use std::{
    io::ErrorKind,
    path::{Path, PathBuf},
    time::Instant,
};

use futures::{StreamExt as _, stream};
use tokio::io::AsyncWriteExt as _;

use crate::{
    install::intent::{
        InstallIntent,
        LEGACY_INTENT_FILE,
        LEGACY_INTENT_TMP_FILE,
        intent_path,
        write_intent,
    },
    local_games::{is_ignored_game_root_name, legacy_library_index_path},
    state_paths::{local_library_index_path, setup_done_path},
};

const LEGACY_LIBRARY_INDEX_DIR: &str = ".lanspread";
const LEGACY_FIRST_START_DONE_FILE: &str = ".softlan_first_start_done";
const LEGACY_SOFTLAN_INSTALL_MARKER: &str = ".softlan_game_installed";
const MIGRATION_CONCURRENCY: usize = 16;

#[derive(Clone, Copy, Debug, Default, Eq, PartialEq, serde::Serialize)]
pub struct MigrationReport {
    pub games_checked: usize,
    pub library_index_migrated: bool,
    pub install_intents_migrated: usize,
    pub setup_markers_migrated: usize,
    pub legacy_files_deleted: usize,
    pub unknown_softlan_files: usize,
    pub failures: usize,
}

impl MigrationReport {
    fn merge(&mut self, other: Self) {
        self.games_checked += other.games_checked;
        self.library_index_migrated |= other.library_index_migrated;
        self.install_intents_migrated += other.install_intents_migrated;
        self.setup_markers_migrated += other.setup_markers_migrated;
        self.legacy_files_deleted += other.legacy_files_deleted;
        self.unknown_softlan_files += other.unknown_softlan_files;
        self.failures += other.failures;
    }
}

/// Migrates legacy app-owned files out of the configured game directory.
///
/// This is intentionally separate from normal operation: callers should run it
/// before starting the peer runtime for a game directory.
pub async fn migrate_legacy_state(game_dir: &Path, state_dir: &Path) -> MigrationReport {
    let started = Instant::now();
    let mut report = MigrationReport::default();

    report.merge(migrate_library_index(game_dir, state_dir).await);

    let game_roots = match collect_game_roots(game_dir).await {
        Ok(game_roots) => game_roots,
        Err(err) => {
            if err.kind() != ErrorKind::NotFound {
                log::warn!(
                    "Failed to enumerate game roots for legacy state migration in {}: {err}",
                    game_dir.display()
                );
                report.failures += 1;
            }
            log_migration_report(&report, started);
            return report;
        }
    };

    let game_reports = stream::iter(game_roots)
        .map(|(id, root)| async move { migrate_game_root(state_dir, id, root).await })
        .buffer_unordered(MIGRATION_CONCURRENCY)
        .collect::<Vec<_>>()
        .await;

    for game_report in game_reports {
        report.merge(game_report);
    }

    log_migration_report(&report, started);
    report
}

async fn collect_game_roots(game_dir: &Path) -> std::io::Result<Vec<(String, PathBuf)>> {
    let mut roots = Vec::new();
    let mut entries = tokio::fs::read_dir(game_dir).await?;
    while let Some(entry) = entries.next_entry().await? {
        if !entry.file_type().await?.is_dir() {
            continue;
        }

        let Some(id) = entry.file_name().to_str().map(ToOwned::to_owned) else {
            continue;
        };
        if is_ignored_game_root_name(&id) {
            continue;
        }

        roots.push((id, entry.path()));
    }
    Ok(roots)
}

async fn migrate_library_index(game_dir: &Path, state_dir: &Path) -> MigrationReport {
    let mut report = MigrationReport::default();
    let legacy_path = legacy_library_index_path(game_dir);
    let target_path = local_library_index_path(state_dir);

    match migrate_raw_file(&legacy_path, &target_path).await {
        Ok(MigrationOutcome::Migrated) => {
            report.library_index_migrated = true;
            report.legacy_files_deleted += 1;
        }
        Ok(MigrationOutcome::TargetAlreadyExists) => {
            report.legacy_files_deleted += 1;
        }
        Ok(MigrationOutcome::SourceMissing) => {}
        Err(err) => {
            log::warn!(
                "Failed to migrate legacy library index {} to {}: {err}",
                legacy_path.display(),
                target_path.display()
            );
            report.failures += 1;
        }
    }

    report.merge(delete_if_exists(&library_index_tmp_path(&legacy_path)).await);
    report.merge(remove_empty_legacy_library_dir(game_dir).await);
    report
}

async fn migrate_game_root(state_dir: &Path, id: String, root: PathBuf) -> MigrationReport {
    let mut report = MigrationReport {
        games_checked: 1,
        ..MigrationReport::default()
    };

    report.merge(migrate_install_intent(state_dir, &id, &root).await);
    report.merge(delete_if_exists(&root.join(LEGACY_INTENT_TMP_FILE)).await);
    report.merge(migrate_setup_marker(state_dir, &id, &root).await);
    report.merge(delete_if_exists(&root.join(LEGACY_SOFTLAN_INSTALL_MARKER)).await);
    report.merge(note_unknown_softlan_files(&root).await);

    report
}

async fn migrate_install_intent(state_dir: &Path, id: &str, root: &Path) -> MigrationReport {
    let mut report = MigrationReport::default();
    let legacy_path = root.join(LEGACY_INTENT_FILE);
    let target_path = intent_path(state_dir, id);

    match path_exists(&legacy_path).await {
        Ok(false) => return report,
        Ok(true) => {}
        Err(err) => {
            log::warn!(
                "Failed to inspect legacy install intent {}: {err}",
                legacy_path.display()
            );
            report.failures += 1;
            return report;
        }
    }

    match path_exists(&target_path).await {
        Ok(true) => {
            report.merge(delete_file(&legacy_path).await);
            return report;
        }
        Ok(false) => {}
        Err(err) => {
            log::warn!(
                "Failed to inspect app-state install intent {}: {err}",
                target_path.display()
            );
            report.failures += 1;
            return report;
        }
    }

    let data = match tokio::fs::read_to_string(&legacy_path).await {
        Ok(data) => data,
        Err(err) => {
            log::warn!(
                "Failed to read legacy install intent {}: {err}",
                legacy_path.display()
            );
            report.failures += 1;
            return report;
        }
    };

    let intent = match serde_json::from_str::<InstallIntent>(&data) {
        Ok(intent) if intent.is_current_for(id) => intent,
        Ok(intent) => {
            log::warn!(
                "Leaving legacy install intent {} in place because it belongs to id {} schema {}",
                legacy_path.display(),
                intent.id,
                intent.schema_version
            );
            report.failures += 1;
            return report;
        }
        Err(err) => {
            log::warn!(
                "Leaving corrupt legacy install intent {} in place: {err}",
                legacy_path.display()
            );
            report.failures += 1;
            return report;
        }
    };

    if let Err(err) = write_intent(state_dir, id, &intent).await {
        log::warn!(
            "Failed to write migrated install intent {}: {err}",
            target_path.display()
        );
        report.failures += 1;
        return report;
    }

    report.install_intents_migrated += 1;
    report.merge(delete_file(&legacy_path).await);
    report
}

async fn migrate_setup_marker(state_dir: &Path, id: &str, root: &Path) -> MigrationReport {
    let mut report = MigrationReport::default();
    let legacy_path = root.join("local").join(LEGACY_FIRST_START_DONE_FILE);
    let target_path = setup_done_path(state_dir, id);

    match migrate_empty_marker(&legacy_path, &target_path).await {
        Ok(MigrationOutcome::Migrated) => {
            report.setup_markers_migrated += 1;
            report.legacy_files_deleted += 1;
        }
        Ok(MigrationOutcome::TargetAlreadyExists) => {
            report.legacy_files_deleted += 1;
        }
        Ok(MigrationOutcome::SourceMissing) => {}
        Err(err) => {
            log::warn!(
                "Failed to migrate legacy setup marker {} to {}: {err}",
                legacy_path.display(),
                target_path.display()
            );
            report.failures += 1;
        }
    }

    report
}

async fn note_unknown_softlan_files(root: &Path) -> MigrationReport {
    let mut report = MigrationReport::default();
    report.unknown_softlan_files += count_unknown_softlan_files(root).await;
    report.unknown_softlan_files += count_unknown_softlan_files(&root.join("local")).await;
    report
}

async fn count_unknown_softlan_files(dir: &Path) -> usize {
    let mut count = 0;
    let mut entries = match tokio::fs::read_dir(dir).await {
        Ok(entries) => entries,
        Err(err) if err.kind() == ErrorKind::NotFound => return 0,
        Err(err) => {
            log::warn!(
                "Failed to inspect {} for legacy .softlan files: {err}",
                dir.display()
            );
            return 0;
        }
    };

    while let Ok(Some(entry)) = entries.next_entry().await {
        let Some(name) = entry.file_name().to_str().map(ToOwned::to_owned) else {
            continue;
        };
        if !name.starts_with(".softlan_")
            || name == LEGACY_SOFTLAN_INSTALL_MARKER
            || name == LEGACY_FIRST_START_DONE_FILE
        {
            continue;
        }
        count += 1;
        log::info!(
            "Leaving unknown legacy .softlan file in place: {}",
            entry.path().display()
        );
    }

    count
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum MigrationOutcome {
    SourceMissing,
    TargetAlreadyExists,
    Migrated,
}

async fn migrate_raw_file(
    legacy_path: &Path,
    target_path: &Path,
) -> std::io::Result<MigrationOutcome> {
    if !path_exists(legacy_path).await? {
        return Ok(MigrationOutcome::SourceMissing);
    }

    if path_exists(target_path).await? {
        remove_file_if_exists(legacy_path).await?;
        return Ok(MigrationOutcome::TargetAlreadyExists);
    }

    let data = tokio::fs::read(legacy_path).await?;
    write_bytes_atomically(target_path, &data).await?;
    remove_file_if_exists(legacy_path).await?;
    Ok(MigrationOutcome::Migrated)
}

async fn migrate_empty_marker(
    legacy_path: &Path,
    target_path: &Path,
) -> std::io::Result<MigrationOutcome> {
    if !path_exists(legacy_path).await? {
        return Ok(MigrationOutcome::SourceMissing);
    }

    if path_exists(target_path).await? {
        remove_file_if_exists(legacy_path).await?;
        return Ok(MigrationOutcome::TargetAlreadyExists);
    }

    if let Some(parent) = target_path.parent() {
        tokio::fs::create_dir_all(parent).await?;
    }
    tokio::fs::File::create(target_path)
        .await?
        .sync_all()
        .await?;
    remove_file_if_exists(legacy_path).await?;
    Ok(MigrationOutcome::Migrated)
}

async fn write_bytes_atomically(path: &Path, data: &[u8]) -> std::io::Result<()> {
    if let Some(parent) = path.parent() {
        tokio::fs::create_dir_all(parent).await?;
    }

    let tmp_path = library_index_tmp_path(path);
    let mut file = tokio::fs::File::create(&tmp_path).await?;
    file.write_all(data).await?;
    file.sync_all().await?;
    drop(file);

    tokio::fs::rename(&tmp_path, path).await?;
    sync_parent_dir(path)
}

fn library_index_tmp_path(path: &Path) -> PathBuf {
    let Some(file_name) = path.file_name() else {
        return path.with_extension("tmp");
    };

    let mut tmp_name = file_name.to_os_string();
    tmp_name.push(".tmp");
    path.with_file_name(tmp_name)
}

async fn path_exists(path: &Path) -> std::io::Result<bool> {
    match tokio::fs::metadata(path).await {
        Ok(_) => Ok(true),
        Err(err) if err.kind() == ErrorKind::NotFound => Ok(false),
        Err(err) => Err(err),
    }
}

async fn delete_if_exists(path: &Path) -> MigrationReport {
    match remove_file_if_exists(path).await {
        Ok(true) => MigrationReport {
            legacy_files_deleted: 1,
            ..MigrationReport::default()
        },
        Ok(false) => MigrationReport::default(),
        Err(err) => {
            log::warn!("Failed to delete legacy file {}: {err}", path.display());
            MigrationReport {
                failures: 1,
                ..MigrationReport::default()
            }
        }
    }
}

async fn delete_file(path: &Path) -> MigrationReport {
    match remove_file_if_exists(path).await {
        Ok(true) => MigrationReport {
            legacy_files_deleted: 1,
            ..MigrationReport::default()
        },
        Ok(false) => MigrationReport::default(),
        Err(err) => {
            log::warn!("Failed to delete legacy file {}: {err}", path.display());
            MigrationReport {
                failures: 1,
                ..MigrationReport::default()
            }
        }
    }
}

async fn remove_file_if_exists(path: &Path) -> std::io::Result<bool> {
    if !path_exists(path).await? {
        return Ok(false);
    }

    match tokio::fs::remove_file(path).await {
        Ok(()) => Ok(true),
        Err(err) if err.kind() == ErrorKind::NotFound => Ok(false),
        Err(err) => Err(err),
    }
}

async fn remove_empty_legacy_library_dir(game_dir: &Path) -> MigrationReport {
    let path = game_dir.join(LEGACY_LIBRARY_INDEX_DIR);
    let exists = match path_exists(&path).await {
        Ok(exists) => exists,
        Err(err) => {
            log::warn!(
                "Failed to inspect legacy library index directory {}: {err}",
                path.display()
            );
            return MigrationReport {
                failures: 1,
                ..MigrationReport::default()
            };
        }
    };
    if !exists {
        return MigrationReport::default();
    }

    match tokio::fs::remove_dir(&path).await {
        Ok(()) => MigrationReport {
            legacy_files_deleted: 1,
            ..MigrationReport::default()
        },
        Err(err)
            if err.kind() == ErrorKind::NotFound || err.kind() == ErrorKind::DirectoryNotEmpty =>
        {
            MigrationReport::default()
        }
        Err(err) => {
            log::warn!(
                "Failed to remove empty legacy library index directory {}: {err}",
                path.display()
            );
            MigrationReport {
                failures: 1,
                ..MigrationReport::default()
            }
        }
    }
}

fn log_migration_report(report: &MigrationReport, started: Instant) {
    log::info!(
        "Legacy state migration finished in {:?}: games_checked={}, library_index_migrated={}, \
         install_intents_migrated={}, setup_markers_migrated={}, legacy_files_deleted={}, \
         unknown_softlan_files={}, failures={}",
        started.elapsed(),
        report.games_checked,
        report.library_index_migrated,
        report.install_intents_migrated,
        report.setup_markers_migrated,
        report.legacy_files_deleted,
        report.unknown_softlan_files,
        report.failures
    );
}

#[cfg(unix)]
fn sync_parent_dir(path: &Path) -> std::io::Result<()> {
    if let Some(parent) = path.parent() {
        std::fs::File::open(parent)?.sync_all()?;
    }
    Ok(())
}

#[cfg(not(unix))]
fn sync_parent_dir(_path: &Path) -> std::io::Result<()> {
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{
        install::intent::{InstallIntentState, read_intent},
        test_support::TempDir,
    };

    fn write_file(path: &Path, bytes: &[u8]) {
        if let Some(parent) = path.parent() {
            std::fs::create_dir_all(parent).expect("parent dir should be created");
        }
        std::fs::write(path, bytes).expect("file should be written");
    }

    #[tokio::test]
    async fn migrates_legacy_library_index_to_app_state() {
        let games = TempDir::new("lanspread-migration-games");
        let state = TempDir::new("lanspread-migration-state");
        let legacy_path = legacy_library_index_path(games.path());
        let target_path = local_library_index_path(state.path());
        let legacy_tmp_path = library_index_tmp_path(&legacy_path);

        write_file(&legacy_path, br#"{"revision":7,"games":{}}"#);
        write_file(&legacy_tmp_path, b"tmp");

        let report = migrate_legacy_state(games.path(), state.path()).await;

        assert!(report.library_index_migrated);
        assert_eq!(
            std::fs::read_to_string(&target_path).expect("index should migrate"),
            r#"{"revision":7,"games":{}}"#
        );
        assert!(!legacy_path.exists());
        assert!(!legacy_tmp_path.exists());
        assert!(!games.path().join(LEGACY_LIBRARY_INDEX_DIR).exists());
    }

    #[tokio::test]
    async fn migrates_per_game_intent_and_setup_marker() {
        let games = TempDir::new("lanspread-migration-games");
        let state = TempDir::new("lanspread-migration-state");
        let root = games.path().join("game");
        let intent = InstallIntent::new(
            "game",
            InstallIntentState::Updating,
            Some("20250101".to_string()),
        );
        let legacy_intent = root.join(LEGACY_INTENT_FILE);
        let legacy_tmp = root.join(LEGACY_INTENT_TMP_FILE);
        let legacy_setup = root.join("local").join(LEGACY_FIRST_START_DONE_FILE);
        let legacy_marker = root.join(LEGACY_SOFTLAN_INSTALL_MARKER);

        write_file(
            &legacy_intent,
            &serde_json::to_vec_pretty(&intent).expect("intent should serialize"),
        );
        write_file(&legacy_tmp, b"tmp");
        write_file(&legacy_setup, b"");
        write_file(&legacy_marker, b"");

        let report = migrate_legacy_state(games.path(), state.path()).await;

        assert_eq!(report.install_intents_migrated, 1);
        assert_eq!(report.setup_markers_migrated, 1);
        let migrated_intent = read_intent(state.path(), "game").await;
        assert_eq!(migrated_intent.state, InstallIntentState::Updating);
        assert_eq!(migrated_intent.eti_version.as_deref(), Some("20250101"));
        assert!(setup_done_path(state.path(), "game").is_file());
        assert!(!legacy_intent.exists());
        assert!(!legacy_tmp.exists());
        assert!(!legacy_setup.exists());
        assert!(!legacy_marker.exists());
    }

    #[tokio::test]
    async fn app_state_wins_over_legacy_per_game_state() {
        let games = TempDir::new("lanspread-migration-games");
        let state = TempDir::new("lanspread-migration-state");
        let root = games.path().join("game");
        let app_intent = InstallIntent::none("game", Some("app".to_string()));
        let legacy_intent = InstallIntent::new(
            "game",
            InstallIntentState::Installing,
            Some("legacy".to_string()),
        );
        let legacy_intent_path = root.join(LEGACY_INTENT_FILE);
        let legacy_setup = root.join("local").join(LEGACY_FIRST_START_DONE_FILE);

        write_intent(state.path(), "game", &app_intent)
            .await
            .expect("app-state intent should be written");
        write_file(
            &legacy_intent_path,
            &serde_json::to_vec_pretty(&legacy_intent).expect("intent should serialize"),
        );
        write_file(&setup_done_path(state.path(), "game"), b"");
        write_file(&legacy_setup, b"");

        let report = migrate_legacy_state(games.path(), state.path()).await;

        assert_eq!(report.install_intents_migrated, 0);
        assert_eq!(report.setup_markers_migrated, 0);
        let intent = read_intent(state.path(), "game").await;
        assert_eq!(intent.state, InstallIntentState::None);
        assert_eq!(intent.eti_version.as_deref(), Some("app"));
        assert!(!legacy_intent_path.exists());
        assert!(!legacy_setup.exists());
    }
}