use std::collections::VecDeque;
use std::iter::Peekable;
use yara_x_parser::GrammarRule;

use crate::tokens::{categories, Token, TokenStream};

// The line below could be uncommented and all occurrences of
// Iterator<Item = Token<'a>> replaced with TokenIterator<'a>, which would
// improve legibility, however, trait aliases are still experimental.
// See https://github.com/rust-lang/rust/issues/41517> for more information.
//
// trait TokenIterator<'a> = Iterator<Item = Token<'a>>;

/// Processor represents a step in a processing pipeline. Each processor
/// receives a stream of input tokens, process them one by one, and produces
/// a stream of output tokens, that could be used as the input to the next
/// processor in the pipeline. The input is provided in the form of a token
/// iterator that is passed as an argument to [`Processor::new`]. The processor
/// itself implements the Iterator trait, and this iterator will return the
/// output tokens.
///
/// The input tokens are transformed into the output tokens according to rules
/// that are added to the processor using the [`Processor::add_rule`] method.
/// Each rule consists of a condition and an action. Conditions are functions
/// that receive a [`Context`] and return a boolean, the context provides access
/// to the next and previous tokens, and the returned boolean controls whether
/// the action associated to the rule will be executed or not.
///
/// The rules are tried in the order they were added to the processor. Once a
/// condition function returns true, the corresponding action is executed and
/// no more rules are tried. If none of the conditions are true the default
/// action will be executed, which is moving the token from the input to the
/// output (i.e: executing the "copy" action).
///
/// The main idea is described in [`A Pretty Good Formatting Pipeline`], but
/// some modifications has been made.
///
/// [`A Pretty Good Formatting Pipeline`]: https://bora.uib.no/bora-xmlui/handle/1956/8915
pub(crate) struct Processor<'a, T>
where
    T: TokenStream<'a>,
{
    context: Context<'a, T>,
    rules: Vec<(ConditionFn<'a, T>, ActionFn<'a, T>)>,
    debug: bool,
}

type ConditionFn<'a, T> = Box<dyn Fn(&Context<'a, T>) -> bool + 'a>;
type ActionFn<'a, T> = Box<dyn Fn(&mut Context<'a, T>) + 'a>;

impl<'a, T> Processor<'a, T>
where
    T: TokenStream<'a>,
{
    /// Creates a new processor that will process the given input.
    pub fn new(input: T) -> Self {
        Self {
            context: Context {
                input: input.peekable(),
                output: VecDeque::new(),
                stack: Vec::new(),
                prev_tokens: VecDeque::new(),
                next_tokens: VecDeque::new(),
                passthrough: *categories::NONE,
            },
            rules: Vec::new(),
            debug: false,
        }
    }

    /// Sets a category of tokens that are copied directly from input to output
    /// without being processed by rules. All tokens in this category will be
    /// completely invisible to rules, and rules can be created as if those
    /// tokens don't exist at all. This means that if we have the sequence
    /// *foo*, *bar*, *baz*, where *bar* belongs to the pass-through category,
    /// from the rules standpoint the input sequence is *foo*, *baz* and
    /// therefore rule conditions that checks for *baz* being right after *foo*
    /// will be true.
    pub fn set_passthrough(mut self, category: categories::Category) -> Self {
        self.context.passthrough = category;
        self
    }

    /// Add a new processing rule. The order in which this method is called is
    /// relevant, rules are tried in the order they were added.
    pub fn add_rule<C, A>(mut self, condition: C, action: A) -> Self
    where
        C: Fn(&Context<'a, T>) -> bool + 'a,
        A: Fn(&mut Context<'a, T>) + 'a,
    {
        self.rules.push((Box::new(condition), Box::new(action)));
        self
    }

    /// Prints the processor's output to stdout for debugging purposes.
    #[allow(dead_code)]
    pub fn debug(mut self) -> Self {
        self.debug = true;
        self
    }
}

impl<'a, T> Iterator for Processor<'a, T>
where
    T: TokenStream<'a>,
{
    type Item = Token<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            // If there are pending tokens in the output buffer, return one
            // of the pending tokens.
            if let Some(output) = self.context.output.pop_front() {
                if self.debug {
                    dbg!(&output);
                }
                return Some(output);
            }

            // `advance` returns false if there are no more tokens in the
            // input to be processed, or in the output buffer to be
            // returned. In that case the iterator can return None, as
            // there's nothing else to do.
            if self.context.advance() {
                return None;
            }

            let mut rule_found = false;

            for (condition, action) in self.rules.iter() {
                if (condition)(&self.context) {
                    rule_found = true;
                    (action)(&mut self.context);
                    break;
                }
            }

            // If no rule matched the current state perform the default
            // action, which is moving the token from the input to the
            // output.
            if !rule_found {
                let next = self.context.pop_input_token();
                self.context.push_output_token(next);
            }
        }
    }
}

/// Context is the structure passed to conditions and actions.
///
/// The context has methods that allows conditions to peek into the next
/// input tokens and the past output tokens. It also has methods that actions
/// use for removing a token from the input and putting tokens into the output.
#[derive(Debug)]
pub(crate) struct Context<'a, T>
where
    T: TokenStream<'a>,
{
    input: Peekable<T>,
    output: VecDeque<Token<'a>>,
    stack: Vec<super::GrammarRule>,
    /// A list with the last N tokens that has been written into the output
    /// (N <= MAX_PREV_TOKENS). Contains only non-pass-through tokens.
    prev_tokens: VecDeque<Token<'a>>,
    /// A list where input tokens are buffered, allowing to peek into the
    /// next few input tokens. This list accumulates tokens from the input
    /// until it contains MAX_NEXT_TOKENS non-pass-through tokens. Its size
    /// can be larger though, because it may contain pass-through tokens too.
    next_tokens: VecDeque<Token<'a>>,
    /// Categories of tokens that are considered pass-through tokens.
    passthrough: categories::Category,
}

impl<'a, T> Context<'a, T>
where
    T: TokenStream<'a>,
{
    const MAX_PREV_TOKENS: usize = 3;
    const MAX_NEXT_TOKENS: usize = 3;

    /// Returns the next non-pass-through token from the input. Returns None
    /// if the end of the input has being reached. Tokens that match the
    /// pass-through category are copied directly to the output when found.
    pub fn pop_input_token(&mut self) -> Option<Token<'a>> {
        self.advance();
        self.next_tokens.pop_front()
    }

    /// Adds a token to the output.
    ///
    /// If `token` is `None` it does nothing. The reason for accepting an
    /// [`Option`] is allowing to pass the value returned by
    /// [`Context::pop_input_token`] directly to this function.
    pub fn push_output_token(&mut self, token: Option<Token<'a>>) {
        let token = match token {
            Some(token) => token,
            None => return,
        };

        match token {
            Token::Begin(rule) => self.stack.push(rule),
            Token::End(rule) => {
                if let Some(top) = self.stack.pop() {
                    assert_eq!(top, rule);
                }
            }
            _ => {}
        }

        // Store outputted token in prev_tokens, but only if it is a
        // non-pass-through token.
        if !token.is(self.passthrough) {
            self.prev_tokens.push_front(token.clone());
        }

        self.output.push_back(token);

        // Keep up to MAX_PREV_TOKENS in the prev_tokens, remove the oldest
        // one if necessary.
        if self.prev_tokens.len() > Self::MAX_PREV_TOKENS {
            self.prev_tokens.pop_back();
        }
    }

    /// Allows to peek into the next tokens in the input or the tokens recently
    /// put into the output. For example token(1) returns the next token in the
    /// input while token(2) returns the one that comes after token(1). In the
    /// other hand, token(-1) returns the most recent output and token(-2) the
    /// one that was outputted before token(-1).
    ///
    /// Valid values for n are in the range [-MAX_PREV_TOKENS..MAX_NEXT_TOKENS]
    /// (both inclusive) except 0, which is not valid. Invalid values of n
    /// make the function panic.
    ///
    /// Notice that pass-through tokens are completely ignored, which means that
    /// token(1) actually returns the first non-pass-through token in the input.
    ///
    /// If the requested token can't be returned because the end of the input
    /// is reached, or no token has been outputted yet, the result will be
    /// Token::None.
    pub fn token(&self, n: i8) -> &Token<'a> {
        // n is 1,2,3,4 ... MAX_NEXT_TOKENS
        if n >= 1 && n <= Self::MAX_NEXT_TOKENS as i8 {
            // Return Nth token that is not a pass-through token.
            self.next_tokens
                .iter()
                .filter(|token| !token.is(self.passthrough))
                .nth((n - 1) as usize)
                .unwrap_or(&Token::None)
        }
        // n is -1,-2,-3 ... -MAX_PREV_TOKENS
        else if n <= -1 && n >= -(Self::MAX_PREV_TOKENS as i8) {
            self.prev_tokens.get((-n - 1) as usize).unwrap_or(&Token::None)
        }
        // n out of the valid range or zero.
        else {
            panic!(
                "n must be in the range [-MAX_PREV_TOKENS..MAX_NEXT_TOKENS], \
                 both inclusive, and can't be 0"
            )
        }
    }

    /// Swaps the tokens at positions i and j in the input. Pass-through tokens
    /// are ignored, which means that position i means the i-th non-pass-through
    /// token in the input.
    #[allow(dead_code)]
    pub fn swap(&mut self, i: i8, j: i8) {
        if i < 1
            || i > Self::MAX_NEXT_TOKENS as i8
            || j < 1
            || j > Self::MAX_NEXT_TOKENS as i8
        {
            panic!(
                "i and j must be in the range [1..MAX_NEXT_TOKENS], both inclusive"
            )
        }

        // Compute the actual index in the input for the i-th non-pass-through
        // token. The actual index can be larger than i, as the input may
        // contain pass-through tokens that should be ignored.
        let (index_i, _) = self
            .next_tokens
            .iter()
            .enumerate()
            .filter(|(_, token)| !token.is(self.passthrough))
            .nth((i - 1) as usize)
            .unwrap();

        // Compute the actual index in the input for the j-th non-pass-through
        // token
        let (index_j, _) = self
            .next_tokens
            .iter()
            .enumerate()
            .filter(|(_, token)| !token.is(self.passthrough))
            .nth((j - 1) as usize)
            .unwrap();

        self.next_tokens.swap(index_i, index_j)
    }

    /// Returns true if the the next token is within a given grammar rule.
    ///
    /// If `deep` is `false` this function returns true only when the token is
    /// a direct child of the grammar rule, tokens within sub-rules won't be
    /// taken into account. When `deep` is true, the result of the function
    /// is true if the token appears in the given rule or any of its sub-rules.
    ///
    /// For example, the code "rule test { condition: true }" produces the
    /// following sequence of tokens (indentation was added for highlighting
    /// the hierarchical structure of parsing rules):
    ///
    ///   Begin(Rule::rule_decl),
    ///      Keyword("rule"),
    ///      Identifier("test"),
    ///      Grouping("{"),
    ///      Keyword("condition"),
    ///      Punctuation(":"),
    ///      Begin(Rule::boolean_expr),
    ///         Keyword("true"),
    ///      End(Rule::boolean_expr),
    ///      Grouping("}"),
    ///   End(Rule::rule_decl),
    ///
    /// Now let's suppose the next token is the `true` keyword. In that state,
    /// `in_rule(Rule::rule_decl, false)` returns `false` because the `true`
    /// keyword it not a direct child of `Rule::rule_decl`.  
    ///
    /// However, calling `in_rule(Rule::rule_decl, true)` returns `true`,
    /// because the `true` keyword is somewhere inside `Rule::rule_decl`,
    /// although not directly.
    pub fn in_rule(&self, rule: GrammarRule, deep: bool) -> bool {
        // The logic for determining if the next token is within the scope of
        // a rule goes as follows:
        //
        // * deep == true  => the rule is anywhere in the stack.
        // * deep == false => the rule is at the top of the stack.
        //
        // However, there's an edge-case when the next token is End(some_rule).
        // In that case the rule at the top of the stack must be `some_rule`,
        // but we don't want End(some_rule) to be considered within the scope
        // of `some_rule`. Both Begin(some_rule) and End(some_rule) are within
        // the scope of their parent rule.
        //
        // This means that when the next token is End(some_rule), the top of the
        // stack is ignored.

        // First step is creating an iterator for traversing the stack starting
        // at the top and going downwards.
        let mut stack_iter = self.stack.iter().rev();

        // If the next token indicates the end of a rule, make sure that the
        // top of the stack is that same rule, but remove it from the iterator
        // so that it won't taken into account in next step.
        if let Some(&Token::End(rule)) = self.next_tokens.front() {
            let top = stack_iter.next();
            debug_assert_eq!(*top.unwrap(), rule);
        }

        // If deep is true look for the rule in the whole stack (the top
        // was removed if necessary), if not look only at the top.
        if deep {
            for r in stack_iter {
                if *r == rule {
                    return true;
                }
            }
        } else if let Some(r) = stack_iter.next() {
            return *r == rule;
        }

        false
    }

    /// Helper function that reads tokens from input and puts them into
    /// `next_tokens` until we have enough non-pass-through tokens to satisfy
    /// calls to `token(n)`. The function guarantees that the first token
    /// in `next_tokens` is a non-pass-through token.
    ///
    /// Returns true if there are token in `next_tokens` or if there are
    /// any pending tokens in the output buffer.
    fn advance(&mut self) -> bool {
        // Count the number of non-pass-through tokens already in the input
        // buffer.
        let mut non_passthrough_tokens = self
            .next_tokens
            .iter()
            .filter(|token| !token.is(self.passthrough))
            .count();
        // Keep reading tokens from the input and putting them in next_tokens
        // until next_tokens contains at least MAX_NEXT_TOKENS non-pass-through
        // tokens. Break the loop if the end of the input is reached.
        while non_passthrough_tokens < Self::MAX_NEXT_TOKENS {
            if let Some(next) = self.input.next() {
                if !next.is(self.passthrough) {
                    non_passthrough_tokens += 1;
                }
                self.next_tokens.push_back(next);
            } else {
                break;
            }
        }
        // Check if the tokens at the front of the input queue are pass-through
        // tokens. In that case copy them directly to the output until the we
        // have a non-pass-through token at the front of the queue.
        while let Some(token) = self.next_tokens.front() {
            if token.is(self.passthrough) {
                let token = self.next_tokens.pop_front().unwrap();
                // Adjust the stack if the pass-through token indicates the
                // start or the end of a rule.
                if let Token::Begin(rule) = token {
                    self.stack.push(rule)
                } else if let Token::End(rule) = token {
                    if let Some(top) = self.stack.pop() {
                        assert_eq!(top, rule);
                    }
                }
                self.output.push_back(token);
            } else {
                break;
            }
        }
        self.next_tokens.is_empty() && self.output.is_empty()
    }
}

pub(crate) mod actions {
    use crate::processor::{ActionFn, Context};
    use crate::tokens::{Token, TokenStream};

    /// Action that removes the next token from the input without producing
    /// any output.
    pub(crate) fn drop<'a, I>(ctx: &mut Context<'a, I>)
    where
        I: TokenStream<'a>,
    {
        // Removes next input token...
        ctx.pop_input_token();
        // ...but does not put it in the output, so it's dropped.
    }

    /// Action that removes the next token from the input and puts it into
    /// the output. Notice that this action is named as "move" in
    /// [`A Pretty Good Formatting Pipeline`], but "move" is reserved keyword
    /// in Rust.
    #[allow(dead_code)]
    pub(crate) fn copy<'a, I>(ctx: &mut Context<'a, I>)
    where
        I: TokenStream<'a>,
    {
        // Removes next input token and returns it, so it gets copied to the
        // output.
        let token = ctx.pop_input_token();
        ctx.push_output_token(token);
    }

    /// Action that puts a space token into the output without removing
    /// the next token from the input.
    pub(crate) fn space<'a, I>(ctx: &mut Context<'a, I>)
    where
        I: TokenStream<'a>,
    {
        ctx.push_output_token(Some(Token::Whitespace));
    }

    /// Action that puts a line break token into the output without
    /// removing the next token from the input.
    pub(crate) fn newline<'a, I>(ctx: &mut Context<'a, I>)
    where
        I: TokenStream<'a>,
    {
        ctx.push_output_token(Some(Token::Newline));
    }

    /// Action that puts the specified token into the output without
    /// removing the next token from the input.
    pub(crate) fn insert<'a, I>(token: Token<'a>) -> ActionFn<'a, I>
    where
        I: TokenStream<'a>,
    {
        Box::new(move |ctx| ctx.push_output_token(Some(token.clone())))
    }

    /// Action that swaps the tokens at positions i and j in the input.
    /// Both i and j most be in the range [1..MAX_NEXT_TOKENS].
    #[allow(dead_code)]
    pub(crate) fn swap<'a, I>(i: i8, j: i8) -> ActionFn<'a, I>
    where
        I: TokenStream<'a>,
    {
        Box::new(move |ctx| ctx.swap(i, j))
    }
}

#[cfg(test)]
mod tests;
