86 lines
2 KiB
TypeScript
86 lines
2 KiB
TypeScript
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// Returns a new string comprised of every characters in `xs` except for the
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// character at `i`.
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function everyOtherChar(xs: string, i: number): string[] {
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const result = [];
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for (let j = 0; j < xs.length; j += 1) {
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if (i !== j) {
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result.push(xs[j]);
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}
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}
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return [xs[i], result.join('')];
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}
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function getPermutations(xs: string): Set<string> {
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if (xs === '') {
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return new Set(['']);
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}
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const result: Set<string> = new Set;
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for (let i = 0; i < xs.length; i += 1) {
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const [char, rest] = everyOtherChar(xs, i);
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const perms = getPermutations(rest);
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for (const perm of perms) {
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result.add(char + perm);
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}
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}
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return result;
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}
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// Tests
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let desc = 'empty string';
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let input = '';
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let actual = getPermutations(input);
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let expected = new Set(['']);
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assert(isSetsEqual(actual, expected), desc);
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desc = 'one character string';
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input = 'a';
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actual = getPermutations(input);
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expected = new Set(['a']);
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assert(isSetsEqual(actual, expected), desc);
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desc = 'two character string';
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input = 'ab';
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actual = getPermutations(input);
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expected = new Set(['ab', 'ba']);
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assert(isSetsEqual(actual, expected), desc);
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desc = 'three character string';
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input = 'abc';
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actual = getPermutations(input);
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expected = new Set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba']);
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assert(isSetsEqual(actual, expected), desc);
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desc = 'four character string';
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input = 'abca';
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actual = getPermutations(input);
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expected = new Set([
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'abca', 'abac', 'acba', 'acab', 'aabc', 'aacb', 'baca', 'baac', 'bcaa',
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'bcaa', 'baac', 'baca', 'caba', 'caab', 'cbaa', 'cbaa', 'caab', 'caba',
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'aabc', 'aacb', 'abac', 'abca', 'acab', 'acba'
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]);
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assert(isSetsEqual(actual, expected), desc);
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function isSetsEqual(as, bs) {
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if (as.size !== bs.size) {
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return false;
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}
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for (let a of as) {
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if (!bs.has(a)) return false;
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}
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return true;
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}
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function assert(condition, desc) {
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if (condition) {
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console.log(`${desc} ... PASS`);
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} else {
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console.log(`${desc} ... FAIL`);
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}
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}
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