feat: Added checkSubsequence func and tests for it

README.md

@@ -111,6 +111,7 @@ const count = stringzy.analyze.wordCount('Hello world'); // 2
 - [patternCount](#patterncount) - calculates the number of times a specific pattern occurs in a given text
 - [vowelConsonantCount](#vowelconsonantcount) - Counts the number of vowels and consonants in a given string
 - [checkMultiplePatterns](#checkmultiplepatterns) - Finds occurrences of multiple patterns within a given text using Rabin–Karp algorithm (case sensitive)
+- [checkSubsequence](#checksubsequence) - Checks whether the second string is a subsequence of the first string (case sensitive)
 
 ### Formatting
 
@@ -993,6 +994,37 @@ checkMultiplePatterns('hello world', ['xyz', '123']);
 | text      | string    | required | The text to search within.                                  |
 | patterns  | string\[ ] | required | An array of patterns to search for (each must be a string). |
 
+#### <a id="checksubsequence"></a>checkSubsequence(str, sub)
+
+Checks whether a given string sub is a subsequence of another string str.
+A subsequence maintains the relative order of characters, but they do not need to be consecutive.
+Case-sensitive comparison is performed.
+Spaces and all characters are treated literally.
+
+```javascript
+isSubsequence('abcde', 'ace');
+// true  → 'a', 'c', 'e' appear in order
+
+isSubsequence('abracadabra', 'aaa');
+// true  → multiple 'a's in correct order
+
+isSubsequence('abcde', 'aec');
+// false → order is broken (e comes before c)
+
+isSubsequence('anything', '');
+// true  → empty subsequence is always valid
+
+isSubsequence('AbC', 'AC');
+// true  → exact case matches
+
+isSubsequence('a b c', 'abc');
+// false → spaces count as characters
+```
+| Parameter | Type   | Default  | Description                                     |
+| --------- | ------ | -------- | ----------------------------------------------- |
+| str       | string | required | The main string to check within.                |
+| sub       | string | required | The subsequence string to verify against `str`. |
+
 
 ---
 

src/analyzing/checkSubsequence.ts

@@ -0,0 +1,33 @@
+/**
+ * Checks whether the second string is a subsequence of the first string.
+ *
+ * A subsequence means all characters of the second string appear in the first string
+ * in the same relative order, but not necessarily consecutively.
+ * 
+ * Is case sensitive
+ *
+ * @param {string} str1 - The string to check against.
+ * @param {string} str2 - The candidate subsequence.
+ * @returns {boolean} True if str2 is a subsequence of str1, otherwise false.
+ * @throws {TypeError} If either input is not a string.
+ */
+export function checkSubsequence(str1: string, str2: string): boolean {
+  if (typeof str1 !== "string" || typeof str2 !== "string") {
+    throw new TypeError("Both inputs must be strings");
+  }
+
+  // empty subsequence is always valid
+  if (str2 === "") return true;
+
+  let i = 0; // pointer for str2 (the subsequence)
+  let j = 0; // pointer for str1 (the main string)
+
+  while (i < str2.length && j < str1.length) {
+    if (str2[i] === str1[j]) {
+      i++;
+    }
+    j++;
+  }
+
+  return i === str2.length;
+}

src/analyzing/index.ts

@@ -7,6 +7,7 @@ export { stringSimilarity } from './stringSimilarity';
 export { patternCount } from './patternCount';
 export { vowelConsonantCount } from './vowelConsonantCount';
 export { checkMultiplePatterns } from './checkMultiplePatterns';
+export { checkSubsequence } from './checkSubsequence';
 
 export { functionWordCount } from './functionWordCount';
 export { contentWordCount } from './contentWordCount';
@@ -21,6 +22,7 @@ import { stringSimilarity } from './stringSimilarity';
 import { patternCount } from './patternCount';
 import { vowelConsonantCount } from './vowelConsonantCount';
 import { checkMultiplePatterns } from './checkMultiplePatterns';
+import { checkSubsequence } from './checkSubsequence';
 
 
 
@@ -33,5 +35,6 @@ export const analyzing = {
   stringSimilarity,
   patternCount,
   vowelConsonantCount,
-  checkMultiplePatterns
+  checkMultiplePatterns,
+  checkSubsequence
 };

src/tests/analyzing/checkSubsequence.test.ts

@@ -0,0 +1,35 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import { checkSubsequence } from '../../analyzing/checkSubsequence';
+
+describe('checkSubsequence', () => {
+  it('returns true for valid subsequences', () => {
+    assert.strictEqual(checkSubsequence('abcde', 'ace'), true);
+    assert.strictEqual(checkSubsequence('abracadabra', 'aaa'), true);
+    assert.strictEqual(checkSubsequence('hello world', 'hlo'), true);
+  });
+
+  it('returns false when order is broken', () => {
+    assert.strictEqual(checkSubsequence('abcde', 'aec'), false);
+    assert.strictEqual(checkSubsequence('abcdef', 'z'), false);
+  });
+
+  it('handles empty subsequence', () => {
+    assert.strictEqual(checkSubsequence('anything', ''), true);
+  });
+
+  it('is case-sensitive', () => {
+    assert.strictEqual(checkSubsequence('abc', 'A'), false);
+    assert.strictEqual(checkSubsequence('AbC', 'AC'), true);
+  });
+
+  it('handles spaces as normal characters', () => {
+    assert.strictEqual(checkSubsequence('a b c', 'abc'), true); 
+    assert.strictEqual(checkSubsequence('a b c', 'a c'), true);
+  });
+
+  it('throws if inputs are not strings', () => {
+    assert.throws(() => checkSubsequence(123 as any, 'abc'), /Both inputs must be strings/);
+    assert.throws(() => checkSubsequence('abc', null as any), /Both inputs must be strings/);
+  });
+});