bitar 0.14.0

//! Archive header structure.
//!
//! | Offset | Size | Description                                                         |
//! |--------|------|---------------------------------------------------------------------|
//! |      0 |    6 | Archive file magic (BITA1\0).                                       |
//! |      6 |    8 | Dictionary size (u64 le).                                           |
//! |     14 |    n | Protobuf encoded dictionary.                                        |
//! |      n |    8 | Chunk data offset in archive, absolute from archive start (u64 le). |
//! |  n + 8 |   64 | Full header checksum (blake2), from offset 0 to n + 8.              |

use blake2::{Blake2b512, Digest};
use prost::Message;

use crate::chunk_dictionary::ChunkDictionary;

/// Archive file magic
pub const ARCHIVE_MAGIC: &[u8; 6] = b"BITA1\0";

/// Pre header is the file magic + the size of the dictionary length value (u64)
pub const PRE_HEADER_SIZE: usize = 6 + std::mem::size_of::<u64>();

/// Build an archive header from dictionary.
pub fn build(
    dictionary: &ChunkDictionary,
    chunk_data_offset: Option<u64>,
) -> Result<Vec<u8>, std::io::Error> {
    let mut header: Vec<u8> = vec![];
    let mut hasher = Blake2b512::new();
    let mut dictionary_buf: Vec<u8> = Vec::new();

    dictionary.encode(&mut dictionary_buf)?;

    // File magic indicating bita archive version 1
    header.extend(ARCHIVE_MAGIC);

    // Chunk dictionary size
    header.extend((dictionary_buf.len() as u64).to_le_bytes());

    // The chunk dictionary
    header.extend(dictionary_buf);

    // Start of archive chunk data, absolute to the archive start
    let offset = match chunk_data_offset {
        Some(o) => o,
        None => header.len() as u64 + 8 + 64,
    };
    header.extend(offset.to_le_bytes());

    // Create and store hash of full header
    hasher.update(&header);
    header.extend(hasher.finalize());

    Ok(header)
}