use std::cmp::min;
use std::collections::VecDeque;
use std::error::Error;
use std::future::Future;
use std::pin::{pin, Pin};
use std::task::{Context, Poll};
use curl::easy::{Easy2, Handler, WriteError};
use curl::multi::{Easy2Handle, EasyHandle, Multi};
use tokio::task::spawn_blocking;
use crate::launcher::constants::USER_AGENT;

trait Download {
    async fn prepare(&mut self, easy: &EasyHandle) -> Result<bool, Box<dyn Error>>;
    async fn handle_chunk(&mut self, data: &[u8]) -> Result<(), Box<dyn Error>>;
    async fn finish(&mut self) -> Result<(), Box<dyn Error>>;
}

#[derive(Clone, Copy)]
enum MultiDownloaderState {
    Primed,
    Running,
    Complete
}

struct MultiDownloader<T>
where
    T: Download + Send + Unpin + 'static
{
    state: MultiDownloaderState,
    nhandles: usize,
    jobs: Option<VecDeque<T>>
}

impl<T> MultiDownloader<T>
where
    T: Download + Send + Unpin + 'static
{
    // TODO: this interface is kind of weird (we have to take ownership of the jobs, but maybe this isn't the best way to do that)

    pub fn new(jobs: Vec<T>, nhandles: usize) -> MultiDownloader<T> {
        assert!(nhandles > 0);

        MultiDownloader {
            state: MultiDownloaderState::Primed,
            nhandles,
            jobs: Some(jobs.into())
        }
    }
}

struct MultiDownloadResult {

}

impl<T> Future for MultiDownloader<T>
where
    T: Download + Send + Unpin + 'static
{
    type Output = MultiDownloadResult;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let self_mut = self.get_mut();
        match self_mut.state {
            MultiDownloaderState::Primed => {
                self_mut.state = MultiDownloaderState::Running;
                let jobs = self_mut.jobs.take().unwrap();
                let nhandles = self_mut.nhandles;

                spawn_blocking(move || {
                    // TODO
                    MultiDownloadBlocking::new(jobs, nhandles).perform();
                });

                Poll::Pending
            },
            MultiDownloaderState::Running => {
                Poll::Pending // TODO
            },
            MultiDownloaderState::Complete => panic!("multi download polled after completion")
        }
    }
}

struct MultiHandler<T: Download> {
    job: Option<T>
}

impl<T: Download> Handler for MultiHandler<T> {
    fn write(&mut self, data: &[u8]) -> Result<usize, WriteError> {
        todo!()
    }
}

struct MultiDownloadBlocking<T: Download> {
    multi: Multi,
    easy_handles: Vec<Easy2Handle<MultiHandler<T>>>,
    jobs: VecDeque<T>
}

impl<'a, T: Download> MultiDownloadBlocking<T> {
    fn new(jobs: VecDeque<T>, nhandles: usize) -> MultiDownloadBlocking<T> {
        assert!(nhandles > 0);

        let nhandles = min(nhandles, jobs.len());

        let multi = Multi::new();
        let mut easy_handles = Vec::with_capacity(nhandles);

        for n in 0..nhandles {
            let mut easy = Easy2::new(MultiHandler { job: None });
            easy.useragent(USER_AGENT).expect("setting user agent shouldn't fail");

            let mut handle = multi.add2(easy).expect("adding easy handle cannot fail");
            handle.set_token(n).expect("setting token cannot fail");
            easy_handles.push(handle);
        }

        MultiDownloadBlocking {
            multi,
            easy_handles,
            jobs
        }
    }
    
    fn prepare_job(&mut self, easy: &mut Easy2Handle<MultiHandler<T>>, job: T) {
        let handler = easy.get_mut();

        todo!()
    }

    fn perform(&mut self) -> MultiDownloadResult {
        todo!()
    }
}

// 
// pub struct MultiDownloader<'j> {
//     state: MultiDownloaderState,
//     jobs: VecDeque<DownloadJob>,
//     multi: Multi,
//     handles: Vec<Easy2Handle<EasyHandler<'j>>>
// }
// 
// pub struct EasyHandler<'j> {
//     job: Option<&'j DownloadJob>
// }
// 
// impl<'j> Handler for EasyHandler<'j> {
//     fn write(&mut self, data: &[u8]) -> Result<usize, WriteError> {
// 
//     }
// }
// 
// impl<'j, 'js, T> MultiDownloader<'j, 'js, T>
// where
//     T: Download + Sync + Send + ?Sized
// {
//     pub fn new(jobs: &'js [&'j T]) -> MultiDownloader<'j, 'js, T> {
//         Self::with_handles(jobs, 8)
//     }
// 
//     pub fn with_handles(jobs: &'js [&'j T], nhandles: usize) -> MultiDownloader<'j, 'js, T> {
//         assert!(nhandles > 0);
// 
//         let mut handles = Vec::with_capacity(nhandles);
//         let multi = Multi::new();
// 
//         for n in 0..nhandles {
//             let mut easy = multi.add2(Easy2::new(EasyHandler { job: None })).expect("adding easy handle should not fail");
//             easy.set_token(n).expect("setting token should not fail");
// 
//             handles.push(easy);
//         }
// 
//         MultiDownloader {
//             state: MultiDownloaderState::Primed,
//             
//             multi,
//             handles
//         }
//     }
// 
//     fn next_job(&mut self) -> Option<&T> {
//         let oj = self.jobs.get(self.job_idx).cloned();
//         self.job_idx += 1;
//         oj
//     }
// }
// 
// pub struct MultiDownloadResult {
// 
// }
// 
// impl<'j, 'js, T> Future for MultiDownloader<'j, 'js, T>
// where
//     T: Download + Sync + Send + ?Sized
// {
//     type Output = MultiDownloadResult;
// 
//     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
//         let self_mut = self.get_mut();
//         match self_mut.state {
//             MultiDownloaderState::Primed => {
//                 self_mut.state = MultiDownloaderState::Running;
// 
//                 // TODO: assign download jobs to each
// 
//                 Poll::Pending
//             },
//             MultiDownloaderState::Running => {
//                 todo!()
//             },
//             MultiDownloaderState::Complete => panic!("multi downloader polled after completion")
//         }
//     }
// }

// fn sample() {
//     struct AbcdDownloader {
//         file: Option<File>,
//         path: PathBuf,
//         url: String,
//         expect_size: usize,
//         expect_sha1: Digest
//     }
//
//     impl Download for AbcdDownloader {
//         async fn before_download(&mut self) -> Result<bool, Box<dyn Error>> {
//             let mut file = match File::open(&self.path).await {
//                 Ok(f) => f,
//                 Err(e) => return match e.kind() {
//                     ErrorKind::NotFound => {
//                         // TODO: ensure parent dirs exist
//
//                         Ok(true)
//                     },
//                     _ => Err(e.into())
//                 }
//             };
//
//             let mut buf = [0u8; 4096];
//             let mut sha1 = Sha1::new();
//             let mut tally = 0usize;
//
//             loop {
//                 let n = file.read(&mut buf[..]).await?;
//                 if n == 0 {
//                     file.seek(SeekFrom::Start(0)).await?;
//                     break;
//                 }
//
//                 sha1.update(&buf[..n]);
//                 tally += n;
//             }
//
//             Ok(tally != self.expect_size || sha1.digest() != self.expect_sha1)
//         }
//
//         async fn handle_chunk(&mut self, data: &[u8]) -> Result<usize, WriteError> {
//             self.file.unwrap().write_all(data).await
//         }
//
//         async fn after_download(&mut self) -> Result<(), Box<dyn Error>> {
//             todo!()
//         }
//     }
//
//     let mut jobs = Vec::new();
//     jobs.push(AbcdDownloader {
//         file: None,
//         path: PathBuf::from("/home/bigfoot/test.words"),
//         url: String::from("https://github.com/"),
//         expect_size: 10,
//         expect_sha1: Digest::default()
//     });
//
//     let abcd = MultiDownloader::new(jobs.iter().map(|a| a).collect::<Vec<&AbcdDownloader>>().as_slice());
// }