//! Fail-safe file sequence
//!
//! Works by versioning values of sequences and throwing away all versions,
//! but the current and the previous one.
//!
//! Inspired by [this Java implementation](https://commons.apache.org/proper/commons-transaction/apidocs/org/apache/commons/transaction/file/FileSequence.html)
//!
//! # Usage
//!
//! ```
//! use file_seq::FileSeq;
//! use std::path::Path;
//!
//! let dir = Path::new("/tmp/example");
//! let initial_value = 1;
//!
//! let seq = FileSeq::new(&dir, initial_value).unwrap();
//!
//! // Get current value
//! assert_eq!(initial_value, seq.value().unwrap());
//!
//! // Increment and get
//! assert_eq!(initial_value + 1, seq.increment_and_get(1).unwrap());
//!
//! // Get and then increment
//! assert_eq!(initial_value + 1, seq.get_and_increment(1).unwrap());
//! assert_eq!(initial_value + 2, seq.value().unwrap());
//! ```

use std::fs;
use std::io::{Error, ErrorKind, Read};
use std::path::{Path, PathBuf};

use log::warn;

#[derive(Debug)]
pub struct FileSeq {
    path_1: PathBuf,
    path_2: PathBuf,
}

impl FileSeq {
    pub fn new<P: AsRef<Path>>(store_dir: P, initial_value: u64) -> std::io::Result<Self> {
        let store_path = store_dir.as_ref();

        fs::create_dir_all(store_path)?;
        let store_path_buf = store_path.to_path_buf();
        let path_1 = store_path_buf.join("_1.seq");
        let path_2 = store_path_buf.join("_2.seq");

        let seq = Self { path_1, path_2 };

        seq.initialize_if_necessary(initial_value)?;

        Ok(seq)
    }

    fn initialize_if_necessary(&self, initial_value: u64) -> std::io::Result<()> {
        if fs::metadata(&self.path_1).is_ok() || fs::metadata(&self.path_2).is_ok() {
            Ok(())
        } else {
            self.write(initial_value)
        }
    }

    /// Deletes this sequence
    ///
    /// Once deleted, the sequence must be recreated
    ///
    /// # Example
    ///
    /// ```
    /// use file_seq::FileSeq;
    /// use std::path::Path;
    ///
    /// let dir = Path::new("/tmp/example_delete");
    /// let initial_value = 1;
    ///
    /// let seq = FileSeq::new(&dir, initial_value).unwrap();
    ///
    /// // Get current value
    /// assert_eq!(initial_value, seq.value().unwrap());
    ///
    /// seq.delete();
    ///
    /// // Attempts to read the sequence after it's deleted returns an error
    /// assert_eq!(seq.value().is_err(), true)
    /// ```
    pub fn delete(&self) -> () {
        // The files might not exist already
        let _ = fs::remove_file(&self.path_1);
        let _ = fs::remove_file(&self.path_2);
    }

    /// Returns the current value of the sequence and then increments it.
    ///
    /// # Example
    ///
    /// ```
    /// use file_seq::FileSeq;
    /// use std::path::Path;
    ///
    /// let dir = Path::new("/tmp/example_get_and_increment");
    /// let initial_value = 1;
    ///
    /// let seq = FileSeq::new(&dir, initial_value).unwrap();
    ///
    /// assert_eq!(initial_value, seq.get_and_increment(1).unwrap());
    /// assert_eq!(initial_value + 1, seq.value().unwrap());
    ///
    /// ```
    pub fn get_and_increment(&self, increment: u64) -> std::io::Result<u64> {
        let value = self.read()?;
        self.write(value + increment)?;
        Ok(value)
    }

    /// Increments the sequence and return the value.
    ///
    /// # Example
    ///
    /// ```
    /// use file_seq::FileSeq;
    /// use std::path::Path;
    ///
    /// let dir = Path::new("/tmp/example_increment_and_get");
    /// let initial_value = 1;
    ///
    /// let seq = FileSeq::new(&dir, initial_value).unwrap();
    ///
    /// assert_eq!(initial_value + 1, seq.increment_and_get(1).unwrap());
    /// assert_eq!(initial_value + 1, seq.value().unwrap());
    ///
    /// ```
    pub fn increment_and_get(&self, increment: u64) -> std::io::Result<u64> {
        let value = self.get_and_increment(increment)?;
        Ok(value + increment)
    }

    /// Returns the current value of the sequence.
    ///
    /// # Example
    ///
    /// ```
    /// use file_seq::FileSeq;
    /// use std::path::Path;
    ///
    /// let dir = Path::new("/tmp/example_value");
    /// let initial_value = 1;
    ///
    /// let seq = FileSeq::new(&dir, initial_value).unwrap();
    ///
    /// assert_eq!(initial_value, seq.value().unwrap());
    ///
    /// ```
    pub fn value(&self) -> std::io::Result<u64> {
        self.read()
    }

    fn read(&self) -> std::io::Result<u64> {
        let mut value1: Option<u64> = None;
        if fs::metadata(&self.path_1).is_ok() {
            let value = self.read_from_path(&self.path_1)?;
            value1 = Some(value);
        }

        let mut value2: Option<u64> = None;
        if fs::metadata(&self.path_2).is_ok() {
            value2 = self.read_from_path(&self.path_2).ok();
        }

        match value2 {
            Some(v2) => match value1 {
                Some(v1) => {
                    if v2 > v1 {
                        Ok(v2)
                    } else {
                        warn!("Latest sequence value is smaller than backup, using backup.");
                        fs::remove_file(&self.path_2).ok();
                        Ok(v1)
                    }
                }
                None => Ok(v2),
            },
            None => {
                fs::remove_file(&self.path_2).ok();

                match value1 {
                    Some(v1) => Ok(v1),
                    None => Err(Error::new(
                        ErrorKind::InvalidData,
                        "Looks like both backup and latest sequence files are corrupted.",
                    )),
                }
            }
        }
    }

    fn read_from_path<P: AsRef<Path>>(&self, path: P) -> std::io::Result<u64> {
        let mut buff = [0; 8];
        let mut f = fs::File::open(path.as_ref())?;
        f.read_exact(&mut buff)?;
        let value = u64::from_be_bytes(buff);
        Ok(value)
    }

    fn write(&self, value: u64) -> std::io::Result<()> {
        if fs::metadata(&self.path_2).is_ok() {
            fs::rename(&self.path_2, &self.path_1)?;
        }
        self.write_to_path(&self.path_2, value)
    }

    fn write_to_path<P: AsRef<Path>>(&self, path: P, value: u64) -> std::io::Result<()> {
        fs::write(path.as_ref(), value.to_be_bytes())
    }
}

#[cfg(test)]
mod tests {
    use std::env;
    use std::fs;
    use std::path::PathBuf;

    use rand::RngCore;

    use crate::FileSeq;

    pub fn tmpdir() -> PathBuf {
        let p = env::temp_dir();
        let mut r = rand::thread_rng();
        let ret = p.join(&format!("file-seq-{}", r.next_u32()));
        fs::create_dir(&ret).unwrap();
        ret
    }

    #[test]
    fn should_store_initial_seq_correctly() {
        let dir = tmpdir();
        let seq = FileSeq::new(&dir, 1).unwrap();
        assert!(std::fs::metadata(dir).is_ok());
        assert!(std::fs::metadata(seq.path_2).is_ok());
    }

    #[test]
    fn should_cycle_seq_files() {
        let dir = tmpdir();
        let seq = FileSeq::new(&dir, 1).unwrap();
        assert!(std::fs::metadata(dir).is_ok());
        assert!(std::fs::metadata(&seq.path_2).is_ok());
        let path_2_value = std::fs::read(&seq.path_2).unwrap();
        seq.increment_and_get(1).unwrap();
        let path_1_value = std::fs::read(&seq.path_1).unwrap();
        assert_eq!(path_2_value, path_1_value);
    }

    #[test]
    fn should_delete() {
        let dir = tmpdir();
        let seq = FileSeq::new(&dir, 1).unwrap();
        assert!(std::fs::metadata(dir).is_ok());
        assert!(std::fs::metadata(&seq.path_2).is_ok());
        seq.increment_and_get(1).unwrap();
        seq.delete();
        assert!(!std::fs::metadata(&seq.path_1).is_ok());
        assert!(!std::fs::metadata(&seq.path_2).is_ok());
    }

    #[test]
    fn should_increment_and_get() {
        let dir = tmpdir();
        let seq = FileSeq::new(dir, 1).unwrap();
        let prev_value = seq.value().unwrap();
        let curr_value = seq.increment_and_get(1).unwrap();
        assert_eq!(prev_value + 1, curr_value);
        assert_eq!(curr_value, seq.value().unwrap());
    }

    #[test]
    fn should_get_and_increment() {
        let dir = tmpdir();
        let seq = FileSeq::new(dir, 1).unwrap();
        let prev_value = seq.value().unwrap();
        let curr_value = seq.get_and_increment(1).unwrap();
        assert_eq!(prev_value, curr_value);
        assert_eq!(curr_value + 1, seq.value().unwrap())
    }
}