use std::borrow::Cow;
use std::collections::BTreeSet;
use std::io;
use std::result::Result as StdResult;

use roaring::RoaringBitmap;

use crate::heed_codec::CboRoaringBitmapCodec;
use crate::update::del_add::{DelAdd, KvReaderDelAdd, KvWriterDelAdd};
use crate::update::index_documents::transform::Operation;
use crate::Result;

pub type MergeFn = for<'a> fn(&[u8], &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>>;

pub fn serialize_roaring_bitmap(bitmap: &RoaringBitmap, buffer: &mut Vec<u8>) -> io::Result<()> {
    buffer.clear();
    buffer.reserve(bitmap.serialized_size());
    bitmap.serialize_into(buffer)
}

pub fn merge_roaring_bitmaps<'a>(_key: &[u8], values: &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>> {
    if values.len() == 1 {
        Ok(values[0].clone())
    } else {
        let merged = values
            .iter()
            .map(AsRef::as_ref)
            .map(RoaringBitmap::deserialize_from)
            .map(StdResult::unwrap)
            .reduce(|a, b| a | b)
            .unwrap();
        let mut buffer = Vec::new();
        serialize_roaring_bitmap(&merged, &mut buffer)?;
        Ok(Cow::Owned(buffer))
    }
}

pub fn keep_first<'a>(_key: &[u8], values: &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>> {
    Ok(values[0].clone())
}

/// Only the last value associated with an id is kept.
pub fn keep_latest_obkv<'a>(_key: &[u8], obkvs: &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>> {
    Ok(obkvs.last().unwrap().clone())
}

pub fn merge_two_del_add_obkvs(
    base: obkv::KvReaderU16<'_>,
    update: obkv::KvReaderU16<'_>,
    merge_additions: bool,
    buffer: &mut Vec<u8>,
) {
    use itertools::merge_join_by;
    use itertools::EitherOrBoth::{Both, Left, Right};

    buffer.clear();

    let mut writer = obkv::KvWriter::new(buffer);
    let mut value_buffer = Vec::new();
    for eob in merge_join_by(base.iter(), update.iter(), |(b, _), (u, _)| b.cmp(u)) {
        match eob {
            Left((k, v)) => {
                if merge_additions {
                    writer.insert(k, v).unwrap()
                } else {
                    // If merge_additions is false, recreate an obkv keeping the deletions only.
                    value_buffer.clear();
                    let mut value_writer = KvWriterDelAdd::new(&mut value_buffer);
                    let base_reader = KvReaderDelAdd::new(v);

                    if let Some(deletion) = base_reader.get(DelAdd::Deletion) {
                        value_writer.insert(DelAdd::Deletion, deletion).unwrap();
                        value_writer.finish().unwrap();
                        writer.insert(k, &value_buffer).unwrap()
                    }
                }
            }
            Right((k, v)) => writer.insert(k, v).unwrap(),
            Both((k, base), (_, update)) => {
                // merge deletions and additions.
                value_buffer.clear();
                let mut value_writer = KvWriterDelAdd::new(&mut value_buffer);
                let base_reader = KvReaderDelAdd::new(base);
                let update_reader = KvReaderDelAdd::new(update);

                // keep newest deletion.
                if let Some(deletion) = update_reader
                    .get(DelAdd::Deletion)
                    .or_else(|| base_reader.get(DelAdd::Deletion))
                {
                    value_writer.insert(DelAdd::Deletion, deletion).unwrap();
                }

                // keep base addition only if merge_additions is true.
                let base_addition =
                    merge_additions.then(|| base_reader.get(DelAdd::Addition)).flatten();
                // keep newest addition.
                // TODO use or_else
                if let Some(addition) = update_reader.get(DelAdd::Addition).or(base_addition) {
                    value_writer.insert(DelAdd::Addition, addition).unwrap();
                }

                value_writer.finish().unwrap();
                writer.insert(k, &value_buffer).unwrap()
            }
        }
    }

    writer.finish().unwrap();
}

/// Merge all the obkvs from the newest to the oldest.
fn inner_merge_del_add_obkvs<'a>(
    obkvs: &[Cow<'a, [u8]>],
    merge_additions: bool,
) -> Result<Cow<'a, [u8]>> {
    // pop the newest operation from the list.
    let (newest, obkvs) = obkvs.split_last().unwrap();
    // keep the operation type for the returned value.
    let newest_operation_type = newest[0];

    // treat the newest obkv as the starting point of the merge.
    let mut acc_operation_type = newest_operation_type;
    let mut acc = newest[1..].to_vec();
    let mut buffer = Vec::new();
    // reverse iter from the most recent to the oldest.
    for current in obkvs.iter().rev() {
        // if in the previous iteration there was a complete deletion,
        // stop the merge process.
        if acc_operation_type == Operation::Deletion as u8 {
            break;
        }

        let newest = obkv::KvReader::new(&acc);
        let oldest = obkv::KvReader::new(&current[1..]);
        merge_two_del_add_obkvs(oldest, newest, merge_additions, &mut buffer);

        // we want the result of the merge into our accumulator.
        std::mem::swap(&mut acc, &mut buffer);
        acc_operation_type = current[0];
    }

    acc.insert(0, newest_operation_type);
    Ok(Cow::from(acc))
}

/// Merge all the obkvs from the newest to the oldest.
pub fn obkvs_merge_additions_and_deletions<'a>(
    _key: &[u8],
    obkvs: &[Cow<'a, [u8]>],
) -> Result<Cow<'a, [u8]>> {
    inner_merge_del_add_obkvs(obkvs, true)
}

/// Merge all the obkvs deletions from the newest to the oldest and keep only the newest additions.
pub fn obkvs_keep_last_addition_merge_deletions<'a>(
    _key: &[u8],
    obkvs: &[Cow<'a, [u8]>],
) -> Result<Cow<'a, [u8]>> {
    inner_merge_del_add_obkvs(obkvs, false)
}

/// Do a union of all the CboRoaringBitmaps in the values.
pub fn merge_cbo_roaring_bitmaps<'a>(
    _key: &[u8],
    values: &[Cow<'a, [u8]>],
) -> Result<Cow<'a, [u8]>> {
    if values.len() == 1 {
        Ok(values[0].clone())
    } else {
        let mut vec = Vec::new();
        CboRoaringBitmapCodec::merge_into(values, &mut vec)?;
        Ok(Cow::from(vec))
    }
}

/// Do a union of CboRoaringBitmaps on both sides of a DelAdd obkv
/// separately and outputs a new DelAdd with both unions.
pub fn merge_deladd_cbo_roaring_bitmaps<'a>(
    _key: &[u8],
    values: &[Cow<'a, [u8]>],
) -> Result<Cow<'a, [u8]>> {
    if values.len() == 1 {
        Ok(values[0].clone())
    } else {
        // Retrieve the bitmaps from both sides
        let mut del_bitmaps_bytes = Vec::new();
        let mut add_bitmaps_bytes = Vec::new();
        for value in values {
            let obkv = KvReaderDelAdd::new(value);
            if let Some(bitmap_bytes) = obkv.get(DelAdd::Deletion) {
                del_bitmaps_bytes.push(bitmap_bytes);
            }
            if let Some(bitmap_bytes) = obkv.get(DelAdd::Addition) {
                add_bitmaps_bytes.push(bitmap_bytes);
            }
        }

        let mut output_deladd_obkv = KvWriterDelAdd::memory();
        let mut buffer = Vec::new();
        CboRoaringBitmapCodec::merge_into(del_bitmaps_bytes, &mut buffer)?;
        output_deladd_obkv.insert(DelAdd::Deletion, &buffer)?;
        buffer.clear();
        CboRoaringBitmapCodec::merge_into(add_bitmaps_bytes, &mut buffer)?;
        output_deladd_obkv.insert(DelAdd::Addition, &buffer)?;
        output_deladd_obkv.into_inner().map(Cow::from).map_err(Into::into)
    }
}

/// A function that merges a DelAdd of bitmao into an already existing bitmap.
///
/// The first argument is the DelAdd obkv of CboRoaringBitmaps and
/// the second one is the CboRoaringBitmap to merge into.
pub fn merge_deladd_cbo_roaring_bitmaps_into_cbo_roaring_bitmap<'a>(
    deladd_obkv: &[u8],
    previous: &[u8],
    buffer: &'a mut Vec<u8>,
) -> Result<Option<&'a [u8]>> {
    Ok(CboRoaringBitmapCodec::merge_deladd_into(
        KvReaderDelAdd::new(deladd_obkv),
        previous,
        buffer,
    )?)
}

/// Do a union of BtreeSet on both sides of a DelAdd obkv
/// separately and outputs a new DelAdd with both unions.
pub fn merge_deladd_btreeset_string<'a>(
    _key: &[u8],
    values: &[Cow<'a, [u8]>],
) -> Result<Cow<'a, [u8]>> {
    if values.len() == 1 {
        Ok(values[0].clone())
    } else {
        // Retrieve the bitmaps from both sides
        let mut del_set = BTreeSet::new();
        let mut add_set = BTreeSet::new();
        for value in values {
            let obkv = KvReaderDelAdd::new(value);
            if let Some(bytes) = obkv.get(DelAdd::Deletion) {
                let set = serde_json::from_slice::<BTreeSet<String>>(bytes).unwrap();
                for value in set {
                    del_set.insert(value);
                }
            }
            if let Some(bytes) = obkv.get(DelAdd::Addition) {
                let set = serde_json::from_slice::<BTreeSet<String>>(bytes).unwrap();
                for value in set {
                    add_set.insert(value);
                }
            }
        }

        let mut output_deladd_obkv = KvWriterDelAdd::memory();
        let del = serde_json::to_vec(&del_set).unwrap();
        output_deladd_obkv.insert(DelAdd::Deletion, &del)?;
        let add = serde_json::to_vec(&add_set).unwrap();
        output_deladd_obkv.insert(DelAdd::Addition, &add)?;
        output_deladd_obkv.into_inner().map(Cow::from).map_err(Into::into)
    }
}