238 lines
10 KiB
Markdown
238 lines
10 KiB
Markdown
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---
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id: 5956795bc9e2c415eb244de1
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title: Hash join
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challengeType: 5
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forumTopicId: 302284
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dashedName: hash-join
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---
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# --description--
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An [inner join](https://en.wikipedia.org/wiki/Join_(SQL)#Inner_join "wp: Join\_(SQL)#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](https://en.wikipedia.org/wiki/Nested loop join "wp: Nested loop join") algorithm, but a more scalable alternative is the [hash join](https://en.wikipedia.org/wiki/hash join "wp: hash join") algorithm.
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The "hash join" algorithm consists of two steps:
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<ol>
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<li><strong>Hash phase:</strong> Create a <a href='https://en.wikipedia.org/wiki/Multimap' title='wp: Multimap' target='_blank'>multimap</a> from one of the two tables, mapping from each join column value to all the rows that contain it.</li>
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<ul>
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<li>The multimap must support hash-based lookup which scales better than a simple linear search, because that's the whole point of this algorithm.</li>
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<li>Ideally we should create the multimap for the smaller table, thus minimizing its creation time and memory size.</li>
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</ul>
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<li><strong>Join phase:</strong> Scan the other table, and find matching rows by looking in the multimap created before.</li>
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</ol>
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In pseudo-code, the algorithm could be expressed as follows:
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<pre><strong>let</strong> <i>A</i> = the first input table (or ideally, the larger one)
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<strong>let</strong> <i>B</i> = the second input table (or ideally, the smaller one)
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<strong>let</strong> <i>j<sub>A</sub></i> = the join column ID of table <i>A</i>
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<strong>let</strong> <i>j<sub>B</sub></i> = the join column ID of table <i>B</i>
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<strong>let</strong> <i>M<sub>B</sub></i> = a multimap for mapping from single values to multiple rows of table <i>B</i> (starts out empty)
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<strong>let</strong> <i>C</i> = the output table (starts out empty)
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<strong>for each</strong> row <i>b</i> in table <i>B</i>:
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<strong>place</strong> <i>b</i> in multimap <i>M<sub>B</sub></i> under key <i>b(j<sub>B</sub>)</i>
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<strong>for each</strong> row <i>a</i> in table <i>A</i>:
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<strong>for each</strong> row <i>b</i> in multimap <i>M<sub>B</sub></i> under key <i>a(j<sub>A</sub>)</i>:
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<strong>let</strong> <i>c</i> = the concatenation of row <i>a</i> and row <i>b</i>
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<strong>place</strong> row <i>c</i> in table <i>C</i>
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</pre>
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# --instructions--
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Implement the "hash join" algorithm as a function and demonstrate that it passes the test-case listed below. The function should accept two arrays of objects and return an array of combined objects.
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**Input**
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<table>
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<tr>
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<td style="padding: 4px; margin: 5px;">
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<table style="border:none; border-collapse:collapse;">
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<tr>
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<td style="border:none"><i>A =</i></td>
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<td style="border:none">
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<table>
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<tr>
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<th style="padding: 4px; margin: 5px;">Age</th>
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<th style="padding: 4px; margin: 5px;">Name</th>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">27</td>
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<td style="padding: 4px; margin: 5px;">Jonah</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">18</td>
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<td style="padding: 4px; margin: 5px;">Alan</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">28</td>
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<td style="padding: 4px; margin: 5px;">Glory</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">18</td>
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<td style="padding: 4px; margin: 5px;">Popeye</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">28</td>
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<td style="padding: 4px; margin: 5px;">Alan</td>
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</tr>
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</table>
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</td>
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<td style="border:none; padding-left:1.5em;" rowspan="2"></td>
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<td style="border:none"><i>B =</i></td>
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<td style="border:none">
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<table>
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<tr>
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<th style="padding: 4px; margin: 5px;">Character</th>
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<th style="padding: 4px; margin: 5px;">Nemesis</th>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">Jonah</td>
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<td style="padding: 4px; margin: 5px;">Whales</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">Jonah</td>
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<td style="padding: 4px; margin: 5px;">Spiders</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">Alan</td>
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<td style="padding: 4px; margin: 5px;">Ghosts</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">Alan</td>
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<td style="padding: 4px; margin: 5px;">Zombies</td>
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</tr>
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<tr>
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<td style="padding: 4px; margin: 5px;">Glory</td>
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<td style="padding: 4px; margin: 5px;">Buffy</td>
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</tr>
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</table>
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</td>
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</tr>
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<tr>
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<td style="border:none">
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<i>j<sub>A</sub> =</i>
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</td>
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<td style="border:none">
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<i><code>Name</code> (i.e. column 1)</i>
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</td>
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<td style="border:none">
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<i>j<sub>B</sub> =</i>
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</td>
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<td style="border:none">
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<i><code>Character</code> (i.e. column 0)</i>
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</td>
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</tr>
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</table>
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</td>
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</tr>
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</table>
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**Output**
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| A_age | A_name | B_character | B_nemesis |
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| ----- | ------ | ----------- | --------- |
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| 27 | Jonah | Jonah | Whales |
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| 27 | Jonah | Jonah | Spiders |
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| 18 | Alan | Alan | Ghosts |
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| 18 | Alan | Alan | Zombies |
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| 28 | Glory | Glory | Buffy |
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| 28 | Alan | Alan | Ghosts |
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| 28 | Alan | Alan | Zombies |
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The order of the rows in the output table is not significant.
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# --hints--
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`hashJoin` should be a function.
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```js
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assert(typeof hashJoin === 'function');
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```
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`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" }])` should return `[{"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"}]`
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```js
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assert.deepEqual(hashJoin(hash1, hash2), res);
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```
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# --seed--
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## --after-user-code--
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```js
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const hash1 = [
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{ age: 27, name: 'Jonah' },
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{ age: 18, name: 'Alan' },
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{ age: 28, name: 'Glory' },
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{ age: 18, name: 'Popeye' },
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{ age: 28, name: 'Alan' }
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];
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const hash2 = [
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{ character: 'Jonah', nemesis: 'Whales' },
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{ character: 'Jonah', nemesis: 'Spiders' },
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{ character: 'Alan', nemesis: 'Ghosts' },
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{ character: 'Alan', nemesis: 'Zombies' },
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{ character: 'Glory', nemesis: 'Buffy' },
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{ character: 'Bob', nemesis: 'foo' }
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];
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const res = [
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{ A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Whales' },
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{ A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Spiders' },
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{ A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
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{ A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' },
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{ A_age: 28, A_name: 'Glory', B_character: 'Glory', B_nemesis: 'Buffy' },
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{ A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
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{ A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' }
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];
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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 }];
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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 }];
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```
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## --seed-contents--
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```js
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function hashJoin(hash1, hash2) {
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return [];
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}
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```
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# --solutions--
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```js
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function hashJoin(hash1, hash2) {
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const hJoin = (tblA, tblB, strJoin) => {
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const [jA, jB] = strJoin.split('=');
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const M = tblB.reduce((a, x) => {
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const id = x[jB];
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return (
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a[id] ? a[id].push(x) : (a[id] = [x]),
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a
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);
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}, {});
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return tblA.reduce((a, x) => {
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const match = M[x[jA]];
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return match ? (
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a.concat(match.map(row => dictConcat(x, row)))
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) : a;
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}, []);
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};
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const dictConcat = (dctA, dctB) => {
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const ok = Object.keys;
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return ok(dctB).reduce(
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(a, k) => (a[`B_${k}`] = dctB[k]) && a,
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ok(dctA).reduce(
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(a, k) => (a[`A_${k}`] = dctA[k]) && a, {}
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)
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);
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};
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return hJoin(hash1, hash2, 'name=character');
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}
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```
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