use futures::Stream;
pub use futures::StreamExt;
pub use reqwest::header::{HeaderMap, HeaderValue};
use reqwest::header::{ACCEPT, AUTHORIZATION};
pub use reqwest::{StatusCode, Url};
use serde::de::DeserializeOwned;
use std::panic::{RefUnwindSafe, UnwindSafe};
use std::sync::atomic::AtomicU64;
use std::sync::atomic::Ordering::SeqCst;
use std::sync::Arc;
use std::time::{Duration, SystemTime};

/// Response from the API
pub struct Response {
    res: reqwest::Response,
    client: Arc<ClientInner>,
    config: ConfigInner,
}

impl Response {
    /// Fetch a single JSON object from the API
    pub async fn obj<T: DeserializeOwned>(self) -> Result<T, GHError> {
        Ok(self.res.json().await?)
    }

    /// Stream an array of objects from the API
    pub fn array<T: DeserializeOwned + Unpin + 'static>(self) -> impl Stream<Item = Result<T, GHError>> {
        let mut res = self.res;
        let client = self.client;
        let config = self.config;

        // Pin is required for easy iteration, otherwise the caller would have to pin it
        Box::pin(async_stream::try_stream! {
            #[allow(clippy::large_futures)]
            loop {
                let next_link = res.headers().get("link")
                    .and_then(|h| h.to_str().ok())
                    .and_then(parse_next_link);
                let items = res.json::<Vec<T>>().await?;
                for item in items {
                    yield item;
                }
                match next_link {
                    Some(url) => res = client.raw_get(&url, &config).await?,
                    None => break,
                }
            }
        })
    }

    /// Response headers
    pub fn headers(&self) -> &HeaderMap {
        self.res.headers()
    }

    /// Response status
    pub fn status(&self) -> StatusCode {
        self.res.status()
    }

    /// Final URL
    pub fn url(&self) -> &Url {
        self.res.url()
    }
}

/// See `Client::get()`
///
/// Make a new request by constructing the request URL bit by bit
pub struct Builder {
    client: Arc<ClientInner>,
    config: ConfigInner,
    url: String,
    query_string_started: bool,
}

impl Builder {
    /// Add a constant path to the request, e.g. `.path("users")`
    ///
    /// Inner slashes are OK, but the string must not start or end with a slash.
    ///
    /// Panics if query string has been added.
    ///
    /// It's appended raw, so must be URL-safe.
    #[must_use]
    pub fn path(mut self, url_part: &'static str) -> Self {
        debug_assert_eq!(url_part, url_part.trim_matches('/'));
        assert!(!self.query_string_started);

        self.url.push('/');
        self.url.push_str(url_part);
        self
    }

    /// Add a user-supplied argument to the request path, e.g. `.path("users").arg(username)`,
    /// or after a call to `query()`, starts adding fragments to the query string with no delimiters.
    ///
    /// The arg is URL-escaped, so it's safe to use any user-supplied data.
    #[must_use]
    pub fn arg(mut self, arg: &str) -> Self {
        if !self.query_string_started {
            self.url.push('/');
        }
        use std::fmt::Write;
        write!(&mut self.url, "{}", urlencoding::Encoded(arg)).unwrap();
        self
    }

    /// Add a raw unescaped query string. The string must *not* start with `?`
    ///
    /// ```rust
    /// # github_v3::Client::new(None)
    /// .get().path("search/users").query("q=").arg("somestring");
    /// ```
    #[must_use]
    pub fn query(mut self, query_string: &str) -> Self {
        debug_assert!(!query_string.starts_with('?'));
        debug_assert!(!query_string.starts_with('&'));
        self.url.push(if self.query_string_started { '&' } else { '?' });
        self.url.push_str(query_string);
        self.query_string_started = true;
        self
    }

    /// Make the request
    pub async fn send(self) -> Result<Response, GHError> {
        let res = Box::pin(self.client.raw_get(&self.url, &self.config)).await?;
        Ok(Response {
            client: self.client,
            config: self.config,
            res,
        })
    }
}

struct ClientInner {
    client: reqwest::Client,
    // FIXME: this should be per endpoint, because search and others have different throttling
    wait_until_timestamp_ms: AtomicU64,
}

#[derive(Clone)]
struct ConfigInner {
    authorization_header: Option<HeaderValue>,
}

impl UnwindSafe for Client {}
impl RefUnwindSafe for Client {}

/// API Client. Start here.
#[derive(Clone)]
pub struct Client {
    inner: Arc<ClientInner>,
    config: ConfigInner,
}

impl Client {
    /// Reads `GITHUB_TOKEN` env var.
    #[must_use] pub fn new_from_env() -> Self {
        Self::new(std::env::var("GITHUB_TOKEN").ok().as_deref())
    }

    /// Takes API token for authenticated requests (make the token in GitHub settings)
    #[must_use]
    pub fn new(token: Option<&str>) -> Self {
        let mut default_headers = HeaderMap::with_capacity(2);
        default_headers.insert(ACCEPT, HeaderValue::from_static("application/vnd.github.v3+json"));
        default_headers.insert("X-GitHub-Api-Version", HeaderValue::from_static("2022-11-28"));

        Self {
            config: ConfigInner {
                authorization_header: token.and_then(|token| HeaderValue::from_str(&format!("token {token}")).ok()),
            },
            inner: Arc::new(ClientInner {
                client: reqwest::Client::builder()
                    .user_agent(concat!("rust-github-v3/{}", env!("CARGO_PKG_VERSION")))
                    .default_headers(default_headers)
                    .connect_timeout(Duration::from_secs(4))
                    .timeout(Duration::from_secs(20))
                    .build()
                    .unwrap(),
                wait_until_timestamp_ms: AtomicU64::new(0),
            }),
        }
    }

    /// Takes raw header value (e.g. "Bearer …")
    #[must_use]
    pub fn with_authorization(&self, header: Option<&str>) -> Self {
        Self {
            config: ConfigInner {
                authorization_header: header.and_then(|header| HeaderValue::from_str(header).ok()),
            },
            inner: self.inner.clone(),
        }
    }

    /// Make a new request to the API.
    #[must_use]
    pub fn get(&self) -> Builder {
        let mut url = String::with_capacity(100);
        url.push_str("https://api.github.com");
        Builder {
            client: self.inner.clone(),
            config: self.config.clone(),
            url,
            query_string_started: false,
        }
    }

    pub fn wait_time(&self) -> Duration {
        self.inner.wait_time()
    }
}

impl ClientInner {
    fn wait_time(&self) -> Duration {
        let ts_ms = self.wait_until_timestamp_ms.load(SeqCst);
        let until = SystemTime::UNIX_EPOCH + Duration::from_millis(ts_ms);
        until.duration_since(SystemTime::now()).unwrap_or(Duration::ZERO)
    }

    // Get a single response
    async fn raw_get(&self, url: &str, config: &ConfigInner) -> Result<reqwest::Response, GHError> {
        debug_assert!(url.starts_with("https://api.github.com/"));

        let mut retries = 5u8;
        let mut retry_delay = 1;
        loop {
            let wait_duration = self.wait_time();
            if wait_duration > Duration::ZERO {
                // This has poor behavior with concurrency. It should be pacing all requests.
                tokio::time::sleep(wait_duration).await;
            }

            let mut req = self.client.get(url);
            if let Some(auth) = &config.authorization_header {
                req = req.header(AUTHORIZATION, auth);
            }

            let res = req.send().await?;

            let headers = res.headers();
            let status = res.status();

            let now = SystemTime::now();
            let wait_duration = match (Self::rate_limit_remaining(headers), Self::rate_limit_reset(headers)) {
                (Some(rl), Some(rs)) => {
                    rs.duration_since(now).ok()
                        .and_then(|d| d.checked_div(rl + 2))
                        .unwrap_or(Duration::ZERO)
                        .min(Duration::from_secs(30)) // 30s wait max
                }
                _ => Duration::from_secs(if status == StatusCode::TOO_MANY_REQUESTS {3} else {0}),
            };
            let wait_until = now + wait_duration;
            let wait_until_timestamp_ms = wait_until.duration_since(SystemTime::UNIX_EPOCH).unwrap().as_millis() as u64;
            self.wait_until_timestamp_ms.store(wait_until_timestamp_ms, SeqCst);

            let should_wait_for_content = status == StatusCode::ACCEPTED;
            if should_wait_for_content && retries > 0 {
                tokio::time::sleep(Duration::from_secs(retry_delay)).await;
                retry_delay *= 2;
                retries -= 1;
                continue;
            }

            return if status.is_success() && !should_wait_for_content {
                Ok(res)
            } else {
                Err(error_for_response(res).await)
            };
        }
    }

    /// GitHub's `x-ratelimit-remaining` header
    pub fn rate_limit_remaining(headers: &HeaderMap) -> Option<u32> {
        headers.get("x-ratelimit-remaining")
            .and_then(|s| s.to_str().ok())
            .and_then(|s| s.parse().ok())
    }

    /// GitHub's `x-ratelimit-reset` header
    pub fn rate_limit_reset(headers: &HeaderMap) -> Option<SystemTime> {
        headers
            .get("x-ratelimit-reset")
            .and_then(|s| s.to_str().ok())
            .and_then(|s| s.parse().ok())
            .map(|s| SystemTime::UNIX_EPOCH + Duration::from_secs(s))
    }
}

async fn error_for_response(res: reqwest::Response) -> GHError {
    let status = res.status();
    let mime = res.headers().get("content-type").and_then(|h| h.to_str().ok()).unwrap_or("");
    GHError::Response {
        status,
        message: if mime.starts_with("application/json") {
            res.json::<GitHubErrorResponse>().await.ok().map(|res| res.message)
        } else {
            None
        },
    }
}

fn parse_next_link(link: &str) -> Option<String> {
    for part in link.split(',') {
        if part.contains(r#"; rel="next""#) {
            if let Some(start) = part.find('<') {
                let next_link = &part[start + 1..];
                if let Some(end) = next_link.find('>') {
                    return Some(next_link[..end].to_owned());
                }
            }
        }
    }
    None
}

#[derive(serde::Deserialize)]
struct GitHubErrorResponse {
    message: String,
}

use thiserror::Error;

#[derive(Error, Debug)]
pub enum GHError {
    #[error("Request timed out")]
    Timeout,
    #[error("Request error: {}", _0)]
    Request(String),
    #[error("{} ({})", message.as_deref().unwrap_or("HTTP error"), status)]
    Response { status: StatusCode, message: Option<String> },
    #[error("Internal error")]
    Internal,
}

impl From<reqwest::Error> for GHError {
    fn from(e: reqwest::Error) -> Self {
        if e.is_timeout() {
            return Self::Timeout;
        }
        if let Some(status) = e.status() {
            Self::Response {
                status,
                message: Some(e.to_string()),
            }
        } else {
            Self::Request(e.to_string())
        }
    }
}

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

    #[tokio::test]
    async fn req_test() {
        let gh = Client::new_from_env();
        gh.get().path("users/octocat/orgs").send().await.unwrap();
    }

    #[test]
    fn parse_next_link_test() {
        let example = "\"<https://api.github.com/organizations/fakeid/repos?page=1>; rel=\"prev\", <https://api.github.com/organizations/fakeid/repos?page=3>; rel=\"next\", <https://api.github.com/organizations/fakeid/repos?page=38>; rel=\"last\", <https://api.github.com/organizations/fakeid/repos?page=1>; rel=\"first\"";

        let expected = Some(String::from("https://api.github.com/organizations/fakeid/repos?page=3"));
        assert_eq!(parse_next_link(example), expected);
    }
}