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use std::fs;
#[cfg(unix)]
use std::os::unix::fs::MetadataExt;
#[cfg(windows)]
use std::os::windows::fs::MetadataExt;
use super::Result;
use super::Error;

/// AutoLoader will watch the filesystem for changes in the
/// passed files and will try to load them again using a load function
///
/// The only thing that AutoLoader checks is the date of the files
/// so files that are just touched but not really changed will be
/// considered changed nontheless
///
/// The watch will happen whenever the update function is called
pub struct AutoLoader<T>{
    paths: Vec<String>,
    #[cfg(unix)]
    times: Vec<i64>,
    #[cfg(windows)]
    times: Vec<u64>,
    load_function: Box<dyn Fn() -> Result<T>>,
}

#[cfg(unix)]
fn file_time(file: &str) -> Result<i64>{
    let meta = fs::metadata(file).map_err(|err| Error::with_cause("IO error", err))?;
    let time = meta.atime();
    Ok(time)
}

#[cfg(windows)]
fn file_time(file: &str) -> Result<u64>{
    let meta = fs::metadata(file).map_err(|err| Error::with_cause("IO error", err))?;
    let time = meta.last_write_time();
    Ok(time)
}

impl<T> AutoLoader<T>{
    /// Creates a new AutoLoader that will watch the passed paths
    /// for changes and execute the load_function if any of them
    /// has changed
    pub fn new<F:'static + Fn()->Result<T>>(files: Vec<String>, load_function: F) -> AutoLoader<T>{
        let times = files.iter()
            .map(|file| file_time(file).expect(&format!("Error loading {}", file)))
            .collect();
        AutoLoader{
            paths: files,
            times: times,
            load_function: Box::new(load_function),
        }
    }

    /// Checks the paths for changes and reloads using the load_function
    /// if needed
    ///
    /// If any file changed and the load function returns succesfully, update
    /// returns Ok with the new value
    ///
    /// If any file changed and the load function fails, update will return the
    /// error
    ///
    /// If no file changed update will return Ok(None)
    pub fn update(&mut self) -> Result<Option<T>>{
        let times = self.paths.iter()
            .map(|path| file_time(path).unwrap_or(0))
            .collect::<Vec<_>>();
        let changed = self.times.is_empty () || self.paths.iter()
                            .zip(self.times.iter())
                            .zip(times.iter())
                            .any(|((_path, time), new_time)| *time < *new_time);
        if changed{
            self.load().map(|v| Some(v))
        }else{
            Ok(None)
        }
    }

    /// Forces the execution of the load function and updates the dates of
    /// the files to the current one
    pub fn load(&mut self) -> Result<T>{
        self.times = self.paths.iter().map(|path| file_time(path).unwrap_or(0)).collect();
        (self.load_function)()
    }
}