zipatch_rs/index/

source.rs

//! [`PatchSource`] trait and built-in implementations.
//!
//! [`PatchSource`] is the byte-fetching seam between a [`crate::index::Plan`]
//! and the patch files the plan was built from. [`IndexApplier`](crate::index::IndexApplier)
//! never touches the network or owns a file handle — it calls
//! [`PatchSource::read`] with a `(patch, offset)` tuple and an exact-size
//! buffer, and the caller decides how those reads are served (local files,
//! in-memory cache, HTTP multi-range request, …).
//!
//! # Chain semantics
//!
//! `patch: u32` indexes into [`crate::index::Plan::patches`]. A single-patch
//! plan uses index `0`; multi-patch chains use successive indices in the
//! order patches were fed to [`crate::index::PlanBuilder::add_patch`]. The
//! built-in sources store one file/buffer per patch and reject out-of-range
//! indices with [`crate::ZiPatchError::PatchIndexOutOfRange`].
//!
//! # Caller-fills-buffer
//!
//! The destination buffer is supplied by the applier at the exact length it
//! needs (every region carries its length). Returning bytes via `&[u8]` would
//! force either an extra copy or a `Cow` that allocates for compressed
//! regions; this shape composes cleanly with HTTP multi-range responses and
//! pre-allocated scratch buffers without either compromise.
//!
//! # Built-in implementations
//!
//! - [`FilePatchSource`] — wraps one or more owned [`std::fs::File`]s,
//!   single-threaded. Use [`FilePatchSource::open`] for one patch or
//!   [`FilePatchSource::open_chain`] for many. Callers wanting concurrent
//!   reads should construct one source per thread.
//! - [`MemoryPatchSource`] (test-only) — wraps one or more `Vec<u8>`s.
//!   Gated behind the `test-utils` feature.

use crate::{Result, ZiPatchError};
use std::fs::File;
use std::io::{Read, Seek, SeekFrom};
use std::path::Path;

/// Source of patch-file bytes for an [`IndexApplier`](crate::index::IndexApplier).
///
/// Implementations must fill `dst` completely with `dst.len()` source bytes
/// starting at `offset` in the patch identified by `patch`. A short read —
/// fewer than `dst.len()` bytes available — must surface as
/// [`ZiPatchError::PatchSourceTooShort`], not as a partial fill. An
/// out-of-range `patch` must surface as
/// [`ZiPatchError::PatchIndexOutOfRange`].
pub trait PatchSource {
    /// Fill `dst` with `dst.len()` source bytes starting at `offset` in the
    /// patch indexed by `patch`.
    ///
    /// # Errors
    ///
    /// - [`ZiPatchError::PatchIndexOutOfRange`] — `patch` is `>= count`.
    /// - [`ZiPatchError::PatchSourceTooShort`] — the source has fewer than
    ///   `dst.len()` bytes at `offset` in patch `patch`.
    /// - [`ZiPatchError::Io`] — underlying I/O failure (only meaningful for
    ///   sources backed by real I/O, e.g. [`FilePatchSource`]).
    fn read(&mut self, patch: u32, offset: u64, dst: &mut [u8]) -> Result<()>;
}

/// [`PatchSource`] backed by one or more owned [`std::fs::File`]s, one per
/// patch in the chain.
///
/// Single-threaded — no internal `Mutex`. Callers that need concurrent reads
/// should construct one source per thread.
#[derive(Debug)]
pub struct FilePatchSource {
    files: Vec<File>,
}

impl FilePatchSource {
    /// Open a single patch file and wrap it as a one-patch chain.
    ///
    /// Equivalent to [`FilePatchSource::open_chain`] with a one-element iterator.
    ///
    /// # Errors
    ///
    /// Returns [`ZiPatchError::Io`] if the file cannot be opened.
    pub fn open(path: impl AsRef<Path>) -> Result<Self> {
        let file = File::open(path)?;
        Ok(Self { files: vec![file] })
    }

    /// Open every patch file in `paths` and wrap them as a chain. The first
    /// path becomes patch index `0`, the second index `1`, and so on, matching
    /// the order [`crate::index::PlanBuilder::add_patch`] consumed them.
    ///
    /// # Errors
    ///
    /// Returns [`ZiPatchError::Io`] on the first path that fails to open.
    pub fn open_chain<I, P>(paths: I) -> Result<Self>
    where
        I: IntoIterator<Item = P>,
        P: AsRef<Path>,
    {
        let iter = paths.into_iter();
        // `IntoIterator` doesn't promise `ExactSizeIterator`, but `Vec` /
        // `&[P]` inputs (the common case) do report an accurate lower bound
        // through `size_hint().0` and so skip the geometric regrow.
        let mut files = Vec::with_capacity(iter.size_hint().0);
        for p in iter {
            files.push(File::open(p).map_err(ZiPatchError::Io)?);
        }
        Ok(Self { files })
    }

    /// Wrap an already-open [`File`] as a one-patch chain.
    #[must_use]
    pub fn from_file(file: File) -> Self {
        Self { files: vec![file] }
    }

    /// Number of patch files in the chain.
    #[must_use]
    pub fn patch_count(&self) -> usize {
        self.files.len()
    }
}

impl PatchSource for FilePatchSource {
    fn read(&mut self, patch: u32, offset: u64, dst: &mut [u8]) -> Result<()> {
        let count = self.files.len();
        let file = self
            .files
            .get_mut(patch as usize)
            .ok_or(ZiPatchError::PatchIndexOutOfRange { patch, count })?;
        file.seek(SeekFrom::Start(offset))?;
        match file.read_exact(dst) {
            Ok(()) => Ok(()),
            Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => {
                Err(ZiPatchError::PatchSourceTooShort {
                    offset,
                    requested: dst.len(),
                })
            }
            Err(e) => Err(ZiPatchError::Io(e)),
        }
    }
}

/// [`PatchSource`] backed by one or more in-memory byte buffers, one per
/// patch in the chain.
///
/// Test-only — gated behind the `test-utils` feature.
#[cfg(any(test, feature = "test-utils"))]
#[derive(Debug, Clone)]
pub struct MemoryPatchSource {
    bufs: Vec<std::sync::Arc<[u8]>>,
}

#[cfg(any(test, feature = "test-utils"))]
impl MemoryPatchSource {
    /// Wrap a single `Vec<u8>` as a one-patch chain.
    #[must_use]
    pub fn new(buf: Vec<u8>) -> Self {
        Self {
            bufs: vec![buf.into()],
        }
    }

    /// Wrap a single byte slice as a one-patch chain (copying it).
    #[must_use]
    pub fn from_slice(buf: &[u8]) -> Self {
        Self {
            bufs: vec![Vec::from(buf).into()],
        }
    }

    /// Wrap multiple `Vec<u8>`s as a multi-patch chain.
    #[must_use]
    pub fn new_chain(bufs: Vec<Vec<u8>>) -> Self {
        Self {
            bufs: bufs.into_iter().map(Into::into).collect(),
        }
    }

    /// Wrap multiple byte slices as a multi-patch chain (copying each).
    #[must_use]
    pub fn from_slices(bufs: &[&[u8]]) -> Self {
        Self {
            bufs: bufs.iter().map(|b| Vec::from(*b).into()).collect(),
        }
    }

    /// Number of patches in the chain.
    #[must_use]
    pub fn patch_count(&self) -> usize {
        self.bufs.len()
    }
}

#[cfg(any(test, feature = "test-utils"))]
impl PatchSource for MemoryPatchSource {
    fn read(&mut self, patch: u32, offset: u64, dst: &mut [u8]) -> Result<()> {
        let count = self.bufs.len();
        let buf = self
            .bufs
            .get(patch as usize)
            .ok_or(ZiPatchError::PatchIndexOutOfRange { patch, count })?;
        let start = usize::try_from(offset).map_err(|_| ZiPatchError::PatchSourceTooShort {
            offset,
            requested: dst.len(),
        })?;
        let end = start
            .checked_add(dst.len())
            .ok_or(ZiPatchError::PatchSourceTooShort {
                offset,
                requested: dst.len(),
            })?;
        if end > buf.len() {
            return Err(ZiPatchError::PatchSourceTooShort {
                offset,
                requested: dst.len(),
            });
        }
        dst.copy_from_slice(&buf[start..end]);
        Ok(())
    }
}

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

    #[test]
    fn memory_source_round_trips_arbitrary_ranges() {
        let bytes: Vec<u8> = (0..=255u8).collect();
        let mut src = MemoryPatchSource::new(bytes.clone());

        let mut head = [0u8; 16];
        src.read(0, 0, &mut head).unwrap();
        assert_eq!(&head, &bytes[..16]);

        let mut mid = [0u8; 32];
        src.read(0, 100, &mut mid).unwrap();
        assert_eq!(&mid, &bytes[100..132]);

        let mut tail = [0u8; 16];
        src.read(0, 240, &mut tail).unwrap();
        assert_eq!(&tail, &bytes[240..256]);

        // Zero-length read at any in-range offset is a no-op success.
        let mut empty = [0u8; 0];
        src.read(0, 0, &mut empty).unwrap();
        src.read(0, 256, &mut empty).unwrap();
    }

    #[test]
    fn memory_source_out_of_range_returns_too_short() {
        let mut src = MemoryPatchSource::new(vec![0u8; 16]);
        let mut buf = [0u8; 4];

        let err = src
            .read(0, 15, &mut buf)
            .expect_err("read past end must fail");
        match err {
            ZiPatchError::PatchSourceTooShort { offset, requested } => {
                assert_eq!(offset, 15);
                assert_eq!(requested, 4);
            }
            other => panic!("expected PatchSourceTooShort, got {other:?}"),
        }

        let err = src
            .read(0, 1_000_000, &mut buf)
            .expect_err("read far past end must fail");
        assert!(matches!(err, ZiPatchError::PatchSourceTooShort { .. }));
    }

    #[test]
    fn memory_source_chain_indexes_each_patch() {
        // Two patches: patch 0 = [0xA0, 0xA1, ...], patch 1 = [0xB0, 0xB1, ...].
        let p0: Vec<u8> = (0..16u8).map(|i| 0xA0 | i).collect();
        let p1: Vec<u8> = (0..16u8).map(|i| 0xB0 | i).collect();
        let mut src = MemoryPatchSource::new_chain(vec![p0.clone(), p1.clone()]);

        let mut buf = [0u8; 4];
        src.read(0, 0, &mut buf).unwrap();
        assert_eq!(&buf, &p0[..4]);
        src.read(1, 0, &mut buf).unwrap();
        assert_eq!(&buf, &p1[..4]);
        src.read(0, 12, &mut buf).unwrap();
        assert_eq!(&buf, &p0[12..16]);
    }

    #[test]
    fn memory_source_chain_rejects_out_of_range_patch() {
        let mut src = MemoryPatchSource::new_chain(vec![vec![0u8; 16]]);
        let mut buf = [0u8; 4];
        let err = src
            .read(1, 0, &mut buf)
            .expect_err("patch 1 must be out of range");
        match err {
            ZiPatchError::PatchIndexOutOfRange { patch, count } => {
                assert_eq!(patch, 1);
                assert_eq!(count, 1);
            }
            other => panic!("expected PatchIndexOutOfRange, got {other:?}"),
        }
    }

    #[test]
    fn file_source_round_trips_arbitrary_ranges() {
        let bytes: Vec<u8> = (0..=255u8).collect();
        let tmp = tempfile::tempdir().unwrap();
        let path = tmp.path().join("source.bin");
        std::fs::write(&path, &bytes).unwrap();

        let mut src = FilePatchSource::open(&path).unwrap();

        let mut head = [0u8; 16];
        src.read(0, 0, &mut head).unwrap();
        assert_eq!(&head, &bytes[..16]);

        let mut mid = [0u8; 32];
        src.read(0, 100, &mut mid).unwrap();
        assert_eq!(&mid, &bytes[100..132]);
    }

    #[test]
    fn file_source_short_returns_too_short() {
        let tmp = tempfile::tempdir().unwrap();
        let path = tmp.path().join("source.bin");
        std::fs::write(&path, [0u8; 16]).unwrap();

        let mut src = FilePatchSource::open(&path).unwrap();
        let mut buf = [0u8; 32];
        let err = src
            .read(0, 0, &mut buf)
            .expect_err("read past end must fail");
        assert!(matches!(err, ZiPatchError::PatchSourceTooShort { .. }));
    }

    #[test]
    fn file_source_chain_indexes_each_file() {
        let tmp = tempfile::tempdir().unwrap();
        let p0 = tmp.path().join("p0.bin");
        let p1 = tmp.path().join("p1.bin");
        std::fs::write(&p0, b"AAAAAAAA").unwrap();
        std::fs::write(&p1, b"BBBBBBBB").unwrap();

        let mut src = FilePatchSource::open_chain([&p0, &p1]).unwrap();
        assert_eq!(src.patch_count(), 2);

        let mut buf = [0u8; 4];
        src.read(0, 0, &mut buf).unwrap();
        assert_eq!(&buf, b"AAAA");
        src.read(1, 4, &mut buf).unwrap();
        assert_eq!(&buf, b"BBBB");
    }
}