cw1_reverse_string_BEGIN
```python
def reverse_string(s):
    return s[::-1]
```
cw1_reverse_string_END
cw2_binary_search_BEGIN
```python
def binary_search(arr, target):
    lo, hi = 0, len(arr) - 1
    while lo <= hi:
        mid = (lo + hi) // 2
        if arr[mid] == target:
            return mid
        if arr[mid] < target:
            lo = mid + 1
        else:
            hi = mid - 1
    return -1
```
cw2_binary_search_END
cw3_parse_json_file_BEGIN
```python
import json

def parse_json_file(path):
    with open(path) as f:
        return json.load(f)
```
cw3_parse_json_file_END
cw4_factorial_BEGIN
USE_STDLIB: math.factorial
cw4_factorial_END
cw5_sort_dict_by_value_BEGIN
```python
def sort_dict_by_value(d):
    return dict(sorted(d.items(), key=lambda kv: kv[1], reverse=True))
```
cw5_sort_dict_by_value_END
cw6_debounce_decorator_BEGIN
```python
import threading

def debounce(wait_seconds):
    def decorator(fn):
        timer = None
        lock = threading.Lock()
        def wrapper(*args, **kwargs):
            nonlocal timer
            with lock:
                if timer is not None:
                    timer.cancel()
                timer = threading.Timer(wait_seconds, lambda: fn(*args, **kwargs))
                timer.start()
        return wrapper
    return decorator
```
cw6_debounce_decorator_END
cw7_count_word_frequencies_BEGIN
```python
from collections import Counter

def count_word_frequencies(s):
    return dict(Counter(s.split()))
```
cw7_count_word_frequencies_END
cw8_validate_email_BEGIN
```python
import re

_EMAIL_RE = re.compile(r"^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$")

def is_valid_email(s):
    return bool(_EMAIL_RE.match(s))
```
cw8_validate_email_END
