use std::{
    convert::Infallible,
    path::{Path, PathBuf},
    time::SystemTime,
};

use async_stream::stream;
use axum::{
    extract::Query,
    response::{
        sse::{Event, KeepAlive, Sse},
        IntoResponse,
    },
    Json,
};
use serde::{Deserialize, Serialize};
use tokio::{
    fs,
    io::{AsyncReadExt, AsyncSeekExt, SeekFrom},
    time::{Duration, interval},
};

use crate::util::response::ApiErr;

const LOG_DIR: &str = "./logs";
const DEFAULT_TAIL_LINES: usize = 200;
const MAX_TAIL_LINES: usize = 2000;
const DEFAULT_MAX_BYTES: usize = 64 * 1024;
const STREAM_MAX_BYTES: usize = 32 * 1024;
const MAX_MAX_BYTES: usize = 512 * 1024;

#[derive(Debug, Deserialize)]
pub struct LogQuery {
    pub file: Option<String>,
    pub cursor: Option<u64>,
    pub tail_lines: Option<usize>,
    pub max_bytes: Option<usize>,
}

#[derive(Debug, Serialize)]
pub struct LogChunkResponse {
    pub file: String,
    pub cursor: u64,
    pub lines: Vec<String>,
    pub truncated: bool,
    pub reset: bool,
}

#[derive(Debug, Clone, PartialEq, Eq)]
struct StreamFileState {
    path: PathBuf,
    file_name: String,
    cursor: u64,
}

pub async fn get_logs(Query(query): Query<LogQuery>) -> Result<impl IntoResponse, ApiErr> {
    let path = resolve_log_file(query.file.as_deref()).await?;
    let file_name = file_name_of(&path);
    let max_bytes = query
        .max_bytes
        .unwrap_or(DEFAULT_MAX_BYTES)
        .clamp(1, MAX_MAX_BYTES);

    let response = if let Some(cursor) = query.cursor {
        read_since(&path, &file_name, cursor, max_bytes).await?
    } else {
        let tail_lines = query
            .tail_lines
            .unwrap_or(DEFAULT_TAIL_LINES)
            .clamp(1, MAX_TAIL_LINES);
        read_tail(&path, &file_name, tail_lines).await?
    };

    Ok(Json(response))
}

pub async fn stream_logs(Query(query): Query<LogQuery>) -> Result<impl IntoResponse, ApiErr> {
    let path = resolve_log_file(query.file.as_deref()).await?;
    let file_name = file_name_of(&path);
    let max_bytes = query
        .max_bytes
        .unwrap_or(STREAM_MAX_BYTES)
        .clamp(1, MAX_MAX_BYTES);
    let follow_latest = query.file.is_none();
    let start_cursor = query.cursor.unwrap_or(file_len(&path).await?);

    let event_stream = stream! {
        let mut ticker = interval(Duration::from_millis(800));
        let mut stream_file = StreamFileState {
            path,
            file_name,
            cursor: start_cursor,
        };

        loop {
            ticker.tick().await;
            let switched = if follow_latest {
                match latest_log_file(Path::new(LOG_DIR)).await {
                    Ok(latest_path) => {
                        let latest = StreamFileState {
                            file_name: file_name_of(&latest_path),
                            path: latest_path,
                            cursor: 0,
                        };
                        let (next, switched) = advance_stream_file(&stream_file, &latest);
                        stream_file = next;
                        switched
                    }
                    Err(_) => false,
                }
            } else {
                false
            };

            match read_since(&stream_file.path, &stream_file.file_name, stream_file.cursor, max_bytes).await {
                Ok(chunk) => {
                    stream_file.cursor = chunk.cursor;
                    let chunk = LogChunkResponse {
                        reset: chunk.reset || switched,
                        ..chunk
                    };
                    if chunk.reset || !chunk.lines.is_empty() {
                        match Event::default().event("log").json_data(&chunk) {
                            Ok(event) => yield Ok::<Event, Infallible>(event),
                            Err(_) => {
                                yield Ok::<Event, Infallible>(
                                    Event::default().event("error").data("serialize log event failed")
                                );
                                break;
                            }
                        }
                    }
                }
                Err(_) => {
                    yield Ok::<Event, Infallible>(
                        Event::default().event("error").data("log stream read failed")
                    );
                    break;
                }
            }
        }
    };

    Ok(
        Sse::new(event_stream)
            .keep_alive(KeepAlive::new().interval(Duration::from_secs(10)).text("keepalive")),
    )
}

async fn resolve_log_file(file: Option<&str>) -> Result<PathBuf, ApiErr> {
    let log_dir = PathBuf::from(LOG_DIR);

    if let Some(file_name) = file {
        validate_file_name(file_name)?;
        let path = log_dir.join(file_name);
        ensure_exists(&path).await?;
        return Ok(path);
    }

    latest_log_file(&log_dir).await
}

fn validate_file_name(file_name: &str) -> Result<(), ApiErr> {
    if file_name.is_empty() {
        return Err(ApiErr::BadRequest("file cannot be empty".to_string(), None));
    }
    if file_name.contains(['/', '\\']) || file_name.contains("..") {
        return Err(ApiErr::BadRequest("invalid log file name".to_string(), None));
    }
    if !file_name.starts_with("app.log") {
        return Err(ApiErr::BadRequest("only app.log* files are allowed".to_string(), None));
    }
    Ok(())
}

async fn ensure_exists(path: &Path) -> Result<(), ApiErr> {
    fs::metadata(path).await.map_err(|e| match e.kind() {
        std::io::ErrorKind::NotFound => ApiErr::NotFound("log file not found".to_string(), None),
        _ => ApiErr::Internal("failed to access log file".to_string(), None),
    })?;
    Ok(())
}

async fn latest_log_file(log_dir: &Path) -> Result<PathBuf, ApiErr> {
    let mut entries = fs::read_dir(log_dir).await.map_err(|e| match e.kind() {
        std::io::ErrorKind::NotFound => ApiErr::NotFound("log directory not found".to_string(), None),
        _ => ApiErr::Internal("failed to read log directory".to_string(), None),
    })?;

    let mut latest: Option<(SystemTime, PathBuf)> = None;

    while let Some(entry) = entries
        .next_entry()
        .await
        .map_err(|_| ApiErr::Internal("failed to enumerate log files".to_string(), None))?
    {
        let file_name = entry.file_name();
        let file_name = file_name.to_string_lossy();
        if !file_name.starts_with("app.log") {
            continue;
        }

        let metadata = entry
            .metadata()
            .await
            .map_err(|_| ApiErr::Internal("failed to read log metadata".to_string(), None))?;
        if !metadata.is_file() {
            continue;
        }

        let modified = metadata.modified().unwrap_or(SystemTime::UNIX_EPOCH);
        let path = entry.path();

        match latest.as_ref() {
            Some((latest_modified, _)) if modified <= *latest_modified => {}
            _ => latest = Some((modified, path)),
        }
    }

    latest
        .map(|(_, path)| path)
        .ok_or_else(|| ApiErr::NotFound("no app.log files found".to_string(), None))
}

async fn read_tail(path: &Path, file_name: &str, tail_lines: usize) -> Result<LogChunkResponse, ApiErr> {
    let mut file = fs::File::open(path).await.map_err(map_open_err)?;
    let mut buffer = Vec::new();
    file.read_to_end(&mut buffer)
        .await
        .map_err(|_| ApiErr::Internal("failed to read log file".to_string(), None))?;

    let cursor = buffer.len() as u64;
    let text = String::from_utf8_lossy(&buffer);
    let mut lines: Vec<String> = text.lines().map(|line| line.to_string()).collect();
    let dropped = lines.len().saturating_sub(tail_lines);
    if dropped > 0 {
        lines = lines.into_iter().skip(dropped).collect();
    }

    Ok(LogChunkResponse {
        file: file_name.to_string(),
        cursor,
        lines,
        truncated: false,
        reset: false,
    })
}

async fn read_since(
    path: &Path,
    file_name: &str,
    cursor: u64,
    max_bytes: usize,
) -> Result<LogChunkResponse, ApiErr> {
    let mut file = fs::File::open(path).await.map_err(map_open_err)?;
    let metadata = file
        .metadata()
        .await
        .map_err(|_| ApiErr::Internal("failed to read log metadata".to_string(), None))?;
    let file_size = metadata.len();

    let (start, reset) = if cursor > file_size {
        (0, true)
    } else {
        (cursor, false)
    };
    let end = (start + max_bytes as u64).min(file_size);

    file.seek(SeekFrom::Start(start))
        .await
        .map_err(|_| ApiErr::Internal("failed to seek log file".to_string(), None))?;

    let mut buffer = vec![0u8; (end - start) as usize];
    if !buffer.is_empty() {
        file.read_exact(&mut buffer)
            .await
            .map_err(|_| ApiErr::Internal("failed to read log file chunk".to_string(), None))?;
    }

    let text = String::from_utf8_lossy(&buffer);
    let lines = text.lines().map(|line| line.to_string()).collect();

    Ok(LogChunkResponse {
        file: file_name.to_string(),
        cursor: end,
        lines,
        truncated: end < file_size,
        reset,
    })
}

async fn file_len(path: &Path) -> Result<u64, ApiErr> {
    let metadata = fs::metadata(path).await.map_err(map_open_err)?;
    Ok(metadata.len())
}

fn file_name_of(path: &Path) -> String {
    path.file_name()
        .and_then(|name| name.to_str())
        .unwrap_or_default()
        .to_string()
}

fn advance_stream_file(
    current: &StreamFileState,
    latest: &StreamFileState,
) -> (StreamFileState, bool) {
    if current.path == latest.path {
        return (current.clone(), false);
    }

    (
        StreamFileState {
            path: latest.path.clone(),
            file_name: latest.file_name.clone(),
            cursor: 0,
        },
        true,
    )
}

fn map_open_err(err: std::io::Error) -> ApiErr {
    match err.kind() {
        std::io::ErrorKind::NotFound => ApiErr::NotFound("log file not found".to_string(), None),
        _ => ApiErr::Internal("failed to access log file".to_string(), None),
    }
}

#[cfg(test)]
mod tests {
    use super::{advance_stream_file, StreamFileState};
    use std::path::PathBuf;

    #[test]
    fn advance_stream_file_switches_to_latest_file_and_resets_cursor() {
        let current = StreamFileState {
            path: PathBuf::from("logs/app.log"),
            file_name: "app.log".to_string(),
            cursor: 128,
        };
        let latest = StreamFileState {
            path: PathBuf::from("logs/app.log.1"),
            file_name: "app.log.1".to_string(),
            cursor: 42,
        };

        let (next, switched) = advance_stream_file(&current, &latest);

        assert!(switched);
        assert_eq!(next.path, latest.path);
        assert_eq!(next.file_name, latest.file_name);
        assert_eq!(next.cursor, 0);
    }
}