#![forbid(unsafe_code)]

use std::{
    collections::HashMap,
    num::{NonZeroU64, NonZeroUsize},
    path::PathBuf,
    sync::{atomic::AtomicBool, Arc},
};

use anyhow::{anyhow, Context};
use clap::Parser;
use either::Either;
use flate2::{write::GzEncoder, Compression};
use itertools::Itertools;
use parseq::ParallelIterator;

mod core;
use core::*;

mod fs;
use fs::*;

// std::process::ExitCode::exit_process is unstable
// Exit codes should fit u8.
const EXIT_SUCCESS: i32 = 0;
const EXIT_FAILURE: i32 = 1;
const EXIT_INTERRUPT: i32 = 130;

/// A simple bit rot checker for legacy file systems.
///
/// On the first invocation, Brck records the modification time and a cryptographic hash sum of each regular file in the current working directory to a .brck file.
/// On subsequent invocations, Brck compares the recorded files against the current file system.
///
/// A file can be added, touched, changed, unchanged, removed, or corrupted.
/// A file is corrupted if it's content changed but it's modification did not.
///
/// Without any options, Brck denies corrupted files.
/// If Brck finds one or more denied differences, it prints them to standard output and exits >0.
/// Otherwise, Brck updates the .brck file.
#[derive(Debug, Parser)]
#[command(about, version, author)]
struct Args {
    /// Increase verbosity.
    ///
    /// Specify once to print all changed files.
    /// Specify twice to print unchanged files, too.
    /// By default, only denied differences are printed.
    #[arg(short = 'v', long = "verbose", action = clap::ArgAction::Count)]
    verbosity: u8,

    /// Do not actually modify the database.
    #[arg(short = 'n', long)]
    dry_run: bool,

    /// Specify the number of worker threads.
    ///
    /// Defaults to the available parallelism, typically the number of CPUs.
    #[arg(short, long)]
    jobs: Option<NonZeroUsize>,

    /// Deny select differences only.
    ///
    /// This option accepts a comma-separated list of differences to deny.
    /// Without this option, only corrupted files are denied.
    /// With this option, only the specified differences are denied.
    #[arg(value_enum, short, long, default_values_t = vec![DiffKind::Corrupted], value_delimiter = ',', action = clap::ArgAction::Append)]
    deny: Vec<DiffKind>,

    /// Enable structured output.
    ///
    /// By default, Brck prints differences in a human readable format to standard output.
    /// With this option, Brck prints differences as newline-delimited JSON objects to standard output.
    #[arg(short = 'J', long)]
    json: bool,

    /// Print a summary to standard error.
    #[arg(short, long)]
    summary: bool,

    /// Specify the maximum number of bytes a worker thread may read without checking for an interrupt signal.
    ///
    /// Defaults to 1 GiB.
    #[arg(long)]
    chunk_size: Option<NonZeroU64>,

    /// Specify the maximum number of files to read ahead.
    ///
    /// This should be greater than the number of worker threads.
    ///
    /// Defaults to 1024 files per worker thread.
    #[arg(long)]
    queue_size: Option<NonZeroUsize>,
}

fn main() {
    std::process::exit(match run() {
        Ok(_) => EXIT_SUCCESS,
        Err(err) if err.is::<clap::Error>() => {
            // Use clap's own error formatter
            eprintln!("{err}");
            EXIT_FAILURE
        }
        Err(err) => {
            eprintln!("Error: {:?}", err);
            match err.downcast_ref::<RecordError>() {
                Some(RecordError::Interrupt) => EXIT_INTERRUPT,
                _ => EXIT_FAILURE,
            }
        }
    })
}

fn run() -> Result<(), anyhow::Error> {
    let terminate = Arc::new(AtomicBool::new(false));
    let args = Args::try_parse()?;

    // Paths must begin with ./ for correct filtering.
    let current_dir: PathBuf = std::path::Component::CurDir.as_os_str().into();
    let db_path = current_dir.join(".brck");
    let backup_path = current_dir.join(".brck.bak");

    // Creating temporary files in the same directory instead of std::env_temp_dir() to enable atomic rename(2) on POSIX-conform systems.
    // The drawback is that the operating system won't remove these files automatically.
    let tmp_db_path = current_dir.join(".brck.tmp");
    let tmp_backup_path = current_dir.join(".brck.bak.tmp");

    #[cfg(target_os = "openbsd")]
    {
        use std::{os::unix::ffi::OsStrExt, path::Path};

        pledge::pledge![Stdio Rpath Wpath Cpath Unveil, Stdio].context("Failed to pledge(2)")?;

        fn unveil<P: AsRef<Path>>(path: P, permissions: &str) -> Result<(), anyhow::Error> {
            unveil::unveil(path.as_ref().as_os_str().as_bytes(), permissions).with_context(|| {
                format!(
                    "Failed to unveil(2) {} with permissions {}",
                    path.as_ref().display(),
                    permissions
                )
            })
        }

        unveil(".", "r")?;
        unveil(&db_path, "rwc")?;
        unveil(&tmp_db_path, "rwc")?;
        unveil(&backup_path, "rwc")?;
        unveil(&tmp_backup_path, "rwc")?;
        unveil::unveil("", "").context("Failed to disable unveil(2)")?;
    }

    let t = terminate.clone();
    ctrlc::try_set_handler(move || {
        if t.load(std::sync::atomic::Ordering::SeqCst) {
            eprintln!("Received second interrupt signal");
            std::process::exit(EXIT_INTERRUPT);
        }
        eprintln!("Received interrupt signal");
        t.store(true, std::sync::atomic::Ordering::SeqCst);
    })
    .context("Failed to register interrupt signal handler")?;

    let (db, first_run) = match read_db(&db_path) {
        Ok(db) => Ok((Either::Left(db), false)),
        Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
            Ok((Either::Right(std::iter::empty()), true))
        }
        Err(err) => Err(err),
    }
    .with_context(|| format!("Failed to read database {}", db_path.display()))?;
    let db = db.map(|item| {
        item.map_err(|err| {
            anyhow::Error::new(err).context(format!(
                "Failed to decode record from database {}",
                &db_path.display()
            ))
        })
    });

    let tmp = TmpFile::open(&tmp_db_path).with_context(|| {
        format!(
            "Failed to create temporary database {}",
            tmp_db_path.display()
        )
    })?;
    let mut tmp = GzEncoder::new(tmp.file(), Compression::default());

    let jobs = args
        .jobs
        .or_else(|| std::thread::available_parallelism().ok())
        .map(NonZeroUsize::get)
        .unwrap_or(1);
    let queue_size = args
        .queue_size
        .map(NonZeroUsize::get)
        .unwrap_or_else(|| jobs.saturating_mul(1024));
    let chunk_size = args
        .chunk_size
        .map(NonZeroU64::get)
        .unwrap_or(1024 * 1024 * 1024);

    let fs = find_files(&current_dir)
        .filter_ok(|path| path != &db_path && path != &tmp_db_path && path != &backup_path)
        .map(|item| {
            item.map_err(|err| anyhow::Error::new(err).context("Failed to walk filesystem"))
        })
        .map_parallel_limit(jobs, queue_size, move |item| {
            item.and_then(|path| {
                Record::from_path(&path, chunk_size, terminate.clone())
                    .with_context(|| format!("Failed to read file {}", path.display()))
            })
        });

    let mut counter = HashMap::new();
    let mut denied = 0;

    let print = if args.json {
        |diff: &Diff| println!("{}", serde_json::to_string(&diff).unwrap())
    } else {
        |diff: &Diff| println!("{diff}")
    };

    diff(db, fs).try_for_each(|item| match item {
        Err(Either::Left(err)) => Err(err),
        Err(Either::Right(err)) => Err(err),
        Ok(diff) => {
            *counter.entry(diff.kind()).or_insert(0) += 1;
            record(&diff, &mut tmp).with_context(|| {
                format!(
                    "Failed to write to temporary database {}",
                    tmp_db_path.display()
                )
            })?;
            if args.deny.contains(&diff.kind()) {
                denied += 1;
                print(&diff);
            } else if args.verbosity > 1
                || (diff.kind() != DiffKind::Unchanged && args.verbosity > 0)
            {
                print(&diff);
            }
            Ok(())
        }
    })?;

    if args.summary {
        let total: usize = counter.values().sum();
        let width = usize::try_from(total.checked_ilog10().unwrap_or(0) + 1).unwrap_or(0);
        eprintln!(
            "Removed:   {:>width$}",
            counter.get(&DiffKind::Removed).unwrap_or(&0)
        );
        eprintln!(
            "Added:     {:>width$}",
            counter.get(&DiffKind::Added).unwrap_or(&0)
        );
        eprintln!(
            "Unchanged: {:>width$}",
            counter.get(&DiffKind::Unchanged).unwrap_or(&0)
        );
        eprintln!(
            "Corrupted: {:>width$}",
            counter.get(&DiffKind::Corrupted).unwrap_or(&0)
        );
        eprintln!(
            "Touched:   {:>width$}",
            counter.get(&DiffKind::Touched).unwrap_or(&0)
        );
        eprintln!(
            "Changed:   {:>width$}",
            counter.get(&DiffKind::Changed).unwrap_or(&0)
        );
        eprintln!("Total:     {:>width$}", total);
    }

    if denied > 0 {
        return Err(anyhow!(
            "Found {} denied {}",
            denied,
            if denied == 1 {
                "difference"
            } else {
                "differences"
            }
        ));
    }

    if args.dry_run {
        eprintln!("Exiting due to dry run");
        return Ok(());
    }

    tmp.finish().with_context(|| {
        format!(
            "Failed to synchronize temporary database {}",
            tmp_db_path.display()
        )
    })?;
    if !first_run {
        copy_file(&db_path, tmp_backup_path, &backup_path).with_context(|| {
            format!(
                "Failed to backup database {} to {}",
                db_path.display(),
                backup_path.display()
            )
        })?;
    }
    rename_file(&tmp_db_path, &db_path).with_context(|| {
        format!(
            "Failed to persist temporary database {} to {}",
            tmp_db_path.display(),
            db_path.display()
        )
    })?;

    Ok(())
}

fn record<T: std::io::Write>(diff: &Diff, mut writer: T) -> Result<(), std::io::Error> {
    let file = match diff {
        Diff::Removed { .. } => None,
        Diff::Added { new, .. } => Some(new),
        Diff::Unchanged { new, .. } => Some(new),
        Diff::Corrupted { new, .. } => Some(new),
        Diff::Touched { new, .. } => Some(new),
        Diff::Changed { new, .. } => Some(new),
    };
    if let Some(file) = file {
        let s = serde_json::to_string(file).unwrap();
        writeln!(writer, "{}", s)
    } else {
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn verify_clap_app() {
        use clap::CommandFactory;
        Args::command().debug_assert()
    }
}