use asupersync::time::{sleep, wall_now};
use async_stream::stream;
use chrono::{DateTime, Utc};
use futures::Stream;
use std::pin::Pin;
use std::sync::Arc;
use std::sync::Mutex;
use std::time::{Duration, Instant};
use tracing::debug;

use super::types::{
    GitLabDiscussion, GitLabIssue, GitLabIssueRef, GitLabLabelEvent, GitLabMergeRequest,
    GitLabMilestoneEvent, GitLabMrDiff, GitLabProject, GitLabStateEvent, GitLabUser, GitLabVersion,
};
use crate::core::error::{LoreError, Result};
use crate::http;

struct RateLimiter {
    last_request: Instant,
    min_interval: Duration,
}

impl RateLimiter {
    fn new(requests_per_second: f64) -> Self {
        let rps = requests_per_second.max(0.1);
        Self {
            last_request: Instant::now() - Duration::from_secs(1),
            min_interval: Duration::from_secs_f64(1.0 / rps),
        }
    }

    fn check_delay(&mut self) -> Option<Duration> {
        let elapsed = self.last_request.elapsed();

        if elapsed < self.min_interval {
            let jitter = Duration::from_millis(rand_jitter());
            let delay = self.min_interval - elapsed + jitter;
            self.last_request = Instant::now() + delay;
            Some(delay)
        } else {
            self.last_request = Instant::now();
            None
        }
    }
}

fn rand_jitter() -> u64 {
    use std::sync::atomic::{AtomicU64, Ordering};
    static COUNTER: AtomicU64 = AtomicU64::new(0);
    let n = COUNTER.fetch_add(1, Ordering::Relaxed);
    let nanos = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .unwrap_or_default()
        .subsec_nanos() as u64;
    (n ^ nanos) % 50
}

pub struct GitLabClient {
    client: http::Client,
    base_url: String,
    token: String,
    rate_limiter: Arc<Mutex<RateLimiter>>,
}

impl GitLabClient {
    pub fn new(base_url: &str, token: &str, requests_per_second: Option<f64>) -> Self {
        Self {
            client: http::Client::with_timeout(Duration::from_secs(30)),
            base_url: base_url.trim_end_matches('/').to_string(),
            token: token.to_string(),
            rate_limiter: Arc::new(Mutex::new(RateLimiter::new(
                requests_per_second.unwrap_or(10.0),
            ))),
        }
    }

    pub fn graphql_client(&self) -> crate::gitlab::graphql::GraphqlClient {
        crate::gitlab::graphql::GraphqlClient::new(&self.base_url, &self.token)
    }

    pub async fn get_current_user(&self) -> Result<GitLabUser> {
        self.request("/api/v4/user").await
    }

    pub async fn get_project(&self, path_with_namespace: &str) -> Result<GitLabProject> {
        let encoded = urlencoding::encode(path_with_namespace);
        self.request(&format!("/api/v4/projects/{encoded}")).await
    }

    pub async fn get_version(&self) -> Result<GitLabVersion> {
        self.request("/api/v4/version").await
    }

    pub async fn get_issue_by_iid(&self, project_id: i64, iid: i64) -> Result<GitLabIssue> {
        self.request(&format!("/api/v4/projects/{project_id}/issues/{iid}"))
            .await
    }

    pub async fn get_mr_by_iid(&self, project_id: i64, iid: i64) -> Result<GitLabMergeRequest> {
        self.request(&format!(
            "/api/v4/projects/{project_id}/merge_requests/{iid}"
        ))
        .await
    }

    const MAX_RETRIES: u32 = 3;

    async fn request<T: serde::de::DeserializeOwned>(&self, path: &str) -> Result<T> {
        let url = format!("{}{}", self.base_url, path);
        let mut last_response = None;

        for attempt in 0..=Self::MAX_RETRIES {
            // SAFETY: std::sync::Mutex blocks the executor thread while held. This is safe
            // because the critical section is a single Instant::now() comparison with no I/O.
            // If async work is ever added inside the lock, switch to an async-aware lock.
            let delay = {
                let mut limiter = self
                    .rate_limiter
                    .lock()
                    .unwrap_or_else(|poisoned| poisoned.into_inner());
                limiter.check_delay()
            };
            if let Some(d) = delay {
                sleep(wall_now(), d).await;
            }

            debug!(url = %url, attempt, "GitLab request");

            let response = self
                .client
                .get(
                    &url,
                    &[
                        ("PRIVATE-TOKEN", self.token.as_str()),
                        ("Accept", "application/json"),
                    ],
                )
                .await?;

            if response.status == 429 && attempt < Self::MAX_RETRIES {
                let retry_after = Self::parse_retry_after(&response);
                tracing::info!(
                    path = %path,
                    attempt,
                    retry_after_secs = retry_after,
                    status_code = 429u16,
                    "Rate limited, retrying"
                );
                sleep(wall_now(), Duration::from_secs(retry_after)).await;
                continue;
            }

            last_response = Some(response);
            break;
        }

        self.handle_response(last_response.expect("retry loop ran at least once"), path)
    }

    fn parse_retry_after(response: &http::Response) -> u64 {
        response
            .header("retry-after")
            .and_then(|s| s.parse().ok())
            .unwrap_or(60)
    }

    fn handle_response<T: serde::de::DeserializeOwned>(
        &self,
        response: http::Response,
        path: &str,
    ) -> Result<T> {
        match response.status {
            401 => Err(LoreError::GitLabAuthFailed),
            404 => Err(LoreError::GitLabNotFound {
                resource: path.to_string(),
            }),
            429 => {
                let retry_after = Self::parse_retry_after(&response);
                Err(LoreError::GitLabRateLimited { retry_after })
            }
            _ if response.is_success() => response.json::<T>().map_err(|e| {
                LoreError::Other(format!("Failed to decode response from {path}: {e}"))
            }),
            s => Err(LoreError::Other(format!(
                "GitLab API error: {s} {}",
                response.reason
            ))),
        }
    }

    pub fn paginate_issues(
        &self,
        gitlab_project_id: i64,
        updated_after: Option<i64>,
        cursor_rewind_seconds: u32,
    ) -> Pin<Box<dyn Stream<Item = Result<GitLabIssue>> + Send + '_>> {
        Box::pin(stream! {
            let mut page = 1u32;
            let per_page = 100u32;

            let rewound_cursor = updated_after.map(|ts| {
                let rewind_ms = (cursor_rewind_seconds as i64) * 1000;
                (ts - rewind_ms).max(0)
            });

            loop {
                let mut params = vec![
                    ("scope", "all".to_string()),
                    ("state", "all".to_string()),
                    ("order_by", "updated_at".to_string()),
                    ("sort", "asc".to_string()),
                    ("per_page", per_page.to_string()),
                    ("page", page.to_string()),
                ];

                if let Some(ts_ms) = rewound_cursor
                    && let Some(iso) = ms_to_iso8601(ts_ms)
                {
                    params.push(("updated_after", iso));
                }

                let path = format!("/api/v4/projects/{}/issues", gitlab_project_id);
                let result = self.request_with_headers::<Vec<GitLabIssue>>(&path, &params).await;

                match result {
                    Ok((issues, headers)) => {
                        let is_empty = issues.is_empty();
                        let full_page = issues.len() as u32 == per_page;

                        for issue in issues {
                            yield Ok(issue);
                        }

                        let next_page = header_value(&headers, "x-next-page")
                            .and_then(|s| s.parse::<u32>().ok());

                        match next_page {
                            Some(next) if next > page => {
                                page = next;
                            }
                            _ => {
                                if is_empty || !full_page {
                                    break;
                                }
                                page += 1;
                            }
                        }
                    }
                    Err(e) => {
                        yield Err(e);
                        break;
                    }
                }
            }
        })
    }

    pub fn paginate_issue_discussions(
        &self,
        gitlab_project_id: i64,
        issue_iid: i64,
    ) -> Pin<Box<dyn Stream<Item = Result<GitLabDiscussion>> + Send + '_>> {
        Box::pin(stream! {
            let mut page = 1u32;
            let per_page = 100u32;

            loop {
                let params = vec![
                    ("per_page", per_page.to_string()),
                    ("page", page.to_string()),
                ];

                let path = format!(
                    "/api/v4/projects/{}/issues/{}/discussions",
                    gitlab_project_id, issue_iid
                );
                let result = self.request_with_headers::<Vec<GitLabDiscussion>>(&path, &params).await;

                match result {
                    Ok((discussions, headers)) => {
                        let is_empty = discussions.is_empty();
                        let full_page = discussions.len() as u32 == per_page;

                        for discussion in discussions {
                            yield Ok(discussion);
                        }

                        let next_page = header_value(&headers, "x-next-page")
                            .and_then(|s| s.parse::<u32>().ok());

                        match next_page {
                            Some(next) if next > page => {
                                page = next;
                            }
                            _ => {
                                if is_empty || !full_page {
                                    break;
                                }
                                page += 1;
                            }
                        }
                    }
                    Err(e) => {
                        yield Err(e);
                        break;
                    }
                }
            }
        })
    }

    pub fn paginate_merge_requests(
        &self,
        gitlab_project_id: i64,
        updated_after: Option<i64>,
        cursor_rewind_seconds: u32,
    ) -> Pin<Box<dyn Stream<Item = Result<GitLabMergeRequest>> + Send + '_>> {
        Box::pin(stream! {
            let mut page = 1u32;
            let per_page = 100u32;

            loop {
                let page_result = self
                    .fetch_merge_requests_page(
                        gitlab_project_id,
                        updated_after,
                        cursor_rewind_seconds,
                        page,
                        per_page,
                    )
                    .await;

                match page_result {
                    Ok(mr_page) => {
                        for mr in mr_page.items {
                            yield Ok(mr);
                        }

                        if mr_page.is_last_page {
                            break;
                        }

                        match mr_page.next_page {
                            Some(np) => page = np,
                            None => break,
                        }
                    }
                    Err(e) => {
                        yield Err(e);
                        break;
                    }
                }
            }
        })
    }

    pub async fn fetch_merge_requests_page(
        &self,
        gitlab_project_id: i64,
        updated_after: Option<i64>,
        cursor_rewind_seconds: u32,
        page: u32,
        per_page: u32,
    ) -> Result<MergeRequestPage> {
        let rewound_cursor = updated_after.map(|ts| {
            let rewind_ms = (cursor_rewind_seconds as i64) * 1000;
            (ts - rewind_ms).max(0)
        });

        let mut params = vec![
            ("scope", "all".to_string()),
            ("state", "all".to_string()),
            ("order_by", "updated_at".to_string()),
            ("sort", "asc".to_string()),
            ("per_page", per_page.to_string()),
            ("page", page.to_string()),
        ];

        if let Some(ts_ms) = rewound_cursor
            && let Some(iso) = ms_to_iso8601(ts_ms)
        {
            params.push(("updated_after", iso));
        }

        let path = format!("/api/v4/projects/{}/merge_requests", gitlab_project_id);
        let (items, headers) = self
            .request_with_headers::<Vec<GitLabMergeRequest>>(&path, &params)
            .await?;

        let link_next = parse_link_header_next(&headers);
        let x_next_page = header_value(&headers, "x-next-page").and_then(|s| s.parse::<u32>().ok());
        let full_page = items.len() as u32 == per_page;

        let (next_page, is_last_page) = match (link_next.is_some(), x_next_page, full_page) {
            (true, _, _) => (Some(page + 1), false),
            (false, Some(np), _) => (Some(np), false),
            (false, None, true) => (Some(page + 1), false),
            (false, None, false) => (None, true),
        };

        Ok(MergeRequestPage {
            items,
            next_page,
            is_last_page,
        })
    }

    pub fn paginate_mr_discussions(
        &self,
        gitlab_project_id: i64,
        mr_iid: i64,
    ) -> Pin<Box<dyn Stream<Item = Result<GitLabDiscussion>> + Send + '_>> {
        Box::pin(stream! {
            let mut page = 1u32;
            let per_page = 100u32;

            loop {
                let params = vec![
                    ("per_page", per_page.to_string()),
                    ("page", page.to_string()),
                ];

                let path = format!(
                    "/api/v4/projects/{}/merge_requests/{}/discussions",
                    gitlab_project_id, mr_iid
                );
                let result = self.request_with_headers::<Vec<GitLabDiscussion>>(&path, &params).await;

                match result {
                    Ok((discussions, headers)) => {
                        let is_empty = discussions.is_empty();
                        let full_page = discussions.len() as u32 == per_page;

                        for discussion in discussions {
                            yield Ok(discussion);
                        }

                        let link_next = parse_link_header_next(&headers);
                        let x_next_page = header_value(&headers, "x-next-page")
                            .and_then(|s| s.parse::<u32>().ok());

                        let should_continue = match (link_next.is_some(), x_next_page, full_page) {
                            (true, _, _) => {
                                page += 1;
                                true
                            }
                            (false, Some(np), _) if np > page => {
                                page = np;
                                true
                            }
                            (false, None, true) => {
                                page += 1;
                                true
                            }
                            _ => false,
                        };

                        if !should_continue || is_empty {
                            break;
                        }
                    }
                    Err(e) => {
                        yield Err(e);
                        break;
                    }
                }
            }
        })
    }

    async fn request_with_headers<T: serde::de::DeserializeOwned>(
        &self,
        path: &str,
        params: &[(&str, String)],
    ) -> Result<(T, Vec<(String, String)>)> {
        let url = format!("{}{}", self.base_url, path);
        let mut last_response = None;

        for attempt in 0..=Self::MAX_RETRIES {
            // SAFETY: std::sync::Mutex blocks the executor thread while held. This is safe
            // because the critical section is a single Instant::now() comparison with no I/O.
            // If async work is ever added inside the lock, switch to an async-aware lock.
            let delay = {
                let mut limiter = self
                    .rate_limiter
                    .lock()
                    .unwrap_or_else(|poisoned| poisoned.into_inner());
                limiter.check_delay()
            };
            if let Some(d) = delay {
                sleep(wall_now(), d).await;
            }

            debug!(url = %url, ?params, attempt, "GitLab paginated request");

            let response = self
                .client
                .get_with_query(
                    &url,
                    params,
                    &[
                        ("PRIVATE-TOKEN", self.token.as_str()),
                        ("Accept", "application/json"),
                    ],
                )
                .await?;

            if response.status == 429 && attempt < Self::MAX_RETRIES {
                let retry_after = Self::parse_retry_after(&response);
                tracing::info!(
                    path = %path,
                    attempt,
                    retry_after_secs = retry_after,
                    status_code = 429u16,
                    "Rate limited, retrying"
                );
                sleep(wall_now(), Duration::from_secs(retry_after)).await;
                continue;
            }

            last_response = Some(response);
            break;
        }

        let response = last_response.expect("retry loop ran at least once");
        let headers = response.headers.clone();
        let body = self.handle_response(response, path)?;
        Ok((body, headers))
    }
}

impl GitLabClient {
    pub async fn fetch_all_mr_discussions(
        &self,
        gitlab_project_id: i64,
        mr_iid: i64,
    ) -> Result<Vec<GitLabDiscussion>> {
        use futures::StreamExt;

        let mut discussions = Vec::new();
        let mut stream = self.paginate_mr_discussions(gitlab_project_id, mr_iid);

        while let Some(result) = stream.next().await {
            discussions.push(result?);
        }

        Ok(discussions)
    }

    pub async fn fetch_all_issue_discussions(
        &self,
        gitlab_project_id: i64,
        issue_iid: i64,
    ) -> Result<Vec<GitLabDiscussion>> {
        use futures::StreamExt;

        let mut discussions = Vec::new();
        let mut stream = self.paginate_issue_discussions(gitlab_project_id, issue_iid);

        while let Some(result) = stream.next().await {
            discussions.push(result?);
        }

        Ok(discussions)
    }
}

impl GitLabClient {
    async fn fetch_all_pages<T: serde::de::DeserializeOwned>(&self, path: &str) -> Result<Vec<T>> {
        let mut results = Vec::new();
        let mut page = 1u32;
        let per_page = 100u32;

        loop {
            let params = vec![
                ("per_page", per_page.to_string()),
                ("page", page.to_string()),
            ];

            let (items, headers) = self.request_with_headers::<Vec<T>>(path, &params).await?;

            let is_empty = items.is_empty();
            let full_page = items.len() as u32 == per_page;
            results.extend(items);

            let next_page =
                header_value(&headers, "x-next-page").and_then(|s| s.parse::<u32>().ok());

            match next_page {
                Some(next) if next > page => page = next,
                _ => {
                    if is_empty || !full_page {
                        break;
                    }
                    page += 1;
                }
            }
        }

        Ok(results)
    }

    pub async fn fetch_mr_closes_issues(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabIssueRef>> {
        let path =
            format!("/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/closes_issues");
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_mr_diffs(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabMrDiff>> {
        let path = format!("/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/diffs");
        coalesce_not_found(self.fetch_all_pages(&path).await)
    }

    pub async fn fetch_issue_state_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabStateEvent>> {
        let path =
            format!("/api/v4/projects/{gitlab_project_id}/issues/{iid}/resource_state_events");
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_issue_label_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabLabelEvent>> {
        let path =
            format!("/api/v4/projects/{gitlab_project_id}/issues/{iid}/resource_label_events");
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_issue_milestone_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabMilestoneEvent>> {
        let path =
            format!("/api/v4/projects/{gitlab_project_id}/issues/{iid}/resource_milestone_events");
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_mr_state_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabStateEvent>> {
        let path = format!(
            "/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/resource_state_events"
        );
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_mr_label_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabLabelEvent>> {
        let path = format!(
            "/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/resource_label_events"
        );
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_mr_milestone_events(
        &self,
        gitlab_project_id: i64,
        iid: i64,
    ) -> Result<Vec<GitLabMilestoneEvent>> {
        let path = format!(
            "/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/resource_milestone_events"
        );
        self.fetch_all_pages(&path).await
    }

    pub async fn fetch_all_resource_events(
        &self,
        gitlab_project_id: i64,
        entity_type: &str,
        iid: i64,
    ) -> Result<(
        Vec<GitLabStateEvent>,
        Vec<GitLabLabelEvent>,
        Vec<GitLabMilestoneEvent>,
    )> {
        let (state_res, label_res, milestone_res) = match entity_type {
            "issue" => {
                futures::join!(
                    self.fetch_issue_state_events(gitlab_project_id, iid),
                    self.fetch_issue_label_events(gitlab_project_id, iid),
                    self.fetch_issue_milestone_events(gitlab_project_id, iid),
                )
            }
            "merge_request" => {
                futures::join!(
                    self.fetch_mr_state_events(gitlab_project_id, iid),
                    self.fetch_mr_label_events(gitlab_project_id, iid),
                    self.fetch_mr_milestone_events(gitlab_project_id, iid),
                )
            }
            _ => {
                return Err(LoreError::Other(format!(
                    "Invalid entity type for resource events: {entity_type}"
                )));
            }
        };

        let state = coalesce_not_found(state_res)?;
        let label = coalesce_not_found(label_res)?;
        let milestone = coalesce_not_found(milestone_res)?;

        Ok((state, label, milestone))
    }
}

#[derive(Debug)]
pub struct MergeRequestPage {
    pub items: Vec<GitLabMergeRequest>,
    pub next_page: Option<u32>,
    pub is_last_page: bool,
}

fn header_value<'a>(headers: &'a [(String, String)], name: &str) -> Option<&'a str> {
    headers
        .iter()
        .find(|(k, _)| k.eq_ignore_ascii_case(name))
        .map(|(_, v)| v.as_str())
}

fn parse_link_header_next(headers: &[(String, String)]) -> Option<String> {
    header_value(headers, "link").and_then(|link_str| {
        for part in link_str.split(',') {
            let part = part.trim();
            if (part.contains("rel=\"next\"") || part.contains("rel=next"))
                && let Some(start) = part.find('<')
                && let Some(end) = part.find('>')
            {
                return Some(part[start + 1..end].to_string());
            }
        }
        None
    })
}

fn coalesce_not_found<T>(result: Result<Vec<T>>) -> Result<Vec<T>> {
    match result {
        Ok(v) => Ok(v),
        Err(LoreError::GitLabNotFound { .. }) => Ok(Vec::new()),
        Err(e) => Err(e),
    }
}

fn ms_to_iso8601(ms: i64) -> Option<String> {
    DateTime::<Utc>::from_timestamp_millis(ms)
        .map(|dt| dt.format("%Y-%m-%dT%H:%M:%S%.3fZ").to_string())
}

#[cfg(test)]
#[path = "client_tests.rs"]
mod client_tests;

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

    #[test]
    fn ms_to_iso8601_converts_correctly() {
        let result = ms_to_iso8601(1705312800000);
        assert_eq!(result, Some("2024-01-15T10:00:00.000Z".to_string()));
    }

    #[test]
    fn ms_to_iso8601_handles_zero() {
        let result = ms_to_iso8601(0);
        assert_eq!(result, Some("1970-01-01T00:00:00.000Z".to_string()));
    }

    #[test]
    fn cursor_rewind_clamps_to_zero() {
        let updated_after = 1000i64;
        let cursor_rewind_seconds = 10u32;

        let rewind_ms = i64::from(cursor_rewind_seconds) * 1000;
        let rewound = (updated_after - rewind_ms).max(0);

        assert_eq!(rewound, 0);
    }

    #[test]
    fn cursor_rewind_applies_correctly() {
        let updated_after = 1705312800000i64;
        let cursor_rewind_seconds = 60u32;

        let rewind_ms = i64::from(cursor_rewind_seconds) * 1000;
        let rewound = (updated_after - rewind_ms).max(0);

        assert_eq!(rewound, 1705312740000);
    }

    #[test]
    fn parse_link_header_extracts_next_url() {
        let headers = vec![(
            "link".to_string(),
            r#"<https://gitlab.example.com/api/v4/projects/1/merge_requests?page=2>; rel="next", <https://gitlab.example.com/api/v4/projects/1/merge_requests?page=5>; rel="last""#.to_string(),
        )];

        let result = parse_link_header_next(&headers);
        assert_eq!(
            result,
            Some("https://gitlab.example.com/api/v4/projects/1/merge_requests?page=2".to_string())
        );
    }

    #[test]
    fn parse_link_header_handles_unquoted_rel() {
        let headers = vec![(
            "link".to_string(),
            r#"<https://example.com/next>; rel=next"#.to_string(),
        )];

        let result = parse_link_header_next(&headers);
        assert_eq!(result, Some("https://example.com/next".to_string()));
    }

    #[test]
    fn parse_link_header_returns_none_when_no_next() {
        let headers = vec![(
            "link".to_string(),
            r#"<https://example.com/last>; rel="last""#.to_string(),
        )];

        let result = parse_link_header_next(&headers);
        assert!(result.is_none());
    }

    #[test]
    fn parse_link_header_returns_none_when_missing() {
        let headers: Vec<(String, String)> = vec![];
        let result = parse_link_header_next(&headers);
        assert!(result.is_none());
    }
}