hextree 0.3.2

Location to value mapping.
Documentation 
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use crate::{
    disktree::{dptr::Dp, dtseek::DtSeek, varint},
    error::Result,
};
use byteorder::ReadBytesExt;
use std::{io::Read, mem::size_of, ops::Range};

// Enough bytes to read node tag and 7 child dptrs.
const NODE_BUF_SZ: usize = size_of::<u8>() + 7 * Dp::size();

#[derive(Debug)]
pub(crate) enum Node {
    // value_begin..value_end
    Leaf(Range<usize>),
    // (H3 Cell digit, file position of child's node tag)
    Parent([Option<Dp>; 7]),
}

impl Node {
    pub(crate) fn read<R>(rdr: &mut R) -> Result<Node>
    where
        R: Read + DtSeek,
    {
        let start_pos = rdr.pos()?;
        let mut buf = [0u8; NODE_BUF_SZ];
        let bytes_read = rdr.read(&mut buf)?;
        let buf_rdr = &mut &buf[..bytes_read];
        let node_tag = buf_rdr.read_u8()?;
        if 0 == node_tag & 0b1000_0000 {
            let (val_len, n_read) = varint::read(&mut &buf[..bytes_read])?;
            let begin = start_pos + n_read;
            let end = begin + val_len;
            Ok(Node::Leaf(usize::from(begin)..usize::from(end)))
        } else {
            let mut children: [Option<Dp>; 7] = [None, None, None, None, None, None, None];
            for (_digit, child) in (0..7)
                .zip(children.iter_mut())
                .filter(|(digit, _)| node_tag & (1 << digit) != 0)
            {
                *child = Some(Dp::read(buf_rdr)?);
            }
            Ok(Node::Parent(children))
        }
    }
}