#![deny(rust_2018_idioms)]

#[macro_use]
extern crate log;
//extern crate lru_cache;
pub mod lru_cache;

use std::io::prelude::*;
use std::borrow::Borrow;
use std::boxed::Box;
use std::collections::hash_map::RandomState;
use std::error::Error as StdError;
use std::ffi::{OsStr, OsString};
use std::fmt;
use std::fs::{self,File};
use std::io;
use std::hash::BuildHasher;
use std::path::{Path,PathBuf};
use std::time::{SystemTime, UNIX_EPOCH};

use filetime::{FileTime, set_file_times};
pub use crate::lru_cache::{LruCache,Meter};
use walkdir::WalkDir;

struct FileSize;

/// Given a tuple of (path, filesize), use the filesize for measurement.
impl<K> Meter<K, u64> for FileSize {
    type Measure = usize;
    fn measure<Q: ?Sized>(&self, _: &Q, v: &u64) -> usize
        where K: Borrow<Q>
    {
        *v as usize
    }
}

/// Return an iterator of `(path, size)` of files under `path` sorted by ascending last-modified
/// time, such that the oldest modified file is returned first.
fn get_all_files<P: AsRef<Path>>(path: P) -> Box<dyn Iterator<Item=(PathBuf, u64)>> {
    let mut files: Vec<_> = WalkDir::new(path.as_ref())
        .into_iter()
        .filter_map(|e| e.ok()
                    .and_then(|f| {
                        // Only look at files
                        if f.file_type().is_file() {
                            // Get the last-modified time, size, and the full path.
                            f.metadata().ok().and_then(|m| m.modified().ok().map(|mtime| (mtime, f.path().to_owned(), m.len())))
                        } else {
                            None
                        }
                    }))
        .collect();
    // Sort by last-modified-time, so oldest file first.
    files.sort_by_key(|k| k.0);
    Box::new(files.into_iter().map(|(_mtime, path, size)| (path, size)))
}

/// An LRU cache of files on disk.
pub struct LruDiskCache<S: BuildHasher = RandomState> {
    lru: LruCache<OsString, u64, S, FileSize>,
    root: PathBuf,
}

/// Errors returned by this crate.
#[derive(Debug)]
pub enum Error {
    /// The file was too large to fit in the cache.
    FileTooLarge,
    /// The file was not in the cache.
    FileNotInCache,
    /// An IO Error occurred.
    Io(io::Error),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.description())
    }
}

impl StdError for Error {
    fn description(&self) -> &str {
        match self {
            &Error::FileTooLarge => "File too large",
            &Error::FileNotInCache => "File not in cache",
            &Error::Io(ref e) => e.description(),
        }
    }

    fn cause(&self) -> Option<&dyn StdError> {
        match self {
            &Error::Io(ref e) => Some(e),
            _ => None,
        }
    }
}

impl From<io::Error> for Error {
    fn from(e: io::Error) -> Error {
        Error::Io(e)
    }
}

/// A convenience `Result` type
pub type Result<T> = std::result::Result<T, Error>;

/// Trait objects can't be bounded by more than one non-builtin trait.
pub trait ReadSeek: Read + Seek + Send {}

impl<T: Read + Seek + Send> ReadSeek for T {}

fn filetime_now() -> FileTime {
    let d = SystemTime::now().duration_since(UNIX_EPOCH).unwrap();
    FileTime::from_seconds_since_1970(d.as_secs(), d.subsec_nanos())
}

enum AddFile<'a> {
    AbsPath(PathBuf),
    RelPath(&'a OsStr),
}

impl LruDiskCache {
    /// Create an `LruDiskCache` that stores files in `path`, limited to `size` bytes.
    ///
    /// Existing files in `path` will be stored with their last-modified time from the filesystem
    /// used as the order for the recency of their use. Any files that are individually larger
    /// than `size` bytes will be removed.
    ///
    /// The cache is not observant of changes to files under `path` from external sources, it
    /// expects to have sole maintence of the contents.
    pub fn new<T>(path: T, size: u64) -> Result<Self>
        where PathBuf: From<T>
    {
        LruDiskCache {
            lru: LruCache::with_meter(size, FileSize),
            root: PathBuf::from(path),
        }.init()
    }

    /// Return the current size of all the files in the cache.
    pub fn size(&self) -> u64 { self.lru.size() }

    /// Return the maximum size of the cache.
    pub fn capacity(&self) -> u64 { self.lru.capacity() }

    /// Return the path in which the cache is stored.
    pub fn path(&self) -> &Path { self.root.as_path() }

    /// Return the path that `key` would be stored at.
    fn rel_to_abs_path<K: AsRef<Path>>(&self, rel_path: K) -> PathBuf { self.root.join(rel_path) }

    /// Scan `self.root` for existing files and store them.
    fn init(mut self) -> Result<Self> {
        fs::create_dir_all(&self.root)?;
        for (file, size) in get_all_files(&self.root) {
            if !self.can_store(size) {
                fs::remove_file(file).unwrap_or_else(|e| error!("Error removing file `{}` which is too large for the cache ({} bytes)", e, size));
            } else {
                self.add_file(AddFile::AbsPath(file), size)
                    .unwrap_or_else(|e| error!("Error adding file: {}", e));
            }
        }
        Ok(self)
    }

    /// Returns `true` if the disk cache can store a file of `size` bytes.
    pub fn can_store(&self, size: u64) -> bool {
        size <= self.lru.capacity() as u64
    }

    /// Add the file at `path` of size `size` to the cache.
    fn add_file(&mut self, addfile_path: AddFile<'_>, size: u64) -> Result<()> {
        if !self.can_store(size) {
            return Err(Error::FileTooLarge);
        }
        let rel_path = match addfile_path {
            AddFile::AbsPath(ref p) => p.strip_prefix(&self.root).expect("Bad path?").as_os_str(),
            AddFile::RelPath(p) => p,
        };
        //TODO: ideally LRUCache::insert would give us back the entries it had to remove.
        while self.lru.size() as u64 + size > self.lru.capacity() as u64 {
            let (rel_path,  _) = self.lru.remove_lru().expect("Unexpectedly empty cache!");
            let remove_path = self.rel_to_abs_path(rel_path);
            //TODO: check that files are removable during `init`, so that this is only
            // due to outside interference.
            fs::remove_file(&remove_path).unwrap_or_else(|e| panic!("Error removing file from cache: `{:?}`: {}", remove_path, e));
        }
        self.lru.insert(rel_path.to_owned(), size);
        Ok(())
    }

    fn insert_by<K: AsRef<OsStr>, F: FnOnce(&Path) -> io::Result<()>>(&mut self, key: K, size: Option<u64>, by: F) -> Result<()> {
        if let Some(size) = size {
            if !self.can_store(size) {
                return Err(Error::FileTooLarge);
            }
        }
        let rel_path = key.as_ref();
        let path = self.rel_to_abs_path(rel_path);
        fs::create_dir_all(path.parent().expect("Bad path?"))?;
        by(&path)?;
        let size = match size {
            Some(size) => size,
            None => fs::metadata(path)?.len(),
        };
        self.add_file(AddFile::RelPath(rel_path), size)
            .or_else(|e| {
                error!("Failed to insert file `{}`: {}", rel_path.to_string_lossy(), e);
                fs::remove_file(&self.rel_to_abs_path(rel_path)).expect("Failed to remove file we just created!");
                Err(e)
            })
    }

    /// Add a file by calling `with` with the open `File` corresponding to the cache at path `key`.
    pub fn insert_with<K: AsRef<OsStr>, F: FnOnce(File) -> io::Result<()>>(&mut self, key: K, with: F) -> Result<()> {
        self.insert_by(key, None, |path| with(File::create(&path)?))
    }

    /// Add a file with `bytes` as its contents to the cache at path `key`.
    pub fn insert_bytes<K: AsRef<OsStr>>(&mut self, key: K, bytes: &[u8]) -> Result<()> {
        self.insert_by(key, Some(bytes.len() as u64), |path| {
            let mut f = File::create(&path)?;
            f.write_all(bytes)?;
            Ok(())
        })
    }

    /// Add an existing file at `path` to the cache at path `key`.
    pub fn insert_file<K: AsRef<OsStr>, P: AsRef<OsStr>>(&mut self, key: K, path: P) -> Result<()> {
        let size = fs::metadata(path.as_ref())?.len();
        self.insert_by(key, Some(size), |new_path| {
            fs::rename(path.as_ref(), new_path)
                .or_else(|_| {
                    warn!("fs::rename failed, falling back to copy!");
                    fs::copy(path.as_ref(), new_path)?;
                    fs::remove_file(path.as_ref())
                        .unwrap_or_else(|e| error!("Failed to remove original file in insert_file: {}", e));
                    Ok(())
                })
        })
    }

    /// Return `true` if a file with path `key` is in the cache.
    pub fn contains_key<K: AsRef<OsStr>>(&self, key: K) -> bool {
        self.lru.contains_key(key.as_ref())
    }

    /// Get an opened `File` for `key`, if one exists and can be opened. Updates the LRU state
    /// of the file if present. Avoid using this method if at all possible, prefer `.get`.
    pub fn get_file<K: AsRef<OsStr>>(&mut self, key: K) -> Result<File> {
        let rel_path = key.as_ref();
        let path = self.rel_to_abs_path(rel_path);
        self.lru.get(rel_path)
            .ok_or(Error::FileNotInCache)
            .and_then(|_| {
                let t = filetime_now();
                set_file_times(&path, t, t)?;
                File::open(path).map_err(Into::into)
            })
    }

    /// Get an opened readable and seekable handle to the file at `key`, if one exists and can
    /// be opened. Updates the LRU state of the file if present.
    pub fn get<K: AsRef<OsStr>>(&mut self, key: K) -> Result<Box<dyn ReadSeek>> {
        self.get_file(key).map(|f| Box::new(f) as Box<dyn ReadSeek>)
    }
}

#[cfg(test)]
mod tests {
    use super::{LruDiskCache, Error};

    use filetime::{FileTime, set_file_times};
    use std::fs::{self,File};
    use std::io::{self, Read, Write};
    use std::path::{Path,PathBuf};
    use tempdir::TempDir;

    struct TestFixture {
        /// Temp directory.
        pub tempdir: TempDir,
    }

    fn create_file<T: AsRef<Path>, F: FnOnce(File) -> io::Result<()>>(dir: &Path, path: T, fill_contents: F) -> io::Result<PathBuf> {
        let b = dir.join(path);
        fs::create_dir_all(b.parent().unwrap())?;
        let f = fs::File::create(&b)?;
        fill_contents(f)?;
        b.canonicalize()
    }

    /// Set the last modified time of `path` backwards by `seconds` seconds.
    fn set_mtime_back<T: AsRef<Path>>(path: T, seconds: usize) {
        let m = fs::metadata(path.as_ref()).unwrap();
        let t = FileTime::from_last_modification_time(&m);
        let t = FileTime::from_seconds_since_1970(t.seconds_relative_to_1970() - seconds as u64, t.nanoseconds());
        set_file_times(path, t, t).unwrap();
    }

    fn read_all<R: Read>(r: &mut R) -> io::Result<Vec<u8>> {
        let mut v = vec!();
        r.read_to_end(&mut v)?;
        Ok(v)
    }

    impl TestFixture {
        pub fn new() -> TestFixture {
            TestFixture {
                tempdir: TempDir::new("lru-disk-cache-test").unwrap(),
            }
        }

        pub fn tmp(&self) -> &Path { self.tempdir.path() }

        pub fn create_file<T: AsRef<Path>>(&self, path: T, size: usize) -> PathBuf {
            create_file(self.tempdir.path(), path, |mut f| f.write_all(&vec![0; size])).unwrap()
        }
    }

    #[test]
    fn test_empty_dir() {
        let f = TestFixture::new();
        LruDiskCache::new(f.tmp(), 1024).unwrap();
    }

    #[test]
    fn test_missing_root() {
        let f = TestFixture::new();
        LruDiskCache::new(f.tmp().join("not-here"), 1024).unwrap();
    }

    #[test]
    fn test_some_existing_files() {
        let f = TestFixture::new();
        f.create_file("file1", 10);
        f.create_file("file2", 10);
        let c = LruDiskCache::new(f.tmp(), 20).unwrap();
        assert_eq!(c.size(), 20);
    }

    #[test]
    fn test_existing_file_too_large() {
        let f = TestFixture::new();
        // Create files explicitly in the past.
        set_mtime_back(f.create_file("file1", 10), 10);
        set_mtime_back(f.create_file("file2", 10), 5);
        let c = LruDiskCache::new(f.tmp(), 15).unwrap();
        assert_eq!(c.size(), 10);
        assert!(!c.contains_key("file1"));
        assert!(c.contains_key("file2"));
    }

    #[test]
    fn test_existing_files_lru_mtime() {
        let f = TestFixture::new();
        // Create files explicitly in the past.
        set_mtime_back(f.create_file("file1", 10), 5);
        set_mtime_back(f.create_file("file2", 10), 10);
        let mut c = LruDiskCache::new(f.tmp(), 25).unwrap();
        assert_eq!(c.size(), 20);
        c.insert_bytes("file3", &vec![0; 10]).unwrap();
        assert_eq!(c.size(), 20);
        // The oldest file on disk should have been removed.
        assert!(!c.contains_key("file2"));
        assert!(c.contains_key("file1"));
    }

    #[test]
    fn test_insert_bytes() {
        let f = TestFixture::new();
        let mut c = LruDiskCache::new(f.tmp(), 25).unwrap();
        c.insert_bytes("a/b/c", &vec![0; 10]).unwrap();
        assert!(c.contains_key("a/b/c"));
        c.insert_bytes("a/b/d", &vec![0; 10]).unwrap();
        assert_eq!(c.size(), 20);
        // Adding this third file should put the cache above the limit.
        c.insert_bytes("x/y/z", &vec![0; 10]).unwrap();
        assert_eq!(c.size(), 20);
        // The least-recently-used file should have been removed.
        assert!(!c.contains_key("a/b/c"));
        assert!(!f.tmp().join("a/b/c").exists());
    }

    #[test]
    fn test_insert_bytes_exact() {
        // Test that files adding up to exactly the size limit works.
        let f = TestFixture::new();
        let mut c = LruDiskCache::new(f.tmp(), 20).unwrap();
        c.insert_bytes("file1", &vec![1; 10]).unwrap();
        c.insert_bytes("file2", &vec![2; 10]).unwrap();
        assert_eq!(c.size(), 20);
        c.insert_bytes("file3", &vec![3; 10]).unwrap();
        assert_eq!(c.size(), 20);
        assert!(!c.contains_key("file1"));
    }

    #[test]
    fn test_add_get_lru() {
        let f = TestFixture::new();
        {
            let mut c = LruDiskCache::new(f.tmp(), 25).unwrap();
            c.insert_bytes("file1", &vec![1; 10]).unwrap();
            c.insert_bytes("file2", &vec![2; 10]).unwrap();
            // Get the file to bump its LRU status.
            assert_eq!(read_all(&mut c.get("file1").unwrap()).unwrap(), vec![1u8; 10]);
            // Adding this third file should put the cache above the limit.
            c.insert_bytes("file3", &vec![3; 10]).unwrap();
            assert_eq!(c.size(), 20);
            // The least-recently-used file should have been removed.
            assert!(!c.contains_key("file2"));
        }
        // Get rid of the cache, to test that the LRU persists on-disk as mtimes.
        // This is hacky, but mtime resolution on my mac with HFS+ is only 1 second, so we either
        // need to have a 1 second sleep in the test (boo) or adjust the mtimes back a bit so
        // that updating one file to the current time actually works to make it newer.
        set_mtime_back(f.tmp().join("file1"), 5);
        set_mtime_back(f.tmp().join("file3"), 5);
        {
            let mut c = LruDiskCache::new(f.tmp(), 25).unwrap();
            // Bump file1 again.
            c.get("file1").unwrap();
        }
        // Now check that the on-disk mtimes were updated and used.
        {
            let mut c = LruDiskCache::new(f.tmp(), 25).unwrap();
            assert!(c.contains_key("file1"));
            assert!(c.contains_key("file3"));
            assert_eq!(c.size(), 20);
            // Add another file to bump out the least-recently-used.
            c.insert_bytes("file4", &vec![4; 10]).unwrap();
            assert_eq!(c.size(), 20);
            assert!(!c.contains_key("file3"));
            assert!(c.contains_key("file1"));
        }
    }

    #[test]
    fn test_insert_bytes_too_large() {
        let f = TestFixture::new();
        let mut c = LruDiskCache::new(f.tmp(), 1).unwrap();
        match c.insert_bytes("a/b/c", &vec![0; 2]) {
            Err(Error::FileTooLarge) => assert!(true),
            x @ _ => panic!("Unexpected result: {:?}", x),
        }
    }

    #[test]
    fn test_insert_file() {
        let f = TestFixture::new();
        let p1 = f.create_file("file1", 10);
        let p2 = f.create_file("file2", 10);
        let p3 = f.create_file("file3", 10);
        let mut c = LruDiskCache::new(f.tmp().join("cache"), 25).unwrap();
        c.insert_file("file1", &p1).unwrap();
        c.insert_file("file2", &p2).unwrap();
        // Get the file to bump its LRU status.
        assert_eq!(read_all(&mut c.get("file1").unwrap()).unwrap(), vec![0u8; 10]);
        // Adding this third file should put the cache above the limit.
        c.insert_file("file3", &p3).unwrap();
        assert_eq!(c.size(), 20);
        // The least-recently-used file should have been removed.
        assert!(!c.contains_key("file2"));
        assert!(!p1.exists());
        assert!(!p2.exists());
        assert!(!p3.exists());
    }
}