use std::{convert::TryInto, ops::Range}; mod error {    /// The value returned by [crate::FileRef::from_bytes()    #[derive(Debug, thiserror::Error)]    #[allow(missing_docs)]    pub enum Error {        #[error("Sentinel value encountered while still processing chunks.")]        EarlySentinelValue,        #[error("Sentinel value wasn't found, saw {:?}", std::str::from_utf8(actual.as_ref()).unwrap_or("<non-ascii>"))]        MissingSentinelValue { actual: crate::Id },        #[error("The chunk offset {offset} went past the file of length {file_length} - was it truncated?")]        ChunkSizeOutOfBounds {            offset: crate::file::Offset,            file_length: u64,        },        #[error("All chunk offsets must be incrementing.")]        NonIncrementalChunkOffsets,        #[error("The chunk of kind {:?} was encountered more than once", std::str::from_utf8(kind.as_ref()).unwrap_or("<non-ascii>"))]        DuplicateChunk { kind: crate::Id },        #[error("The table of contents would be {expected} bytes, but got only {actual}")]        TocTooSmall { actual: usize, expected: usize },        #[error("Empty chunk indices are not allowed as the point of chunked files is to have chunks.")]        Empty,    }}pub use error::Error; use crate::{file, file::index}; impl file::Index {    /// Provided a mapped file at the beginning via `data`, starting at `toc_offset` decode all chunk information to return    /// an index with `num_chunks` chunks.    pub fn from_bytes(data: &[u8], toc_offset: usize, num_chunks: u32) -> Result<Self, Error> {        if num_chunks == 0 {            return Err(Error::Empty);        }         let data_len: u64 = data.len() as u64;        let mut chunks = Vec::with_capacity(num_chunks as usize);        let mut toc_entry = &data[toc_offset..];        let expected_min_size = (num_chunks as usize + 1) * file::Index::ENTRY_SIZE;        if toc_entry.len() < expected_min_size {            return Err(Error::TocTooSmall {                expected: expected_min_size,                actual: toc_entry.len(),            });        }         for _ in 0..num_chunks {            let (kind, offset) = toc_entry.split_at(4);            let kind = to_kind(kind);            if kind == crate::SENTINEL {                return Err(Error::EarlySentinelValue);            }            if chunks.iter().any(|c: &index::Entry| c.kind == kind) {                return Err(Error::DuplicateChunk { kind });            }             let offset = be_u64(offset);            if offset > data_len {                return Err(Error::ChunkSizeOutOfBounds {                    offset,                    file_length: data_len,                });            }            toc_entry = &toc_entry[file::Index::ENTRY_SIZE..];            let next_offset = be_u64(&toc_entry[4..]);            if next_offset > data_len {                return Err(Error::ChunkSizeOutOfBounds {                    offset: next_offset,                    file_length: data_len,                });            }            if next_offset <= offset {                return Err(Error::NonIncrementalChunkOffsets);            }            chunks.push(index::Entry {                kind,                offset: Range {                    start: offset,                    end: next_offset,                },            })        }         let sentinel = to_kind(&toc_entry[..4]);        if sentinel != crate::SENTINEL {            return Err(Error::MissingSentinelValue { actual: sentinel });        }         Ok(file::Index {            chunks,            will_write: false,        })    }} fn to_kind(data: &[u8]) -> crate::Id {    data[..4].try_into().unwrap()} fn be_u64(data: &[u8]) -> u64 {    u64::from_be_bytes(data[..8].try_into().unwrap())}