use std::fs::File;

use std::io;

use std::mem::ManuallyDrop;

use std::os::unix::io::{FromRawFd, RawFd};

use std::sync::atomic::{AtomicUsize, Ordering};

#[cfg(any(

    all(target_os = "linux", not(target_arch = "mips")),

    target_os = "freebsd",

    target_os = "android"

))]

const MAP_STACK: libc::c_int = libc::MAP_STACK;

#[cfg(not(any(

    all(target_os = "linux", not(target_arch = "mips")),

    target_os = "freebsd",

    target_os = "android"

)))]

const MAP_STACK: libc::c_int = 0;

#[cfg(any(target_os = "linux", target_os = "android"))]

const MAP_POPULATE: libc::c_int = libc::MAP_POPULATE;

#[cfg(not(any(target_os = "linux", target_os = "android")))]

const MAP_POPULATE: libc::c_int = 0;

#[cfg(any(target_os = "linux", target_os = "android"))]

const MAP_HUGETLB: libc::c_int = libc::MAP_HUGETLB;

#[cfg(target_os = "linux")]

const MAP_HUGE_MASK: libc::c_int = libc::MAP_HUGE_MASK;

#[cfg(any(target_os = "linux", target_os = "android"))]

const MAP_HUGE_SHIFT: libc::c_int = libc::MAP_HUGE_SHIFT;

#[cfg(not(any(target_os = "linux", target_os = "android")))]

const MAP_HUGETLB: libc::c_int = 0;

#[cfg(not(target_os = "linux"))]

const MAP_HUGE_MASK: libc::c_int = 0;

#[cfg(not(any(target_os = "linux", target_os = "android")))]

const MAP_HUGE_SHIFT: libc::c_int = 0;

#[cfg(any(

    target_os = "linux",

    target_os = "android",

    target_vendor = "apple",

    target_os = "netbsd",

    target_os = "solaris",

    target_os = "illumos",

))]

const MAP_NORESERVE: libc::c_int = libc::MAP_NORESERVE;

#[cfg(not(any(

    target_os = "linux",

    target_os = "android",

    target_vendor = "apple",

    target_os = "netbsd",

    target_os = "solaris",

    target_os = "illumos",

)))]

const MAP_NORESERVE: libc::c_int = 0;

#[cfg(any(

    target_os = "android",

    all(target_os = "linux", not(target_env = "musl"))

))]

use libc::{mmap64 as mmap, off64_t as off_t};

#[cfg(not(any(

    target_os = "android",

    all(target_os = "linux", not(target_env = "musl"))

)))]

use libc::{mmap, off_t};

pub struct MmapInner {

    ptr: *mut libc::c_void,

    len: usize,

}

impl MmapInner {

    /// Creates a new `MmapInner`.

    ///

    /// This is a thin wrapper around the `mmap` system call.

    fn new(

        len: usize,

        prot: libc::c_int,

        flags: libc::c_int,

        file: RawFd,

        offset: u64,

    ) -> io::Result<MmapInner> {

        let alignment = offset % page_size() as u64;

        let aligned_offset = offset - alignment;

        let (map_len, map_offset) = Self::adjust_mmap_params(len, alignment as usize)?;

        // SAFETY: creating a new memory map with a nullptr as address hint is always sound:

        // it does not modify any existing mapping or memory contents.

        let ptr = unsafe {

            mmap(

                std::ptr::null_mut(),

                map_len as libc::size_t,

                prot,

                flags,

                file,

                aligned_offset as off_t,

            )

        };

        if ptr == libc::MAP_FAILED {

            Err(io::Error::last_os_error())

        } else {

            // SAFETY: The ptr and len have been checked,

            // and the offset has been calculated as required.

            Ok(unsafe { Self::from_raw_parts(ptr, len, map_offset) })

        }

    }

    fn adjust_mmap_params(len: usize, alignment: usize) -> io::Result<(usize, usize)> {

        // Rust's slice cannot be larger than isize::MAX.

        // See https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html

        //

        // This is not a problem on 64-bit targets, but on 32-bit one

        // having a file or an anonymous mapping larger than 2GB is quite normal

        // and we have to prevent it.

        //

        // The code below is essentially the same as in Rust's std:

        // https://github.com/rust-lang/rust/blob/db78ab70a88a0a5e89031d7ee4eccec835dcdbde/library/alloc/src/raw_vec.rs#L495

        if std::mem::size_of::<usize>() < 8 && len > isize::MAX as usize {

            return Err(io::Error::new(

                io::ErrorKind::InvalidData,

                "memory map length overflows isize",

            ));

        }

        let map_len = len + alignment;

        let map_offset = alignment;

        // `libc::mmap` does not support zero-size mappings. POSIX defines:

        //

        // https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html

        // > If `len` is zero, `mmap()` shall fail and no mapping shall be established.

        //

        // So if we would create such a mapping, crate a one-byte mapping instead:

        let map_len = map_len.max(1);

        // Note that in that case `MmapInner::len` is still set to zero,

        // and `Mmap` will still dereferences to an empty slice.

        //

        // If this mapping is backed by an empty file, we create a mapping larger than the file.

        // This is unusual but well-defined. On the same man page, POSIX further defines:

        //

        // > The `mmap()` function can be used to map a region of memory that is larger

        // > than the current size of the object.

        //

        // (The object here is the file.)

        //

        // > Memory access within the mapping but beyond the current end of the underlying

        // > objects may result in SIGBUS signals being sent to the process. The reason for this

        // > is that the size of the object can be manipulated by other processes and can change

        // > at any moment. The implementation should tell the application that a memory reference

        // > is outside the object where this can be detected; otherwise, written data may be lost

        // > and read data may not reflect actual data in the object.

        //

        // Because `MmapInner::len` is not incremented, this increment of `aligned_len`

        // will not allow accesses past the end of the file and will not cause SIGBUS.

        //

        // (SIGBUS is still possible by mapping a non-empty file and then truncating it

        // to a shorter size, but that is unrelated to this handling of empty files.)

        Ok((map_len, map_offset))

    }

    /// Get the current memory mapping as a `(ptr, map_len, offset)` tuple.

    ///

    /// Note that `map_len` is the length of the memory mapping itself and

    /// _not_ the one that would be passed to `from_raw_parts`.

    fn as_mmap_params(&self) -> (*mut libc::c_void, usize, usize) {

        let offset = self.ptr as usize % page_size();

        let len = self.len + offset;

        // There are two possible memory layouts we could have, depending on

        // the length and offset passed when constructing this instance:

        //

        // 1. The "normal" memory layout looks like this:

        //

        //         |<------------------>|<---------------------->|

        //     mmap ptr    offset      ptr     public slice

        //

        //    That is, we have

        //    - The start of the page-aligned memory mapping returned by mmap,

        //      followed by,

        //    - Some number of bytes that are memory mapped but ignored since

        //      they are before the byte offset requested by the user, followed

        //      by,

        //    - The actual memory mapped slice requested by the user.

        //

        //    This maps cleanly to a (ptr, len, offset) tuple.

        //

        // 2. Then, we have the case where the user requested a zero-length

        //    memory mapping. mmap(2) does not support zero-length mappings so

        //    this crate works around that by actually making a mapping of

        //    length one. This means that we have

        //    - A length zero slice, followed by,

        //    - A single memory mapped byte

        //

        //    Note that this only happens if the offset within the page is also

        //    zero. Otherwise, we have a memory map of offset bytes and not a

        //    zero-length memory map.

        //

        //    This doesn't fit cleanly into a (ptr, len, offset) tuple. Instead,

        //    we fudge it slightly: a zero-length memory map turns into a

        //    mapping of length one and can't be told apart outside of this

        //    method without knowing the original length.

        if len == 0 {

            (self.ptr, 1, 0)

        } else {

            let offset = self.ptr as usize % page_size();

            // SAFETY: MmapInner guarantees that rounding `self.ptr` down to a page boundary gives the real address of the memory map.

            // This means that it points into the same allocation as `self.ptr`.

            let ptr = unsafe { self.ptr.sub(offset) };

            (ptr, len, offset)

        }

    }

    /// Construct this `MmapInner` from its raw components

    ///

    /// # Safety

    ///

    /// - `ptr` must point to the start of memory mapping that can be freed

    ///   using `munmap(2)` (i.e. returned by `mmap(2)` or `mremap(2)`)

    /// - The memory mapping at `ptr` must have a length of `len + offset`.

    /// - If `len + offset == 0` then the memory mapping must be of length 1.

    /// - `offset` must be less than the current page size.

    unsafe fn from_raw_parts(ptr: *mut libc::c_void, len: usize, offset: usize) -> Self {

        debug_assert_eq!(ptr as usize % page_size(), 0, "ptr not page-aligned");

        debug_assert!(offset < page_size(), "offset larger than page size");

        Self {

            ptr: unsafe { ptr.add(offset) },

            len,

        }

    }

    pub fn map(

        len: usize,

        file: RawFd,

        offset: u64,

        populate: bool,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let populate = if populate { MAP_POPULATE } else { 0 };

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ,

            libc::MAP_SHARED | populate | no_reserve,

            file,

            offset,

        )

    }

    pub fn map_exec(

        len: usize,

        file: RawFd,

        offset: u64,

        populate: bool,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let populate = if populate { MAP_POPULATE } else { 0 };

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ | libc::PROT_EXEC,

            libc::MAP_SHARED | populate | no_reserve,

            file,

            offset,

        )

    }

    pub fn map_mut(

        len: usize,

        file: RawFd,

        offset: u64,

        populate: bool,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let populate = if populate { MAP_POPULATE } else { 0 };

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ | libc::PROT_WRITE,

            libc::MAP_SHARED | populate | no_reserve,

            file,

            offset,

        )

    }

    pub fn map_copy(

        len: usize,

        file: RawFd,

        offset: u64,

        populate: bool,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let populate = if populate { MAP_POPULATE } else { 0 };

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ | libc::PROT_WRITE,

            libc::MAP_PRIVATE | populate | no_reserve,

            file,

            offset,

        )

    }

    pub fn map_copy_read_only(

        len: usize,

        file: RawFd,

        offset: u64,

        populate: bool,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let populate = if populate { MAP_POPULATE } else { 0 };

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ,

            libc::MAP_PRIVATE | populate | no_reserve,

            file,

            offset,

        )

    }

    /// Open an anonymous memory map.

    pub fn map_anon(

        len: usize,

        stack: bool,

        populate: bool,

        huge: Option<u8>,

        no_reserve: bool,

    ) -> io::Result<MmapInner> {

        let stack = if stack { MAP_STACK } else { 0 };

        let populate = if populate { MAP_POPULATE } else { 0 };

        let hugetlb = if huge.is_some() { MAP_HUGETLB } else { 0 };

        let hugetlb_size = huge.map_or(0, |mask| {

            (u64::from(mask) & (MAP_HUGE_MASK as u64)) << MAP_HUGE_SHIFT

        }) as i32;

        let no_reserve = if no_reserve { MAP_NORESERVE } else { 0 };

        MmapInner::new(

            len,

            libc::PROT_READ | libc::PROT_WRITE,

            libc::MAP_PRIVATE

                | libc::MAP_ANON

                | stack

                | populate

                | hugetlb

                | hugetlb_size

                | no_reserve,

            -1,

            0,

        )

    }

    pub fn flush(&self, offset: usize, len: usize) -> io::Result<()> {

        if offset > self.len || len > self.len - offset {

            return Err(io::ErrorKind::InvalidInput.into());

        }

        let alignment = (self.ptr as usize + offset) % page_size();

        let offset = offset as isize - alignment as isize;

        let len = len + alignment;

        let result =

            // SAFETY: We've checked that offset and len fall within the mapped region.

            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_SYNC) };

        if result == 0 {

            Ok(())

        } else {

            Err(io::Error::last_os_error())

        }

    }

    pub fn flush_async(&self, offset: usize, len: usize) -> io::Result<()> {

        if offset > self.len || len > self.len - offset {

            return Err(io::ErrorKind::InvalidInput.into());

        }

        let alignment = (self.ptr as usize + offset) % page_size();

        let offset = offset as isize - alignment as isize;

        let len = len + alignment;

        let result =

            // SAFETY: We've checked that offset and len fall within the mapped region.

            unsafe { libc::msync(self.ptr.offset(offset), len as libc::size_t, libc::MS_ASYNC) };

        if result == 0 {

            Ok(())

        } else {

            Err(io::Error::last_os_error())

        }

    }

    fn mprotect(&mut self, prot: libc::c_int) -> io::Result<()> {

        let alignment = self.ptr as usize % page_size();

        // SAFETY: rounding self.ptr down to the previous page boundary gives the pointer of the actual memory map.

        let ptr = unsafe { self.ptr.sub(alignment) };

        let len = self.len + alignment;

        let len = len.max(1);

        // SAFETY: the contract of MmapInner guarantees ptr and len are valid.

        if unsafe { libc::mprotect(ptr, len, prot) } == 0 {

            Ok(())

        } else {

            Err(io::Error::last_os_error())

        }

    }

    pub fn make_read_only(&mut self) -> io::Result<()> {

        self.mprotect(libc::PROT_READ)

    }

    pub fn make_exec(&mut self) -> io::Result<()> {

        self.mprotect(libc::PROT_READ | libc::PROT_EXEC)

    }

    pub fn make_mut(&mut self) -> io::Result<()> {

        self.mprotect(libc::PROT_READ | libc::PROT_WRITE)

    }

    #[inline]

    pub fn ptr(&self) -> *const u8 {

        self.ptr as *const u8

    }

    #[inline]

    pub fn mut_ptr(&mut self) -> *mut u8 {

        self.ptr.cast()

    }

    #[inline]

    pub fn len(&self) -> usize {

        self.len

    }

    /// Perform an `madvise()`.

    ///

    /// # Safety

    ///

    /// Some `advise` values can be unsound depending on the situation.

    /// It is up to the caller to only perform sound madvise() calls on the memory range.

    pub unsafe fn advise(&self, advice: libc::c_int, offset: usize, len: usize) -> io::Result<()> {

        if offset > self.len || len > self.len {

            return Err(std::io::ErrorKind::InvalidInput.into());

        }

        let alignment = (self.ptr as usize + offset) % page_size();

        let offset = offset as isize - alignment as isize;

        let len = len + alignment;

        // SAFETY: We've checked that offset is within the mapped region.

        let ptr = unsafe { self.ptr.offset(offset) };

        // The AIX signature of 'madvise()' differs from the POSIX

        // specification, which expects 'void *' as the type of the

        // 'addr' argument, whereas AIX uses 'caddr_t' (i.e., 'char *').

        #[cfg(target_os = "aix")]

        let ptr = self.ptr.offset(offset).cast();

        // SAFETY: ptr and len are valid. The burden of giving a safe `advice` value is on the caller.

        if unsafe { libc::madvise(ptr, len, advice) } != 0 {

            Err(io::Error::last_os_error())

        } else {

            Ok(())

        }

    }

    #[cfg(target_os = "linux")]

    pub fn remap(&mut self, new_len: usize, options: crate::RemapOptions) -> io::Result<()> {

        let (old_ptr, old_len, offset) = self.as_mmap_params();

        let (map_len, offset) = Self::adjust_mmap_params(new_len, offset)?;

        // SAFETY: we hold a mutable reference to self, so we can adjust the location and size of the mapping.

        let new_ptr = unsafe { libc::mremap(old_ptr, old_len, map_len, options.into_flags()) };

        if new_ptr == libc::MAP_FAILED {

            Err(io::Error::last_os_error())

        } else {

            // SAFETY: The pointer and length passed to `from_raw_parts` have just been obtained from a real map, so they must be valid.

            let new_map = unsafe { Self::from_raw_parts(new_ptr, new_len, offset) };

            // We explicitly don't drop self since the pointer within is no longer valid.

            // Instead, swap the new map into `self` and forget the old one.

            let old_map = std::mem::replace(self, new_map);

            std::mem::forget(old_map);

            Ok(())

        }

    }

    pub fn lock(&self) -> io::Result<()> {

        unsafe {

            if libc::mlock(self.ptr, self.len) != 0 {

                Err(io::Error::last_os_error())

            } else {

                Ok(())

            }

        }

    }

    pub fn unlock(&self) -> io::Result<()> {

        unsafe {

            if libc::munlock(self.ptr, self.len) != 0 {

                Err(io::Error::last_os_error())

            } else {

                Ok(())

            }

        }

    }

}

impl Drop for MmapInner {

    fn drop(&mut self) {

        let (ptr, len, _) = self.as_mmap_params();

        // Any errors during unmapping/closing are ignored as the only way

        // to report them would be through panicking which is highly discouraged

        // in Drop impls, c.f. https://github.com/rust-lang/lang-team/issues/97

        unsafe { libc::munmap(ptr, len as libc::size_t) };

    }

}

unsafe impl Sync for MmapInner {}

unsafe impl Send for MmapInner {}

fn page_size() -> usize {

    static PAGE_SIZE: AtomicUsize = AtomicUsize::new(0);

    match PAGE_SIZE.load(Ordering::Relaxed) {

        0 => {

            let page_size = unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize };

            PAGE_SIZE.store(page_size, Ordering::Relaxed);

            page_size

        }

        page_size => page_size,

    }

}

pub fn file_len(file: RawFd) -> io::Result<u64> {

    // SAFETY: We must not close the passed-in fd by dropping the File we create,

    // we ensure this by immediately wrapping it in a ManuallyDrop.

    unsafe {

        let file = ManuallyDrop::new(File::from_raw_fd(file));

        Ok(file.metadata()?.len())

    }

}