Bit Manipulation - Reverse Bits
Identify the bug in the following code snippet that attempts to reverse bits of a 32-bit unsigned integer using a cache lookup:
Identify the bug in the following code snippet that attempts to reverse bits of a 32-bit unsigned integer using a cache lookup:
medium🐞 Bug Identification Q7 of Q15
Step-by-Step Solution
Solution:
Step 1: Analyze byte extraction
Extracting byte uses (n >> (i*8)) & 0xff, which correctly masks the byte.Step 2: Check cache lookup
If masking was missing, higher bits could leak, corrupting cache index.Step 3: Identify bug
In this code, masking is present, so no bug there. The subtle bug is elsewhere.Step 4: Check loop and shifts
Looping 4 times is correct for 32 bits; final masking is done implicitly.Step 5: Final bug
The bug is not masking the final result to 32 bits after shifts, which can cause overflow.Final Answer:
Option A -> Option AQuick Check:
Final result should be masked to 32 bits to avoid overflow [OK]
Quick Trick: Always mask final result to 32 bits after shifts [OK]
Common Mistakes:
MISTAKES- Forgetting final mask
- Incorrect loop count
- Using signed shifts
Trap Explanation:
PITFALL- Candidates often overlook final masking, causing wrong results on large inputs.
Interviewer Note:
CONTEXT- Tests attention to detail in bitwise operations and masking.
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