pub fn line_start_byte(source: &[u8], row: usize) -> usize {

    let mut current_row = 0usize;

    let mut line_start = 0usize;

    for (idx, b) in source.iter().enumerate() {

        if current_row == row {

            break;

        }

        if *b == b'\n' {

            current_row += 1;

            line_start = idx + 1;

        }

    line_start

}

fn point_range_at(source: &str, row: usize, column: usize) -> tree_sitter::Range {

    let line_start = line_start_byte(source.as_bytes(), row);

    let byte = line_start + column;

    tree_sitter::Range {

        start_byte: byte,

        end_byte: byte,

        start_point: tree_sitter::Point { row, column },

        end_point: tree_sitter::Point { row, column },

    }

}

fn is_int_keyword_suffix(prefix: &str) -> bool {

    let prefix = prefix.trim_end();

    if !prefix.ends_with("int") {

        return false;

    }

    let bytes = prefix.as_bytes();

    bytes.len() == 3 || {

        let b = bytes[bytes.len() - 4];

        !(b.is_ascii_alphanumeric() || b == b'_')

}

fn int_domain_missing_rparen_line(line: &str, start_col: usize, end_col: usize) -> bool {

    line.as_bytes().get(start_col) == Some(&b'(')

        && line[end_col..].trim().is_empty()

        && !line[start_col..].contains(')')

        && is_int_keyword_suffix(&line[..start_col])

}

fn clamp_range_before_line_comment(range: &mut tree_sitter::Range, source: &str) {

    let Some(line) = source.lines().nth(range.start_point.row) else {

    let Some(dollar_idx) = line.find('$') else {

        return;

    let prefix = &line[..dollar_idx];

    let clamped_col = prefix.trim_end().len();

    if range.start_point.column > clamped_col {

    }

    if range.end_point.row == range.start_point.row && range.end_point.column > clamped_col {

        range.end_point.column = clamped_col;

    }

    if range.end_point.row > range.start_point.row {

    }

    let line_start = line_start_byte(source.as_bytes(), range.start_point.row);

    range.start_byte = line_start + range.start_point.column;

    range.end_byte = line_start + range.end_point.column;

}

pub fn detect_syntactic_errors(

    source: &str,

    tree: &tree_sitter::Tree,

    errors: &mut Vec<RecoverableParseError>,

) {

    let mut malformed_lines_reported = HashSet::new();

    let root_node = tree.root_node();

    let retract: &dyn Fn(&tree_sitter::Node) -> bool = &|node: &tree_sitter::Node| {

        node.is_missing() || node.is_error() || node.start_position() == node.end_position()

    };

    for node in WalkDFS::with_retract(&root_node, &retract) {

        if node.start_position() == node.end_position() {

            errors.push(classify_missing_token(node, source));

            continue;

        }

        if node.is_error() {

            let line = node.start_position().row;

            if malformed_lines_reported.contains(&line) {

            }

            if let Some(line_str) = source.lines().nth(line)

                && let Some(dollar_idx) = line_str.find('$')

                && node.start_position().column >= dollar_idx

            }

            if let Some(missing_expression) = classify_such_that_missing_expression(&node, source) {

            }

            if is_malformed_line_error(&node, source) {

                malformed_lines_reported.insert(line);

                let start_byte = node.start_byte();

                let end_byte = node.end_byte();

                let last_char = source.lines().nth(line).map_or(0, |l| l.len());

                errors.push(RecoverableParseError::new(

                    generate_malformed_line_message(line, source),

                    Some(tree_sitter::Range {

                        start_byte,

                        end_byte,

                        start_point: tree_sitter::Point {

                            row: line,

                            column: 0,

                        },

                        end_point: tree_sitter::Point {

                            row: line,

                            column: last_char,

                        },

                    }),

                ));

                continue;

                if let Some(missing_rparen) = classify_int_domain_missing_rparen(&node, source) {

                    errors.push(missing_rparen);

                    continue;

                }

                errors.push(classify_unexpected_token_error(node, source));

            continue;

        }

}

fn classify_int_domain_missing_rparen(

    node: &tree_sitter::Node,

    source: &str,

) -> Option<RecoverableParseError> {

    let start = node.start_position();

    let end = node.end_position();

    let line = source.lines().nth(start.row)?;

    let comment_col = line.find('$').unwrap_or(line.len());

    let line = &line[..comment_col];

    let start_col = start.column.min(line.len());

    let end_col = end.column.min(line.len());

    if !int_domain_missing_rparen_line(line, start_col, end_col) {

        return None;

    }

    let insertion_col = line.trim_end().len();

    Some(RecoverableParseError::new(

        "Missing )".to_string(),

        Some(point_range_at(source, start.row, insertion_col)),

    ))

}

fn classify_such_that_missing_expression(

    node: &tree_sitter::Node,

    source: &str,

) -> Option<RecoverableParseError> {

    let line = source.lines().nth(node.start_position().row)?;

    if source[node.start_byte()..node.end_byte()].trim() != "such that" {

        return None;

}

fn classify_missing_token(node: Node, source: &str) -> RecoverableParseError {

    let mut range = tree_sitter::Range {

        start_byte: node.start_byte(),

        end_byte: node.end_byte(),

        start_point: node.start_position(),

        end_point: node.end_position(),

    };

    clamp_range_before_line_comment(&mut range, source);

    let message = if let Some(parent) = node.parent() {

        match parent.kind() {

            "letting_variable_declaration" => "Missing Expression or Domain".to_string(),

            _ => format!("Missing {}", user_friendly_token_name(node.kind(), false)),

    RecoverableParseError::new(message, Some(range))

}

fn classify_unexpected_token_error(node: Node, source_code: &str) -> RecoverableParseError {

    let mut range = tree_sitter::Range {

        start_byte: node.start_byte().min(source_code.len()),

        end_byte: node.end_byte().min(source_code.len()),

        start_point: node.start_position(),

        end_point: node.end_position(),

    };

    clamp_range_before_line_comment(&mut range, source_code);

    let message = if let Some(parent) = node.parent() {

        let src_token: std::borrow::Cow<'_, str> = source_code

            .as_bytes()

            .get(range.start_byte..range.end_byte)

            .map(String::from_utf8_lossy)

        let src_token = src_token.trim_end();

        if parent.kind() == "program" {

            format!("Unexpected {}", src_token)

            format!(

                src_token,

                user_friendly_token_name(parent.kind(), true)

    RecoverableParseError::new(message, Some(range))

}

pub fn is_malformed_line_error(node: &tree_sitter::Node, source: &str) -> bool {

    let parent = node.parent();

    let grandparent = parent.and_then(|n| n.parent());

    let root = grandparent.and_then(|n| n.parent());

    if let (Some(parent), Some(grandparent), Some(root)) = (parent, grandparent, root)

        && parent.kind() == "set_comparison"

    }

    let mut curr = node.parent();

    while let Some(n) = curr {

        let kind = n.kind();

        if matches!(

            kind,

            return false;

        }

        curr = n.parent();

    let line = source.lines().nth(node.start_position().row).unwrap_or("");

    let first_non_witespace = line

        .as_bytes()

        .iter()

        .take_while(|b| b.is_ascii_whitespace())

    if node.start_position().column <= first_non_witespace || error_node_out_of_range(node, source)

        if first_non_witespace > 0 && is_constraint_continuation(source, node.start_position().row)

        }

        return true;

    }

    false

}

fn is_constraint_continuation(source: &str, row: usize) -> bool {

    let lines: Vec<&str> = source.lines().collect();

    if row == 0 {

        return false;

}

fn user_friendly_token_name(token: &str, article: bool) -> String {

    let capitalized = if token.contains("atom") {

        "Expression".to_string()

    } else if token == "COLON" {

        ":".to_string()

        let friendly_name = token

            .replace("literal", "")

            .replace("int", "Integer")

            .replace("expr", "Expression")

            .replace('_', " ");

        friendly_name

            .split_whitespace()

            .map(|word| word.capitalize())

            .join(" ")

    if !article {

        return capitalized;

    }

    let first_char = capitalized.chars().next().unwrap();

    let article = match first_char.to_ascii_lowercase() {

        'a' | 'e' | 'i' | 'o' | 'u' => "an",

        _ => "a",

    format!("{} {}", article, capitalized)

}

fn generate_malformed_line_message(line: usize, source: &str) -> String {

    let got = source.lines().nth(line).unwrap_or("").trim();

    let got = got.split('$').next().unwrap_or("").trim_end();

    let got = got.replace('"', "\\\"");

    let mut words = got.split_whitespace();

    let first = words.next().unwrap_or("").to_ascii_lowercase();

    let second = words.next().unwrap_or("").to_ascii_lowercase();

    let expected = match first.as_str() {

        "find" => "a find declaration statement",

        "letting" => "a letting declaration statement",

        "given" => "a given declaration statement",

        "where" => "an instantiation condition",

        "minimising" | "maximising" => "an objective statement",

        "such" if second == "that" => "a constraint statement",

        "such" => "a valid top-level statement",

            "a valid top-level statement"

    format!("Expected {}, but got '{}'", expected, got)

}

fn error_node_out_of_range(node: &tree_sitter::Node, source: &str) -> bool {

    let lines: Vec<&str> = source.lines().collect();

    let start = node.start_position();

    let end = node.end_position();

    let start_line_len = lines.get(start.row).map_or(0, |l| l.len());

    let end_line_len = lines.get(end.row).map_or(0, |l| l.len());

    (start.column > start_line_len) || (end.column > end_line_len)

}

    fn assert_essence_parse_error_eq(a: &RecoverableParseError, b: &RecoverableParseError) {

        assert_eq!(a.msg, b.msg, "error messages differ");

        assert_eq!(a.range, b.range, "error ranges differ");

    }

    fn malformed_line() {

        let source = " a,a,b: int(1..3)";

        let (tree, _) = get_tree(source).expect("Should parse");

        let root_node = tree.root_node();

        let error_node = WalkDFS::with_retract(&root_node, &|_node| false)

            .find(|node| node.is_error())

            .expect("Should find an error node");

        assert!(is_malformed_line_error(&error_node, source));

    }

    fn malformed_find_message() {

        let source = "find >=lex,b,c: int(1..3)";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a find declaration statement, but got 'find >=lex,b,c: int(1..3)'"

    }

    fn malformed_top_level_message() {

        let source = "a >=lex,b,c: int(1..3)";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a valid top-level statement, but got 'a >=lex,b,c: int(1..3)'"

    }

    fn malformed_objective_message() {

        let source = "maximising %x";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected an objective statement, but got 'maximising %x'"

    }

    fn malformed_letting_message() {

        let source = "letting % A be 3";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a letting declaration statement, but got 'letting % A be 3'"

    }

    fn malformed_constraint_message() {

        let source = "such that % A > 3";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a constraint statement, but got 'such that % A > 3'"

    }

    fn malformed_top_level_message_2() {

        let source = "such % A > 3";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a valid top-level statement, but got 'such % A > 3'"

    }

    fn malformed_given_message() {

        let source = "given 1..3)";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected a given declaration statement, but got 'given 1..3)'"

    }

    fn malformed_where_message() {

        let source = "where x>6";

        let message = super::generate_malformed_line_message(0, source);

        assert_eq!(

            message,

            "Expected an instantiation condition, but got 'where x>6'"

    }

    fn user_friendly_token_name_article() {

        assert_eq!(

            user_friendly_token_name("int_domain", false),

            "Integer Domain"

        assert_eq!(

            user_friendly_token_name("int_domain", true),

            "an Integer Domain"

        assert_eq!(user_friendly_token_name("COLON", false), ":");

    }

    fn missing_domain() {

        let source = "find x:";

        let (tree, _) = get_tree(source).expect("Should parse");

        let mut errors = vec![];

        detect_syntactic_errors(source, &tree, &mut errors);

        assert_eq!(errors.len(), 1, "Expected exactly one diagnostic");

        let error = &errors[0];

        assert_essence_parse_error_eq(

            error,

            &RecoverableParseError::new(

                "Missing Domain".to_string(),

                Some(tree_sitter::Range {

                    start_byte: 7,

                    end_byte: 7,

                    start_point: tree_sitter::Point { row: 0, column: 7 },

                    end_point: tree_sitter::Point { row: 0, column: 7 },

                }),

            ),

        );

    }

    fn line_start_byte_returns_correct_offsets() {

        let source = "a\nbc\ndef";

        let bytes = source.as_bytes();

        assert_eq!(line_start_byte(bytes, 0), 0);

        assert_eq!(line_start_byte(bytes, 1), 2);

        assert_eq!(line_start_byte(bytes, 2), 5);

    }

    fn point_range_at_returns_correct_zero_length_range() {

        let source = "a\nbc\ndef";

        let range = point_range_at(source, 1, 1); // points to 'c'

        assert_eq!(range.start_point.row, 1);

        assert_eq!(range.start_point.column, 1);

        assert_eq!(range.end_point, range.start_point);

        assert_eq!(range.start_byte, 3);

        assert_eq!(range.end_byte, 3);

    }

    fn clamp_range_before_line_comment_clamps_end_to_before_dollar() {

        let source = "find x: int(1..3 $comment";

        let mut range = tree_sitter::Range {

            start_byte: 0,

            end_byte: source.len(),

            start_point: tree_sitter::Point { row: 0, column: 0 },

            end_point: tree_sitter::Point {

                row: 0,

                column: source.len(),

            },

        };

        clamp_range_before_line_comment(&mut range, source);

        assert_eq!(range.end_point.row, 0);

        assert_eq!(range.end_point.column, 16);

        assert_eq!(range.end_byte, 16);

    }

    fn int_keyword_suffix_checks_word_boundary() {

        assert!(is_int_keyword_suffix("find x: int"));

        assert!(!is_int_keyword_suffix("foo"));

        assert!(!is_int_keyword_suffix("mint"));

    }

    fn int_domain_missing_rparen_line_positive_and_negative_cases() {

        let ok = "find x: int(1..2";

        let start = ok.find('(').unwrap();

        assert!(int_domain_missing_rparen_line(ok, start, ok.len()));

        let has_rparen = "find x: int(1..2)";

        let start = has_rparen.find('(').unwrap();

        assert!(!int_domain_missing_rparen_line(

            has_rparen,

            start,

            has_rparen.len()

        ));

        let trailing = "find x: int(1..2 foo";

        let start = trailing.find('(').unwrap();

        let end = trailing.find(" foo").unwrap();

        assert!(!int_domain_missing_rparen_line(trailing, start, end));

        let print_like = "find x: print(1..2";

        let start = print_like.find('(').unwrap();

        assert!(!int_domain_missing_rparen_line(

            print_like,

            start,

            print_like.len()

        ));

    }