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Fix knuthMorrisPratt for empty word request #101

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merged 2 commits into from
Jul 24, 2018
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Fix knuthMorrisPratt for empty word request #101

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e1949ba
Fix knuthMorrisPratt for empty word request
dubzzz Jul 20, 2018
618d096
Merge branch 'master' into fix-knuth
trekhleb Jul 24, 2018
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3 changes: 3 additions & 0 deletions src/algorithms/string/knuth-morris-pratt/__test__/knuthMorrisPratt.test.js
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Original file line number Diff line number Diff line change
@@ -2,6 +2,9 @@ import knuthMorrisPratt from '../knuthMorrisPratt';

describe('knuthMorrisPratt', () => {
it('should find word position in given text', () => {
expect(knuthMorrisPratt('', '')).toBe(0);
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I think this test is wrong. We should expect -1 here instead of 0 because having zero as a result would mean that I may fetch empty word out of string[0]. But this is not true since the string itself is empty.

expect(knuthMorrisPratt('a', '')).toBe(0);
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This test is actually not correct. It says that when I search for an empty word '' in a string 'a' I should find one and this empty word will be accessible at zero index. But string[0] will return us a which expected but is not correct according to this test. So I assume that we should expect -1 here instead of 0.

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@trekhleb Actually I was understanding it a bit differently.

In my understanding I was expecting knuthMorrisPratt to return the same as String.prototype.indexOf - more a less in a sense.
In other word I considered the output as being: index within text of the first occurrence of word [official reference].

// definition in terms of code
const startIdx = knuthMorrisPratt(text, word);
text.substr(startIdx, word.length) === word // should be true for startIdx !== -1

Actually both "".indexOf("") === 0 and "a".indexOf("") === 0.

expect(knuthMorrisPratt('a', 'a')).toBe(0);
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This test is already valid without main function code changes.

expect(knuthMorrisPratt('abcbcglx', 'abca')).toBe(-1);
expect(knuthMorrisPratt('abcbcglx', 'bcgl')).toBe(3);
expect(knuthMorrisPratt('abcxabcdabxabcdabcdabcy', 'abcdabcy')).toBe(15);
4 changes: 4 additions & 0 deletions src/algorithms/string/knuth-morris-pratt/knuthMorrisPratt.js
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Original file line number Diff line number Diff line change
@@ -30,6 +30,10 @@ function buildPatternTable(word) {
* @return {number}
*/
export default function knuthMorrisPratt(text, word) {
if (word.length === 0) {
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Please take a look at the comments to the tests above. No need to do this checking.

return 0;
}

let textIndex = 0;
let wordIndex = 0;