use super::*;
use crate::db::errors::{DataError, MigrationError};
use crate::db::Database;
use byteorder::LittleEndian;
use sled::transaction::TransactionError;
use sled::{Batch, IVec};
use zerocopy::byteorder::U32;
use zerocopy::AsBytes;

//Not to be confused with the super::RoomAndUser delineator.
#[derive(PartialEq, Eq, std::hash::Hash)]
struct RoomAndUser {
    room_id: String,
    username: String,
}

/// Create a version 0 user variable key.
fn v0_variable_key(info: &RoomAndUser, variable_name: &str) -> Vec<u8> {
    let mut key = vec![];
    key.extend_from_slice(info.room_id.as_bytes());
    key.extend_from_slice(info.username.as_bytes());
    key.extend_from_slice(variable_name.as_bytes());
    key
}

fn map_value_to_room_and_user(
    entry: sled::Result<(IVec, IVec)>,
) -> Result<RoomAndUser, MigrationError> {
    if let Ok((key, _)) = entry {
        let keys: Vec<Result<&str, _>> = key
            .split(|&b| b == 0xff)
            .map(|b| str::from_utf8(b))
            .collect();

        if let &[_, Ok(room_id), Ok(username), Ok(_variable)] = keys.as_slice() {
            Ok(RoomAndUser {
                room_id: room_id.to_owned(),
                username: username.to_owned(),
            })
        } else {
            Err(MigrationError::MigrationFailed(
                "a key violates utf8 schema".to_string(),
            ))
        }
    } else {
        Err(MigrationError::MigrationFailed(
            "encountered unexpected key".to_string(),
        ))
    }
}

pub(in crate::db) fn add_room_user_variable_count(db: &Database) -> Result<(), DataError> {
    let tree = &db.variables.0;
    let prefix = variables_space_prefix("");

    //Extract a vec of tuples, consisting of room id + username.
    let results: Vec<RoomAndUser> = tree
        .scan_prefix(prefix)
        .map(map_value_to_room_and_user)
        .collect::<Result<Vec<_>, MigrationError>>()?;

    let counts: HashMap<RoomAndUser, u32> =
        results
            .into_iter()
            .fold(HashMap::new(), |mut count_map, room_and_user| {
                let count = count_map.entry(room_and_user).or_insert(0);
                *count += 1;
                count_map
            });

    //Start a transaction on the variables tree.
    let tx_result: Result<_, TransactionError<DataError>> = db.variables.0.transaction(|tx_vars| {
        let batch = counts.iter().fold(Batch::default(), |mut batch, entry| {
            let (info, count) = entry;

            //Add variable count according to new schema.
            let delineator = super::RoomAndUser(&info.room_id, &info.username);
            let key = variables_space_key(delineator, VARIABLE_COUNT_KEY);
            let db_value: U32<LittleEndian> = U32::new(*count);
            batch.insert(key, db_value.as_bytes());

            //Delete the old variable_count variable if exists.
            let old_key = v0_variable_key(&info, "variable_count");
            batch.remove(old_key);
            batch
        });

        tx_vars.apply_batch(&batch)?;
        Ok(())
    });

    tx_result?; //For some reason, it cannot infer the type
    Ok(())
}

pub(in crate::db) fn delete_v0_schema(db: &Database) -> Result<(), DataError> {
    let mut vars = db.variables.0.scan_prefix("");
    let mut batch = Batch::default();

    while let Some(Ok((key, _))) = vars.next() {
        let key = key.to_vec();

        if !key.contains(&0xff) {
            batch.remove(key);
        }
    }

    db.variables.0.apply_batch(batch)?;
    Ok(())
}