extern crate libc;

use std::mem::MaybeUninit;
use std::os::unix::io::RawFd;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::{io, ptr};

use crate::advice::Advice;

#[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;

pub struct MmapInner {
    ptr: *mut libc::c_void,
    len: usize,
}

impl MmapInner {
    /// Creates a new `MmapInner`.
    ///
    /// This is a thin wrapper around the `mmap` sytem 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 aligned_len = len + alignment as usize;

        // `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 aligned_len = aligned_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.)

        unsafe {
            let ptr = libc::mmap(
                ptr::null_mut(),
                aligned_len as libc::size_t,
                prot,
                flags,
                file,
                aligned_offset as libc::off_t,
            );

            if ptr == libc::MAP_FAILED {
                Err(io::Error::last_os_error())
            } else {
                Ok(MmapInner {
                    ptr: ptr.offset(alignment as isize),
                    len,
                })
            }
        }
    }

    pub fn map(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_exec(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_EXEC,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_mut(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_SHARED | populate,
            file,
            offset,
        )
    }

    pub fn map_copy(len: usize, file: RawFd, offset: u64, populate: bool) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_PRIVATE | populate,
            file,
            offset,
        )
    }

    pub fn map_copy_read_only(
        len: usize,
        file: RawFd,
        offset: u64,
        populate: bool,
    ) -> io::Result<MmapInner> {
        let populate = if populate { MAP_POPULATE } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ,
            libc::MAP_PRIVATE | populate,
            file,
            offset,
        )
    }

    /// Open an anonymous memory map.
    pub fn map_anon(len: usize, stack: bool) -> io::Result<MmapInner> {
        let stack = if stack { MAP_STACK } else { 0 };
        MmapInner::new(
            len,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_PRIVATE | libc::MAP_ANON | stack,
            -1,
            0,
        )
    }

    pub fn flush(&self, offset: usize, len: usize) -> io::Result<()> {
        let alignment = (self.ptr as usize + offset) % page_size();
        let offset = offset as isize - alignment as isize;
        let len = len + alignment;
        let result =
            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<()> {
        let alignment = (self.ptr as usize + offset) % page_size();
        let offset = offset as isize - alignment as isize;
        let len = len + alignment;
        let result =
            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<()> {
        unsafe {
            let alignment = self.ptr as usize % page_size();
            let ptr = self.ptr.offset(-(alignment as isize));
            let len = self.len + alignment;
            let len = len.max(1);
            if 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 as *mut u8
    }

    #[inline]
    pub fn len(&self) -> usize {
        self.len
    }

    pub fn advise(&self, advice: Advice) -> io::Result<()> {
        unsafe {
            if libc::madvise(self.ptr, self.len, advice as i32) != 0 {
                Err(io::Error::last_os_error())
            } else {
                Ok(())
            }
        }
    }
}

impl Drop for MmapInner {
    fn drop(&mut self) {
        let alignment = self.ptr as usize % page_size();
        let len = self.len + alignment;
        let len = len.max(1);
        // 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 {
            let ptr = self.ptr.offset(-(alignment as isize));
            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> {
    #[cfg(not(any(target_os = "linux", target_os = "emscripten", target_os = "l4re")))]
    use libc::{fstat, stat};
    #[cfg(any(target_os = "linux", target_os = "emscripten", target_os = "l4re"))]
    use libc::{fstat64 as fstat, stat64 as stat};

    unsafe {
        let mut stat = MaybeUninit::<stat>::uninit();

        let result = fstat(file, stat.as_mut_ptr());
        if result == 0 {
            Ok(stat.assume_init().st_size as u64)
        } else {
            Err(io::Error::last_os_error())
        }
    }
}