use alloc::vec::Vec;

use core::convert::TryInto;

use crate::decoding::errors::DictionaryDecodeError;

use crate::decoding::scratch::FSEScratch;

use crate::decoding::scratch::HuffmanScratch;

/// Zstandard includes support for "raw content" dictionaries, that store bytes optionally used

/// during sequence execution.

///

/// <https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#dictionary-format>

pub struct Dictionary {

    /// A 4 byte value used by decoders to check if they can use

    /// the correct dictionary. This value must not be zero.

    pub id: u32,

    /// A dictionary can contain an entropy table, either FSE or

    /// Huffman.

    pub fse: FSEScratch,

    /// A dictionary can contain an entropy table, either FSE or

    /// Huffman.

    pub huf: HuffmanScratch,

    /// The content of a dictionary acts as a "past" in front of data

    /// to compress or decompress,

    /// so it can be referenced in sequence commands.

    /// As long as the amount of data decoded from this frame is less than or

    /// equal to Window_Size, sequence commands may specify offsets longer than

    /// the total length of decoded output so far to reference back to the

    /// dictionary, even parts of the dictionary with offsets larger than Window_Size.

    /// After the total output has surpassed Window_Size however,

    /// this is no longer allowed and the dictionary is no longer accessible

    pub dict_content: Vec<u8>,

    /// The 3 most recent offsets are stored so that they can be used

    /// during sequence execution, see

    /// <https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#repeat-offsets>

    /// for more.

    pub offset_hist: [u32; 3],

}

/// This 4 byte (little endian) magic number refers to the start of a dictionary

pub const MAGIC_NUM: [u8; 4] = [0x37, 0xA4, 0x30, 0xEC];

impl Dictionary {

    /// Parses the dictionary from `raw` and set the tables

    /// it returns the dict_id for checking with the frame's `dict_id``

    pub fn decode_dict(raw: &[u8]) -> Result<Dictionary, DictionaryDecodeError> {

        let mut new_dict = Dictionary {

            id: 0,

            fse: FSEScratch::new(),

            huf: HuffmanScratch::new(),

            dict_content: Vec::new(),

            offset_hist: [2, 4, 8],

        };

        let magic_num: [u8; 4] = raw[..4].try_into().expect("optimized away");

        if magic_num != MAGIC_NUM {

            return Err(DictionaryDecodeError::BadMagicNum { got: magic_num });

        }

        let dict_id = raw[4..8].try_into().expect("optimized away");

        let dict_id = u32::from_le_bytes(dict_id);

        new_dict.id = dict_id;

        let raw_tables = &raw[8..];

        let huf_size = new_dict.huf.table.build_decoder(raw_tables)?;

        let raw_tables = &raw_tables[huf_size as usize..];

        let of_size = new_dict.fse.offsets.build_decoder(

            raw_tables,

            crate::decoding::sequence_section_decoder::OF_MAX_LOG,

        )?;

        let raw_tables = &raw_tables[of_size..];

        let ml_size = new_dict.fse.match_lengths.build_decoder(

            raw_tables,

            crate::decoding::sequence_section_decoder::ML_MAX_LOG,

        )?;

        let raw_tables = &raw_tables[ml_size..];

        let ll_size = new_dict.fse.literal_lengths.build_decoder(

            raw_tables,

            crate::decoding::sequence_section_decoder::LL_MAX_LOG,

        )?;

        let raw_tables = &raw_tables[ll_size..];

        let offset1 = raw_tables[0..4].try_into().expect("optimized away");

        let offset1 = u32::from_le_bytes(offset1);

        let offset2 = raw_tables[4..8].try_into().expect("optimized away");

        let offset2 = u32::from_le_bytes(offset2);

        let offset3 = raw_tables[8..12].try_into().expect("optimized away");

        let offset3 = u32::from_le_bytes(offset3);

        new_dict.offset_hist[0] = offset1;

        new_dict.offset_hist[1] = offset2;

        new_dict.offset_hist[2] = offset3;

        let raw_content = &raw_tables[12..];

        new_dict.dict_content.extend(raw_content);

        Ok(new_dict)

    }

}