// * This file is part of the uutils coreutils package.
// *
// * (c) Michael Rosenberg <42micro@gmail.com>
// * (c) Fort <forticulous@gmail.com>
// *
// * For the full copyright and license information, please view the LICENSE
// * file that was distributed with this source code.

// spell-checker:ignore (words) writeback wipesync

use clap::{crate_version, Arg, ArgAction, Command};
use rand::prelude::SliceRandom;
use rand::Rng;
use std::cell::{Cell, RefCell};
use std::fs;
use std::fs::{File, OpenOptions};
use std::io;
use std::io::prelude::*;
use std::io::SeekFrom;
use std::path::{Path, PathBuf};
use uucore::display::Quotable;
use uucore::error::{FromIo, UResult, USimpleError, UUsageError};
use uucore::{format_usage, show, show_if_err, util_name};

const BLOCK_SIZE: usize = 512;
const NAME_CHARSET: &[u8] = b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_.";

// Patterns as shown in the GNU coreutils shred implementation
const PATTERNS: [&[u8]; 22] = [
    b"\x00",
    b"\xFF",
    b"\x55",
    b"\xAA",
    b"\x24\x92\x49",
    b"\x49\x24\x92",
    b"\x6D\xB6\xDB",
    b"\x92\x49\x24",
    b"\xB6\xDB\x6D",
    b"\xDB\x6D\xB6",
    b"\x11",
    b"\x22",
    b"\x33",
    b"\x44",
    b"\x66",
    b"\x77",
    b"\x88",
    b"\x99",
    b"\xBB",
    b"\xCC",
    b"\xDD",
    b"\xEE",
];

#[derive(Clone, Copy)]
enum PassType<'a> {
    Pattern(&'a [u8]),
    Random,
}

// Used to generate all possible filenames of a certain length using NAME_CHARSET as an alphabet
struct FilenameGenerator {
    name_len: usize,
    name_charset_indices: RefCell<Vec<usize>>, // Store the indices of the letters of our filename in NAME_CHARSET
    exhausted: Cell<bool>,
}

impl FilenameGenerator {
    fn new(name_len: usize) -> Self {
        let indices: Vec<usize> = vec![0; name_len];
        Self {
            name_len,
            name_charset_indices: RefCell::new(indices),
            exhausted: Cell::new(false),
        }
    }
}

impl Iterator for FilenameGenerator {
    type Item = String;

    fn next(&mut self) -> Option<String> {
        if self.exhausted.get() {
            return None;
        }

        let mut name_charset_indices = self.name_charset_indices.borrow_mut();

        // Make the return value, then increment
        let mut ret = String::new();
        for i in name_charset_indices.iter() {
            let c = char::from(NAME_CHARSET[*i]);
            ret.push(c);
        }

        if name_charset_indices[0] == NAME_CHARSET.len() - 1 {
            self.exhausted.set(true);
        }
        // Now increment the least significant index
        for i in (0..self.name_len).rev() {
            if name_charset_indices[i] == NAME_CHARSET.len() - 1 {
                name_charset_indices[i] = 0; // Carry the 1
                continue;
            } else {
                name_charset_indices[i] += 1;
                break;
            }
        }

        Some(ret)
    }
}

// Used to generate blocks of bytes of size <= BLOCK_SIZE based on either a give pattern
// or randomness
struct BytesGenerator<'a> {
    total_bytes: u64,
    bytes_generated: Cell<u64>,
    block_size: usize,
    exact: bool, // if false, every block's size is block_size
    gen_type: PassType<'a>,
    rng: Option<RefCell<rand::rngs::ThreadRng>>,
    bytes: [u8; BLOCK_SIZE],
}

impl<'a> BytesGenerator<'a> {
    fn new(total_bytes: u64, gen_type: PassType<'a>, exact: bool) -> BytesGenerator {
        let rng = match gen_type {
            PassType::Random => Some(RefCell::new(rand::thread_rng())),
            PassType::Pattern(_) => None,
        };

        let bytes = [0; BLOCK_SIZE];

        BytesGenerator {
            total_bytes,
            bytes_generated: Cell::new(0u64),
            block_size: BLOCK_SIZE,
            exact,
            gen_type,
            rng,
            bytes,
        }
    }

    pub fn reset(&mut self, total_bytes: u64, gen_type: PassType<'a>) {
        if let PassType::Random = gen_type {
            if self.rng.is_none() {
                self.rng = Some(RefCell::new(rand::thread_rng()));
            }
        }

        self.total_bytes = total_bytes;
        self.gen_type = gen_type;

        self.bytes_generated.set(0);
    }

    pub fn next(&mut self) -> Option<&[u8]> {
        // We go over the total_bytes limit when !self.exact and total_bytes isn't a multiple
        // of self.block_size
        if self.bytes_generated.get() >= self.total_bytes {
            return None;
        }

        let this_block_size = if !self.exact {
            self.block_size
        } else {
            let bytes_left = self.total_bytes - self.bytes_generated.get();
            if bytes_left >= self.block_size as u64 {
                self.block_size
            } else {
                (bytes_left % self.block_size as u64) as usize
            }
        };

        let bytes = &mut self.bytes[..this_block_size];

        match self.gen_type {
            PassType::Random => {
                let mut rng = self.rng.as_ref().unwrap().borrow_mut();
                rng.fill(bytes);
            }
            PassType::Pattern(pattern) => {
                let skip = if self.bytes_generated.get() == 0 {
                    0
                } else {
                    (pattern.len() as u64 % self.bytes_generated.get()) as usize
                };

                // Copy the pattern in chunks rather than simply one byte at a time
                let mut i = 0;
                while i < this_block_size {
                    let start = (i + skip) % pattern.len();
                    let end = (this_block_size - i).min(pattern.len());
                    let len = end - start;

                    bytes[i..i + len].copy_from_slice(&pattern[start..end]);

                    i += len;
                }
            }
        };

        let new_bytes_generated = self.bytes_generated.get() + this_block_size as u64;
        self.bytes_generated.set(new_bytes_generated);

        Some(bytes)
    }
}

static ABOUT: &str = "Overwrite the specified FILE(s) repeatedly, in order to make it harder\n\
for even very expensive hardware probing to recover the data.
";
const USAGE: &str = "{} [OPTION]... FILE...";

static AFTER_HELP: &str =
    "Delete FILE(s) if --remove (-u) is specified.  The default is not to remove\n\
     the files because it is common to operate on device files like /dev/hda,\n\
     and those files usually should not be removed.\n\
     \n\
     CAUTION: Note that shred relies on a very important assumption:\n\
     that the file system overwrites data in place.  This is the traditional\n\
     way to do things, but many modern file system designs do not satisfy this\n\
     assumption.  The following are examples of file systems on which shred is\n\
     not effective, or is not guaranteed to be effective in all file system modes:\n\
     \n\
     * log-structured or journal file systems, such as those supplied with\n\
     AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)\n\
     \n\
     * file systems that write redundant data and carry on even if some writes\n\
     fail, such as RAID-based file systems\n\
     \n\
     * file systems that make snapshots, such as Network Appliance's NFS server\n\
     \n\
     * file systems that cache in temporary locations, such as NFS\n\
     version 3 clients\n\
     \n\
     * compressed file systems\n\
     \n\
     In the case of ext3 file systems, the above disclaimer applies\n\
     and shred is thus of limited effectiveness) only in data=journal mode,\n\
     which journals file data in addition to just metadata.  In both the\n\
     data=ordered (default) and data=writeback modes, shred works as usual.\n\
     Ext3 journal modes can be changed by adding the data=something option\n\
     to the mount options for a particular file system in the /etc/fstab file,\n\
     as documented in the mount man page (man mount).\n\
     \n\
     In addition, file system backups and remote mirrors may contain copies\n\
     of the file that cannot be removed, and that will allow a shredded file\n\
     to be recovered later.\n\
     ";

pub mod options {
    pub const FORCE: &str = "force";
    pub const FILE: &str = "file";
    pub const ITERATIONS: &str = "iterations";
    pub const SIZE: &str = "size";
    pub const REMOVE: &str = "remove";
    pub const VERBOSE: &str = "verbose";
    pub const EXACT: &str = "exact";
    pub const ZERO: &str = "zero";
}

#[uucore::main]
pub fn uumain(args: impl uucore::Args) -> UResult<()> {
    let args = args.collect_ignore();

    let matches = uu_app().try_get_matches_from(args)?;

    if !matches.contains_id(options::FILE) {
        return Err(UUsageError::new(1, "missing file operand"));
    }

    let iterations = match matches.get_one::<String>(options::ITERATIONS) {
        Some(s) => match s.parse::<usize>() {
            Ok(u) => u,
            Err(_) => {
                return Err(USimpleError::new(
                    1,
                    format!("invalid number of passes: {}", s.quote()),
                ))
            }
        },
        None => unreachable!(),
    };

    // TODO: implement --remove HOW
    //       The optional HOW parameter indicates how to remove a directory entry:
    //         - 'unlink' => use a standard unlink call.
    //         - 'wipe' => also first obfuscate bytes in the name.
    //         - 'wipesync' => also sync each obfuscated byte to disk.
    //       The default mode is 'wipesync', but note it can be expensive.

    // TODO: implement --random-source

    let force = matches.get_flag(options::FORCE);
    let remove = matches.get_flag(options::REMOVE);
    let size_arg = matches
        .get_one::<String>(options::SIZE)
        .map(|s| s.to_string());
    let size = get_size(size_arg);
    let exact = matches.get_flag(options::EXACT) && size.is_none(); // if -s is given, ignore -x
    let zero = matches.get_flag(options::ZERO);
    let verbose = matches.get_flag(options::VERBOSE);

    for path_str in matches.get_many::<String>(options::FILE).unwrap() {
        show_if_err!(wipe_file(
            path_str, iterations, remove, size, exact, zero, verbose, force,
        ));
    }
    Ok(())
}

pub fn uu_app() -> Command {
    Command::new(uucore::util_name())
        .version(crate_version!())
        .about(ABOUT)
        .after_help(AFTER_HELP)
        .override_usage(format_usage(USAGE))
        .infer_long_args(true)
        .arg(
            Arg::new(options::FORCE)
                .long(options::FORCE)
                .short('f')
                .help("change permissions to allow writing if necessary")
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::ITERATIONS)
                .long(options::ITERATIONS)
                .short('n')
                .help("overwrite N times instead of the default (3)")
                .value_name("NUMBER")
                .default_value("3"),
        )
        .arg(
            Arg::new(options::SIZE)
                .long(options::SIZE)
                .short('s')
                .value_name("N")
                .help("shred this many bytes (suffixes like K, M, G accepted)"),
        )
        .arg(
            Arg::new(options::REMOVE)
                .short('u')
                .long(options::REMOVE)
                .help("truncate and remove file after overwriting;  See below")
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::VERBOSE)
                .long(options::VERBOSE)
                .short('v')
                .help("show progress")
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::EXACT)
                .long(options::EXACT)
                .short('x')
                .help(
                    "do not round file sizes up to the next full block;\n\
                     this is the default for non-regular files",
                )
                .action(ArgAction::SetTrue),
        )
        .arg(
            Arg::new(options::ZERO)
                .long(options::ZERO)
                .short('z')
                .help("add a final overwrite with zeros to hide shredding")
                .action(ArgAction::SetTrue),
        )
        // Positional arguments
        .arg(
            Arg::new(options::FILE)
                .action(ArgAction::Append)
                .value_hint(clap::ValueHint::FilePath),
        )
}

// TODO: Add support for all postfixes here up to and including EiB
//       http://www.gnu.org/software/coreutils/manual/coreutils.html#Block-size
fn get_size(size_str_opt: Option<String>) -> Option<u64> {
    size_str_opt.as_ref()?;

    let mut size_str = size_str_opt.as_ref().unwrap().clone();
    // Immutably look at last character of size string
    let unit = match size_str.chars().last().unwrap() {
        'K' => {
            size_str.pop();
            1024u64
        }
        'M' => {
            size_str.pop();
            (1024 * 1024) as u64
        }
        'G' => {
            size_str.pop();
            (1024 * 1024 * 1024) as u64
        }
        _ => 1u64,
    };

    let coefficient = match size_str.parse::<u64>() {
        Ok(u) => u,
        Err(_) => {
            println!(
                "{}: {}: Invalid file size",
                util_name(),
                size_str_opt.unwrap().maybe_quote()
            );
            std::process::exit(1);
        }
    };

    Some(coefficient * unit)
}

fn pass_name(pass_type: PassType) -> String {
    match pass_type {
        PassType::Random => String::from("random"),
        PassType::Pattern(bytes) => {
            let mut s: String = String::new();
            while s.len() < 6 {
                for b in bytes {
                    let readable: String = format!("{:x}", b);
                    s.push_str(&readable);
                }
            }
            s
        }
    }
}

#[allow(clippy::too_many_arguments)]
fn wipe_file(
    path_str: &str,
    n_passes: usize,
    remove: bool,
    size: Option<u64>,
    exact: bool,
    zero: bool,
    verbose: bool,
    force: bool,
) -> UResult<()> {
    // Get these potential errors out of the way first
    let path: &Path = Path::new(path_str);
    if !path.exists() {
        return Err(USimpleError::new(
            1,
            format!("{}: No such file or directory", path.maybe_quote()),
        ));
    }
    if !path.is_file() {
        return Err(USimpleError::new(
            1,
            format!("{}: Not a file", path.maybe_quote()),
        ));
    }

    // If force is true, set file permissions to not-readonly.
    if force {
        let metadata = fs::metadata(path).map_err_context(String::new)?;
        let mut perms = metadata.permissions();
        perms.set_readonly(false);
        fs::set_permissions(path, perms).map_err_context(String::new)?;
    }

    // Fill up our pass sequence
    let mut pass_sequence: Vec<PassType> = Vec::new();

    if n_passes <= 3 {
        // Only random passes if n_passes <= 3
        for _ in 0..n_passes {
            pass_sequence.push(PassType::Random);
        }
    }
    // First fill it with Patterns, shuffle it, then evenly distribute Random
    else {
        let n_full_arrays = n_passes / PATTERNS.len(); // How many times can we go through all the patterns?
        let remainder = n_passes % PATTERNS.len(); // How many do we get through on our last time through?

        for _ in 0..n_full_arrays {
            for p in &PATTERNS {
                pass_sequence.push(PassType::Pattern(p));
            }
        }
        for pattern in PATTERNS.iter().take(remainder) {
            pass_sequence.push(PassType::Pattern(pattern));
        }
        let mut rng = rand::thread_rng();
        pass_sequence.shuffle(&mut rng); // randomize the order of application

        let n_random = 3 + n_passes / 10; // Minimum 3 random passes; ratio of 10 after
                                          // Evenly space random passes; ensures one at the beginning and end
        for i in 0..n_random {
            pass_sequence[i * (n_passes - 1) / (n_random - 1)] = PassType::Random;
        }
    }

    // --zero specifies whether we want one final pass of 0x00 on our file
    if zero {
        pass_sequence.push(PassType::Pattern(b"\x00"));
    }

    {
        let total_passes: usize = pass_sequence.len();
        let mut file: File = OpenOptions::new()
            .write(true)
            .truncate(false)
            .open(path)
            .map_err_context(|| format!("{}: failed to open for writing", path.maybe_quote()))?;

        // NOTE: it does not really matter what we set for total_bytes and gen_type here, so just
        //       use bogus values
        let mut generator = BytesGenerator::new(0, PassType::Pattern(&[]), exact);

        for (i, pass_type) in pass_sequence.iter().enumerate() {
            if verbose {
                let pass_name: String = pass_name(*pass_type);
                if total_passes.to_string().len() == 1 {
                    println!(
                        "{}: {}: pass {}/{} ({})... ",
                        util_name(),
                        path.maybe_quote(),
                        i + 1,
                        total_passes,
                        pass_name
                    );
                } else {
                    println!(
                        "{}: {}: pass {:2.0}/{:2.0} ({})... ",
                        util_name(),
                        path.maybe_quote(),
                        i + 1,
                        total_passes,
                        pass_name
                    );
                }
            }
            // size is an optional argument for exactly how many bytes we want to shred
            show_if_err!(do_pass(&mut file, path, &mut generator, *pass_type, size)
                .map_err_context(|| format!("{}: File write pass failed", path.maybe_quote())));
            // Ignore failed writes; just keep trying
        }
    }

    if remove {
        do_remove(path, path_str, verbose)
            .map_err_context(|| format!("{}: failed to remove file", path.maybe_quote()))?;
    }
    Ok(())
}

fn do_pass<'a>(
    file: &mut File,
    path: &Path,
    generator: &mut BytesGenerator<'a>,
    generator_type: PassType<'a>,
    given_file_size: Option<u64>,
) -> Result<(), io::Error> {
    file.seek(SeekFrom::Start(0))?;

    // Use the given size or the whole file if not specified
    let size: u64 = given_file_size.unwrap_or(get_file_size(path)?);

    generator.reset(size, generator_type);

    while let Some(block) = generator.next() {
        file.write_all(block)?;
    }

    file.sync_data()?;

    Ok(())
}

fn get_file_size(path: &Path) -> Result<u64, io::Error> {
    let size: u64 = fs::metadata(path)?.len();

    Ok(size)
}

// Repeatedly renames the file with strings of decreasing length (most likely all 0s)
// Return the path of the file after its last renaming or None if error
fn wipe_name(orig_path: &Path, verbose: bool) -> Option<PathBuf> {
    let file_name_len: usize = orig_path.file_name().unwrap().to_str().unwrap().len();

    let mut last_path: PathBuf = PathBuf::from(orig_path);

    for length in (1..=file_name_len).rev() {
        for name in FilenameGenerator::new(length) {
            let new_path: PathBuf = orig_path.with_file_name(name);
            // We don't want the filename to already exist (don't overwrite)
            // If it does, find another name that doesn't
            if new_path.exists() {
                continue;
            }
            match fs::rename(&last_path, &new_path) {
                Ok(()) => {
                    if verbose {
                        println!(
                            "{}: {}: renamed to {}",
                            util_name(),
                            last_path.maybe_quote(),
                            new_path.quote()
                        );
                    }

                    // Sync every file rename
                    {
                        let new_file: File = File::open(new_path.clone())
                            .expect("Failed to open renamed file for syncing");
                        new_file.sync_all().expect("Failed to sync renamed file");
                    }

                    last_path = new_path;
                    break;
                }
                Err(e) => {
                    println!(
                        "{}: {}: Couldn't rename to {}: {}",
                        util_name(),
                        last_path.maybe_quote(),
                        new_path.quote(),
                        e
                    );
                    return None;
                }
            }
        } // If every possible filename already exists, just reduce the length and try again
    }

    Some(last_path)
}

fn do_remove(path: &Path, orig_filename: &str, verbose: bool) -> Result<(), io::Error> {
    if verbose {
        println!("{}: {}: removing", util_name(), orig_filename.maybe_quote());
    }

    let renamed_path: Option<PathBuf> = wipe_name(path, verbose);
    if let Some(rp) = renamed_path {
        fs::remove_file(rp)?;
    }

    if verbose {
        println!("{}: {}: removed", util_name(), orig_filename.maybe_quote());
    }

    Ok(())
}
