#!/usr/bin/env python3
"""Checksum verification tool - verifies SHA256 checksums of files.

Usage:
    python3 checksum_verify.py <checksum_file>

Format of checksum_file:
    <sha256_hash>  <filename>
    (same as sha256sum output format)
"""

import hashlib
import sys
from pathlib import Path


def compute_sha256(filepath: str) -> str:
    """Compute SHA256 hash of a file."""
    sha256_hash = hashlib.sha256()
    with open(filepath, "rb") as f:
        for chunk in iter(lambda: f.read(8192), b""):
            sha256_hash.update(chunk)
    return sha256_hash.hexdigest()


def verify_checksums(checksum_file: str) -> bool:
    """Verify all checksums in the file. Returns True if all pass."""
    all_passed = True
    with open(checksum_file, "r") as f:
        for line_num, line in enumerate(f, 1):
            line = line.strip()
            if not line:
                continue
            parts = line.split()
            if len(parts) < 2:
                print(f"Line {line_num}: Invalid format - skipping")
                all_passed = False
                continue
            expected_hash = parts[0]
            filepath = " ".join(parts[1:])
            
            if not Path(filepath).exists():
                print(f"FAIL: {filepath} - file not found")
                all_passed = False
                continue
            
            actual_hash = compute_sha256(filepath)
            if actual_hash == expected_hash:
                print(f"OK: {filepath}")
            else:
                print(f"FAIL: {filepath}")
                print(f"  Expected: {expected_hash}")
                print(f"  Actual:   {actual_hash}")
                all_passed = False
    
    return all_passed


def main():
    if len(sys.argv) < 2:
        print(__doc__)
        sys.exit(1)
    
    checksum_file = sys.argv[1]
    if verify_checksums(checksum_file):
        print("\nAll checksums verified successfully!")
        sys.exit(0)
    else:
        print("\nSome checksums failed!")
        sys.exit(1)


if __name__ == "__main__":
    main()