Don't generate the same character twice in a row

src/censor/BuiltinStrategies.ts

@@ -139,25 +139,37 @@ export function fixedCharCensorStrategy(char: string): TextCensorStrategy {
 
 /**
  * A text censoring strategy that generates replacement strings made up of
- * random characters from the set of characters provided.
+ * random characters from the set of characters provided. The strings never
+ * contain two of the same character in a row.
  *
  * @example
  * ```typescript
  * const strategy = randomCharFromSetCensorStrategy('$#!');
  * const censor = new TextCensor().setStrategy(strategy);
  * // Before: 'fuck you!'
- * // After: '!##$ you!'
+ * // After: '!#$# you!'
  * ```
  * @param charset - Set of characters from which the replacement string should
- * be constructed. Must not be empty.
+ * be constructed. Must have at least two characters.
  * @returns A [[TextCensorStrategy]] for use with the [[TextCensor]].
  */
 export function randomCharFromSetCensorStrategy(charset: string): TextCensorStrategy {
 	const chars = [...charset];
-	if (chars.length === 0) throw new Error('The character set passed must not be empty.');
+	if (chars.length < 2) throw new Error('The character set passed must have at least 2 characters.');
 	return (ctx: CensorContext) => {
-		let censored = '';
-		for (let i = 0; i < ctx.matchLength; i++) censored += chars[Math.floor(Math.random() * chars.length)];
+		if (ctx.matchLength === 0) return '';
+
+		let lastIdx = Math.floor(Math.random() * chars.length);
+		let censored = chars[lastIdx];
+		for (let i = 1; i < ctx.matchLength; i++) {
+			let idx = Math.floor(Math.random() * (chars.length - 1));
+			// Transform the distribution for idx from [0, len-1) to
+			// [0, lastIdx) ∪ (lastIdx, len) to exclude lastIdx while
+			// ensuring a uniform distribution of generated characters.
+			if (idx >= lastIdx) idx++;
+			lastIdx = idx;
+			censored += chars[idx];
+		}
 		return censored;
 	};
 }

test/censor/BuiltinStrategies.test.ts

@@ -130,8 +130,13 @@ describe('fixedCharCensorStrategy()', () => {
 });
 
 describe('randomCharFromSetCensorStrategy()', () => {
-	it('should throw if the charset is empty', () => {
-		expect(() => randomCharFromSetCensorStrategy('')).toThrow(new Error('The character set passed must not be empty.'));
+	it('should throw if the charset has less than 2 characters', () => {
+		expect(() => randomCharFromSetCensorStrategy('')).toThrow(
+			new Error('The character set passed must have at least 2 characters.'),
+		);
+		expect(() => randomCharFromSetCensorStrategy('a')).toThrow(
+			new Error('The character set passed must have at least 2 characters.'),
+		);
 	});
 
 	it('should work for matchLength 0', () => {
@@ -144,4 +149,11 @@ describe('randomCharFromSetCensorStrategy()', () => {
 		const strategy = randomCharFromSetCensorStrategy(charset);
 		expect([...strategy({ ...partialCtx, matchLength: 5 })].every((c) => charset.includes(c))).toBeTruthy();
 	});
+
+	it('should not repeat the same character twice in a row', () => {
+		const strategy = randomCharFromSetCensorStrategy('ab');
+		for (let i = 0; i < 100; i++) {
+			expect(['aba', 'bab']).toContain(strategy({ ...partialCtx, matchLength: 3 }));
+		}
+	});
 });