rustla/parser/

mod.rs

/*!
This is the `parser` module of ruSTLa.
It contains the `Parser` type and the `state_machine` submodule,
that hosts the state transition functions of different states.

Copyright © 2020 Santtu Söderholm
*/
// =========
//  Imports
// =========

// Standard library
use std::cmp;

use std::collections;
use std::str;

// External crates
// ---------------
use regex::Regex;

// Own modules
// -----------

use super::*;
use crate::parser::types_and_aliases::IndentedBlockResult;


pub mod automata;
mod regex_patterns;

mod converters;

pub mod types_and_aliases;
use types_and_aliases::*;

pub mod line_cursor;
use line_cursor::{Line, LineCursor};

pub mod state_machine;
use state_machine::{State, COMPILED_INLINE_TRANSITIONS};

pub mod directive_parsers;
mod table_parsers;

use crate::common::{
    EnumDelims, EnumKind, FootnoteKind, ParsingResult, SectionLineStyle,
};
use crate::parser::regex_patterns::Pattern;
use crate::doctree::tree_node::TreeNode;
use crate::doctree::tree_node_types::TreeNodeType;
use crate::doctree::DocTree;

mod tests;

// ==========================
//  The Parser specification
// ==========================

/// The parser type. Contains an optional
/// source line vector and a document tree.
/// These are optional to facilitate their passing
/// to any transitions functions via
/// `std::option::Option::take`
/// without invalidating the fields.
pub struct Parser {

    /// The source `String` converted to a vector of owned `String`s.
    src_lines: Vec<String>,

    /// The absolute line index of src_lines.
    line_cursor: LineCursor,

    /// The level of basic indentation that the parser is working with.
    /// This is useful information during nested parsing sessions, where
    /// the level of indentation of the incoming block of text to be parsed
    /// needs to be passed to the nested parser for node comparison.
    base_indent: usize,

    /// Keeps track of the section level the parser is currently focused on.
    /// Level 0 indicates document root.
    section_level: usize,

    /// An `Option`al document tree. The optionality is necessary,
    /// as this needs to be given to transition functions for modification
    /// via `Option::take`.
    doctree: Option<DocTree>,

    /// A stack of states that function as keys to vectors of state transitions.
    /// The set of transitios is chosen based on the current state on top of the stack.
    state_stack: Vec<State>,
}

impl Parser {
    /// The `Parser` constructor. Transforms a given source string
    /// into a vector of lines and wraps this and a given `DocTree`
    /// in `Option`s. This wrapping allows the passing of these to owned
    /// state machnes via swapping the optional contents
    /// to `None` before granting ownership of the original contents.
    pub fn new(
        src: Vec<String>,
        doctree: DocTree,
        base_indent: usize,
        base_line: Line,
        initial_state: State,
        section_level: usize,
    ) -> Self {
        Self {
            src_lines: src, //.lines().map(|s| s.to_string()).collect::<Vec<String>>(),
            line_cursor: LineCursor::new(0, base_line),
            base_indent: base_indent,
            section_level: section_level,
            doctree: Some(doctree),
            state_stack: vec![initial_state],
        }
    }

    /// Starts the parsing process for a single file.
    /// Returns the `DocTree` generated by the `StateMachine`s.
    pub fn parse(&mut self) -> ParsingResult {
        // println!("=====================\n Initiating parse...\n=====================\n");

        // eprintln!("... with base indent {:#?} on line {:#?}\n", self.base_indent, self.line_cursor.sum_total());

        let mut line_changed: bool = false;
        let mut line_not_changed_count: u32 = 0;

        // The parsing loop
        loop {
            // eprintln!("Section level: {:#?}", self.section_level);
            // eprintln!("Line {:#?} state stack: {:#?}\n", self.line_cursor.sum_total(), self.state_stack);
            // eprintln!("Focused on {:#?}\n", self.doctree.as_ref().unwrap().shared_node_data());

            if ! line_changed && line_not_changed_count >= 10 {
                return ParsingResult::Failure {
                    message: format!("Line not advanced even after {} iterations of the parsing loop on line {}. Clearly something is amiss...", line_not_changed_count, self.line_cursor.sum_total()),
                    doctree: if let Some(doctree) = self.doctree.take() {
                        doctree
                    } else {
                        panic!(
                            "Doctree lost during parsing process around line {}. Computer says no...",
                            self.line_cursor.sum_total()
                        )
                    }
                };
            }

            line_changed = false;

            let mut match_found = false;

            // Retrieving a clone of the transitions stored in the latest state
            // A clone is needed because the below for loop takes
            // ownership of a reference given to it, which would prevent us from
            // modifying the machine stack.
            let latest_state_transitions = if let Some(machine) = self.state_stack.last() {
                match machine {
                    State::EOF => {
                        match self.doctree.take() {
                            Some(doctree) => {
                                return ParsingResult::EOF {
                                    doctree: doctree,
                                    state_stack: self
                                        .state_stack
                                        .drain(..self.state_stack.len() - 1)
                                        .collect(),
                                }
                            }
                            None => {
                                panic!("Tree should not be in the possession of a transition method after moving past EOF...")
                            }
                        };
                    }

                    State::Failure { .. } => {
                        return ParsingResult::Failure {
                            message: String::from("Parsing ended in Failure state...\n"),
                            doctree: if let Some(doctree) = self.doctree.take() {
                                doctree
                            } else {
                                panic!("Lost doctree inside parsing function before line {}. Computer says no...", self.line_cursor.sum_total())
                            },
                        }
                    }

                    _ => {
                        if let Ok(transitions_ref) = machine.get_transitions(&self.line_cursor) {
                            transitions_ref
                        } else {
                            return ParsingResult::Failure {
                                message: String::from("No transitions for this state...\n"),
                                doctree: if let Some(doctree) = self.doctree.take() {
                                    doctree
                                } else {
                                    panic!("Lost doctree inside parsing function before line {}. Computer says no...", self.line_cursor.sum_total())
                                },
                            };
                        }
                    }
                }
            } else {
                return ParsingResult::EmptyStateStack {
                    state_stack: Vec::new(),
                    doctree: if let Some(doctree) = self.doctree.take() {
                        doctree
                    } else {
                        panic!("Doctree lost during parsing process around line {}. Computer says no...", self.line_cursor.sum_total())
                    },
                };
            };

            // Iterating over a clone of the transitions
            for (pattern_name, regex, method) in latest_state_transitions.iter() {
                // Fetching a reference to current line
                let src_line: &str = match Parser::get_source_from_line(&self.src_lines, self.line_cursor.relative_offset()) {
                    Some(line) => line,
                    None => {
                        return ParsingResult::Failure {
                            message: String::from("Parsing ended prematurely because of an unqualified move past EOF..."),
                            doctree: if let Some(doctree) = self.doctree.take() { doctree } else {
                                panic!(
                                    "Lost doctree inside parsing function before line {}. Computer says no...",
                                    self.line_cursor.sum_total()
                                )
                            }
                        }
                    }
                };

                // Running the current line of text through a DFA compiled from a regex
                if let Some(captures) = regex.captures(src_line) {
                    // eprintln!("Found match for {:?}...\n", pattern_name);

                    match_found = true;

                    // eprintln!("Match: {:#?}", captures.get(0).unwrap().as_str());
                    // eprintln!("Executing transition method...\n");

                    let line_before_transition = self.line_cursor.sum_total();

                    let doctree = if let Some(doctree) = self.doctree.take() {
                        doctree
                    } else {
                        panic!(
                            "Doctree lost inside transition function around line {}? Computer says no...",
                            self.line_cursor.sum_total()
                        )
                    };
                    self.doctree = match method(
                        &self.src_lines,
                        self.base_indent,
                        &mut self.section_level,
                        &mut self.line_cursor,
                        doctree,
                        &captures,
                        pattern_name,
                    ) {
                        TransitionResult::Success {
                            doctree,
                            push_or_pop,
                            line_advance,
                        } => {
                            match push_or_pop {
                                PushOrPop::Push(mut states) => {
                                    self.state_stack.append(&mut states);
                                }
                                PushOrPop::Pop => {
                                    match self.state_stack.pop() {
                                        Some(machine) => (),
                                        None => {
                                            return ParsingResult::Failure {
                                                message: String::from(
                                                    "Can't pop from empty stack...\n",
                                                ),
                                                doctree: if let Some(doctree) = self.doctree.take() {
                                                    doctree
                                                } else {
                                                    panic!("Lost doctree inside parsing function before line {}. Computer says no...", self.line_cursor.sum_total())
                                                },
                                            }
                                        }
                                    };
                                }

                                PushOrPop::Neither => {} // No need to do anything to the stack...
                            };

                            if let LineAdvance::Some(offset) = line_advance {
                                self.line_cursor.increment_by(offset);
                            }

                            // Incrementing the line_not_changed counter, if match was found but no incrementing occurred
                            if self.line_cursor.sum_total() == line_before_transition {
                                line_not_changed_count += 1;
                            } else {
                                line_changed = true;
                                line_not_changed_count = 0;
                            }

                            Some(doctree)
                        }

                        TransitionResult::Failure { message, doctree } => {
                            return ParsingResult::Failure {
                                message: message,
                                doctree: doctree,
                            }
                        }
                    };

                    break; // Match found so stop looking for matches
                }
            }

            // No matches in current state so pop from state stack and attempt
            // parsing in the previous state down stack
            if ! match_found {
                if let None = self.state_stack.pop() {
                    return ParsingResult::EmptyStateStack {
                        doctree: self.doctree.take().unwrap(),
                        state_stack: self.state_stack.drain(..self.state_stack.len()).collect(),
                    };
                };
                if let Some(doctree) = self.doctree.take() {
                    self.doctree = Some(doctree.focus_on_parent());
                } else {
                    return ParsingResult::Failure {
                        message: format!(
                            "Doctree in possession of transition method after transition on line {}. Computer says no...",
                            self.line_cursor.sum_total()
                        ),
                        doctree: if let Some(doctree) = self.doctree.take() { doctree } else {
                            panic!(
                                "Lost doctree inside parsing function before line {}. Computer says no...",
                                self.line_cursor.sum_total()
                            )
                        }
                    };
                }
            }

            if self.line_cursor.relative_offset() >= self.src_lines.len() {
                self.state_stack.push(State::EOF);
            }
        }
    }

    /// Attempts to move `self.current_line` to the given index.
    /// Return an `Err` if not successful.
    fn jump_to_line(&mut self, line: usize) -> Result<(), &'static str> {
        if line < self.src_lines.len() {
            *self.line_cursor.relative_offset_mut_ref() = line;
        } else {
            return Err("Attempted a move to a non-existent line.\nComputer says  no...\n");
        }

        Ok(())
    }

    /// Attempts to increment `self.current_line` by `n`.
    /// Returns nothing if successful, otherwise returns `Err(&str)`.
    /// The called must handle the `Err` case.
    fn nth_next_line(&mut self, n: usize) -> Result<(), &'static str> {
        *self.line_cursor.relative_offset_mut_ref() =
            match self.line_cursor.relative_offset().checked_add(n) {
                Some(value) => value,
                None => {
                    return Err("Attempted indexing with integer overflow.\nComputer says no...\n")
                }
            };

        if self.line_cursor.relative_offset() > self.src_lines.len() {
            return Err("No such line number.\nComputer says no...\n");
        }

        Ok(())
    }

    /// Attempts to decrement `self.current_line` by `n`.
    /// Returns nothing if successful, otherwise returns `Err(&str)`.
    /// The called must handle the `Err` case.
    fn nth_previous_line(&mut self, n: usize) -> Result<(), &'static str> {
        *self.line_cursor.relative_offset_mut_ref() =
            match self.line_cursor.relative_offset().checked_sub(n) {
                Some(value) => value,
                None => {
                    return Err("Attempted indexing with integer overflow. Computer says no...")
                }
            };

        if self.line_cursor.relative_offset() > self.src_lines.len() {
            return Err("No such line number. Computer says no...");
        }

        Ok(())
    }

    /// Attempts to retrieve the source from a given line number.
    /// Returns an `Ok` clone of it if successful, else
    /// returns and `Err` with a message.
    fn get_source_from_line<'src_lines>(src_lines: &Vec<String>, line_num: usize) -> Option<&str> {
        let src = match src_lines.get(line_num) {
            Some(line) => line.as_str(),
            None => {
                eprintln!(
                    "No such line number ({} out of bounds). Computer says no...",
                    line_num
                );
                return None;
            }
        };

        Some(src)
    }

    /// A function that parses inline text. Returns the nodes generated,
    /// if there are any.
    fn inline_parse(
        inline_src_block: String,
        mut doctree: Option<&mut DocTree>,
        line_cursor: &mut LineCursor,
    ) -> InlineParsingResult {
        let mut nodes_data: Vec<TreeNodeType> = Vec::new();

        let mut col: usize = 0;

        let src_chars = &mut inline_src_block.chars();

        loop {
            match Parser::match_inline_str(&mut doctree, &src_chars) {
                Some((mut node_data, offset)) => {
                    nodes_data.append(&mut node_data);

                    // Move iterator to start of next possible match
                    for _ in 0..offset {
                        if let Some(c) = src_chars.next() {
                            col += 1;
                            if c == '\n' {
                                line_cursor.increment_by(1);
                                col = 0;
                            }
                        } else {
                            break;
                        }
                    }
                }

                // No match.
                // This should not happen, as plain text should always be usable as a last resort.
                // Return with no nodes if this should occur.
                None => break,
            }
        }

        if nodes_data.is_empty() {
            return InlineParsingResult::NoNodes;
        } else {
            return InlineParsingResult::Nodes(nodes_data);
        }
    }

    /// A function for checking the string representation of
    /// a given `Chars` iterator for a regex match and executing
    /// the corresponding parsing method. Returns the `Option`al
    /// generated node if successful, otherwise returns with `None`.
    fn match_inline_str<'chars>(
        opt_doctree_ref: &mut Option<&mut DocTree>,
        chars_iter: &'chars str::Chars,
    ) -> Option<(Vec<TreeNodeType>, usize)> {
        let src_str = chars_iter.as_str();
        if src_str.is_empty() {
            return None;
        }
        for (pattern_name, regexp, parsing_function) in COMPILED_INLINE_TRANSITIONS.iter() {
            match regexp.captures(src_str) {
                Some(capts) => {
                    let (node_type, offset) =
                        parsing_function(opt_doctree_ref, *pattern_name, &capts);
                    return Some((node_type, offset));
                }
                None => continue, // no match, do nothing
            };
        }
        None
    }

    /// Parses the first block of a node, in case it contains body level nodes
    /// right after a marker such as an enumerator, on the same line.
    fn parse_first_node_block(
        doctree: DocTree,
        src_lines: &Vec<String>,
        base_indent: usize,
        line_cursor: &mut LineCursor,
        text_indent: usize,
        first_indent: Option<usize>,
        start_state: State,
        section_level: &mut usize,
        force_alignment: bool,
    ) -> Result<(ParsingResult, usize), ParsingResult> {
        let relative_first_indent = first_indent.unwrap_or(text_indent) - base_indent;
        let relative_block_indent = text_indent - base_indent;

        // Read indented block here. Notice we need to subtract base indent from assumed indent for this to work with nested parsers.
        let (block, line_offset) = match Parser::read_indented_block(
            src_lines,
            line_cursor.relative_offset(),
            true,
            true,
            Some(relative_block_indent),
            Some(relative_first_indent),
            force_alignment,
        ) {
            IndentedBlockResult::Ok {lines, minimum_indent, offset, blank_finish } => (lines, offset),
            _ => {
                return Err(ParsingResult::Failure {
                    message: format!(
                        "Error when reading in a block of text for nested parse in line {}.",
                        line_cursor.sum_total()
                    ),
                    doctree: doctree,
                })
            }
        };

        // Run a nested `Parser` over the first indented block with base indent set to `text_indent`.
        match Parser::new(
            block,
            doctree,
            text_indent,
            line_cursor.sum_total(),
            start_state,
            *section_level,
        ).parse() {
            ParsingResult::EOF {
                doctree,
                state_stack,
            } => {
                return Ok((
                    ParsingResult::EOF {
                        doctree: doctree,
                        state_stack: state_stack,
                    },
                    line_offset,
                ))
            }
            ParsingResult::EmptyStateStack {
                doctree,
                state_stack,
            } => {
                return Ok((
                    ParsingResult::EmptyStateStack {
                        doctree: doctree,
                        state_stack: state_stack,
                    },
                    line_offset,
                ))
            }
            ParsingResult::Failure { message, doctree } => {
                return Err(ParsingResult::Failure {
                    message: format!("Nested parse ended in failure: {}", message),
                    doctree: doctree,
                })
            }
        };
    }

    /// Skips empty lines until a non-empty one is found.
    /// If the end of input is encountered, returns `None`, else returns `Some(())`.
    fn skip_to_next_block(src_lines: &Vec<String>, line_cursor: &mut LineCursor) -> Option<()> {
        loop {
            if let Some(line) = src_lines.get(line_cursor.relative_offset()) {
                if line.trim().is_empty() {
                    line_cursor.increment_by(1);
                } else {
                    break Some(())
                }
            } else {
                break None
            }
        }
    }

    /// Reads in an contiguous set of lines of text.
    /// A text block in rST terms is a set of lines
    /// separated from other elements by empty lines above and below.
    /// Checks for indentation:
    /// if indentation is not allowed but indentation is found,
    /// returns an error message in an `Err`.
    fn read_text_block(
        src_lines: &Vec<String>,
        start_line: usize,
        indent_allowed: bool,
        remove_indent: bool,
        alignment: Option<usize>,
        until_blank: bool
    ) -> TextBlockResult {

        let mut line_num = start_line;
        let last_line = src_lines.len();
        let mut lines: Vec<String> = Vec::with_capacity(last_line - start_line);

        while line_num < last_line {
            let mut line: String = match src_lines.get(line_num) {
                Some(line) => line.clone(),
                None => return TextBlockResult::Err {
                    offset: {
                        lines.shrink_to_fit();
                        lines.len()
                    },
                    lines: lines,
                }
            };

            if line.trim().is_empty() {
                if until_blank {
                    break
                } else {
                    lines.push(line.trim().to_string());
                    line_num += 1;
                    continue
                }
            }

            let line_indent = line
                .as_str()
                .chars()
                .take_while(|c| c.is_whitespace())
                .count();

            if !indent_allowed && line_indent > 0 {
                break;
            }

            if let Some(alignment) = alignment {
                if alignment != line_indent {
                    break;
                }
            }

            if remove_indent {
                line = line.as_str().trim_start().to_string();
            }

            lines.push(line);
            line_num += 1;
        }

        lines.shrink_to_fit();
        let offset = lines.len();
        TextBlockResult::Ok {
            lines: lines,
            offset: offset
        }
    }

    /// Reads in a block of indented lines text.
    /// Determines the minimum level of indentation
    /// and uses it as a reference for ending the block.
    fn read_indented_block(
        src_lines: &Vec<String>,
        start_line: usize,
        until_blank: bool,
        strip_indent: bool,
        block_indent: Option<usize>,
        first_indent: Option<usize>,
        force_alignment: bool,
    ) -> IndentedBlockResult {

        if src_lines.is_empty() {
            return IndentedBlockResult::EmptyLinesErr
        }

        let mut line_num = start_line;
        let last_line_num = src_lines.len();

        let mut block_lines: Vec<String> = Vec::with_capacity(last_line_num - start_line);

        // Setting the initial level of minimal indentation
        let mut minimal_indent = match block_indent {
            Some(indent) => Some(indent),
            None => None,
        };

        // If there is block indentation but no predetermined indentation for the first line,
        // set the indentation of the first line equal to block indentation.
        let first_indent = if let (Some(block_indent), None) = (block_indent, first_indent) {
            Some(block_indent)
        } else {
            first_indent
        };

        // Push first line into `block_lines` and increment
        // line number to ignore indentation (for now) if first_indent was set
        if first_indent.is_some() {
            let line = src_lines.get(line_num).unwrap().to_owned();
            block_lines.push(line);
            line_num += 1;
        }

        let mut blank_finish: bool = false;
        let mut loop_broken: bool = false; // Used to detect whether the below while loop was broken out of

        while line_num < last_line_num {
            let line: String = match src_lines.get(line_num) {
                Some(line) => line.clone(),
                None => {
                    loop_broken = true;
                    break
                }
            };

            let line_is_empty = line.trim().is_empty();

            // Check for sufficient (or correct if block alignment was forced) indentation if line isn't empty
            let line_indent = line
                .as_str()
                .chars()
                .take_while(|c| c.is_whitespace())
                .count();

            let break_when_not_aligned: bool = if block_indent.is_some() && force_alignment {
                line_indent != block_indent.unwrap()
            } else if block_indent.is_some() {
                line_indent < block_indent.unwrap()
            } else {
                false
            };

            if !line_is_empty && (line_indent < 1 || break_when_not_aligned) {
                // Ended with a blank finish if the last line before unindent was blank
                blank_finish = (line_num > start_line)
                    && src_lines.get(line_num - 1).unwrap().trim().is_empty();
                loop_broken = true;
                break;
            }

            // Updating the minimal level of indentation, if line isn't blank
            // and there isn't predetermined block indentation
            if line_is_empty && until_blank {
                blank_finish = true;
                break;
            } else if block_indent.is_none() {
                if minimal_indent.is_none() {
                    minimal_indent = Some(line_indent);
                } else if line_indent > 0 {
                    minimal_indent = Some(cmp::min(minimal_indent.unwrap(), line_indent));
                }
            }

            block_lines.push(line);
            line_num += 1;
        }

        if !loop_broken {
            blank_finish = true;
        } // Made it to the end of input

        // Strip all minimal indentation from each line
        if let Some(min_indent) = minimal_indent {
            if strip_indent {
                for (index, line) in block_lines.iter_mut().enumerate() {
                    let indent = if first_indent.is_some() && index == 0 {
                        first_indent.unwrap()
                    } else {
                        min_indent
                    };
                    *line = line.chars().skip(indent).collect::<String>();
                }
            }
        }

        block_lines.shrink_to_fit(); // Free unnecessary used memory
        let line_diff = block_lines.len();

        IndentedBlockResult::Ok {
            lines: block_lines,
            minimum_indent: minimal_indent.unwrap(),
            offset: line_diff,
            blank_finish: blank_finish
        }
    }

    /// Checks how a given indentation matches with the indentation of a given parent node type.
    fn parent_indent_matches(
        parent: &TreeNodeType,
        relevant_child_indent: usize,
    ) -> IndentationMatch {
        if let Some(indent) = parent.body_indent() {
            if indent > relevant_child_indent {
                IndentationMatch::TooLittle
            } else if indent == relevant_child_indent {
                IndentationMatch::JustRight
            } else {
                IndentationMatch::TooMuch
            }
        } else {
            panic!("Asked for parent indentation inside a \"{}\" that is not a container. Computer says no...", parent)
        }
    }

    /// Scans the source lines until it finds a non-empty line and returns the `Option`al indent of it.
    fn indent_on_subsequent_lines(
        src_lines: &Vec<String>,
        mut current_line: usize,
    ) -> Option<(usize, usize)> {
        loop {
            if let Some(line) = src_lines.get(current_line + 1) {
                if line.trim().is_empty() {
                    current_line += 1;
                    continue;
                } else {
                    break Some((
                        line.chars().take_while(|c| c.is_whitespace()).count(),
                        current_line - current_line,
                    ));
                }
            } else {
                break None;
            }
        }
    }

    /// Takes characters from a given string, until a given index, or until a newline is encountered.
    fn line_prefix(line: &str, end_index: usize) -> String {
        let prefix = line
            .chars()
            .enumerate()
            .take_while(|(i, c)| *c == '\n' || *i < end_index)
            .map(|(i, c)| c)
            .collect::<String>();

        if prefix.chars().count() < end_index {
            eprintln!("Encountered a newline or line shorter than given...")
        }

        prefix
    }

    /// Skips the first `start_index` characters of the given `line`
    /// and returns the remainder as a string. If a new
    fn line_suffix(line: &str, start_index: usize) -> String {
        let suffix_len = match line.chars().count().checked_sub(start_index) {
            Some(len) => len,
            None => {
                panic!("Cannot scan line suffix whose start index is greater than the line length")
            }
        };

        let suffix = line
            .chars()
            .enumerate()
            .skip_while(|(i, c)| *c == '\n' || *i < start_index)
            .map(|(i, c)| c)
            .collect::<String>();

        if suffix.chars().count() > suffix_len {
            eprintln!("Encountered a newline before reaching suffix. Returning an empty string...");
            String::new()
        } else {
            suffix
        }
    }

    /// Increments the given line cursor while empty lines are found.
    /// Returns the number of lines skipped.
    fn skip_empty_lines(src_lines: &Vec<String>, line_cursor: &mut LineCursor) -> usize {
        let mut lines_skipped = 0 as usize;

        while let Some(line) = src_lines.get(line_cursor.relative_offset()) {
            if line.trim().is_empty() {
                line_cursor.increment_by(1); // Jump over empty lines
                lines_skipped += 1;
            } else {
                break;
            }
        }

        lines_skipped
    }

    /// Returns an indentation level based on the indentation of the next line.
    /// If the next line is empty or `None` (at the end of input),
    /// returns the given indent after a markup (footnote, citation, field list)
    /// marker. If the line is *not* empty, scans it for indentation and if it is greater than
    /// the given marker indent, returns it as is. Otherwise returns the indentation after the marker.
    ///
    /// This is mainly useful with markup elements like footnotes, citations and field list items,
    /// that decide their body indentation based on the line directly after their respecive markup marker.
    fn indent_from_next_line (
        src_lines: &Vec<String>,
        base_indent: usize,
        marker_indent: usize,
        indent_after_marker: usize,
        line_cursor: &LineCursor
    ) -> usize {
        match src_lines.get(line_cursor.relative_offset() + 1) {
            Some(line) => if line.trim().is_empty() {
                indent_after_marker
            } else {
                let indent = line
                    .chars()
                    .take_while(|c| c.is_whitespace())
                    .count() + base_indent;
                if indent < marker_indent + 1 {
                    indent_after_marker
                } else {
                    indent
                }
            }
            None => indent_after_marker
        }
    }
}