//! Things for dealing with memory.

use crate::Result;
use crate::{BinaryData, ErrorCode};
use std::error::Error;
use std::ffi::{c_char, CStr, CString};
use std::fmt::{Display, Formatter, Result as FmtResult};
use std::marker::{PhantomData, PhantomPinned};
use std::mem::{offset_of, ManuallyDrop};
use std::ops::{Deref, DerefMut};
use std::ptr::NonNull;
use std::result::Result as StdResult;
use std::{mem, ptr, slice};

/// Raised from `calloc` when you have no memory!
#[derive(Debug)]
pub struct NoMem;

impl Display for NoMem {
    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
        write!(f, "out of memory!")
    }
}

impl Error for NoMem {}

impl From<NoMem> for ErrorCode {
    fn from(_: NoMem) -> Self {
        ErrorCode::BufferError
    }
}

/// Allocates `count` elements to hold `T`.
#[inline]
pub fn calloc<T>(count: usize) -> StdResult<NonNull<T>, NoMem> {
    // SAFETY: it's always safe to allocate! Leaking memory is fun!
    NonNull::new(unsafe { libc::calloc(count, size_of::<T>()) }.cast()).ok_or(NoMem)
}

/// Wrapper for [`libc::free`] to make debugging calls/frees easier.
///
/// # Safety
///
/// If you double-free, it's all your fault.
#[inline]
pub unsafe fn free<T>(p: *mut T) {
    libc::free(p.cast())
}

/// Makes whatever it's in not [`Send`], [`Sync`], or [`Unpin`].
#[repr(C)]
#[derive(Debug, Default)]
pub struct Immovable(pub PhantomData<(*mut u8, PhantomPinned)>);

/// Safely converts a `&str` option to a `CString` option.
pub fn option_cstr(prompt: Option<&str>) -> Result<Option<CString>> {
    prompt
        .map(CString::new)
        .transpose()
        .map_err(|_| ErrorCode::ConversationError)
}

/// Gets the pointer to the given CString, or a null pointer if absent.
pub fn prompt_ptr(prompt: Option<&CString>) -> *const c_char {
    match prompt {
        Some(c_str) => c_str.as_ptr(),
        None => ptr::null(),
    }
}

/// It's like a [`Box`], but C heap managed.
#[derive(Debug)]
#[repr(transparent)]
pub struct CHeapBox<T>(NonNull<T>);

// Lots of "as" and "into" associated functions.
#[allow(clippy::wrong_self_convention)]
impl<T> CHeapBox<T> {
    /// Creates a new CHeapBox holding the given data.
    pub fn new(value: T) -> Result<Self> {
        let memory = calloc(1)?;
        unsafe { ptr::write(memory.as_ptr(), value) }
        // SAFETY: We literally just allocated this.
        Ok(Self(memory))
    }

    /// Takes ownership of the given pointer.
    ///
    /// # Safety
    ///
    /// You have to provide a valid pointer to the start of an allocation
    /// that was made with `malloc`.
    pub unsafe fn from_ptr(ptr: NonNull<T>) -> Self {
        Self(ptr)
    }

    /// Converts this CBox into a raw pointer.
    pub fn into_ptr(this: Self) -> NonNull<T> {
        ManuallyDrop::new(this).0
    }

    /// Gets a pointer from this but doesn't convert this into a raw pointer.
    ///
    /// You are responsible for ensuring the CHeapBox lives long enough.
    pub fn as_ptr(this: &Self) -> NonNull<T> {
        this.0
    }

    /// Converts this into a Box of a different type.
    ///
    /// # Safety
    ///
    /// The different type has to be compatible in size/alignment and drop behavior.
    pub unsafe fn cast<R>(this: Self) -> CHeapBox<R> {
        mem::transmute(this)
    }
}

impl<T: Default> Default for CHeapBox<T> {
    fn default() -> Self {
        Self::new(Default::default()).expect("allocation should not fail")
    }
}

impl<T> Deref for CHeapBox<T> {
    type Target = T;
    fn deref(&self) -> &Self::Target {
        // SAFETY: We own this pointer and it is guaranteed valid.
        unsafe { Self::as_ptr(self).as_ref() }
    }
}

impl<T> DerefMut for CHeapBox<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        // SAFETY: We own this pointer and it is guaranteed valid.
        unsafe { Self::as_ptr(self).as_mut() }
    }
}

impl<T> Drop for CHeapBox<T> {
    fn drop(&mut self) {
        // SAFETY: We own a valid pointer, and will never use it after this.
        unsafe {
            let ptr = self.0.as_ptr();
            ptr::drop_in_place(ptr);
            free(ptr)
        }
    }
}

/// A null-terminated string allocated on the C heap.
///
/// Basically [`CString`], but managed by malloc.
#[derive(Debug)]
#[repr(transparent)]
pub struct CHeapString(CHeapBox<c_char>);

impl CHeapString {
    /// Creates a new C heap string with the given contents.
    pub fn new(text: &str) -> Result<Self> {
        let data = text.as_bytes();
        if data.contains(&0) {
            return Err(ErrorCode::ConversationError);
        }
        // +1 for the null terminator
        let data_alloc: NonNull<c_char> = calloc(data.len() + 1)?;
        // SAFETY: we just allocated this and we have enough room.
        unsafe {
            libc::memcpy(data_alloc.as_ptr().cast(), data.as_ptr().cast(), data.len());
            Ok(Self(CHeapBox::from_ptr(data_alloc)))
        }
    }

    /// Converts this C heap string into a raw pointer.
    ///
    /// You are responsible for freeing it later.
    pub fn into_ptr(self) -> NonNull<c_char> {
        let this = ManuallyDrop::new(self);
        CHeapBox::as_ptr(&this.0)
    }

    /// Converts this into a dumb box. It will no longer be zeroed upon drop.
    pub fn into_box(self) -> CHeapBox<c_char> {
        unsafe { mem::transmute(self) }
    }

    /// Takes ownership of a C heap string.
    ///
    /// # Safety
    ///
    /// You have to provide a pointer to the start of an allocation that is
    /// a valid 0-terminated C string.
    unsafe fn from_ptr(ptr: *mut c_char) -> Option<Self> {
        NonNull::new(ptr).map(|p| unsafe { Self(CHeapBox::from_ptr(p)) })
    }

    unsafe fn from_box<T>(bx: CHeapBox<T>) -> Self {
        Self(CHeapBox::cast(bx))
    }

    /// Zeroes the contents of a C string.
    ///
    /// # Safety
    ///
    /// You have to provide a valid pointer to a null-terminated C string.
    pub unsafe fn zero(ptr: NonNull<c_char>) {
        let cstr = ptr.as_ptr();
        let len = libc::strlen(cstr.cast());
        for x in 0..len {
            ptr::write_volatile(cstr.byte_offset(x as isize), mem::zeroed())
        }
    }
}

impl Drop for CHeapString {
    fn drop(&mut self) {
        // SAFETY: We own a valid C String
        unsafe { Self::zero(CHeapBox::as_ptr(&self.0)) }
    }
}

impl Deref for CHeapString {
    type Target = CStr;

    fn deref(&self) -> &Self::Target {
        // SAFETY: We know we own a valid C string pointer.
        let ptr = CHeapBox::as_ptr(&self.0).as_ptr();
        unsafe { CStr::from_ptr(ptr) }
    }
}

/// Creates an owned copy of a string that is returned from a
/// <code>pam_get_<var>whatever</var></code> function.
///
/// # Safety
///
/// It's on you to provide a valid string.
pub unsafe fn copy_pam_string(result_ptr: *const c_char) -> Result<Option<String>> {
    let borrowed = match NonNull::new(result_ptr.cast_mut()) {
        Some(data) => Some(
            CStr::from_ptr(data.as_ptr())
                .to_str()
                .map_err(|_| ErrorCode::ConversationError)?,
        ),
        None => return Ok(None),
    };
    Ok(borrowed.map(String::from))
}

/// Binary data used in requests and responses.
///
/// This is an unsized data type whose memory goes beyond its data.
/// This must be allocated on the C heap.
///
/// A Linux-PAM extension.
#[repr(C)]
pub struct CBinaryData {
    /// The total length of the structure; a u32 in network byte order (BE).
    total_length: [u8; 4],
    /// A tag of undefined meaning.
    data_type: u8,
    /// Pointer to an array of length [`length`](Self::length) − 5
    data: [u8; 0],
    _marker: Immovable,
}

impl CBinaryData {
    /// Copies the given data to a new BinaryData on the heap.
    pub fn alloc((data, data_type): (&[u8], u8)) -> Result<CHeapBox<CBinaryData>> {
        let buffer_size =
            u32::try_from(data.len() + 5).map_err(|_| ErrorCode::ConversationError)?;
        // SAFETY: We're only allocating here.
        unsafe {
            let mut dest_buffer: NonNull<Self> = calloc::<u8>(buffer_size as usize)?.cast();
            let dest = dest_buffer.as_mut();
            dest.total_length = buffer_size.to_be_bytes();
            dest.data_type = data_type;
            libc::memcpy(
                Self::data_ptr(dest_buffer).cast(),
                data.as_ptr().cast(),
                data.len(),
            );
            Ok(CHeapBox::from_ptr(dest_buffer))
        }
    }

    fn length(&self) -> usize {
        u32::from_be_bytes(self.total_length).saturating_sub(5) as usize
    }

    fn data_ptr(ptr: NonNull<Self>) -> *mut u8 {
        unsafe {
            ptr.as_ptr()
                .cast::<u8>()
                .byte_offset(offset_of!(Self, data) as isize)
        }
    }

    unsafe fn data_slice<'a>(ptr: NonNull<Self>) -> &'a mut [u8] {
        unsafe { slice::from_raw_parts_mut(Self::data_ptr(ptr), ptr.as_ref().length()) }
    }

    pub unsafe fn data<'a>(ptr: NonNull<Self>) -> (&'a [u8], u8) {
        unsafe { (Self::data_slice(ptr), ptr.as_ref().data_type) }
    }

    pub unsafe fn zero_contents(ptr: NonNull<Self>) {
        for byte in Self::data_slice(ptr) {
            ptr::write_volatile(byte as *mut u8, mem::zeroed());
        }
        ptr::write_volatile(ptr.as_ptr(), mem::zeroed());
    }

    #[allow(clippy::wrong_self_convention)]
    pub unsafe fn as_binary_data(ptr: NonNull<Self>) -> BinaryData {
        let (data, data_type) = unsafe { (CBinaryData::data_slice(ptr), ptr.as_ref().data_type) };
        (Vec::from(data), data_type).into()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::hint;
    #[test]
    fn test_box() {
        #[allow(non_upper_case_globals)]
        static mut drop_count: u32 = 0;

        struct Dropper(i32);

        impl Drop for Dropper {
            fn drop(&mut self) {
                unsafe { drop_count += 1 }
            }
        }

        let mut dropbox = CHeapBox::new(Dropper(9)).unwrap();
        hint::black_box(dropbox.0);
        dropbox = CHeapBox::new(Dropper(10)).unwrap();
        assert_eq!(1, unsafe { drop_count });
        hint::black_box(dropbox.0);
        drop(dropbox);
        assert_eq!(2, unsafe { drop_count });
    }
    #[test]
    fn test_strings() {
        let str = CHeapString::new("hello there").unwrap();
        let str_ptr = str.into_ptr().as_ptr();
        CHeapString::new("hell\0 there").unwrap_err();
        unsafe {
            let copied = copy_pam_string(str_ptr).unwrap();
            assert_eq!("hello there", copied.unwrap());
            CHeapString::zero(NonNull::new(str_ptr).unwrap());
            let idx_three = str_ptr.add(3).as_mut().unwrap();
            *idx_three = 0x80u8 as i8;
            let zeroed = copy_pam_string(str_ptr).unwrap().unwrap();
            assert!(zeroed.is_empty());
            let _ = CHeapString::from_ptr(str_ptr);
        }
    }

    #[test]
    fn test_option_str() {
        let good = option_cstr(Some("whatever")).unwrap();
        assert_eq!("whatever", good.unwrap().to_str().unwrap());
        let no_str = option_cstr(None).unwrap();
        assert!(no_str.is_none());
        let bad_str = option_cstr(Some("what\0ever")).unwrap_err();
        assert_eq!(ErrorCode::ConversationError, bad_str);
    }

    #[test]
    fn test_prompt() {
        let prompt_cstr = CString::new("good").ok();
        let prompt = prompt_ptr(prompt_cstr.as_ref());
        assert!(!prompt.is_null());
        let no_prompt = prompt_ptr(None);
        assert!(no_prompt.is_null());
    }
}