feat: adds stretch assignment with test cases

Author: Edge-J

lesson_06/expression/src/alpha_abbreviation.ts

@@ -0,0 +1,36 @@
+export function isValidAlphaAbbreviation(word: string, abbr: string): boolean {
+  let i = 0; // position in word
+  let j = 0; // position in abbreviation
+
+  while (j < abbr.length) {
+    const ch = abbr[j];
+
+    if (ch >= "0" && ch <= "9") {
+      // Leading zero check
+      if (ch === "0") return false;
+
+      // Parse complete number
+      let num = 0;
+      while (j < abbr.length && abbr[j] >= "0" && abbr[j] <= "9") {
+        num = num * 10 + (abbr[j].charCodeAt(0) - "0".charCodeAt(0));
+        j++;
+      }
+
+      // Apply skip
+      i += num;
+
+      // Bounds check
+      if (i > word.length) return false;
+    } else {
+      // Literal character match
+      if (i >= word.length || word[i] !== ch) {
+        return false;
+      }
+      i++;
+      j++;
+    }
+  }
+
+  // Must consume entire word
+  return i === word.length;
+}

lesson_06/expression/src/test_alpha.ts

@@ -0,0 +1,58 @@
+import { isValidAlphaAbbreviation } from "./alpha_abbreviation.js";
+
+function test(word: string, abbr: string, expected: boolean) {
+  const result = isValidAlphaAbbreviation(word, abbr);
+  const status = result === expected ? "✅ PASS" : "❌ FAIL";
+  console.log(
+    `${status} | Word: "${word}" | Abbr: "${abbr}" → Expected: ${expected}, Got: ${result}`,
+  );
+}
+
+// === Basic valid cases ===
+test("internationalization", "i18n", true);
+test("substitution", "s10n", true);
+test("hello", "4o", true); // skip "e", "l", "l", "o"
+test("abcde", "4e", true); // skip "b", "c", "d", arrive at "e"
+
+// === Literal-only match ===
+test("dog", "dog", true);
+test("cat", "cut", false); // mismatch at "a" vs "u"
+
+// === Skip everything ===
+test("abcdef", "6", true); // skip entire word
+test("abcdef", "7", false); // skip past word length
+test("abcdef", "5g", true); // skip to last letter and match
+
+// === Mixed skip + literal ===
+test("compression", "c9n", true);
+test("transformation", "t12n", true);
+test("transformation", "t13n", false); // too much skip
+
+// === Zero padding (invalid) ===
+test("test", "01st", false);
+test("skiptest", "s00t", false);
+
+// === Leading and trailing skips ===
+test("abcdef", "1bcdef", false); // can't skip and expect literal 'b' immediately
+test("abcdef", "a5", true); // literal 'a', skip 5 to end
+test("abcdef", "a6", false); // skip past end
+
+// === Multiple number segments ===
+test("abcdefg", "1b1d1f1g", true); // skips then matches
+test("abcdefg", "1b2e1g", true);
+test("abcdefg", "1b3f1g", false); // overshoots
+
+// === Full numeric — skips entire word
+test("skipped", "7", true);
+test("skipped", "8", false); // overshoot
+
+// === Empty abbreviation or word ===
+test("word", "", false); // must match full
+test("", "", true); // both empty is valid
+test("", "1", false); // can't skip into empty
+test("nonempty", "", false); // abbreviation is empty
+
+// === Misc edge junk ===
+test("abc", "a01c", false); // invalid leading 0
+test("abc", "a0b", false); // zero is illegal
+test("abc", "a1b", true); // skip one