//! Tokio-based event loop.
//!
//! A blocking task reads crossterm events and forwards them onto
//! an `mpsc` channel as `AppEvent`s. The main loop awaits events,
//! feeds them to `App::update`, enacts any returned `Action`s,
//! and redraws the terminal. DSL execution is dispatched onto
//! the database worker (see `db::Database`), and its result is
//! posted back as a new `AppEvent`. Future async work (snapshot
//! capture, auto-save) joins the same event channel as
//! additional producers.

use std::io;
use std::time::Duration;

use anyhow::{Context, Result};
use crossterm::event::{Event as CtEvent, EventStream};
use crossterm::execute;
use crossterm::terminal::{
    EnterAlternateScreen, LeaveAlternateScreen, disable_raw_mode, enable_raw_mode,
};
use futures_util::StreamExt;
use ratatui::Terminal;
use ratatui::backend::CrosstermBackend;
use tokio::sync::mpsc;
use tracing::{debug, error, info, warn};

use crate::action::Action;
use crate::app::App;
use crate::cli::Args;
use crate::db::{
    DataResult, Database, DbError, DeleteResult, InsertResult, TableDescription, UpdateResult,
};
use crate::dsl::Command;
use crate::event::AppEvent;
use crate::project::open_or_create;
use crate::theme::Theme;
use crate::ui;

const EVENT_CHANNEL_CAPACITY: usize = 64;
const SHUTDOWN_GRACE: Duration = Duration::from_millis(100);

/// Run the application until a `Quit` action is enacted or the
/// terminal closes.
pub async fn run(args: Args) -> Result<()> {
    let project = open_or_create(args.project_path.as_deref(), args.data_dir.as_deref())
        .context("open or create project")?;
    let db_path = project.db_path();
    let display_name = project.display_name().to_string();
    let project_path = project.path().to_path_buf();
    let persistence = crate::persistence::Persistence::new(project_path.clone());
    // Capture whether the .db file existed BEFORE we open it —
    // sqlite creates it on connect, so this is the only honest
    // signal that we need to rebuild from text (ADR-0015 §7).
    let db_existed = db_path.exists();
    let database = Database::open_with_persistence(db_path.as_path(), persistence)
        .context("open database")?;
    if !db_existed
        && let Err(e) = database
            .rebuild_from_text(project_path.clone(), None)
            .await
    {
        // The terminal is still in cooked mode here (we haven't
        // entered the alternate screen yet), so writing to
        // stderr lands directly in the user's shell. Drop the
        // project to release the lock first.
        drop(project);
        if matches!(
            e,
            DbError::PersistenceFatal { .. } | DbError::RebuildRowFailed { .. }
        ) {
            eprintln!("rdbms-playground: {}", e.friendly_message());
            return Ok(());
        }
        return Err(anyhow::anyhow!(e.friendly_message())).context("rebuild from text");
    }

    let mut terminal = setup_terminal().context("setup terminal")?;
    let result = run_loop(
        &mut terminal,
        args.theme,
        database,
        display_name,
        project_path,
    )
    .await;
    if let Err(e) = teardown_terminal(&mut terminal) {
        // Teardown failures should not mask the primary error.
        warn!(error = %e, "terminal teardown failed");
    }
    // `project` (and the lock it holds) is dropped here, releasing
    // the lock file *after* the terminal has been restored.
    drop(project);

    // ADR-0015 §8: a fatal persistence failure makes its
    // banner visible above the shell prompt by writing to
    // stderr after the alternate screen has been left.
    if let Ok(Some(banner)) = &result {
        eprintln!("{banner}");
    }
    result.map(|_| ())
}

async fn run_loop(
    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
    theme: Theme,
    database: Database,
    project_display_name: String,
    project_path: std::path::PathBuf,
) -> Result<Option<String>> {
    let (event_tx, mut event_rx) = mpsc::channel::<AppEvent>(EVENT_CHANNEL_CAPACITY);
    let reader_handle = spawn_event_reader(event_tx.clone());

    let mut app = App::new();
    app.project_name = Some(project_display_name);

    // Seed the table list with whatever the database currently
    // shows. For a fresh in-memory DB this is empty, but doing
    // it explicitly means file-backed databases (track 2) will
    // show their tables on launch without changes here.
    seed_initial_tables(&database, &event_tx).await;

    terminal
        .draw(|f| ui::render(&mut app, &theme, f))
        .context("initial draw")?;

    info!("entering main event loop");
    while let Some(event) = event_rx.recv().await {
        let actions = app.update(event);
        let mut should_quit = false;
        for action in actions {
            match action {
                Action::Quit => {
                    debug!("quit action received");
                    should_quit = true;
                }
                Action::ExecuteDsl { command, source } => {
                    spawn_dsl_dispatch(database.clone(), event_tx.clone(), command, source);
                }
                Action::PrepareRebuild => {
                    spawn_prepare_rebuild(project_path.clone(), event_tx.clone());
                }
                Action::Rebuild { source } => {
                    spawn_rebuild(
                        database.clone(),
                        project_path.clone(),
                        event_tx.clone(),
                        source,
                    );
                }
            }
        }
        terminal
            .draw(|f| ui::render(&mut app, &theme, f))
            .context("redraw")?;
        if should_quit {
            break;
        }
    }

    let _ = tokio::time::timeout(SHUTDOWN_GRACE, reader_handle).await;

    info!("event loop exited");
    Ok(app.fatal_message.clone())
}

async fn seed_initial_tables(database: &Database, event_tx: &mpsc::Sender<AppEvent>) {
    match database.list_tables().await {
        Ok(tables) => {
            let _ = event_tx.send(AppEvent::TablesRefreshed(tables)).await;
        }
        Err(e) => {
            error!(error = %e, "failed to seed initial table list");
        }
    }
}

/// Read `project.yaml` + `data/` to compute the rebuild
/// summary that the confirmation modal shows. Runs off the
/// event loop so the brief I/O doesn't stall input handling
/// even on slow filesystems.
fn spawn_prepare_rebuild(
    project_path: std::path::PathBuf,
    event_tx: mpsc::Sender<AppEvent>,
) {
    tokio::spawn(async move {
        let summary = match summarize_project(&project_path) {
            Ok(s) => s,
            Err(e) => format!("(could not read project sources: {e})"),
        };
        let _ = event_tx.send(AppEvent::RebuildPrepared { summary }).await;
    });
}

fn summarize_project(project_path: &std::path::Path) -> Result<String, String> {
    let yaml_path = project_path.join(crate::project::PROJECT_YAML);
    let yaml = std::fs::read_to_string(&yaml_path).map_err(|e| e.to_string())?;
    let snapshot = crate::persistence::parse_schema(&yaml).map_err(|e| e.to_string())?;
    let table_count = snapshot.tables.len();
    let data_dir = project_path.join(crate::project::DATA_DIR);
    let mut row_count: usize = 0;
    for table in &snapshot.tables {
        let csv_path = data_dir.join(format!("{}.csv", table.name));
        let Ok(body) = std::fs::read_to_string(&csv_path) else {
            continue;
        };
        // Header line + one line per row (per Iteration 2's
        // "no CSV when empty" rule, this is exact).
        row_count += body.lines().count().saturating_sub(1);
    }
    Ok(format!(
        "{table_count} table{} and {row_count} row{} will be reconstructed; \
         the existing playground.db will be replaced",
        if table_count == 1 { "" } else { "s" },
        if row_count == 1 { "" } else { "s" },
    ))
}

/// Spawn the actual rebuild and forward the typed outcome
/// back as an `AppEvent`.
fn spawn_rebuild(
    database: Database,
    project_path: std::path::PathBuf,
    event_tx: mpsc::Sender<AppEvent>,
    source: String,
) {
    tokio::spawn(async move {
        match database
            .rebuild_from_text(project_path.clone(), Some(source))
            .await
        {
            Ok(()) => {
                let summary = summarize_project(&project_path)
                    .unwrap_or_else(|_| "rebuild complete".to_string());
                let _ = event_tx
                    .send(AppEvent::RebuildSucceeded { summary })
                    .await;
                // Refresh the table list so the items panel
                // reflects whatever the rebuild produced.
                if let Ok(tables) = database.list_tables().await {
                    let _ = event_tx.send(AppEvent::TablesRefreshed(tables)).await;
                }
            }
            Err(DbError::PersistenceFatal {
                operation,
                path,
                message,
            }) => {
                let _ = event_tx
                    .send(AppEvent::PersistenceFatal {
                        operation: operation.to_string(),
                        path,
                        message,
                    })
                    .await;
            }
            Err(other) => {
                let _ = event_tx
                    .send(AppEvent::RebuildFailed {
                        error: other.friendly_message(),
                    })
                    .await;
            }
        }
    });
}

/// Spawn a task that runs a DSL command against the database
/// and forwards the result back as an `AppEvent`.
fn spawn_dsl_dispatch(
    database: Database,
    event_tx: mpsc::Sender<AppEvent>,
    command: Command,
    source: String,
) {
    tokio::spawn(async move {
        let outcome = execute_command_typed(&database, command.clone(), source).await;
        let event = match outcome {
            Ok(CommandOutcome::Schema(description)) => AppEvent::DslSucceeded {
                command: command.clone(),
                description,
            },
            Ok(CommandOutcome::Query(data)) => AppEvent::DslDataSucceeded {
                command: command.clone(),
                data,
            },
            Ok(CommandOutcome::Insert(result)) => AppEvent::DslInsertSucceeded {
                command: command.clone(),
                result,
            },
            Ok(CommandOutcome::Update(result)) => AppEvent::DslUpdateSucceeded {
                command: command.clone(),
                result,
            },
            Ok(CommandOutcome::Delete(result)) => AppEvent::DslDeleteSucceeded {
                command: command.clone(),
                result,
            },
            Err(DbError::PersistenceFatal {
                operation,
                path,
                message,
            }) => AppEvent::PersistenceFatal {
                operation: operation.to_string(),
                path,
                message,
            },
            Err(error) => AppEvent::DslFailed {
                command: command.clone(),
                error: error.friendly_message(),
            },
        };
        if event_tx.send(event).await.is_err() {
            return;
        }
        // Refresh the table list after every DDL operation so
        // the items panel reflects reality. A failed list_tables
        // here is logged but not surfaced to the user — they
        // already saw the primary outcome.
        match database.list_tables().await {
            Ok(tables) => {
                let _ = event_tx.send(AppEvent::TablesRefreshed(tables)).await;
            }
            Err(e) => warn!(error = %e, "post-list_tables failed"),
        }
    });
}

enum CommandOutcome {
    Schema(Option<TableDescription>),
    Query(DataResult),
    Insert(InsertResult),
    Update(UpdateResult),
    Delete(DeleteResult),
}

/// Execute a parsed user command and return either a typed
/// `CommandOutcome` or the raw `DbError`. Keeping the typed
/// error here lets us distinguish persistence-fatal failures
/// from ordinary user errors at dispatch time (ADR-0015 §8).
async fn execute_command_typed(
    database: &Database,
    command: Command,
    source: String,
) -> Result<CommandOutcome, DbError> {
    let src = Some(source);
    match command {
        Command::CreateTable {
            name,
            columns,
            primary_key,
        } => database
            .create_table(name, columns, primary_key, src)
            .await
            .map(|d| CommandOutcome::Schema(Some(d))),
        Command::DropTable { name } => database
            .drop_table(name, src)
            .await
            .map(|()| CommandOutcome::Schema(None)),
        Command::AddColumn { table, column, ty } => database
            .add_column(table, column, ty, src)
            .await
            .map(|d| CommandOutcome::Schema(Some(d))),
        Command::AddRelationship {
            name,
            parent_table,
            parent_column,
            child_table,
            child_column,
            on_delete,
            on_update,
            create_fk,
        } => database
            .add_relationship(
                name,
                parent_table,
                parent_column,
                child_table,
                child_column,
                on_delete,
                on_update,
                create_fk,
                src,
            )
            .await
            .map(|d| CommandOutcome::Schema(Some(d))),
        Command::DropRelationship { selector } => database
            .drop_relationship(selector, src)
            .await
            .map(CommandOutcome::Schema),
        Command::ShowTable { name } => database
            .describe_table(name, src)
            .await
            .map(|d| CommandOutcome::Schema(Some(d))),
        Command::Insert {
            table,
            columns,
            values,
        } => database
            .insert(table, columns, values, src)
            .await
            .map(CommandOutcome::Insert),
        Command::Update {
            table,
            assignments,
            filter,
        } => database
            .update(table, assignments, filter, src)
            .await
            .map(CommandOutcome::Update),
        Command::Delete { table, filter } => database
            .delete(table, filter, src)
            .await
            .map(CommandOutcome::Delete),
        Command::ShowData { name } => database
            .query_data(name, src)
            .await
            .map(CommandOutcome::Query),
    }
}

fn spawn_event_reader(tx: mpsc::Sender<AppEvent>) -> tokio::task::JoinHandle<()> {
    tokio::spawn(async move {
        let mut stream = EventStream::new();
        while let Some(maybe_event) = stream.next().await {
            match maybe_event {
                Ok(CtEvent::Key(key)) => {
                    if tx.send(AppEvent::Key(key)).await.is_err() {
                        break;
                    }
                }
                Ok(CtEvent::Resize(cols, rows)) => {
                    if tx.send(AppEvent::Resize { cols, rows }).await.is_err() {
                        break;
                    }
                }
                Ok(_) => {
                    // Ignore other event kinds (paste, focus, mouse) for now.
                }
                Err(e) => {
                    error!(error = %e, "crossterm event stream error");
                    break;
                }
            }
        }
        debug!("event reader exiting");
    })
}

fn setup_terminal() -> Result<Terminal<CrosstermBackend<io::Stdout>>> {
    enable_raw_mode().context("enable raw mode")?;
    let mut stdout = io::stdout();
    // Mouse capture is intentionally NOT enabled: it would prevent the
    // host terminal's native text selection (the cost of capturing every
    // mouse event), which we don't currently use for anything in-app.
    // If we ever want click-to-select panes or scroll wheel handling,
    // we'll need a different strategy than blanket capture.
    execute!(stdout, EnterAlternateScreen).context("enter alternate screen")?;
    let backend = CrosstermBackend::new(stdout);
    let terminal = Terminal::new(backend).context("construct terminal")?;
    Ok(terminal)
}

fn teardown_terminal(
    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
) -> Result<()> {
    disable_raw_mode().context("disable raw mode")?;
    execute!(terminal.backend_mut(), LeaveAlternateScreen)
        .context("leave alternate screen")?;
    terminal.show_cursor().context("show cursor")?;
    Ok(())
}