//! A wrapper type providing random access to binary data.
//!
//! See the `ByteView` struct for more documentation.

use std::borrow::Cow;
use std::fs::File;
use std::io;
use std::ops::Deref;
use std::path::Path;

use memmap::Mmap;
use owning_ref::OwningHandle;

enum ByteViewInner<'bytes> {
    Buf(Cow<'bytes, [u8]>),
    Mmap(Mmap),
}

/// A smart pointer for byte data.
///
/// This type can be used to uniformly access bytes that were created
/// either from mmapping in a path, a vector or a borrowed slice.  A
/// byteview derefs into a `&[u8]` and is used in symbolic in most
/// situations where binary files are worked with.
///
/// A `ByteView` can be constructed from borrowed slices, vectors or
/// mmaped from the file system directly.
///
/// Example use:
///
/// ```
/// # use symbolic_common::byteview::ByteView;
/// let bv = ByteView::from_slice(b"1234");
/// ```
pub struct ByteView<'bytes> {
    inner: ByteViewInner<'bytes>,
}

impl<'bytes> ByteView<'bytes> {
    /// Constructs a `ByteView` from a `Cow`.
    fn from_cow(cow: Cow<'bytes, [u8]>) -> ByteView<'bytes> {
        ByteView {
            inner: ByteViewInner::Buf(cow),
        }
    }

    /// Constructs a `ByteView` from a byte slice.
    pub fn from_slice(buffer: &'bytes [u8]) -> ByteView<'bytes> {
        ByteView::from_cow(Cow::Borrowed(buffer))
    }

    /// Constructs a `ByteView` from a vector of bytes.
    pub fn from_vec(buffer: Vec<u8>) -> ByteView<'static> {
        ByteView::from_cow(Cow::Owned(buffer))
    }

    /// Constructs a `ByteView` from any `std::io::Reader`.
    ///
    /// This currently consumes the entire reader and stores its data in an
    /// internal buffer. Prefer `ByteView::from_path` when reading from the file
    /// system or `ByteView::from_slice`/`ByteView::from_vec` for in-memory
    /// operations. This behavior might change in the future.
    pub fn from_reader<R: io::Read>(mut reader: R) -> Result<ByteView<'static>, io::Error> {
        let mut buffer = vec![];
        reader.read_to_end(&mut buffer)?;
        Ok(ByteView::from_vec(buffer))
    }

    /// Constructs a `ByteView` from a file path.
    pub fn from_path<P: AsRef<Path>>(path: P) -> Result<ByteView<'static>, io::Error> {
        let file = File::open(path)?;
        let inner = match unsafe { Mmap::map(&file) } {
            Ok(mmap) => ByteViewInner::Mmap(mmap),
            Err(err) => {
                // this is raised on empty mmaps which we want to ignore.  The
                // 1006 windows error looks like "The volume for a file has been externally
                // altered so that the opened file is no longer valid."
                if err.kind() == io::ErrorKind::InvalidInput
                    || (cfg!(windows) && err.raw_os_error() == Some(1006))
                {
                    ByteViewInner::Buf(Cow::Borrowed(b""))
                } else {
                    return Err(err);
                }
            }
        };
        Ok(ByteView { inner })
    }

    #[inline(always)]
    fn buffer(&self) -> &[u8] {
        match self.inner {
            ByteViewInner::Buf(ref buf) => buf,
            ByteViewInner::Mmap(ref mmap) => mmap.deref(),
        }
    }
}

impl<'bytes> Deref for ByteView<'bytes> {
    type Target = [u8];

    fn deref(&self) -> &[u8] {
        self.buffer()
    }
}

impl<'bytes> AsRef<[u8]> for ByteView<'bytes> {
    fn as_ref(&self) -> &[u8] {
        self.buffer()
    }
}

/// A smart pointer for byte data that owns a derived object.
///
/// In some situations symbolic needs to deal with types that are
/// based on potentially owned or borrowed bytes and wants to provide
/// another view at them.  This for instance is used when symbolic
/// works with partially parsed files (like headers) of byte data.
///
/// Upon `deref` the inner type is returned.  Additionally the bytes
/// are exposed through the static `get_bytes` method.
pub struct ByteViewHandle<'bytes, T> {
    inner: OwningHandle<Box<ByteView<'bytes>>, Box<(&'bytes [u8], T)>>,
}

impl<'bytes, T> ByteViewHandle<'bytes, T> {
    /// Creates a new `ByteViewHandle` from a `ByteView`.
    ///
    /// The closure is invoked with the borrowed bytes from the original
    /// byte view and the return value is retained in the handle.
    pub fn from_byteview<F, E>(view: ByteView<'bytes>, f: F) -> Result<ByteViewHandle<'bytes, T>, E>
    where
        F: FnOnce(&'bytes [u8]) -> Result<T, E>,
    {
        // note on the safety here.  This unsafe pointer juggling causes some
        // issues.  The underlying OwningHandle is fundamentally unsafe because
        // it lets you do terrible things with the lifetimes.  A common problem
        // here is that ByteViewHandle can return views to the underlying data
        // that outlive it when not written well.
        //
        // In particular if you load a ByteView::from_path the lifetime of that
        // byteview will be 'static.  However the data within that byteview cannot
        // outlife the byteview.  As such even though the ByteViewHandle is
        // &'static it must not give out borrows that are that long lasting.
        //
        // As such `get_bytes` for instance will only return a borrow scoped
        // to the lifetime of the `ByteViewHandle`.
        Ok(ByteViewHandle {
            inner: OwningHandle::try_new(Box::new(view), |bv| -> Result<_, _> {
                let bytes: &[u8] = unsafe { &*bv };
                Ok(Box::new((bytes, f(bytes)?)))
            })?,
        })
    }

    /// Constructs a `ByteViewHandle` from a byte slice.
    pub fn from_slice<F, E>(buffer: &'bytes [u8], f: F) -> Result<ByteViewHandle<'bytes, T>, E>
    where
        F: FnOnce(&'bytes [u8]) -> Result<T, E>,
    {
        ByteViewHandle::from_byteview(ByteView::from_slice(buffer), f)
    }

    /// Constructs a `ByteViewHandle` from a vector of bytes.
    pub fn from_vec<F, E>(vec: Vec<u8>, f: F) -> Result<ByteViewHandle<'static, T>, E>
    where
        F: FnOnce(&'static [u8]) -> Result<T, E>,
    {
        ByteViewHandle::from_byteview(ByteView::from_vec(vec), f)
    }

    /// Constructs a `ByteViewHandle` from a file path.
    pub fn from_reader<F, R, E>(reader: R, f: F) -> Result<ByteViewHandle<'static, T>, E>
    where
        F: FnOnce(&'static [u8]) -> Result<T, E>,
        E: From<io::Error>,
        R: io::Read,
    {
        ByteViewHandle::from_byteview(ByteView::from_reader(reader)?, f)
    }

    /// Constructs a `ByteViewHandle` from a file path.
    pub fn from_path<F, P, E>(path: P, f: F) -> Result<ByteViewHandle<'static, T>, E>
    where
        F: FnOnce(&'static [u8]) -> Result<T, E>,
        E: From<io::Error>,
        P: AsRef<Path>,
    {
        ByteViewHandle::from_byteview(ByteView::from_path(path)?, f)
    }

    /// Returns the underlying storage (byte slice).
    pub fn get_bytes<'b>(this: &'b ByteViewHandle<'bytes, T>) -> &'b [u8] {
        this.inner.0
    }
}

impl<'bytes, T> Deref for ByteViewHandle<'bytes, T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.inner.1
    }
}

impl<'bytes, T> AsRef<T> for ByteViewHandle<'bytes, T> {
    fn as_ref(&self) -> &T {
        &self.inner.1
    }
}