//! Core functionality.
//!
//! This module contains Brck's core types and functions.

use std::{
    cmp::Ordering,
    fs::OpenOptions,
    io::{BufReader, Read},
    path::{Path, PathBuf},
    sync::{atomic::AtomicBool, Arc},
};

use chrono::{DateTime, Utc};
use either::Either;
use flate2::bufread::GzDecoder;
use itertools::{EitherOrBoth, Itertools};
use serde::{Deserialize, Serialize};
use sha2::{Digest, Sha256};
use walkdir::WalkDir;

/// Produces an iterator over the regular files below the given path.
///
/// The files are sorted by their full path.
/// Symlinks are not followed, except for the original `path`.
pub fn find_files<P: AsRef<Path>>(
    path: P,
) -> impl Iterator<Item = Result<PathBuf, walkdir::Error>> {
    WalkDir::new(path.as_ref())
        .sort_by_path()
        .into_iter()
        .filter_ok(|entry| entry.file_type().is_file())
        .map_ok(|entry| entry.into_path())
}

/// Extension for [WalkDir].
trait WalkDirExt {
    /// Sort directory entries by full path.
    ///
    /// Yiedls "foo bar/buzz" before "foo/buzz".
    fn sort_by_path(self) -> WalkDir;
}

impl WalkDirExt for WalkDir {
    fn sort_by_path(self) -> WalkDir {
        self.sort_by_key(|a| {
            if a.file_type().is_dir() {
                let mut name = a.file_name().to_os_string();
                name.push(std::path::MAIN_SEPARATOR.to_string());
                name
            } else {
                a.file_name().to_os_string()
            }
        })
    }
}

/// Returns an iterator over the records of the given database.
pub fn read_db<P: AsRef<Path>>(
    path: P,
) -> Result<impl Iterator<Item = Result<Record, serde_json::Error>>, std::io::Error> {
    let file = OpenOptions::new().read(true).open(path)?;
    let buf = BufReader::new(file);
    let dec = GzDecoder::new(buf);
    let iter = serde_json::Deserializer::from_reader(dec).into_iter::<Record>();
    Ok(iter)
}

/// A record of a file's modification time and hash sum.
#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
pub struct Record {
    pub path: PathBuf,
    pub modified: DateTime<Utc>,
    #[serde(with = "hex::serde")]
    pub sha256: Vec<u8>,
}

impl Record {
    /// Constructs a `Record` of the given file.
    ///
    /// This function reads the given file in chunks of at most `chunk_size` bytes. It checks the
    /// `terminate` flag before reading each chunk, returning the `RecordError::Interrupt` error
    /// when set.
    pub fn from_path<P: AsRef<Path>>(
        path: P,
        chunk_size: u64,
        terminate: Arc<AtomicBool>,
    ) -> Result<Record, RecordError> {
        let modified = std::fs::symlink_metadata(&path)?.modified()?;
        let mut file = std::fs::File::open(&path)?;
        let mut hasher = Sha256::new();
        loop {
            if terminate.load(std::sync::atomic::Ordering::SeqCst) {
                return Err(RecordError::Interrupt);
            }
            let mut reader = std::io::Read::by_ref(&mut file).take(chunk_size);
            if std::io::copy(&mut reader, &mut hasher)? == 0 {
                break;
            }
        }
        let hash = hasher.finalize();
        Ok(Record {
            path: path.as_ref().into(),
            modified: modified.into(),
            sha256: hash.as_slice().into(),
        })
    }
}

/// The possible errors of [Record::from_path].
#[derive(Debug)]
pub enum RecordError {
    IO(std::io::Error),
    Interrupt,
}

impl std::fmt::Display for RecordError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            RecordError::IO(err) => write!(f, "{}", err),
            RecordError::Interrupt => write!(f, "Received interrupt signal"),
        }
    }
}

impl std::error::Error for RecordError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            RecordError::IO(err) => Some(err),
            RecordError::Interrupt => None,
        }
    }
}

impl From<std::io::Error> for RecordError {
    fn from(value: std::io::Error) -> Self {
        RecordError::IO(value)
    }
}

/// Returns an iterator over the difference of the given files.
pub fn diff<I, J, E, F>(cached: I, found: J) -> impl Iterator<Item = Result<Diff, Either<E, F>>>
where
    I: Iterator<Item = Result<Record, E>>,
    J: Iterator<Item = Result<Record, F>>,
{
    cached
        .merge_join_by(found, |cached, found| match (cached, found) {
            (Err(_), _) => Ordering::Less,
            (Ok(_), Err(_)) => Ordering::Greater,
            (Ok(cached), Ok(found)) => cached.path.cmp(&found.path),
        })
        .map(|item| match item {
            EitherOrBoth::Left(Err(err)) => Err(Either::Left(err)),
            EitherOrBoth::Right(Err(err)) => Err(Either::Right(err)),
            EitherOrBoth::Left(Ok(old)) => Ok(Diff::Removed { old }),
            EitherOrBoth::Right(Ok(new)) => Ok(Diff::Added { new }),
            EitherOrBoth::Both(Ok(old), Ok(new)) => {
                if old.modified == new.modified {
                    if old.sha256 == new.sha256 {
                        Ok(Diff::Unchanged { old, new })
                    } else {
                        Ok(Diff::Corrupted { old, new })
                    }
                } else if old.sha256 == new.sha256 {
                    Ok(Diff::Touched { old, new })
                } else {
                    Ok(Diff::Changed { old, new })
                }
            }
            _ => unreachable!(),
        })
}

/// The list of possible types of differences.
#[derive(Debug, Clone, Hash, Eq, PartialEq, clap::ValueEnum)]
pub enum DiffKind {
    Added,
    Touched,
    Changed,
    Unchanged,
    Removed,
    Corrupted,
}

/// The difference between two records of a file.
#[derive(Debug, Clone, Eq, PartialEq, Serialize)]
#[serde(tag = "type", rename_all = "lowercase")]
pub enum Diff {
    // Modeled variants as structs to improve their JSON representation.
    Added { new: Record },
    Touched { old: Record, new: Record },
    Changed { old: Record, new: Record },
    Unchanged { old: Record, new: Record },
    Removed { old: Record },
    Corrupted { old: Record, new: Record },
}

impl Diff {
    pub fn kind(&self) -> DiffKind {
        match self {
            Diff::Added { .. } => DiffKind::Added,
            Diff::Touched { .. } => DiffKind::Touched,
            Diff::Changed { .. } => DiffKind::Changed,
            Diff::Unchanged { .. } => DiffKind::Unchanged,
            Diff::Removed { .. } => DiffKind::Removed,
            Diff::Corrupted { .. } => DiffKind::Corrupted,
        }
    }
}

impl std::fmt::Display for Diff {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Diff::Added { new, .. } => write!(f, "added: {}", new.path.display()),
            Diff::Touched { old, .. } => write!(f, "touched: {}", old.path.display()),
            Diff::Changed { old, .. } => write!(f, "changed: {}", old.path.display()),
            Diff::Unchanged { old, .. } => write!(f, "unchanged: {}", old.path.display()),
            Diff::Removed { old, .. } => write!(f, "removed: {}", old.path.display()),
            Diff::Corrupted { old, .. } => write!(f, "corrupted: {}", old.path.display()),
        }
    }
}

#[cfg(test)]
mod tests {
    use testresult::TestResult;

    use super::*;

    fn record(path: &str, modified: &str, sha256: &str) -> Record {
        Record {
            path: path.into(),
            modified: DateTime::parse_from_rfc3339(modified).unwrap().into(),
            sha256: sha256.into(),
        }
    }

    #[test]
    fn diff_works() -> TestResult {
        let a = record("a", "2024-03-22T00:00:00Z", "apple");
        let b = record("b", "2024-03-22T00:00:00Z", "banana");
        let b_touched = record("b", "2024-03-22T01:00:00Z", "banana");
        let c = record("c", "2024-03-22T00:00:00Z", "cherry");
        let c_changed = record("c", "2024-03-22T01:00:00Z", "cashew");
        let d = record("d", "2024-03-22T00:00:00Z", "date");
        let e = record("e", "2024-03-22T00:00:00Z", "elderberry");
        let f = record("f", "2024-03-22T00:00:00Z", "fig");
        let f_corrupted = record("f", "2024-03-22T00:00:00Z", "feijoa");

        let db = [
            Err("foo"),
            Ok(b.clone()),
            Ok(c.clone()),
            Ok(d.clone()),
            Ok(e.clone()),
            Ok(f.clone()),
        ];

        let fs = [
            Ok(a.clone()),
            Ok(b_touched.clone()),
            Err("bar"),
            Ok(c_changed.clone()),
            Ok(d.clone()),
            Ok(f_corrupted.clone()),
        ];

        let mut result = diff(db.into_iter(), fs.into_iter());

        assert_eq!(result.next(), Some(Err(Either::Left("foo"))));
        assert_eq!(result.next(), Some(Ok(Diff::Added { new: a })));
        assert_eq!(
            result.next(),
            Some(Ok(Diff::Touched {
                old: b,
                new: b_touched
            }))
        );
        assert_eq!(result.next(), Some(Err(Either::Right("bar"))));
        assert_eq!(
            result.next(),
            Some(Ok(Diff::Changed {
                old: c,
                new: c_changed
            }))
        );
        assert_eq!(
            result.next(),
            Some(Ok(Diff::Unchanged {
                old: d.clone(),
                new: d
            }))
        );
        assert_eq!(result.next(), Some(Ok(Diff::Removed { old: e })));
        assert_eq!(
            result.next(),
            Some(Ok(Diff::Corrupted {
                old: f,
                new: f_corrupted
            }))
        );
        assert_eq!(result.next(), None);

        Ok(())
    }
}