diff --git a/packages/core/tests/index.test.ts b/packages/core/tests/index.test.ts
index 5a54f08..663db79 100644
--- a/packages/core/tests/index.test.ts
+++ b/packages/core/tests/index.test.ts
@@ -1,4 +1,27 @@
-import {encode, decode, getCharCodesFromSource} from '../src';
+import {
+    decode,
+    encode,
+    getCharCodesFromSource,
+    getCharsFrequency,
+    getEntropyOfText,
+    getRelativeFrequency,
+    getTree,
+} from '../src';
+
+const roundTrip = (text: string): string => {
+    const codes = getCharCodesFromSource(text);
+    const encoded = encode(text, codes);
+    return decode(encoded, codes);
+};
+
+const isPrefixFree = (codes: Map<string, string>): boolean => {
+    const values = Array.from(codes.values());
+    return values.every(
+        (code, index) =>
+            code === '' ||
+            values.every((other, otherIndex) => index === otherIndex || !other.startsWith(code)),
+    );
+};
 
 describe('Check Encode/decode', () => {
     it('abra', () => {
@@ -9,13 +32,9 @@ describe('Check Encode/decode', () => {
         expect(decoded).toEqual(text);
     });
 
-    // it('aaaaaaaaaa', () => {
-    //     const text = 'aaaaaaaaaa';
-    //     const codes = getCodesFromText(text);
-    //     const encoded = encode(text, codes);
-    //     const decoded = decode(encoded, codes);
-    //     expect(decoded).toEqual(text);
-    // });
+    it('round-trips a single repeated symbol', () => {
+        expect(roundTrip('aaaaaaaaaa')).toEqual('aaaaaaaaaa');
+    });
 
     it('1234', () => {
         const text = '1234123121';
@@ -25,11 +44,61 @@ describe('Check Encode/decode', () => {
         expect(decoded).toEqual(text);
     });
 
-    // it('', () => {
-    //     const text = '';
-    //     const codes = getCodesFromText(text);
-    //     const encoded = encode(text, codes);
-    //     const decoded = decode(encoded, codes);
-    //     expect(decoded).toEqual(text);
-    // });
+    it('round-trips empty text', () => {
+        const text = '';
+        const codes = getCharCodesFromSource(text);
+        const encoded = encode(text, codes);
+        const decoded = decode(encoded, codes);
+        expect(encoded).toEqual([]);
+        expect(decoded).toEqual(text);
+    });
+
+    it('round-trips punctuation, whitespace, and unicode text', () => {
+        expect(roundTrip('hello, huffman!\nhello, compression!')).toEqual(
+            'hello, huffman!\nhello, compression!',
+        );
+        expect(roundTrip('banana bandana 123')).toEqual('banana bandana 123');
+    });
+});
+
+describe('Frequency helpers', () => {
+    it('counts character frequency in descending order', () => {
+        expect(getCharsFrequency('banana')).toEqual([
+            ['a', 3],
+            ['n', 2],
+            ['b', 1],
+        ]);
+    });
+
+    it('calculates relative frequencies from absolute counts', () => {
+        expect(getRelativeFrequency([
+            ['a', 3],
+            ['b', 1],
+        ])).toEqual([
+            ['a', 0.75],
+            ['b', 0.25],
+        ]);
+    });
+
+    it('calculates entropy for common simple distributions', () => {
+        expect(getEntropyOfText('aaaa')).toBe(-0);
+        expect(getEntropyOfText('abab')).toBe(1);
+    });
+});
+
+describe('Huffman tree and code generation', () => {
+    it('creates a root node weighted by the source length', () => {
+        const tree = getTree(getCharsFrequency('mississippi'));
+        expect(tree.weight).toBe('mississippi'.length);
+    });
+
+    it('creates one code per unique character', () => {
+        const codes = getCharCodesFromSource('mississippi');
+        expect(Array.from(codes.keys()).sort()).toEqual(['i', 'm', 'p', 's']);
+    });
+
+    it('generates prefix-free codes for multi-character input', () => {
+        const codes = getCharCodesFromSource('mississippi river');
+        expect(isPrefixFree(codes)).toBe(true);
+    });
 });