grovedb-path 2.0.3

// MIT LICENSE
//
// Copyright (c) 2023 Dash Core Group
//
// Permission is hereby granted, free of charge, to any
// person obtaining a copy of this software and associated
// documentation files (the "Software"), to deal in the
// Software without restriction, including without
// limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of
// the Software, and to permit persons to whom the Software
// is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice
// shall be included in all copies or substantial portions
// of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
// ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
// PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
// SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
// IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

//! Compact two dimenstional bytes array structure.

use std::mem;

/// Bytes vector wrapper to have multiple byte arrays allocated continuosuly.
#[derive(Debug, Default)]
pub(crate) struct CompactBytes {
    n_segments: usize,
    data: Vec<u8>,
}

impl CompactBytes {
    /// Create empty structure.
    pub fn new() -> Self {
        Self::default()
    }

    /// Append bytes segment.
    pub fn add_segment(&mut self, segment: &[u8]) {
        // Because the iteration will go backwards, a segment is inserted an odd way:
        // First it goes the segment itself
        self.data.extend_from_slice(segment);
        // Next it will be it's length
        self.data.extend_from_slice(&segment.len().to_ne_bytes());
        self.n_segments += 1;
    }

    pub fn reverse_iter(&self) -> CompactBytesIter {
        CompactBytesIter {
            bytes: self,
            offset_back: self.data.len(),
            n_segments_left: self.n_segments,
        }
    }

    pub fn len(&self) -> usize {
        self.n_segments
    }
}

#[derive(Debug, Clone)]
pub(crate) struct CompactBytesIter<'a> {
    bytes: &'a CompactBytes,
    offset_back: usize,
    n_segments_left: usize,
}

impl<'a> Iterator for CompactBytesIter<'a> {
    type Item = &'a [u8];

    fn next(&mut self) -> Option<Self::Item> {
        if self.offset_back == 0 {
            return None;
        }

        // Like [Self::add_segment], but reverse: first we get N bytes, where N
        // is the size of usize and build segment length
        let length_size = mem::size_of::<usize>();
        let segment_length = usize::from_ne_bytes(
            self.bytes.data[self.offset_back - length_size..self.offset_back]
                .try_into()
                .expect("internal structure bug"),
        );
        // Read segment length, moving offset backwards
        self.offset_back -= length_size;

        // Reading segment data starting from new offset
        let segment = &self.bytes.data[self.offset_back - segment_length..self.offset_back];

        // Move offset to the next (according to iteration direction) segment data (must
        // point to it's size)
        self.offset_back -= segment_length;

        // Decrease iterator's size
        self.n_segments_left -= 1;

        Some(segment)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.n_segments_left, Some(self.n_segments_left))
    }
}

impl ExactSizeIterator for CompactBytesIter<'_> {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn empty_two_dimensional_bytes() {
        let bytes = CompactBytes::new();
        assert_eq!(bytes.len(), 0);

        let mut iter = bytes.reverse_iter();
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn non_empty_two_dimensional_bytes_backward_iterator() {
        let mut bytes = CompactBytes::default();
        bytes.add_segment(b"ayya");
        bytes.add_segment(b"ayyb");
        bytes.add_segment(b"didn'texpectthat!");
        bytes.add_segment(b"ayyd");

        assert_eq!(bytes.len(), 4);

        let mut iter = bytes.reverse_iter();
        assert_eq!(iter.next(), Some(b"ayyd".as_ref()));
        assert_eq!(iter.next(), Some(b"didn'texpectthat!".as_ref()));

        assert_eq!(iter.len(), 2);

        assert_eq!(iter.next(), Some(b"ayyb".as_ref()));
        assert_eq!(iter.next(), Some(b"ayya".as_ref()));
        assert_eq!(iter.next(), None);
        assert_eq!(iter.next(), None);

        // Can do it twice
        assert_eq!(bytes.len(), 4);

        let mut iter = bytes.reverse_iter();
        assert_eq!(iter.next(), Some(b"ayyd".as_ref()));

        assert_eq!(iter.len(), 3);

        assert_eq!(iter.next(), Some(b"didn'texpectthat!".as_ref()));
        assert_eq!(iter.next(), Some(b"ayyb".as_ref()));
        assert_eq!(iter.next(), Some(b"ayya".as_ref()));
        assert_eq!(iter.next(), None);
        assert_eq!(iter.next(), None);
    }
}