesis: 'foo' }
];

const res = [
    { A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Whales' },
    { A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Spiders' },
    { A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
    { A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' },
    { A_age: 28, A_name: 'Glory', B_character: 'Glory', B_nemesis: 'Buffy' },
    { A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
    { A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' }
];

const bench1 = [{ name: 'u2v7v', num: 1 }, { name: 'n53c8', num: 10 }, { name: 'oysce', num: 9 }, { name: '0mto2s', num: 1 }, { name: 'vkh5id', num: 4 }, { name: '5od0cf', num: 8 }, { name: 'uuulue', num: 10 }, { name: '3rgsbi', num: 9 }, { name: 'kccv35r', num: 4 }, { name: '80un74', num: 9 }, { name: 'h4pp3', num: 6 }, { name: '51bit', num: 7 }, { name: 'j9ndf', num: 8 }, { name: 'vf3u1', num: 10 }, { name: 'g0bw0om', num: 10 }, { name: 'j031x', num: 7 }, { name: 'ij3asc', num: 9 }, { name: 'byv83y', num: 8 }, { name: 'bjzp4k', num: 4 }, { name: 'f3kbnm', num: 10 }];
const bench2 = [{ friend: 'o8b', num: 8 }, { friend: 'ye', num: 2 }, { friend: '32i', num: 5 }, { friend: 'uz', num: 3 }, { friend: 'a5k', num: 4 }, { friend: 'uad', num: 7 }, { friend: '3w5', num: 10 }, { friend: 'vw', num: 10 }, { friend: 'ah', num: 4 }, { friend: 'qv', num: 7 }, { friend: 'ozv', num: 2 }, { friend: '9ri', num: 10 }, { friend: '7nu', num: 4 }, { friend: 'w3', num: 9 }, { friend: 'tgp', num: 8 }, { friend: 'ibs', num: 1 }, { friend: 'ss7', num: 6 }, { friend: 'g44', num: 9 }, { friend: 'tab', num: 9 }, { friend: 'zem', num: 10 }];

--seed-contents--

function hashJoin(hash1, hash2) {

  return [];
}

--solutions--

function hashJoin(hash1, hash2) {
  const hJoin = (tblA, tblB, strJoin) => {
    const [jA, jB] = strJoin.split('=');
    const M = tblB.reduce((a, x) => {
      const id = x[jB];
      return (
        a[id] ? a[id].push(x) : (a[id] = [x]),
        a
      );
    }, {});

    return tblA.reduce((a, x) => {
      const match = M[x[jA]];
      return match ? (
                a.concat(match.map(row => dictConcat(x, row)))
            ) : a;
    }, []);
  };

  const dictConcat = (dctA, dctB) => {
    const ok = Object.keys;
    return ok(dctB).reduce(
            (a, k) => (a[`B_${k}`] = dctB[k]) && a,
            ok(dctA).reduce(
                (a, k) => (a[`A_${k}`] = dctA[k]) && a, {}
            )
        );
  };

  return hJoin(hash1, hash2, 'name=character');
}