Debug Round Questions - Find and Fix Bugs in Given Code Snippets

Question

## Round 1 - Coding

## Problem

You are given three buggy code snippets. For each, identify the bug(s) and produce the corrected version.

**Snippet 1 - Off-by-one in binary search:**
```python
def binary_search(arr, target):
    lo, hi = 0, len(arr)  # BUG: hi should be len(arr) - 1
    while lo < hi:         # BUG: should be lo <= hi
        mid = (lo + hi) // 2
        if arr[mid] == target: return mid
        elif arr[mid] < target: lo = mid   # BUG: should be mid + 1
        else: hi = mid
    return -1
```

**Snippet 2 - Mutable default argument:**
```python
def append_to(val, lst=[]):  # BUG: mutable default shared across calls
    lst.append(val)
    return lst
```

**Snippet 3 - Race condition in counter:**
```python
count = 0
def increment():
    global count
    count = count + 1   # BUG: non-atomic read-modify-write in multithreaded context
```

## Follow-ups

1. For Snippet 1: what test case exposes each of the three bugs?
2. For Snippet 2: what is the fix? What is the idiomatic Python default for mutable defaults?
3. For Snippet 3: what synchronization primitive fixes the race? Show the corrected code.
4. How would you add a regression test that would have caught Snippet 2's bug?

Debug Round Questions - Find and Fix Bugs in Given Code Snippets

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