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use std::fs;
use std::os::unix::fs::MetadataExt;
use ::Result;
use ::Error;

pub struct AutoLoader<T>{
    paths: Vec<String>,
    times: Vec<i64>,
    load_function: Box<Fn() -> Result<T>>,
}

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)
}

impl<T> AutoLoader<T>{
    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).unwrap()).collect();
        AutoLoader{
            paths: files,
            times: times,
            load_function: Box::new(load_function),
        }
    }

    pub fn update(&mut self) -> Result<Option<T>>{
        let times: Vec<i64> = self.paths.iter().map(|path| file_time(path).unwrap_or(0)).collect();
        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)
        }
    }

    pub fn load(&mut self) -> Result<T>{
        self.times = self.paths.iter().map(|path| file_time(path).unwrap_or(0)).collect();
        (self.load_function)()
    }
}