#!/bin/bash

# This script manages CRC32 checksums for files and directories.
# It can create a checksum file for specified files or directories and verify them later.
#
# Usage:
# To create a checksum file for a file or directory:
# ./crc32_checksum_manager.sh --create /path/to/file_or_directory [output_file.txt]
# To verify checksums using a checksum file:
# ./crc32_checksum_manager.sh --verify /path/to/checksums.txt
# Use -h or --help for usage information.

# Error Messages:
# ERROR: INVALID_PATH: $path
#   Indicates that the specified path is neither a file nor a directory.
# ERROR: CHECKSUM_FILE_NOT_FOUND: $checksum_file
#   Indicates that the specified checksum file does not exist.
# ERROR: CHECKSUM_INVALID: $file
#   Indicates that the calculated checksum for the file does not match the expected checksum.
# ERROR: MISSING_ARGUMENT: [specific_message]
#   Indicates that a required argument (file or directory or checksum file) is missing.
# ERROR: INVALID_OPTION: $1
#   Indicates that an invalid option was provided to the script.

# Function to display a help message
show_help() {
    echo "Usage: $0 [OPTION] [FILE_OR_DIRECTORY] [OUTPUT_FILE]"
    echo "Options:"
    echo "  -c, --create   Create a CRC32 checksum file for the specified file or directory."
    echo "                 Optionally specify an output file name for the checksums (default: checksums.txt)."
    echo "  -v, --verify   Verify the CRC32 checksums of the specified files or directory."
    echo "  -h, --help     Display this help message."
}

# Function to create a CRC32 checksum file
create_checksum() {
    local path="$1"                 # Path to the file or directory to create checksums for
    local output_file="${2:-checksums.txt}"  # Name of the output file where checksums will be saved, defaulting to 'checksums.txt'

    if [ -d "$path" ]; then            # Check if the path is a directory
        # Find all files in the directory and calculate their CRC32 checksums
        find "$path" -type f -exec sh -c 'crc32 "$1" | awk "{print \$1, \"$1\"}"' _ {} \; > "$output_file"
    elif [ -f "$path" ]; then          # Check if the path is a file
        # Calculate the CRC32 checksum for the file and save it to the output file
        crc32 "$path" > "$output_file"
    else                               # If the path is neither a file nor a directory
        echo "ERROR: INVALID_PATH: $path"
        exit 1
    fi
}

# Function to verify CRC32 checksums
verify_checksum() {
    local checksum_file="$1"           # Path to the checksum file

    if [ ! -f "$checksum_file" ]; then # Check if the checksum file exists
        echo "ERROR: CHECKSUM_FILE_NOT_FOUND: $checksum_file"
        exit 1
    fi

    # Read each line of the checksum file
    while read -r line; do
        checksum=$(echo "$line" | awk '{print $1}')    # Extract the checksum from the line
        file=$(echo "$line" | awk '{print $2}')        # Extract the file path from the line
        # Compare the calculated checksum with the stored checksum
        if [ "$(crc32 "$file")" != "$checksum" ]; then
            echo "ERROR: CHECKSUM_INVALID: $file"
        fi
    done < "$checksum_file"  # Redirect the checksum file to the while loop
}

# Check the script's arguments and execute the appropriate functions
case "$1" in
    -c|--create)
        if [ -z "$2" ]; then                # Check if a file or directory was provided
            echo "ERROR: MISSING_ARGUMENT: No file or directory specified."
            show_help
            exit 1
        fi
        
        # Call the create_checksum function with the provided path and optionally a custom output file
        create_checksum "$2" "$3"
        ;;
    -v|--verify)
        if [ -z "$2" ]; then                # Check if a checksum file was provided
            echo "ERROR: MISSING_ARGUMENT: No checksum file specified."
            show_help
            exit 1
        fi
        
        verify_checksum "$2"                 # Call the verify_checksum function with the provided checksum file
        ;;
    -h|--help)
        show_help                          # Display the help message
        ;;
    *)
        echo "ERROR: INVALID_OPTION: $1"
        show_help
        exit 1
        ;;
esac