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use std::ops::{self, Index};

use anyhow::Result;

use crate::merkle::Element;
use crate::store::{Store, StoreConfig};

#[derive(Debug, Clone, Default)]
pub struct VecStore<E: Element>(Vec<E>);

impl<E: Element> ops::Deref for VecStore<E> {
    type Target = [E];

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<E: Element> Store<E> for VecStore<E> {
    fn new_with_config(size: usize, _branches: usize, _config: StoreConfig) -> Result<Self> {
        Self::new(size)
    }

    fn new(size: usize) -> Result<Self> {
        Ok(VecStore(Vec::with_capacity(size)))
    }

    fn write_at(&mut self, el: E, index: usize) -> Result<()> {
        if self.0.len() <= index {
            self.0.resize(index + 1, E::default());
        }

        self.0[index] = el;
        Ok(())
    }

    // NOTE: Performance regression. To conform with the current API we are
    // unnecessarily converting to and from `&[u8]` in the `VecStore` which
    // already stores `E` (in contrast with the `mmap` versions). We are
    // prioritizing performance for the `mmap` case which will be used in
    // production (`VecStore` is mainly for testing and backwards compatibility).
    fn copy_from_slice(&mut self, buf: &[u8], start: usize) -> Result<()> {
        ensure!(
            buf.len() % E::byte_len() == 0,
            "buf size must be a multiple of {}",
            E::byte_len()
        );
        let num_elem = buf.len() / E::byte_len();

        if self.0.len() < start + num_elem {
            self.0.resize(start + num_elem, E::default());
        }

        self.0.splice(
            start..start + num_elem,
            buf.chunks_exact(E::byte_len()).map(E::from_slice),
        );
        Ok(())
    }

    fn new_from_slice_with_config(
        size: usize,
        _branches: usize,
        data: &[u8],
        _config: StoreConfig,
    ) -> Result<Self> {
        Self::new_from_slice(size, data)
    }

    fn new_from_slice(size: usize, data: &[u8]) -> Result<Self> {
        let mut v: Vec<_> = data
            .chunks_exact(E::byte_len())
            .map(E::from_slice)
            .collect();
        let additional = size - v.len();
        v.reserve(additional);

        Ok(VecStore(v))
    }

    fn new_from_disk(_size: usize, _branches: usize, _config: &StoreConfig) -> Result<Self> {
        unimplemented!("Cannot load a VecStore from disk");
    }

    fn read_at(&self, index: usize) -> Result<E> {
        Ok(self.0[index].clone())
    }

    fn read_into(&self, index: usize, buf: &mut [u8]) -> Result<()> {
        self.0[index].copy_to_slice(buf);
        Ok(())
    }

    fn read_range_into(&self, _start: usize, _end: usize, _buf: &mut [u8]) -> Result<()> {
        unimplemented!("Not required here");
    }

    fn read_range(&self, r: ops::Range<usize>) -> Result<Vec<E>> {
        Ok(self.0.index(r).to_vec())
    }

    fn len(&self) -> usize {
        self.0.len()
    }

    fn loaded_from_disk(&self) -> bool {
        false
    }

    fn compact(
        &mut self,
        _branches: usize,
        _config: StoreConfig,
        _store_version: u32,
    ) -> Result<bool> {
        self.0.shrink_to_fit();

        Ok(true)
    }

    fn delete(_config: StoreConfig) -> Result<()> {
        Ok(())
    }

    fn is_empty(&self) -> bool {
        self.0.is_empty()
    }

    fn push(&mut self, el: E) -> Result<()> {
        self.0.push(el);
        Ok(())
    }
}