//! Texlang commands API
//!
//! # Texcraft commands API
//!
//! One of the most important parts of any TeX engine is the primitives that it provides.
//!  This documentation describes the *Texcraft commands API*,
//! which is the mechanism by which TeX engines add new primitives.
//!
//! A note on terminology: *commands* can be categorized into primitives,
//! which are implemented in the TeX engine, and user defined macros,
//!  which are created in specific TeX documents using primitives like `\def`.
//! We often use the word command and primitive interchangeably here because in the context
//! of implementing TeX engines they’re basically synonymous.
//! A TeX engine could theoretically provide a native user defined macro...but it’s unlikely.
//!
//! ## Expansion vs execution
//!
//! Expansion and execution commands seem similar because they both optionally
//! read input tokens and then make changes to the VM.
//! However the differences are pretty significant in practice:
//!
//! |                                          | Expansion | Execution
//! |------------------------------------------|-----------|-----------
//! Can read tokens from the input stream?     | Yes       | Yes
//! Can add tokens to the input stream>        | Yes       | It’s possible, but the API discourages it.[^futurelet]
//! Can make changes to the state?             | No        | Yes
//! Is evaluated when tokens are only being expanded, like in `\edef` | Yes | No
//!
//!
//! [^futurelet]: `\futurelet` is an example of an execution command that does this.
//!

use crate::texmacro;
use crate::token;
use crate::types;
use crate::variable;
use crate::vm;
use once_cell::sync::OnceCell;
use std::num;
use std::rc;
use std::sync;

pub(crate) mod map;

pub use crate::error::Error;
pub use map::Map;

pub type Result<T> = std::result::Result<T, Box<Error>>;

/// The Rust type of expansion primitive functions.
pub type ExpansionFn<S> = fn(token: token::Token, input: &mut vm::ExpansionInput<S>) -> Result<()>;

/// The Rust type of execution primitive functions.
pub type ExecutionFn<S> = fn(token: token::Token, input: &mut vm::ExecutionInput<S>) -> Result<()>;

/// A TeX command.
pub enum Command<S> {
    /// An expansion primitive that is implemented in the engine.
    ///
    /// Examples: `\the`, `\ifnum`.
    Expansion(ExpansionFn<S>, Option<Tag>),

    /// A user defined macro.
    ///
    /// Examples: `\newcommand` and `\include` in LaTeX.
    Macro(rc::Rc<texmacro::Macro>),

    /// A non-expansion primitive that performs operations on the state.
    ///
    /// Examples: `\def`, `\par`.
    Execution(ExecutionFn<S>, Option<Tag>),

    /// A command that is used to reference a variable, like a parameter or a register.
    ///
    /// Such a command is *resolved* to get the variable using the function pointer it holds.
    ///
    /// Examples: `\count`, `\year`.
    Variable(rc::Rc<variable::Command<S>>),

    /// A command that aliases a character token.
    ///
    /// Depending on the context in which this command appears it may behave like a
    ///   character (when typesetting) or like an unexpandable command (when parsing integers).
    /// Created using `\let\cmd=<character>`.
    CharacterTokenAlias(token::Value),

    /// A command that references a character.
    ///
    /// These commands are generally created using `\countdef`.
    /// In the main inner loop they result in a character being typeset.
    /// In other contexts they are interpreted as numbers.
    /// In Plain TeX, `\countdef 255` is used as a more efficient version of `\def{255 }`.
    Character(char),

    /// A command that references a math character.
    ///
    /// These commands are generally created using `\mathchardef`.
    MathCharacter(types::MathCode),
}

impl<S> std::fmt::Display for Command<S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Command::Expansion(_, _) => write![f, "an expansion command"],
            Command::Macro(_) => write![f, "a user-defined macro"],
            Command::Execution(_, _) => write![f, "an execution command"],
            Command::Variable(_) => write![f, "a variable command"],
            Command::CharacterTokenAlias(_) => write![f, "a character token alias"],
            Command::Character(_) => write![f, "a character command"],
            Command::MathCharacter(_) => write![f, "a math character command"],
        }
    }
}

impl<S> Command<S> {
    /// Gets the tag associated to this command, or [None] if the command has no tag.
    pub fn tag(&self) -> Option<Tag> {
        match self {
            Command::Expansion(_, tag) => *tag,
            Command::Execution(_, tag) => *tag,
            Command::Macro(_)
            | Command::Variable(_)
            | Command::CharacterTokenAlias(_)
            | Command::Character(_)
            | Command::MathCharacter(_) => None,
        }
    }
}

/// A built-in command. This is a command provided at VM initialization.
///
/// This struct is simply a combination of a [Command] and a documentation string for the command.
/// It is used when providing the built-in commands for a VM.
pub struct BuiltIn<S> {
    cmd: Command<S>,
    doc: Option<&'static str>,
}

impl<S> BuiltIn<S> {
    /// Create a new expansion built-in command.
    pub fn new_expansion(t: ExpansionFn<S>) -> BuiltIn<S> {
        t.into()
    }

    /// Create a new expansion built-in command.
    pub fn new_execution(t: ExecutionFn<S>) -> BuiltIn<S> {
        t.into()
    }

    /// Create a new variable built-in command.
    pub fn new_variable(cmd: variable::Command<S>) -> BuiltIn<S> {
        Command::Variable(rc::Rc::new(cmd)).into()
    }

    /// Set the tag for this built-in command.
    pub fn with_tag(mut self, tag: Tag) -> BuiltIn<S> {
        match &mut self.cmd {
            Command::Expansion(_, t) => *t = Some(tag),
            Command::Execution(_, t) => *t = Some(tag),
            Command::Macro(_)
            | Command::Variable(_)
            | Command::CharacterTokenAlias(_)
            | Command::Character(_)
            | Command::MathCharacter(_) => {
                panic!("cannot add a tag to this type of command")
            }
        }
        self
    }

    // Set the doc for this built-in command.
    pub fn with_doc(mut self, doc: &'static str) -> BuiltIn<S> {
        self.doc = Some(doc);
        self
    }

    pub fn cmd(&self) -> &Command<S> {
        &self.cmd
    }

    pub fn doc(&self) -> Option<&'static str> {
        self.doc
    }
}

// We need to implement Clone manually as the derived implementation requires S to be Clone.
impl<S> Clone for Command<S> {
    fn clone(&self) -> Self {
        match self {
            Command::Expansion(e, t) => Command::Expansion::<S>(*e, *t),
            Command::Macro(m) => Command::Macro(m.clone()),
            Command::Execution(e, t) => Command::Execution(*e, *t),
            Command::Variable(v) => Command::Variable(v.clone()),
            Command::CharacterTokenAlias(tv) => Command::CharacterTokenAlias(*tv),
            Command::Character(c) => Command::Character(*c),
            Command::MathCharacter(c) => Command::MathCharacter(*c),
        }
    }
}

// We need to implement Clone manually as the derived implementation requires S to be Clone.
impl<S> Clone for BuiltIn<S> {
    fn clone(&self) -> Self {
        Self {
            cmd: self.cmd.clone(),
            doc: self.doc,
        }
    }
}

impl<S> From<ExpansionFn<S>> for BuiltIn<S> {
    fn from(cmd: ExpansionFn<S>) -> Self {
        Command::Expansion(cmd, None).into()
    }
}

impl<S> From<rc::Rc<texmacro::Macro>> for BuiltIn<S> {
    fn from(cmd: rc::Rc<texmacro::Macro>) -> Self {
        Command::Macro(cmd).into()
    }
}

impl<S> From<ExecutionFn<S>> for BuiltIn<S> {
    fn from(cmd: ExecutionFn<S>) -> Self {
        Command::Execution(cmd, None).into()
    }
}

impl<S> From<variable::Command<S>> for BuiltIn<S> {
    fn from(cmd: variable::Command<S>) -> Self {
        Command::Variable(rc::Rc::new(cmd)).into()
    }
}

impl<S> From<Command<S>> for BuiltIn<S> {
    fn from(cmd: Command<S>) -> Self {
        BuiltIn { cmd, doc: None }
    }
}

/// A tag is a piece of metadata that is optionally attached to a command.
///
/// Tags are used to implement certain TeX language semantics.
/// An example is TeX conditionals.
/// When a TeX conditional statement evaluates to false, the `\if` command must scan
///     the input stream until it finds either an `\else` or `\fi` command.
/// (The tokens scanned in this process are in the true branch of the conditional,
///     and must thus be discarded.)
/// Tags are the mechanism by which the scanning algorithm can
///     determine if a token corresponds to an `\else` of `\fi` command.
/// Concretely, both `\else` of `\fi` command have unique tags associated to them.
/// When scanning the stream,
///     if a token is a command token then the tag for the associated command is
///     compared to the known tags for `\else` and `\fi`.
/// If the tags match, the true branch is finished.
///
/// In general, TeX commands interface with the VM in two ways.
/// The first most common way is when the main VM loop or expansion loop encounters a command.
/// The loop invokes the command's associated Rust function.
/// One can think of the Rust function as providing the behavior of the command in this context.
///
/// The second way is when a different command, like a conditional command, performs some operation
///     that is dependent on the commands it reads out of the input stream.
/// In this context the commands in the input stream provide behavior using tags.
/// The `\else` command having the specific else tag results in the conditional branch processing completing.
///
/// Note that the same tag can be used for multiple commands,
/// but each command can only have one tag.
///
/// ## Implementation details
///
/// Tags are non-zero 32 bit integers.
/// The first tag created has value 1, the second tag has value 2, and so on.
/// A global mutex is used to store the next tag value.
/// Tags have the property that `Option<Tag>` takes up 4 bytes in memory.
#[derive(PartialEq, Eq, Clone, Copy, Debug, PartialOrd, Ord, Hash)]
pub struct Tag(num::NonZeroU32);

static NEXT_TAG_VALUE: sync::Mutex<u32> = sync::Mutex::new(1);

impl Tag {
    /// Creates a new unique tag.
    ///
    /// ```
    /// # use texlang::command::Tag;
    /// let tag_1 = Tag::new();
    /// let tag_2 = Tag::new();
    /// assert_ne!(tag_1, tag_2);
    /// ```
    // We suppress the clippy warning because creating a new tag is a global operation and
    // shouldn't be done without explicit intention.
    #[allow(clippy::new_without_default)]
    pub fn new() -> Tag {
        let mut n = NEXT_TAG_VALUE.lock().unwrap();
        let tag = Tag(num::NonZeroU32::new(*n).unwrap());
        *n = n.checked_add(1).unwrap();
        tag
    }
}

/// A static tag enables creating a tag in a static variable.
///
/// ```
/// # use texlang::command::StaticTag;
/// static TAG: StaticTag = StaticTag::new();
///
/// let first_get = TAG.get();
/// let second_get = TAG.get();
/// assert_eq!(first_get, second_get);
/// ```
pub struct StaticTag(OnceCell<Tag>);

impl StaticTag {
    /// Create a new static tag.
    pub const fn new() -> StaticTag {
        StaticTag(OnceCell::new())
    }

    /// Get the actual [Tag] out of this [StaticTag].
    /// Repeated calls to this function return the same tag.
    ///
    /// This is not a trivial getter.
    /// The [Tag] is lazily constructed so even subsequent calls to this getter must do some work to check if the [Tag]
    ///     exists or not.
    /// For very hot code paths it is advised to cache the return value somewhere, for example in a relevant command's state.
    pub fn get(&self) -> Tag {
        *self.0.get_or_init(Tag::new)
    }
}

/// A primitive key uniquely identifies a primitive.
///
/// If two commands have the same key, they are the same primitive (expansion, execution, or variable primitive)
/// The function returns [None] if the command is not a primitive (a macro or a token alias).
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub(crate) enum PrimitiveKey {
    Execution(usize, Option<Tag>),
    Expansion(usize, Option<Tag>),
    Variable(variable::CommandKey),
}

impl PrimitiveKey {
    pub(crate) fn new<S>(command: &Command<S>) -> Option<Self> {
        match command {
            Command::Expansion(f, tag) => Some(PrimitiveKey::Expansion(*f as usize, *tag)),
            Command::Execution(f, tag) => Some(PrimitiveKey::Execution(*f as usize, *tag)),
            Command::Variable(v) => Some(PrimitiveKey::Variable(v.key())),
            Command::Macro(_)
            | Command::CharacterTokenAlias(_)
            | Command::Character(_)
            | Command::MathCharacter(_) => None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn func_size() {
        assert_eq!(std::mem::size_of::<Command<()>>(), 16);
    }

    static STATIC_TAG_1: StaticTag = StaticTag::new();
    static STATIC_TAG_2: StaticTag = StaticTag::new();

    #[test]
    fn tag() {
        let tag_1_val_1 = STATIC_TAG_1.get();
        let tag_2_val_1 = STATIC_TAG_2.get();
        let other_tag_1 = Tag::new();
        let tag_1_val_2 = STATIC_TAG_1.get();
        let tag_2_val_2 = STATIC_TAG_2.get();
        let other_tag_2 = Tag::new();

        assert_eq!(tag_1_val_1, tag_1_val_2);
        assert_eq!(tag_2_val_1, tag_2_val_2);

        assert_ne!(tag_1_val_1, tag_2_val_2);
        assert_ne!(tag_1_val_1, other_tag_1);
        assert_ne!(tag_1_val_1, other_tag_2);
    }

    #[test]
    fn tag_size() {
        assert_eq!(std::mem::size_of::<Option<Tag>>(), 4);
    }
}