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