use std::collections::HashMap;
use std::num::NonZeroU32;
use std::{cmp, str};

use fst::Streamer;
use grenad::{CompressionType, MergerBuilder};
use heed::types::ByteSlice;
use heed::{BytesDecode, BytesEncode};
use log::debug;
use slice_group_by::GroupBy;

use crate::error::SerializationError;
use crate::heed_codec::StrBEU32Codec;
use crate::index::main_key::WORDS_PREFIXES_FST_KEY;
use crate::update::index_documents::{
    create_sorter, fst_stream_into_hashset, fst_stream_into_vec, merge_cbo_roaring_bitmaps,
    sorter_into_lmdb_database, CursorClonableMmap, MergeFn, WriteMethod,
};
use crate::{Index, Result};

pub struct WordPrefixPositionDocids<'t, 'u, 'i> {
    wtxn: &'t mut heed::RwTxn<'i, 'u>,
    index: &'i Index,
    pub(crate) chunk_compression_type: CompressionType,
    pub(crate) chunk_compression_level: Option<u32>,
    pub(crate) max_nb_chunks: Option<usize>,
    pub(crate) max_memory: Option<usize>,
    level_group_size: NonZeroU32,
    min_level_size: NonZeroU32,
}

impl<'t, 'u, 'i> WordPrefixPositionDocids<'t, 'u, 'i> {
    pub fn new(
        wtxn: &'t mut heed::RwTxn<'i, 'u>,
        index: &'i Index,
    ) -> WordPrefixPositionDocids<'t, 'u, 'i> {
        WordPrefixPositionDocids {
            wtxn,
            index,
            chunk_compression_type: CompressionType::None,
            chunk_compression_level: None,
            max_nb_chunks: None,
            max_memory: None,
            level_group_size: NonZeroU32::new(4).unwrap(),
            min_level_size: NonZeroU32::new(5).unwrap(),
        }
    }

    pub fn level_group_size(&mut self, value: NonZeroU32) -> &mut Self {
        self.level_group_size = NonZeroU32::new(cmp::max(value.get(), 2)).unwrap();
        self
    }

    pub fn min_level_size(&mut self, value: NonZeroU32) -> &mut Self {
        self.min_level_size = value;
        self
    }

    #[logging_timer::time("WordPrefixPositionDocids::{}")]
    pub fn execute<A: AsRef<[u8]>>(
        self,
        new_word_position_docids: Vec<grenad::Reader<CursorClonableMmap>>,
        old_prefix_fst: &fst::Set<A>,
    ) -> Result<()> {
        debug!("Computing and writing the word levels positions docids into LMDB on disk...");

        let prefix_fst = self.index.words_prefixes_fst(self.wtxn)?;

        // We retrieve the common words between the previous and new prefix word fst.
        let common_prefix_fst_keys =
            fst_stream_into_vec(old_prefix_fst.op().add(&prefix_fst).intersection());
        let common_prefix_fst_keys: Vec<_> = common_prefix_fst_keys
            .as_slice()
            .linear_group_by_key(|x| x.chars().nth(0).unwrap())
            .collect();

        // We compute the set of prefixes that are no more part of the prefix fst.
        let suppr_pw = fst_stream_into_hashset(old_prefix_fst.op().add(&prefix_fst).difference());

        let mut prefix_position_docids_sorter = create_sorter(
            merge_cbo_roaring_bitmaps,
            self.chunk_compression_type,
            self.chunk_compression_level,
            self.max_nb_chunks,
            self.max_memory,
        );

        let mut word_position_docids_merger = MergerBuilder::new(merge_cbo_roaring_bitmaps);
        word_position_docids_merger.extend(new_word_position_docids);
        let mut word_position_docids_iter =
            word_position_docids_merger.build().into_merger_iter()?;

        // We fetch all the new common prefixes between the previous and new prefix fst.
        let mut buffer = Vec::new();
        let mut current_prefixes: Option<&&[String]> = None;
        let mut prefixes_cache = HashMap::new();
        while let Some((key, data)) = word_position_docids_iter.next()? {
            let (word, pos) = StrBEU32Codec::bytes_decode(key).ok_or(heed::Error::Decoding)?;

            current_prefixes = match current_prefixes.take() {
                Some(prefixes) if word.starts_with(&prefixes[0]) => Some(prefixes),
                _otherwise => {
                    write_prefixes_in_sorter(
                        &mut prefixes_cache,
                        &mut prefix_position_docids_sorter,
                    )?;
                    common_prefix_fst_keys.iter().find(|prefixes| word.starts_with(&prefixes[0]))
                }
            };

            if let Some(prefixes) = current_prefixes {
                for prefix in prefixes.iter() {
                    if word.starts_with(prefix) {
                        buffer.clear();
                        buffer.extend_from_slice(prefix.as_bytes());
                        buffer.extend_from_slice(&pos.to_be_bytes());
                        match prefixes_cache.get_mut(&buffer) {
                            Some(value) => value.push(data.to_owned()),
                            None => {
                                prefixes_cache.insert(buffer.clone(), vec![data.to_owned()]);
                            }
                        }
                    }
                }
            }
        }

        write_prefixes_in_sorter(&mut prefixes_cache, &mut prefix_position_docids_sorter)?;

        // We fetch the docids associated to the newly added word prefix fst only.
        let db = self.index.word_position_docids.remap_data_type::<ByteSlice>();
        let mut new_prefixes_stream = prefix_fst.op().add(old_prefix_fst).difference();
        while let Some(prefix_bytes) = new_prefixes_stream.next() {
            let prefix = str::from_utf8(prefix_bytes).map_err(|_| {
                SerializationError::Decoding { db_name: Some(WORDS_PREFIXES_FST_KEY) }
            })?;

            // iter over all lines of the DB where the key is prefixed by the current prefix.
            let iter = db
                .remap_key_type::<ByteSlice>()
                .prefix_iter(self.wtxn, prefix_bytes)?
                .remap_key_type::<StrBEU32Codec>();
            for result in iter {
                let ((word, pos), data) = result?;
                if word.starts_with(prefix) {
                    let key = (prefix, pos);
                    let bytes = StrBEU32Codec::bytes_encode(&key).unwrap();
                    prefix_position_docids_sorter.insert(bytes, data)?;
                }
            }
        }

        drop(new_prefixes_stream);

        // We remove all the entries that are no more required in this word prefix position
        // docids database.
        let mut iter =
            self.index.word_prefix_position_docids.iter_mut(self.wtxn)?.lazily_decode_data();
        while let Some(((prefix, _), _)) = iter.next().transpose()? {
            if suppr_pw.contains(prefix.as_bytes()) {
                unsafe { iter.del_current()? };
            }
        }

        drop(iter);

        // We finally write all the word prefix position docids into the LMDB database.
        sorter_into_lmdb_database(
            self.wtxn,
            *self.index.word_prefix_position_docids.as_polymorph(),
            prefix_position_docids_sorter,
            merge_cbo_roaring_bitmaps,
            WriteMethod::GetMergePut,
        )?;

        Ok(())
    }
}

fn write_prefixes_in_sorter(
    prefixes: &mut HashMap<Vec<u8>, Vec<Vec<u8>>>,
    sorter: &mut grenad::Sorter<MergeFn>,
) -> Result<()> {
    for (key, data_slices) in prefixes.drain() {
        for data in data_slices {
            sorter.insert(&key, data)?;
        }
    }

    Ok(())
}