yyyi_ru/src/mtgott/parser.rs

use std::collections::HashMap;
use crate::mtgott::charclasses::{is_bad_name, is_digit, is_illegal_name, is_lnspace, is_normal_word_constituent, is_whitespace};
use std::fmt::{self, Display, Formatter};
use std::error::Error;

#[derive(Debug, PartialEq, Eq)]
pub enum Expression {
    Root,
    Argument(u64),
    Get(Box<Expression>, Box<Expression>),
    Attribute(Box<Expression>, String),
    Call(Box<Expression>, Vec<Expression>),
    Int(u64),
    None,
}

#[derive(Debug, PartialEq, Eq)]
pub struct IfSubElement {
    pub branches: Vec<Element>,
    pub conditions: Vec<Expression>,
}

#[derive(Debug, PartialEq, Eq)]
pub struct ForSubElement {
    pub iterable: Expression,
    pub core: Element,
    // Either "\n", " " or ""
    pub join: String,
}

#[derive(Debug, PartialEq, Eq)]
pub enum SubElement {
    Static(String),
    // ======== Other are dynamic ========
    If(IfSubElement),
    // Both for {{}} and {[]}
    InsertExpr(Expression),
    For(ForSubElement),
    Let(Expression, Element),
}

#[derive(Debug, PartialEq, Eq)]
pub struct Element {
    pub argc: usize,
    pub sub_elements: Vec<SubElement>,
}

#[derive(Debug, PartialEq, Eq)]
pub enum Plemege {
    Element(Element),
    Package(HashMap<String, Plemege>),
}

#[derive(Debug, PartialEq, Eq)]
pub enum FileParsingErrorKind {
    expected_pack_opening_or_element_opening_or_pack_ending,
    expected_pack_opening_or_element_opening_or_eof,
    unmatched_pack_ending_tag,
    expected_pack_name,
    illegal_pack_name,
    pack_member_name_already_occupied,
    expected_pack_opening_tag_end,
    expected_element_name,
    incorrect_block_ending_tag_expected_end_element,
    illegal_element_name,
    expected_argument_name_or_eldef_opening_tag_end,
    illegal_argument_name,
    repeated_argument_name,
    expected_command_name,
    incorrect_block_ending_tag_expected_normal,
    expected_write_tag_end_after_expression,
    expected_roughinsert_tag_end_after_expression,
    illegal_command_name,
    expected_cmd_tag_end,
    expected_variable_name,
    illegal_variable_name,
    expected_assignment_operator,
    expected_cmd_tag_end_after_expression,
    expected_comma_or_colon,
    expected_colon,
    forloop_variable_cant_take_occupied_name,
    incorrect_block_ending_tag_expected_normal_or_lf_gap_nogap_or_forloop,
    incorrect_block_ending_tag_expected_normal_or_endif_or_else_or_else_if,
    incorrect_block_ending_tag_expected_normal_or_endif,
    expected_nonempty_expression,
    expected_closing_round_bracket,
    cant_start_word_immediately_after_digit,
    integer_parsing_error,
    illegal_object_name,
    expected_attribute_name_after_dot,
    illegal_attribute_name,
    expected_closing_square_bracket,
    empty_expression_inside_round_brackets,
    empty_expression_inside_square_brackets,
    unmatched_element_ending_tag,
    unmatched_magic_block_ending_tag,
    recursion_limit_exceeded,
}

use FileParsingErrorKind::*;

#[derive(Debug, PartialEq, Eq)]
pub struct FileParsingError {
    pub kind: FileParsingErrorKind,
    pub p1: usize,
    pub p2: usize,
}

impl FileParsingError {
    fn new(kind: FileParsingErrorKind, p1: usize, p2: usize) -> Self {
        Self{kind, p1, p2}
    }
}

impl Display for FileParsingErrorKind {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        write!(f, "{:?}", self)
    }
}

impl Display for FileParsingError {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        write!(f, "FileParsingError: {} at positions {} to {}", self.kind, self.p1, self.p2)
    }
}

impl Error for FileParsingError {}

struct Parser<'a> {
    text: &'a str,
    p: usize
}

impl<'a> Parser<'a> {
    fn here(&self)->Option<char> {
        self.text[self.p..].chars().next()
    }

    fn is_ahead(&self, substr: &str)->bool {
        self.text[self.p..].starts_with(substr)
    }

    fn is_char_ahead(&self, ch: char) -> bool {
        match self.here() {
            Some(cha) => cha == ch,
            None => false,
        }
    }

    fn is_digit_ahead(&self) -> bool {
        match self.here() {
            Some(ch) => is_digit(ch),
            None => false,
        }
    }

    fn is_word_ahead(&self) -> bool {
        match self.here() {
            Some(ch) => is_normal_word_constituent(ch),
            None => false,
        }
    }

    fn advance(&mut self) {
        self.p += self.text[self.p..].chars().next().unwrap().len_utf8();
    }

    fn skip_whitespace(&mut self) {
        loop {
            match self.here() {
                Some(ch ) => if !is_whitespace(ch) {
                    break
                } else { self.advance(); }
                None => break
            }
        }
    }

    fn skip_normal_word(&mut self){
        loop {
            match self.here() {
                Some(ch ) => if !is_normal_word_constituent(ch) {
                    break
                } else { self.advance(); }
                None => break
            }
        }
    }

    fn new_unexpected_char_error(&self, kind: FileParsingErrorKind) -> FileParsingError {
        match self.text[self.p..].char_indices().next() {
            Some((off, _)) => FileParsingError::new(kind, self.p, self.p + off),
            None => FileParsingError::new(kind, self.p, self.p),
        }
    }

    fn parse_pack_plus_ending(&mut self, top: bool) -> Result<Plemege, FileParsingError> {
        let mut res: HashMap<String, Plemege> = HashMap::new();
        loop {
            self.skip_whitespace();
            if self.p == self.text.len() {
                return if top {
                    Ok(Plemege::Package(res))
                } else {
                    Err(self.new_unexpected_char_error(expected_pack_opening_or_element_opening_or_pack_ending))
                }
            }
            if self.is_ahead("{$}") {
                return if top {
                    Err(FileParsingError::new(unmatched_pack_ending_tag, self.p, self.p + 3))
                } else {
                    self.p += 3;
                    Ok(Plemege::Package(res))
                };
            } else if self.is_ahead("{$") {
                self.p += 2;
                self.skip_whitespace();
                let p1 = self.p;
                self.skip_normal_word();
                if self.p == p1 {
                    return Err(self.new_unexpected_char_error(expected_pack_name))
                }
                let child_name: &str = &self.text[p1..self.p];
                if !is_bad_name(child_name) {
                    return Err(FileParsingError::new(illegal_pack_name, p1, self.p))
                }
                if let Some(_) = res.get(child_name) {
                    return Err(FileParsingError::new(pack_member_name_already_occupied, p1, self.p))
                }
                self.skip_normal_word();
                if !self.is_ahead("$}") {
                    return Err(self.new_unexpected_char_error(expected_pack_opening_tag_end))
                }
                self.p += 2;
                res.insert(String::from(child_name), self.parse_pack_plus_ending(false)?);
            } else if self.is_ahead("{@") {
                self.p += 2;
                self.skip_whitespace();
                let p1 = self.p;
                self.skip_normal_word();
                if p1 == self.p {
                    return Err(FileParsingError::new(expected_element_name, p1, self.p))
                }
                let child_name = &self.text[p1..self.p];
                if is_bad_name(child_name) {
                    return Err(FileParsingError::new(illegal_element_name, p1, self.p))
                }
                if let Some(_) = res.get(child_name) {
                    return Err(FileParsingError::new(pack_member_name_already_occupied, p1, self.p))
                }
                let mut arg_names: Vec<&str> = Vec::new();
                loop {
                    self.skip_whitespace();
                    if self.is_ahead("@}") {
                        self.p += 2;
                        break
                    }
                    let p1 = self.p;
                    self.skip_normal_word();
                    if p1 == self.p {
                        return Err(FileParsingError::new(expected_argument_name_or_eldef_opening_tag_end, p1, self.p))
                    }
                    let arg_name: &str = &self.text[p1..self.p];
                    if is_bad_name(arg_name) {
                        return Err(FileParsingError::new(illegal_argument_name, p1, self.p))
                    }
                    if arg_names.iter().any(|b: &&str| *b == arg_name) {
                        return Err(FileParsingError::new(repeated_argument_name, p1, self.p))
                    }
                    arg_names.push(arg_name);
                }
                let (child_el, end_cmd): (Element, ReasonOfElementEnd) = self.parse_element_plus_ending_tag(&arg_names)?;
                if !matches!(end_cmd.cmd, BlockEndingTag::END_ELEMENT) {
                    return Err(FileParsingError::new(incorrect_block_ending_tag_expected_end_element, end_cmd.p1, self.p))
                }
                res.insert(String::from(child_name), Plemege::Element(child_el));
            } else {
                return Err(self.new_unexpected_char_error(if top {
                    expected_pack_opening_or_element_opening_or_eof
                } else {
                    expected_pack_opening_or_element_opening_or_pack_ending
                }));
            }
        }
    }
}

enum BlockEndingTag {
    EOF,
    /* This is {@} */
    END_ELEMENT,
    /* These tags are command tags */
    /* {%} */
    NORMAL,
    LF,
    GAP,
    NOGAP,
    ENDLOOP,
    /* Incomplete tag (ony `{% else if` is included */
    ELSE_IF,
    ELSE,
    ENDIF
}

struct ReasonOfElementEnd {
    p1: usize,
    cmd: BlockEndingTag,
}

fn fix_whitespaces_in_element(subels: &mut Vec<SubElement>) {
    let n = subels.len();
    if n > 0 {
        match &mut subels[0] {
            SubElement::Static(org) => {
                let mut ta = 0;
                for p in 0..org.len(){
                    if !is_whitespace(org.as_bytes()[p] as char) {
                        ta = p; break
                    }
                    if org.as_bytes()[p] == b'\n' {
                        ta = p + 1;
                    }
                }
                *org = String::from(&org[ta..]);
            },
            _ => {},
        }
        match &mut subels[n - 1] {
            SubElement::Static(org) => {
                while let Some(ch) = org.chars().last() {
                    if is_whitespace(ch) { org.pop(); } else { break }
                }
            },
            _ => {},
        }
    }
    let mut min_offset = usize::MAX;
    for i in 0..subels.len() {
        match &mut subels[i] {
            SubElement::Static(org) => {
                let mut seen_online = i > 0;
                let mut line_bg: usize = 0;
                for p in 0..org.len() {
                    let ch = org.as_bytes()[p] as char;
                    if !is_whitespace(ch) {
                        if !seen_online {
                            seen_online = true;
                            min_offset = std::cmp::min(min_offset, p - line_bg)
                        }
                    } else if ch == '\n' {
                        line_bg = p + 1;
                        seen_online = false;
                    }
                }
                /* This won't cause issues on the .last() because we previously rstripped the last part */
                if !seen_online{
                    min_offset = std::cmp::min(min_offset, org.len() - line_bg)
                }
            },
            _ => {},
        }
    }
    for i in 0..subels.len() {
        match &mut subels[i] {
            SubElement::Static(org) => {
                let mut res: Vec<u8> = Vec::new();
                let mut should_ignore_gap = i > 0;
                let mut line_bg: usize = 0;
                for p in 0..org.len() {
                    let ch = org.as_bytes()[p];
                    if ch == b'\n' {
                        line_bg = p + 1;
                        should_ignore_gap = false;
                        /* We handle trailing whitespaces case here */
                        while let Some(&ch) = res.last() {
                            if is_lnspace(ch as char) { res. pop(); } else { break }
                        }
                    } else if p - line_bg < min_offset {
                        continue
                    }
                    res.push(ch);
                }
                *org = String::from_utf8(res).unwrap();
            },
            _ => {},
        }
    }
}

impl<'a> Parser<'a> {
    /* If BlockEndingCmdTag::ELSE_IF is returned, the ending tag won't be read completely,
     * But in other case it would be read to the end */
    fn parse_element_plus_ending_tag(&mut self, arg_names: &Vec<&str>) -> Result<(Element, ReasonOfElementEnd), FileParsingError> {
        let mut res: Vec<SubElement> = Vec::new();
        let mut tp1 = self.p;

        let fin_static = |p: &Parser, tp1: usize, res: &mut Vec<SubElement>| {
            if tp1 < p.p {
                res.push(SubElement::Static(String::from(&self.text[tp1..p.p])))
            }
        };

        /* Fixes whitespaces in static sub-elements   */
        let finishing_touches = |ree: ReasonOfElementEnd, mut res: Vec<SubElement>| -> Result<(Element, ReasonOfElementEnd), FileParsingError> {
            fix_whitespaces_in_element(&mut res);
            Ok((Element{ argc: arg_names.len(), sub_elements: res }, ree))
        };

        loop {
            if self.p == self.text.len() {
                fin_static(self, tp1, &mut res);
                return finishing_touches(ReasonOfElementEnd{p1: self.p, cmd: BlockEndingTag::EOF}, res);
            } else if self.is_ahead("{{") {
                fin_static(self, tp1, &mut res);
                self.p += 2;
                let expr: Expression = self.parse_expression(arg_names)?;
                if !self.is_ahead("}}") {
                    return Err(FileParsingError::new(expected_write_tag_end_after_expression, self.p - 2, self.p));
                }
                self.p += 2;
                if !matches!(expr, Expression::None){
                    res.push(SubElement::InsertExpr(
                        Expression::Call(
                            Box::new(Expression::Attribute(
                                Box::new(Expression::Root), String::from("sanitize")
                            )),
                            vec![expr])
                    ));
                }
                tp1 = self.p;
            } else if self.is_ahead("{[") {
                fin_static(self, tp1, &mut res);
                self.p += 2;
                let expr: Expression = self.parse_expression(arg_names)?;
                if !self.is_ahead("]}") {
                    return Err(FileParsingError::new(expected_roughinsert_tag_end_after_expression, self.p - 2, self.p))
                }
                self.p += 2;
                if !matches!(expr, Expression::None){
                    res.push(SubElement::InsertExpr(expr));
                }
                tp1 = self.p;
            } else if self.is_ahead("{@}") {
                fin_static(self, tp1, &mut res);
                self.p += 3;
                return finishing_touches(ReasonOfElementEnd{p1: self.p - 3, cmd: BlockEndingTag::END_ELEMENT}, res);
            } else if self.is_ahead("{%}") {
                fin_static(self, tp1, &mut res);
                self.p += 3;
                return finishing_touches(ReasonOfElementEnd{p1: self.p - 3, cmd: BlockEndingTag::NORMAL}, res);
            } else if self.is_ahead("{%") {
                fin_static(self, tp1, &mut res);
                /* Might be needed if this is the ENDING cmd tag */
                let p1 = self.p;
                self.p += 2;
                self.skip_whitespace();
                let pb = self.p;
                self.skip_normal_word();
                if pb == self.p {
                    return Err(self.new_unexpected_char_error(expected_command_name))
                }
                let cmd = &self.text[pb..self.p];

                /* Read space + expect %} and do finishing_touches */
                let just_one_thing = |pelf: &mut Parser, cmd: BlockEndingTag, res: Vec<SubElement>| -> Result<(Element, ReasonOfElementEnd), FileParsingError> {
                    pelf.skip_whitespace();
                    if !pelf.is_ahead("%}") {
                        return Err(pelf.new_unexpected_char_error(expected_cmd_tag_end));
                    }
                    pelf.p += 2;
                    finishing_touches(ReasonOfElementEnd{p1, cmd}, res)
                };

                match cmd {
                    "lf" => return just_one_thing(self, BlockEndingTag::LF, res),
                    "gap" => return just_one_thing(self, BlockEndingTag::GAP, res),
                    "nogap" => return just_one_thing(self, BlockEndingTag::NOGAP, res),
                    "else" => {
                        self.skip_whitespace();
                        let ps = self.p;
                        self.skip_normal_word();
                        if ps == self.p {
                            return just_one_thing(self, BlockEndingTag::ELSE, res)
                        } else if &self.text[ps..self.p] != "if" {
                            return Err(FileParsingError::new(illegal_command_name, pb, self.p))
                        }
                        return finishing_touches(ReasonOfElementEnd{p1, cmd: BlockEndingTag::ELSE_IF}, res);
                    }
                    "endif" => return just_one_thing(self, BlockEndingTag::ENDIF, res),
                    "endloop" => return just_one_thing(self, BlockEndingTag::ENDLOOP, res),
                    "for" => res.push(self.parse_let(arg_names)?),
                    "if" => res.push(self.parse_if(arg_names)?),
                    "let" => res.push(self.parse_let(arg_names)?),
                    _ => return Err(FileParsingError::new(illegal_command_name, pb, self.p)),
                }
                tp1 = self.p;
            } else {
                self.advance();
            }
        }
    }

    fn parse_expression_at_cmd_tag_end(&mut self, arg_names: &Vec<&str>) -> Result<Expression, FileParsingError> {
        let p1 = self.p;
        let expr: Expression = self.parse_expression(arg_names)?;
        if matches!(expr, Expression::None) {
            return Err(FileParsingError::new(expected_nonempty_expression, p1, self.p))
        }
        if !self.is_ahead("%}"){
            return Err(self.new_unexpected_char_error(expected_cmd_tag_end_after_expression));
        }
        Ok(expr)
    }

    /* It turned out to be so complex I put it in a separate function.
     * It parses expr %} block {% else if expr %} block {% else %} block {%}   */
    fn parse_if(&mut self, arg_names: &Vec<&str>) -> Result<SubElement, FileParsingError> {
        let mut conditions: Vec<Expression> = Vec::new();
        let mut blocks: Vec<Element> = Vec::new();
        loop {
            let expr = self.parse_expression_at_cmd_tag_end(arg_names)?;
            let (inner_block, ending_tag) = self.parse_element_plus_ending_tag(arg_names)?;
            conditions.push(expr);
            match ending_tag.cmd {
                BlockEndingTag::ELSE | BlockEndingTag::NORMAL | BlockEndingTag::ENDIF |
                BlockEndingTag::ELSE_IF => blocks.push(inner_block),
                _ => return Err(FileParsingError::new(
                    incorrect_block_ending_tag_expected_normal_or_endif_or_else_or_else_if, ending_tag.p1, self.p)),
            }
            if matches!(ending_tag.cmd, BlockEndingTag::ELSE) {
                let (else_block, the_end) = self.parse_element_plus_ending_tag(arg_names)?;
                if !matches!(the_end.cmd, BlockEndingTag::NORMAL | BlockEndingTag::ENDIF){
                    return Err(FileParsingError::new(incorrect_block_ending_tag_expected_normal_or_endif, the_end.p1, self.p));
                }
                blocks.push(else_block);
                break
            } else if matches!(ending_tag.cmd, BlockEndingTag::NORMAL | BlockEndingTag::ENDIF) {
                break
            }
        }
        Ok(SubElement::If(IfSubElement{branches: blocks, conditions}))
    }

    fn parse_let(&mut self, arg_names: &Vec<&str>) -> Result<SubElement, FileParsingError> {
        self.skip_whitespace();
        let p1 = self.p;
        self.skip_normal_word();
        if p1 == self.p {
            // Ironically, these symbols are actually constants
            return Err(FileParsingError::new(expected_variable_name, p1, self.p));
        }
        let new_variable_name = &self.text[p1..self.p];
        if is_bad_name(new_variable_name){
            return Err(FileParsingError::new(illegal_variable_name, p1, self.p));
        }
        self.skip_whitespace();
        if !self.is_char_ahead('=') {
            return Err(self.new_unexpected_char_error(expected_assignment_operator));
        }
        self.p += 1;
        let expr = self.parse_expression_at_cmd_tag_end(arg_names)?;
        let mut arg_names_extended = arg_names.clone();
        arg_names_extended.push(new_variable_name);
        let (inner_block, ending) = self.parse_element_plus_ending_tag(&arg_names_extended)?;
        if  !matches!(ending.cmd, BlockEndingTag::NORMAL) {
            return Err(FileParsingError::new(incorrect_block_ending_tag_expected_normal, ending.p1, self.p));
        }
        Ok(SubElement::Let(expr, inner_block))
    }

    fn parse_for_new_variable(&mut self, arg_names_extended: &Vec<&str>) -> Result<&'a str, FileParsingError> {
        self.skip_whitespace();
        let t1 = self.p;
        self.skip_normal_word();
        if t1 == self.p {
            return Err(self.new_unexpected_char_error(expected_variable_name));
        }
        let name = &self.text[t1..self.p];
        if is_illegal_name(name) {
            return Err(FileParsingError::new(illegal_variable_name, t1, self.p));
        }
        if name != "_" && arg_names_extended.iter().find(|&&b| b == name).is_some() {
            return Err(FileParsingError::new(forloop_variable_cant_take_occupied_name, t1, self.p));
        }
        Ok(name)
    }

    fn parse_for(&mut self, arg_names: &Vec<&str>) -> Result<SubElement, FileParsingError> {
        let mut arg_names_extended = arg_names.clone();

        let name1 = self.parse_for_new_variable(&arg_names_extended)?;
        arg_names_extended.push(name1);

        let mut name2 = "";
        self.skip_whitespace();
        if self.is_char_ahead(',') {
            self.p += 1;
            name2 = self.parse_for_new_variable(&arg_names_extended)?;
        }
        arg_names_extended.push(name2);

        if !self.is_char_ahead(':'){
            return Err(self.new_unexpected_char_error(
                if name2.len() > 0 { expected_colon } else { expected_comma_or_colon }
            ));
        }
        self.p += 1;

        let expr = self.parse_expression_at_cmd_tag_end(arg_names)?;
        let (inner_block, ending) = self.parse_element_plus_ending_tag(&arg_names_extended)?;
        let separator: String = String::from(match ending.cmd {
            BlockEndingTag::NOGAP => "",
            BlockEndingTag::GAP => " ",
            BlockEndingTag::NORMAL | BlockEndingTag::LF | BlockEndingTag::ENDLOOP => "\n",
            _ => return Err(FileParsingError::new(
                incorrect_block_ending_tag_expected_normal_or_lf_gap_nogap_or_forloop, ending.p1, self.p)),
        });
        Ok(SubElement::For(ForSubElement{iterable: expr, core: inner_block, join: separator}))
    }

    /* Checks for ]} }} and %}. May actually return NoneOfThose */
    fn is_tag_end_ahead(&self) -> bool {
        self.is_ahead("]}") || self.is_ahead("}}") || self.is_ahead("%}")
    }

    /* l1 expression = l2 space l2 space ... space l2 */
    fn parse_expression_l2(&mut self, arg_names: &Vec<&str>) -> Result<Expression, FileParsingError> {
        self.skip_whitespace();
        if self.is_char_ahead('(') {
            let p1 = self.p;
            self.p += 1;
            let expr = self.parse_expression(arg_names)?;
            self.skip_whitespace();
            if !self.is_char_ahead(')') {
                return Err(self.new_unexpected_char_error(expected_closing_round_bracket))
            }
            self.p += 1;
            if matches!(expr, Expression::None){
                return Err(FileParsingError::new(empty_expression_inside_round_brackets, p1, self.p))
            }
            return Ok(expr);
        }
        if self.is_digit_ahead() {
            let p1 = self.p;
            loop {
                if self.is_digit_ahead() {
                    self.p += 1;
                } else if self.is_word_ahead() {
                    return Err(self.new_unexpected_char_error(cant_start_word_immediately_after_digit))
                } else {
                    return match self.text[p1..self.p].parse::<u64>() {
                        Ok(v) => Ok(Expression::Int(v)),
                        Err(_) => Err(FileParsingError::new(integer_parsing_error, p1, self.p)),
                    };
                }
            }
        } else if self.is_word_ahead() {
            let p1 = self.p;
            self.skip_normal_word();
            let toplevel_name = &self.text[p1..self.p];
            if is_bad_name(toplevel_name) {
                return Err(FileParsingError::new(illegal_object_name, p1, self.p));
            }
            let mut bg: Expression = match arg_names.iter().rposition(|&n| n == toplevel_name) {
                Some(i) => Expression::Argument(i as u64),
                None => Expression::Attribute(Box::new(Expression::Root), String::from(toplevel_name))
            };
            loop {
                self.skip_whitespace();
                bg = if self.is_char_ahead('.') {
                    self.p += 1;
                    self.skip_whitespace();
                    let attrp1 = self.p;
                    self.skip_normal_word();
                    if attrp1 == self.p {
                        return Err(self.new_unexpected_char_error(expected_attribute_name_after_dot));
                    }
                    let attr_name = &self.text[attrp1..self.p];
                    if is_bad_name(attr_name) {
                        return Err(FileParsingError::new(illegal_attribute_name, attrp1, self.p));
                    }
                    Expression::Attribute(Box::new(bg), String::from(attr_name))
                } else if self.is_char_ahead('[') {
                    let sqbrp1 = self.p;
                    self.p += 1;
                    let sub_expr = self.parse_expression(arg_names)?;
                    self.skip_whitespace();
                    if !self.is_char_ahead(']') {
                        return Err(self.new_unexpected_char_error(expected_closing_square_bracket))
                    }
                    self.p += 1;
                    if matches!(sub_expr, Expression::None) {
                        return Err(FileParsingError::new(empty_expression_inside_square_brackets, sqbrp1, self.p))
                    }
                    Expression::Get(Box::new(bg), Box::new(sub_expr))
                } else {
                    break
                }
            }
            return Ok(bg)
        } else {
            return Ok(Expression::None)
        }
    }

    fn parse_expression(&mut self, arg_names: &Vec<&str>) -> Result<Expression, FileParsingError> {
        let e1: Expression = self.parse_expression_l2(arg_names)?;
        let mut call_args: Vec<Expression> = Vec::new();
        /* It is okay to enter call_args reading loop even when e1 is None.
         If parse_expression_l2 returned None, subsequent call to parse_expression_l2
         is guaranteed to return None. Arg list will be empty and the final `if` will choose
         the second branch, which will return Expression::None */
        loop {
            let arg_expr: Expression = self.parse_expression_l2(arg_names)?;
            if matches!(arg_expr, Expression::None) {
                break
            }
            call_args.push(arg_expr)
        }
        Ok(if call_args.len() > 0 {
            Expression::Call(Box::new(e1), call_args)
        } else {
            e1
        })
    }
}

/* Parses a file treating it like a package (where other packages and elements could be located) */
pub fn parse_one_file_packed(text: &str) -> Result<Plemege, FileParsingError> {
    let mut parser: Parser = Parser{text, p: 0};
    parser.parse_pack_plus_ending(true)
}

pub fn parse_one_file_simplified(text: &str) -> Result<Plemege, FileParsingError> {
    let mut parser: Parser = Parser{text, p: 0};
    let (el, tt): (Element, ReasonOfElementEnd) = parser.parse_element_plus_ending_tag(&Vec::new())?;
    match tt.cmd {
        BlockEndingTag::EOF => Ok(Plemege::Element(el)),
        BlockEndingTag::END_ELEMENT => Err(FileParsingError::new(unmatched_element_ending_tag, tt.p1, parser.p)),
        _ => Err(FileParsingError::new(unmatched_magic_block_ending_tag, tt.p1, parser.p)),
    }
}