//! Things for dealing with memory.

use crate::ErrorCode;
use crate::Result;
use std::ffi::{c_char, c_void, CStr, CString};
use std::marker::{PhantomData, PhantomPinned};
use std::result::Result as StdResult;
use std::{ptr, slice};

/// Makes whatever it's in not [`Send`], [`Sync`], or [`Unpin`].
pub type Immovable = 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.
pub unsafe fn copy_pam_string(result_ptr: *const libc::c_char) -> Result<String> {
    // We really shouldn't get a null pointer back here, but if we do, return nothing.
    if result_ptr.is_null() {
        return Ok(String::new());
    }
    let bytes = unsafe { CStr::from_ptr(result_ptr) };
    bytes
        .to_str()
        .map(String::from)
        .map_err(|_| ErrorCode::ConversationError)
}

/// Allocates a string with the given contents on the C heap.
///
/// This is like [`CString::new`](std::ffi::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: impl AsRef<str>) -> StdResult<*mut c_char, NulError> {
    let data = text.as_ref().as_bytes();
    if let Some(nul) = data.iter().position(|x| *x == 0) {
        return Err(NulError(nul));
    }
    unsafe {
        let data_alloc = libc::calloc(data.len() + 1, 1);
        libc::memcpy(data_alloc, data.as_ptr() as *const c_void, data.len());
        Ok(data_alloc.cast())
    }
}

/// 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_void) {
    if !cstr.is_null() {
        libc::memset(cstr, 0, libc::strlen(cstr as *const c_char));
    }
}

/// 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(source: &[u8], data_type: u8) -> StdResult<*mut CBinaryData, TooBigError> {
        let buffer_size = u32::try_from(source.len() + 5).map_err(|_| TooBigError {
            max: (u32::MAX - 5) as usize,
            actual: source.len(),
        })?;
        let data = unsafe {
            let dest_buffer = libc::malloc(buffer_size as usize) as *mut CBinaryData;
            let data = &mut *dest_buffer;
            data.total_length = buffer_size.to_be_bytes();
            data.data_type = data_type;
            let dest = data.data.as_mut_ptr();
            libc::memcpy(
                dest as *mut c_void,
                source.as_ptr() as *const c_void,
                source.len(),
            );
            dest_buffer
        };
        Ok(data)
    }

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

    pub fn contents(&self) -> &[u8] {
        unsafe { slice::from_raw_parts(self.data.as_ptr(), self.length()) }
    }
    pub fn data_type(&self) -> u8 {
        self.data_type
    }

    /// Clears this data and frees it.
    pub unsafe fn zero_contents(&mut self) {
        let contents = slice::from_raw_parts_mut(self.data.as_mut_ptr(), self.length());
        for v in contents {
            *v = 0
        }
        self.data_type = 0;
        self.total_length = [0; 4];
    }
}

#[derive(Debug, thiserror::Error)]
#[error("null byte within input at byte {0}")]
pub struct NulError(pub usize);

/// Returned when trying to fit too much data into a binary message.
#[derive(Debug, thiserror::Error)]
#[error("cannot create a message of {actual} bytes; maximum is {max}")]
pub struct TooBigError {
    pub actual: usize,
    pub max: usize,
}

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