use bson::{doc, Bson, Document};
use rustdb_storage::OpType;
use rustdb_txn::{TransactionState, WriteEntry, WriteOp};

use crate::context::CommandContext;
use crate::error::{CommandError, CommandResult};

pub fn command_starts_transaction(cmd: &Document) -> bool {
    matches!(cmd.get("startTransaction"), Some(Bson::Boolean(true)))
}

pub fn command_uses_transaction(cmd: &Document) -> bool {
    command_starts_transaction(cmd) || matches!(cmd.get("autocommit"), Some(Bson::Boolean(false)))
}

pub fn active_transaction_id(ctx: &CommandContext, cmd: &Document) -> Option<String> {
    if !command_uses_transaction(cmd) {
        return None;
    }

    let session_id = cmd
        .get("lsid")
        .and_then(rustdb_txn::SessionEngine::extract_session_id)?;
    ctx.sessions.get_transaction_id(&session_id)
}

pub fn prepare_transaction_for_command(
    ctx: &CommandContext,
    cmd: &Document,
    command_name: &str,
) -> CommandResult<()> {
    if matches!(command_name, "commitTransaction" | "abortTransaction") {
        return Ok(());
    }

    let starts_transaction = command_starts_transaction(cmd);
    let uses_transaction = command_uses_transaction(cmd);
    if !uses_transaction {
        return Ok(());
    }

    let session_id = session_id_from_command(cmd)?;
    require_txn_number(cmd)?;
    ctx.sessions.get_or_create_session(&session_id);

    if starts_transaction {
        let txn_id = ctx.transactions.start_transaction(&session_id)?;
        ctx.sessions.start_transaction(&session_id, &txn_id)?;
        return Ok(());
    }

    if ctx.sessions.get_transaction_id(&session_id).is_none() {
        return Err(CommandError::NoSuchTransaction(format!(
            "session {session_id} has no active transaction"
        )));
    }

    Ok(())
}

pub async fn load_transaction_docs(
    ctx: &CommandContext,
    txn_id: &str,
    db: &str,
    coll: &str,
) -> CommandResult<Vec<Document>> {
    let ns = namespace(db, coll);
    if !ctx.transactions.has_snapshot(txn_id, &ns) {
        let docs = match ctx.storage.collection_exists(db, coll).await {
            Ok(true) => ctx.storage.find_all(db, coll).await?,
            Ok(false) => Vec::new(),
            Err(_) => Vec::new(),
        };
        ctx.transactions.set_snapshot(txn_id, &ns, docs);
    }

    ctx.transactions
        .get_snapshot(txn_id, &ns)
        .ok_or_else(|| CommandError::NoSuchTransaction(txn_id.to_string()))
}

pub async fn record_insert(
    ctx: &CommandContext,
    txn_id: &str,
    db: &str,
    coll: &str,
    doc: Document,
) -> CommandResult<String> {
    let id = document_id_string(&doc)?;
    let docs = load_transaction_docs(ctx, txn_id, db, coll).await?;
    if docs.iter().any(|existing| document_id_string(existing).ok().as_deref() == Some(id.as_str())) {
        return Err(CommandError::DuplicateKey(format!(
            "duplicate _id '{}' in transaction",
            id
        )));
    }

    ctx.transactions.record_write(
        txn_id,
        &namespace(db, coll),
        &id,
        WriteOp::Insert,
        Some(doc),
        None,
    );
    Ok(id)
}

pub async fn record_update(
    ctx: &CommandContext,
    txn_id: &str,
    db: &str,
    coll: &str,
    original: Document,
    updated: Document,
) -> CommandResult<String> {
    let id = document_id_string(&original)?;
    ctx.transactions.record_write(
        txn_id,
        &namespace(db, coll),
        &id,
        WriteOp::Update,
        Some(updated),
        Some(original),
    );
    Ok(id)
}

pub async fn record_delete(
    ctx: &CommandContext,
    txn_id: &str,
    db: &str,
    coll: &str,
    original: Document,
) -> CommandResult<String> {
    let id = document_id_string(&original)?;
    ctx.transactions.record_write(
        txn_id,
        &namespace(db, coll),
        &id,
        WriteOp::Delete,
        None,
        Some(original),
    );
    Ok(id)
}

pub async fn commit_transaction_command(
    cmd: &Document,
    ctx: &CommandContext,
) -> CommandResult<Document> {
    let session_id = session_id_from_command(cmd)?;
    let txn_id = ctx
        .sessions
        .get_transaction_id(&session_id)
        .ok_or_else(|| CommandError::NoSuchTransaction(format!(
            "session {session_id} has no active transaction"
        )))?;
    let state = ctx.transactions.take_transaction(&txn_id)?;

    preflight_transaction(&state, ctx).await?;
    apply_transaction(state, ctx).await?;
    ctx.sessions.end_transaction(&session_id);

    Ok(doc! { "ok": 1.0 })
}

pub fn abort_transaction_command(cmd: &Document, ctx: &CommandContext) -> CommandResult<Document> {
    let session_id = session_id_from_command(cmd)?;
    let txn_id = ctx
        .sessions
        .get_transaction_id(&session_id)
        .ok_or_else(|| CommandError::NoSuchTransaction(format!(
            "session {session_id} has no active transaction"
        )))?;
    ctx.transactions.abort_transaction(&txn_id)?;
    ctx.sessions.end_transaction(&session_id);
    Ok(doc! { "ok": 1.0 })
}

pub fn document_id_string(doc: &Document) -> CommandResult<String> {
    match doc.get("_id") {
        Some(Bson::ObjectId(oid)) => Ok(oid.to_hex()),
        Some(Bson::String(s)) => Ok(s.clone()),
        Some(other) => Ok(format!("{}", other)),
        None => Err(CommandError::InvalidArgument("document missing _id field".into())),
    }
}

fn session_id_from_command(cmd: &Document) -> CommandResult<String> {
    cmd.get("lsid")
        .and_then(rustdb_txn::SessionEngine::extract_session_id)
        .ok_or_else(|| CommandError::InvalidArgument("transaction command requires lsid".into()))
}

fn require_txn_number(cmd: &Document) -> CommandResult<()> {
    match cmd.get("txnNumber") {
        Some(Bson::Int64(_)) | Some(Bson::Int32(_)) => Ok(()),
        _ => Err(CommandError::InvalidArgument(
            "transaction command requires txnNumber".into(),
        )),
    }
}

fn namespace(db: &str, coll: &str) -> String {
    format!("{db}.{coll}")
}

async fn preflight_transaction(state: &TransactionState, ctx: &CommandContext) -> CommandResult<()> {
    for (ns, writes) in &state.write_set {
        let (db, coll) = split_namespace(ns)?;
        drop(ctx.get_or_init_index_engine(db, coll).await);

        for (doc_id, entry) in writes {
            let current = current_doc(ctx, db, coll, doc_id).await?;
            match entry.op {
                WriteOp::Insert => {
                    if current.is_some() {
                        return Err(CommandError::DuplicateKey(format!(
                            "duplicate _id '{}' on transaction commit",
                            doc_id
                        )));
                    }
                    if let Some(ref doc) = entry.doc {
                        if let Some(engine) = ctx.indexes.get(ns) {
                            engine.check_unique_constraints(doc)?;
                        }
                    }
                }
                WriteOp::Update => {
                    assert_unchanged(doc_id, current.as_ref(), entry.original_doc.as_ref())?;
                    if let (Some(current_doc), Some(updated_doc)) = (current.as_ref(), entry.doc.as_ref()) {
                        if let Some(engine) = ctx.indexes.get(ns) {
                            engine.check_unique_constraints_for_update(current_doc, updated_doc)?;
                        }
                    }
                }
                WriteOp::Delete => {
                    assert_unchanged(doc_id, current.as_ref(), entry.original_doc.as_ref())?;
                }
            }
        }
    }

    Ok(())
}

async fn apply_transaction(state: TransactionState, ctx: &CommandContext) -> CommandResult<()> {
    let mut namespaces: Vec<_> = state.write_set.into_iter().collect();
    namespaces.sort_by(|a, b| a.0.cmp(&b.0));

    for (ns, writes) in namespaces {
        let (db, coll) = split_namespace(&ns)?;
        ensure_collection_exists(db, coll, ctx).await?;
        drop(ctx.get_or_init_index_engine(db, coll).await);

        let mut writes: Vec<(String, WriteEntry)> = writes.into_iter().collect();
        writes.sort_by(|a, b| a.0.cmp(&b.0));

        for (doc_id, entry) in writes {
            match entry.op {
                WriteOp::Insert => {
                    let Some(doc) = entry.doc else { continue; };
                    let inserted_id = ctx.storage.insert_one(db, coll, doc.clone()).await?;
                    ctx.oplog.append(OpType::Insert, db, coll, &inserted_id, Some(doc.clone()), None);
                    if let Some(mut engine) = ctx.indexes.get_mut(&ns) {
                        engine.on_insert(&doc)?;
                    }
                }
                WriteOp::Update => {
                    let Some(doc) = entry.doc else { continue; };
                    ctx.storage.update_by_id(db, coll, &doc_id, doc.clone()).await?;
                    ctx.oplog.append(
                        OpType::Update,
                        db,
                        coll,
                        &doc_id,
                        Some(doc.clone()),
                        entry.original_doc.clone(),
                    );
                    if let (Some(mut engine), Some(ref original)) =
                        (ctx.indexes.get_mut(&ns), entry.original_doc.as_ref())
                    {
                        engine.on_update(original, &doc)?;
                    }
                }
                WriteOp::Delete => {
                    ctx.storage.delete_by_id(db, coll, &doc_id).await?;
                    ctx.oplog.append(
                        OpType::Delete,
                        db,
                        coll,
                        &doc_id,
                        None,
                        entry.original_doc.clone(),
                    );
                    if let (Some(mut engine), Some(ref original)) =
                        (ctx.indexes.get_mut(&ns), entry.original_doc.as_ref())
                    {
                        engine.on_delete(original);
                    }
                }
            }
        }
    }

    Ok(())
}

async fn current_doc(
    ctx: &CommandContext,
    db: &str,
    coll: &str,
    doc_id: &str,
) -> CommandResult<Option<Document>> {
    match ctx.storage.collection_exists(db, coll).await {
        Ok(true) => Ok(ctx.storage.find_by_id(db, coll, doc_id).await?),
        Ok(false) => Ok(None),
        Err(_) => Ok(None),
    }
}

fn assert_unchanged(
    doc_id: &str,
    current: Option<&Document>,
    original: Option<&Document>,
) -> CommandResult<()> {
    if current == original {
        return Ok(());
    }

    Err(CommandError::WriteConflict(format!(
        "document '{}' changed during transaction",
        doc_id
    )))
}

async fn ensure_collection_exists(
    db: &str,
    coll: &str,
    ctx: &CommandContext,
) -> CommandResult<()> {
    if let Err(e) = ctx.storage.create_database(db).await {
        let msg = e.to_string();
        if !msg.contains("AlreadyExists") && !msg.contains("already exists") {
            return Err(CommandError::StorageError(msg));
        }
    }

    match ctx.storage.collection_exists(db, coll).await {
        Ok(true) => Ok(()),
        Ok(false) | Err(_) => {
            if let Err(e) = ctx.storage.create_collection(db, coll).await {
                let msg = e.to_string();
                if !msg.contains("AlreadyExists") && !msg.contains("already exists") {
                    return Err(CommandError::StorageError(msg));
                }
            }
            Ok(())
        }
    }
}

fn split_namespace(ns: &str) -> CommandResult<(&str, &str)> {
    ns.split_once('.')
        .ok_or_else(|| CommandError::InvalidArgument(format!("invalid namespace '{ns}'")))
}