use std::marker::PhantomData;

use countio::Counter;
use quick_xml::{events, Reader};
use url::Url;

use crate::{
    attribute as attr,
    parse::XmlParser,
    record::{EntryRecord, IndexRecord},
    Error,
};

// TODO: Limits.
pub(crate) enum SitemapDetector<R> {
    // Plain(TxtParser<R, Url>),
    Entry(XmlParser<R, EntryRecord>),
    Index(XmlParser<R, IndexRecord>),
}

impl<R> SitemapDetector<R> {
    fn is_index(event: events::Event) -> Result<Option<bool>, Error> {
        if let events::Event::Start(bytes) = event {
            let name = bytes.name().into_inner();
            if name.eq_ignore_ascii_case(attr::SITEMAP_INDEX.as_bytes()) {
                return Ok(Some(true));
            } else if name.eq_ignore_ascii_case(attr::URL_SET.as_bytes()) {
                return Ok(Some(false));
            }
        }

        Ok(None)
    }

    fn create(is_index: bool, reader: Reader<Counter<R>>) -> Self {
        if is_index {
            Self::Index(XmlParser::from_wrapper(reader, attr::SITEMAP_INDEX))
        } else {
            Self::Entry(XmlParser::from_wrapper(reader, attr::URL_SET))
        }
    }
}

impl<R: std::io::BufRead> SitemapDetector<R> {
    ///
    pub fn from_sync(reader: R) -> Result<Self, Error> {
        let mut reader = Reader::from_reader(Counter::new(reader));
        let mut buf = Vec::new();

        loop {
            let event = reader.read_event_into(&mut buf)?;
            if let Some(is_index) = Self::is_index(event)? {
                return Ok(Self::create(is_index, reader));
            }
        }
    }
}

#[cfg(feature = "tokio")]
#[cfg_attr(docsrs, doc(cfg(feature = "tokio")))]
impl<R: tokio::io::AsyncBufRead + Unpin + Send> SitemapDetector<R> {
    ///
    pub async fn from_async(reader: R) -> Result<Self, Error> {
        let mut reader = Reader::from_reader(Counter::new(reader));
        let mut buf = Vec::new();

        loop {
            let event = reader.read_event_into_async(&mut buf).await?;
            if let Some(is_index) = Self::is_index(event)? {
                return Ok(Self::create(is_index, reader));
            }
        }
    }
}

/// Automatic sitemap record resolver.
///
/// ```rust
/// #[derive(Debug, thiserror::Error)]
/// enum CustomError {
///     // ..
///     #[error("sitemap error: {0}")]
///     Sitemap(#[from] sitemapo::Error),
///     //..
/// }
///
/// fn main() -> Result<(), CustomError> {
///     type SyncReader = std::io::BufReader<std::io::Cursor<Vec<u8>>>;
///     fn fetch(_: url::Url) -> Result<SyncReader, CustomError> {
///         // ..
///         unreachable!()
///     }
///
///     let sitemaps = Vec::default(); // Sitemaps listed in the robots.txt file.
///     let mut parser = sitemapo::AutoParser::new_sync(&sitemaps, fetch);
///     while let Some(_record) = parser.read_sync()? {
///         // ..
///     }
///
///     Ok(())
/// }
/// ```
pub struct AutoParser<R, E, O, F: Fn(Url) -> O> {
    sitemaps: Vec<Url>,
    // plain: Option<TxtParser<R, Url>>,
    index: Option<XmlParser<R, IndexRecord>>,
    entry: Option<XmlParser<R, EntryRecord>>,

    error_type: PhantomData<E>,
    fetcher: F,
}

impl<R, E: std::error::Error + From<Error>, O, F: Fn(Url) -> O> AutoParser<R, E, O, F> {
    fn new(sitemaps: &[Url], fetcher: F) -> Self {
        Self {
            sitemaps: sitemaps.to_vec(),
            // plain: None,
            index: None,
            entry: None,
            error_type: PhantomData,
            fetcher,
        }
    }

    ///
    fn save_parser(&mut self, detector: SitemapDetector<R>) {
        match detector {
            // SitemapDetector::Plain(parser) => self.plain = Some(parser),
            SitemapDetector::Entry(parser) => self.entry = Some(parser),
            SitemapDetector::Index(parser) => self.index = Some(parser),
        }
    }
}

impl<R: std::io::BufRead, E: std::error::Error + From<Error>, F: Fn(Url) -> Result<R, E>>
    AutoParser<R, E, Result<R, E>, F>
{
    /// Creates a new instance from the `robots.txt` provided list of root sitemaps
    /// (or sitemap indexes) and callback for fetching.
    pub fn new_sync(sitemaps: &[Url], fetcher: F) -> Self {
        Self::new(sitemaps, fetcher)
    }

    ///
    pub fn read_sync(&mut self) -> Result<Option<EntryRecord>, E> {
        while !self.sitemaps.is_empty() || self.index.is_some() || self.entry.is_some() {
            if self.index.is_none() && self.entry.is_none() {
                let sitemap = self.sitemaps.pop().expect("not empty");
                let reader = (self.fetcher)(sitemap)?;
                let detector = SitemapDetector::from_sync(reader)?;
                self.save_parser(detector);
            }

            if let Some(parser) = &mut self.entry {
                use crate::parse::Parser;
                let r = parser.read();
                if let Ok(Some(record)) = r {
                    return Ok(Some(record));
                }

                self.entry.take();
                if let Err(e) = r {
                    return Err(e.into());
                }
            }

            if let Some(parser) = &mut self.index {
                use crate::parse::Parser;
                let record = parser.read();
                if let Ok(Some(r)) = &record {
                    let reader = (self.fetcher)(r.location().clone())?;
                    match SitemapDetector::from_sync(reader)? {
                        SitemapDetector::Index(_) => { /* Ignore nested sitemap index. */ }
                        sitemap => self.save_parser(sitemap),
                    }
                }

                self.index.take();
                if let Err(e) = record {
                    return Err(e.into());
                }
            }
        }

        Ok(None)
    }
}

impl<R: std::io::BufRead, E: std::error::Error + From<Error>, F: Fn(Url) -> Result<R, E>> Iterator
    for AutoParser<R, E, Result<R, E>, F>
{
    type Item = EntryRecord;

    fn next(&mut self) -> Option<Self::Item> {
        self.read_sync().ok().flatten()
    }
}

// TODO: AsyncIterator/Stream.
// https://tokio.rs/tokio/tutorial/streams
#[cfg(feature = "tokio")]
#[cfg_attr(docsrs, doc(cfg(feature = "tokio")))]
impl<
        R: tokio::io::AsyncBufRead + Unpin + Send,
        E: std::error::Error + From<Error>,
        A: std::future::Future<Output = Result<R, E>>,
        F: Fn(Url) -> A,
    > AutoParser<R, E, A, F>
{
    /// Creates a new instance from the `robots.txt` provided list of root sitemaps
    /// (or sitemap indexes) and callback for fetching.
    pub fn new_async(sitemaps: &[Url], fetcher: F) -> Self {
        Self::new(sitemaps, fetcher)
    }

    ///
    pub async fn read_async(&mut self) -> Result<Option<EntryRecord>, E> {
        while !self.sitemaps.is_empty() || self.index.is_some() || self.entry.is_some() {
            if self.index.is_none() && self.entry.is_none() {
                let sitemap = self.sitemaps.pop().expect("not empty");
                let reader = (self.fetcher)(sitemap).await?;
                let detector = SitemapDetector::from_async(reader).await?;
                self.save_parser(detector);
            }

            if let Some(parser) = &mut self.entry {
                use crate::parse::AsyncParser;
                let r = parser.read().await;
                if let Ok(Some(record)) = r {
                    return Ok(Some(record));
                }

                self.entry.take();
                if let Err(e) = r {
                    return Err(e.into());
                }
            }

            if let Some(parser) = &mut self.index {
                use crate::parse::AsyncParser;
                let record = parser.read().await;
                if let Ok(Some(r)) = &record {
                    let reader = (self.fetcher)(r.location().clone()).await?;
                    match SitemapDetector::from_async(reader).await? {
                        SitemapDetector::Index(_) => { /* Ignore nested sitemap index. */ }
                        sitemap => self.save_parser(sitemap),
                    }
                }

                self.index.take();
                if let Err(e) = record {
                    return Err(e.into());
                }
            }
        }

        Ok(None)
    }
}

impl<R, E: std::error::Error + From<Error>, O, F: Fn(Url) -> O> std::fmt::Debug
    for AutoParser<R, E, O, F>
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Sitemap")
            .field("pool", &self.sitemaps.len())
            .field("index", &self.index.is_some())
            .field("entry", &self.entry.is_some())
            .finish()
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[derive(Debug, thiserror::Error)]
    enum CustomError {
        #[error("sitemap error: {0}")]
        Sitemap(#[from] Error),
    }

    #[test]
    fn synk() -> Result<(), Error> {
        type SyncReader = std::io::BufReader<std::io::Cursor<Vec<u8>>>;
        fn sync_fetcher(_: Url) -> Result<SyncReader, CustomError> {
            unreachable!()
        }

        let _ = AutoParser::new_sync(&[], sync_fetcher);
        Ok(())
    }

    #[cfg(feature = "tokio")]
    #[tokio::test]
    async fn asynk() -> Result<(), Error> {
        type AsyncReader = tokio::io::BufReader<std::io::Cursor<Vec<u8>>>;
        async fn async_fetcher(_: Url) -> Result<AsyncReader, test::CustomError> {
            unreachable!()
        }

        let _ = AutoParser::new_async(&[], async_fetcher);
        Ok(())
    }
}