//! Things for dealing with memory.

use crate::Result;
use crate::{BinaryData, ErrorCode};
use std::ffi::{c_char, CStr, CString};
use std::marker::{PhantomData, PhantomPinned};
use std::mem::offset_of;
use std::ptr::NonNull;
use std::{mem, ptr, slice};

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

/// 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(),
    }
}

/// 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))
}

/// Allocates a string with the given contents on the C heap.
///
/// This is like [`CString::new`], but:
///
/// - it allocates data on the C heap with [`libc::malloc`].
/// - it doesn't take ownership of the data passed in.
pub fn malloc_str(text: &str) -> Result<NonNull<c_char>> {
    let data = text.as_bytes();
    if data.contains(&0) {
        return Err(ErrorCode::ConversationError);
    }
    // +1 for the null terminator
    let data_alloc: *mut c_char = calloc(data.len() + 1);
    // SAFETY: we just allocated this and we have enough room.
    unsafe {
        libc::memcpy(data_alloc.cast(), data.as_ptr().cast(), data.len());
        Ok(NonNull::new_unchecked(data_alloc))
    }
}

/// Writes zeroes over the contents of a C string.
///
/// This won't overwrite a null pointer.
///
/// # Safety
///
/// It's up to you to provide a valid C string.
pub unsafe fn zero_c_string(cstr: *mut c_char) {
    if !cstr.is_null() {
        let len = libc::strlen(cstr.cast());
        for x in 0..len {
            ptr::write_volatile(cstr.byte_offset(x as isize), mem::zeroed())
        }
    }
}

/// 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<NonNull<CBinaryData>> {
        let buffer_size =
            u32::try_from(data.len() + 5).map_err(|_| ErrorCode::ConversationError)?;
        // SAFETY: We're only allocating here.
        let dest = unsafe {
            let mut dest_buffer: NonNull<Self> =
                NonNull::new_unchecked(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(),
            );
            dest_buffer
        };
        Ok(dest)
    }

    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::{
        copy_pam_string, free, malloc_str, option_cstr, prompt_ptr, zero_c_string, CString,
        ErrorCode,
    };
    #[test]
    fn test_strings() {
        let str = malloc_str("hello there").unwrap();
        let str = str.as_ptr();
        malloc_str("hell\0 there").unwrap_err();
        unsafe {
            let copied = copy_pam_string(str).unwrap();
            assert_eq!("hello there", copied.unwrap());
            zero_c_string(str);
            let idx_three = str.add(3).as_mut().unwrap();
            *idx_three = 0x80u8 as i8;
            let zeroed = copy_pam_string(str).unwrap().unwrap();
            assert!(zeroed.is_empty());
            free(str);
        }
    }

    #[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());
    }
}