use std::time::{Duration, Instant, SystemTime};

use futures::{sink::SinkExt, TryStreamExt};
use lazy_static::lazy_static;
use reqwest::Url;
use scraper::Selector;
use time::PrimitiveDateTime;
use tokio_util::codec;

lazy_static! {
    static ref CHARTS_URL: Url = Url::parse("https://www.mwra.com/biobot/biobotdata.htm").unwrap();
    static ref PDF_SEL: Selector = Selector::parse(r#"a[href$="-data.pdf"]"#).unwrap();
    static ref MIN_CHECK_INTERVAL: Duration = Duration::from_secs(300);
}

const CACHED_PDF_PATH: &str = "data.pdf";
const LAST_MODIFIED_FORMAT: &[time::format_description::FormatItem] = time::macros::format_description!(
    "[weekday repr:short], [day] [month repr:short] [year] [hour repr:24]:[minute]:[second] GMT"
);

#[derive(thiserror::Error, Debug)]
pub enum Error {
    #[error("HTTP request failed")]
    Http(#[from] reqwest::Error),
    #[error("couldn't parse URL")]
    UrlParse(#[from] url::ParseError),
    #[error("couldn't read PDF")]
    Pdf(#[from] pdf::error::PdfError),
    #[error("I/O error")]
    Io(#[from] std::io::Error),
    #[error("system clock anomaly")]
    SystemClock(#[from] std::time::SystemTimeError),
    #[error("data not found")]
    NotFound,
    #[error("requesting data too soon")]
    TooSoon,
}

pub struct PdfFetcher {
    client: reqwest::Client,
    last_checked: Option<Instant>,
}

impl PdfFetcher {
    pub fn new() -> Result<Self, Error> {
        Ok(Self {
            client: reqwest::ClientBuilder::new().build()?,
            last_checked: None,
        })
    }

    pub async fn fetch(&mut self) -> Result<pdf::file::File<Vec<u8>>, Error> {
        let cache_modtime = match tokio::fs::File::open(CACHED_PDF_PATH).await {
            Ok(file) => Some(
                file.metadata()
                    .await?
                    .modified()?
                    .duration_since(SystemTime::UNIX_EPOCH)?
                    .as_secs(),
            ),
            Err(_) => None,
        };

        if let Some(instant) = self.last_checked {
            if Instant::now() - instant < *MIN_CHECK_INTERVAL {
                return if cache_modtime.is_some() {
                    self.cached_pdf()
                } else {
                    Err(Error::TooSoon)
                };
            }
        }

        let body = self
            .client
            .get(CHARTS_URL.as_ref())
            .send()
            .await?
            .text()
            .await?;
        let document = scraper::Html::parse_document(&body);
        let pdf_href = document
            .select(&PDF_SEL)
            .next()
            .ok_or(Error::NotFound)?
            .value()
            .attr("href")
            .ok_or(Error::NotFound)?;
        let pdf_url = CHARTS_URL.join(pdf_href)?;

        let origin_modtime = self
            .client
            .head(pdf_url.clone())
            .send()
            .await?
            .headers()
            .get(reqwest::header::LAST_MODIFIED)
            .and_then(|val| {
                u64::try_from(
                    PrimitiveDateTime::parse(val.to_str().ok()?, LAST_MODIFIED_FORMAT)
                        .ok()?
                        .assume_utc()
                        .unix_timestamp(),
                )
                .ok()
            });

        let outdated = cache_modtime
            .zip(origin_modtime)
            .map_or(true, |(cache, origin)| origin > cache);

        if outdated {
            let mut pdf_stream = self
                .client
                .get(pdf_url)
                .send()
                .await?
                .bytes_stream()
                .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e));

            let cache_file = tokio::fs::File::create(CACHED_PDF_PATH).await?;
            let mut sink = codec::FramedWrite::new(cache_file, codec::BytesCodec::new());
            sink.send_all(&mut pdf_stream).await?;
            <dyn futures::Sink<bytes::Bytes, Error = std::io::Error> + Unpin>::close(&mut sink)
                .await?;
        }
        self.cached_pdf()
    }

    fn cached_pdf(&self) -> Result<pdf::file::File<Vec<u8>>, Error> {
        Ok(pdf::file::File::open(CACHED_PDF_PATH)?)
    }
}