//! Data and types dealing with PAM messages.

use crate::constants::InvalidEnum;
use crate::conv::{BinaryQAndA, ErrorMsg, InfoMsg, MaskedQAndA, Message, QAndA, RadioQAndA};
use crate::libpam::conversation::OwnedMessage;
use crate::libpam::memory;
use crate::libpam::memory::{CBinaryData, Immovable};
use crate::ErrorCode;
use crate::Result;
use num_derive::FromPrimitive;
use num_traits::FromPrimitive;
use std::ffi::{c_int, c_void, CStr};
use std::result::Result as StdResult;
use std::{iter, ptr, slice};

/// Abstraction of a collection of questions to be sent in a PAM conversation.
///
/// The PAM C API conversation function looks like this:
///
/// ```c
/// int pam_conv(
///     int count,
///     const struct pam_message **questions,
///     struct pam_response **answers,
///     void *appdata_ptr,
/// )
/// ```
///
/// On Linux-PAM and other compatible implementations, `questions`
/// is treated as a pointer-to-pointers, like `int argc, char **argv`.
/// (In this situation, the value of `Questions.indirect` is
/// the pointer passed to `pam_conv`.)
///
/// ```text
/// ╔═ Questions ═╗  points to  ┌─ Indirect ─┐       ╔═ Question ═╗
/// ║ indirect ┄┄┄╫┄┄┄┄┄┄┄┄┄┄┄> │ base[0] ┄┄┄┼┄┄┄┄┄> ║ style      ║
/// ║ count       ║             │ base[1] ┄┄┄┼┄┄┄╮   ║ data ┄┄┄┄┄┄╫┄┄> ...
/// ╚═════════════╝             │ ...        │   ┆   ╚════════════╝
///                                              ┆
///                                              ┆    ╔═ Question ═╗
///                                              ╰┄┄> ║ style      ║
///                                                   ║ data ┄┄┄┄┄┄╫┄┄> ...
///                                                   ╚════════════╝
/// ```
///
/// On OpenPAM and other compatible implementations (like Solaris),
/// `messages` is a pointer-to-pointer-to-array.  This appears to be
/// the correct implementation as required by the XSSO specification.
///
/// ```text
/// ╔═ Questions ═╗  points to  ┌─ Indirect ─┐       ╔═ Question[] ═╗
/// ║ indirect ┄┄┄╫┄┄┄┄┄┄┄┄┄┄┄> │ base ┄┄┄┄┄┄┼┄┄┄┄┄> ║ style        ║
/// ║ count       ║             └────────────┘       ║ data ┄┄┄┄┄┄┄┄╫┄┄> ...
/// ╚═════════════╝                                  ╟──────────────╢
///                                                  ║ style        ║
///                                                  ║ data ┄┄┄┄┄┄┄┄╫┄┄> ...
///                                                  ╟──────────────╢
///                                                  ║ ...          ║
/// ```
///
/// ***THIS LIBRARY CURRENTLY SUPPORTS ONLY LINUX-PAM.***
pub struct Questions {
    /// An indirection to the questions themselves, stored on the C heap.
    indirect: *mut Indirect,
    /// The number of questions.
    count: usize,
}

impl Questions {
    /// Stores the provided questions on the C heap.
    pub fn new(messages: &[Message]) -> Result<Self> {
        let count = messages.len();
        let mut ret = Self {
            indirect: Indirect::alloc(count),
            count,
        };
        // Even if we fail partway through this, all our memory will be freed.
        for (question, message) in iter::zip(ret.iter_mut(), messages) {
            question.fill(message)?
        }
        Ok(ret)
    }

    /// The pointer to the thing with the actual list.
    pub fn indirect(&self) -> *const Indirect {
        self.indirect
    }

    pub fn iter(&self) -> impl Iterator<Item = &Question> {
        // SAFETY: we're iterating over an amount we know.
        unsafe { (*self.indirect).iter(self.count) }
    }
    pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut Question> {
        // SAFETY: we're iterating over an amount we know.
        unsafe { (*self.indirect).iter_mut(self.count) }
    }
}

impl Drop for Questions {
    fn drop(&mut self) {
        // SAFETY: We are valid and have a valid pointer.
        // Once we're done, everything will be safe.
        unsafe {
            if let Some(indirect) = self.indirect.as_mut() {
                indirect.free(self.count)
            }
            memory::free(self.indirect);
            self.indirect = ptr::null_mut();
        }
    }
}

/// An indirect reference to messages.
///
/// This is kept separate to provide a place where we can separate
/// the pointer-to-pointer-to-list from pointer-to-list-of-pointers.
#[repr(transparent)]
pub struct Indirect {
    base: [*mut Question; 0],
    _marker: Immovable,
}

impl Indirect {
    /// Allocates memory for this indirector and all its members.
    fn alloc(count: usize) -> *mut Self {
        // SAFETY: We're only allocating, and when we're done,
        // everything will be in a known-good state.
        let me_ptr: *mut Indirect = memory::calloc::<Question>(count).cast();
        unsafe {
            let me = &mut *me_ptr;
            let ptr_list = slice::from_raw_parts_mut(me.base.as_mut_ptr(), count);
            for entry in ptr_list {
                *entry = memory::calloc(1);
            }
            me
        }
    }

    /// Returns an iterator yielding the given number of messages.
    ///
    /// # Safety
    ///
    /// You have to provide the right count.
    pub unsafe fn iter(&self, count: usize) -> impl Iterator<Item = &Question> {
        (0..count).map(|idx| &**self.base.as_ptr().add(idx))
    }

    /// Returns a mutable iterator yielding the given number of messages.
    ///
    /// # Safety
    ///
    /// You have to provide the right count.
    pub unsafe fn iter_mut(&mut self, count: usize) -> impl Iterator<Item = &mut Question> {
        (0..count).map(|idx| &mut **self.base.as_mut_ptr().add(idx))
    }

    /// Frees everything this points to.
    ///
    /// # Safety
    ///
    /// You have to pass the right size.
    unsafe fn free(&mut self, count: usize) {
        let msgs = slice::from_raw_parts_mut(self.base.as_mut_ptr(), count);
        for msg in msgs {
            if let Some(msg) = msg.as_mut() {
                msg.clear();
            }
            memory::free(*msg);
            *msg = ptr::null_mut();
        }
    }
}

/// The C enum values for messages shown to the user.
#[derive(Debug, PartialEq, FromPrimitive)]
pub enum Style {
    /// Requests information from the user; will be masked when typing.
    PromptEchoOff = 1,
    /// Requests information from the user; will not be masked.
    PromptEchoOn = 2,
    /// An error message.
    ErrorMsg = 3,
    /// An informational message.
    TextInfo = 4,
    /// Yes/No/Maybe conditionals. A Linux-PAM extension.
    RadioType = 5,
    /// For server–client non-human interaction.
    ///
    /// NOT part of the X/Open PAM specification.
    /// A Linux-PAM extension.
    BinaryPrompt = 7,
}

impl TryFrom<c_int> for Style {
    type Error = InvalidEnum<Self>;
    fn try_from(value: c_int) -> StdResult<Self, Self::Error> {
        Self::from_i32(value).ok_or(value.into())
    }
}

impl From<Style> for c_int {
    fn from(val: Style) -> Self {
        val as Self
    }
}

/// A question sent by PAM or a module to an application.
///
/// PAM refers to this as a "message", but we call it a question
/// to avoid confusion with [`Message`].
///
/// This question, and its internal data, is owned by its creator
/// (either the module or PAM itself).
#[repr(C)]
pub struct Question {
    /// The style of message to request.
    style: c_int,
    /// A description of the data requested.
    ///
    /// For most requests, this will be an owned [`CStr`], but for requests
    /// with [`Style::BinaryPrompt`], this will be [`CBinaryData`]
    /// (a Linux-PAM extension).
    data: *mut c_void,
    _marker: Immovable,
}

impl Question {
    /// Replaces the contents of this question with the question
    /// from the message.
    pub fn fill(&mut self, msg: &Message) -> Result<()> {
        let (style, data) = copy_to_heap(msg)?;
        self.clear();
        self.style = style as c_int;
        self.data = data;
        Ok(())
    }

    /// Gets this message's data pointer as a string.
    ///
    /// # Safety
    ///
    /// It's up to you to pass this only on types with a string value.
    unsafe fn string_data(&self) -> Result<&str> {
        if self.data.is_null() {
            Ok("")
        } else {
            CStr::from_ptr(self.data.cast())
                .to_str()
                .map_err(|_| ErrorCode::ConversationError)
        }
    }

    /// Gets this message's data pointer as borrowed binary data.
    unsafe fn binary_data(&self) -> (&[u8], u8) {
        self.data
            .cast::<CBinaryData>()
            .as_ref()
            .map(Into::into)
            .unwrap_or_default()
    }

    /// Zeroes out the data stored here.
    fn clear(&mut self) {
        // SAFETY: We either created this data or we got it from PAM.
        // After this function is done, it will be zeroed out.
        unsafe {
            if let Ok(style) = Style::try_from(self.style) {
                match style {
                    Style::BinaryPrompt => {
                        if let Some(d) = self.data.cast::<CBinaryData>().as_mut() {
                            d.zero_contents()
                        }
                    }
                    Style::TextInfo
                    | Style::RadioType
                    | Style::ErrorMsg
                    | Style::PromptEchoOff
                    | Style::PromptEchoOn => memory::zero_c_string(self.data.cast()),
                }
            };
            memory::free(self.data);
            self.data = ptr::null_mut();
        }
    }
}

impl<'a> TryFrom<&'a Question> for OwnedMessage<'a> {
    type Error = ErrorCode;
    fn try_from(question: &'a Question) -> Result<Self> {
        let style: Style = question
            .style
            .try_into()
            .map_err(|_| ErrorCode::ConversationError)?;
        // SAFETY: In all cases below, we're matching the
        let prompt = unsafe {
            match style {
                Style::PromptEchoOff => {
                    Self::MaskedPrompt(MaskedQAndA::new(question.string_data()?))
                }
                Style::PromptEchoOn => Self::Prompt(QAndA::new(question.string_data()?)),
                Style::ErrorMsg => Self::Error(ErrorMsg::new(question.string_data()?)),
                Style::TextInfo => Self::Info(InfoMsg::new(question.string_data()?)),
                Style::RadioType => Self::RadioPrompt(RadioQAndA::new(question.string_data()?)),
                Style::BinaryPrompt => Self::BinaryPrompt(BinaryQAndA::new(question.binary_data())),
            }
        };
        Ok(prompt)
    }
}

/// Copies the contents of this message to the C heap.
fn copy_to_heap(msg: &Message) -> Result<(Style, *mut c_void)> {
    let alloc = |style, text| Ok((style, memory::malloc_str(text)?.cast()));
    match *msg {
        Message::MaskedPrompt(p) => alloc(Style::PromptEchoOff, p.question()),
        Message::Prompt(p) => alloc(Style::PromptEchoOn, p.question()),
        Message::RadioPrompt(p) => alloc(Style::RadioType, p.question()),
        Message::Error(p) => alloc(Style::ErrorMsg, p.question()),
        Message::Info(p) => alloc(Style::TextInfo, p.question()),
        Message::BinaryPrompt(p) => Ok((
            Style::BinaryPrompt,
            CBinaryData::alloc(p.question())?.cast(),
        )),
    }
}

#[cfg(test)]
mod tests {

    use super::{MaskedQAndA, Questions, Result};
    use crate::conv::{BinaryQAndA, ErrorMsg, InfoMsg, QAndA, RadioQAndA};
    use crate::libpam::conversation::OwnedMessage;

    #[test]
    fn test_round_trip() {
        let interrogation = Questions::new(&[
            MaskedQAndA::new("hocus pocus").message(),
            BinaryQAndA::new((&[5, 4, 3, 2, 1], 66)).message(),
            QAndA::new("what").message(),
            QAndA::new("who").message(),
            InfoMsg::new("hey").message(),
            ErrorMsg::new("gasp").message(),
            RadioQAndA::new("you must choose").message(),
        ])
        .unwrap();
        let indirect = interrogation.indirect();

        let remade = unsafe { indirect.as_ref() }.unwrap();
        let messages: Vec<OwnedMessage> = unsafe { remade.iter(interrogation.count) }
            .map(TryInto::try_into)
            .collect::<Result<_>>()
            .unwrap();
        let [masked, bin, what, who, hey, gasp, choose] = messages.try_into().unwrap();
        macro_rules! assert_matches {
            ($id:ident => $variant:path, $q:expr) => {
                if let $variant($id) = $id {
                    assert_eq!($q, $id.question());
                } else {
                    panic!("mismatched enum variant {x:?}", x = $id);
                }
            };
        }
        assert_matches!(masked => OwnedMessage::MaskedPrompt, "hocus pocus");
        assert_matches!(bin => OwnedMessage::BinaryPrompt, (&[5, 4, 3, 2, 1][..], 66));
        assert_matches!(what => OwnedMessage::Prompt, "what");
        assert_matches!(who => OwnedMessage::Prompt, "who");
        assert_matches!(hey => OwnedMessage::Info, "hey");
        assert_matches!(gasp => OwnedMessage::Error, "gasp");
        assert_matches!(choose => OwnedMessage::RadioPrompt, "you must choose");
    }
}