query-engine/src/frontend/language.rs

//! Minimal command language for the query-engine REPL and GUI.

use crate::chase::rule::RuleBuilder;
use crate::chase::{Atom, Rule, Term};
use crate::sql::ast::Select;
use crate::sql::parser::parse_select;

#[derive(Debug, Clone)]
pub enum Command {
    Fact(Atom),
    Rule(Rule),
    Sql(Select),
    Run,
    Query(Vec<Atom>),
    Explain(Vec<Atom>),
    ShowFacts,
    ShowRules,
    Reset,
    Help,
}

pub fn parse_script(input: &str) -> Result<Vec<Command>, String> {
    let mut commands = Vec::new();

    for (index, raw_line) in input.lines().enumerate() {
        let line = raw_line.trim();
        if line.is_empty() || line.starts_with('#') {
            continue;
        }

        let command = parse_command(line).map_err(|err| format!("line {}: {}", index + 1, err))?;
        commands.push(command);
    }

    Ok(commands)
}

pub fn parse_command(input: &str) -> Result<Command, String> {
    let trimmed = input.trim();

    if trimmed.eq_ignore_ascii_case("run") || trimmed.eq_ignore_ascii_case("run.") {
        return Ok(Command::Run);
    }
    if trimmed.eq_ignore_ascii_case("show facts") || trimmed.eq_ignore_ascii_case("show facts.") {
        return Ok(Command::ShowFacts);
    }
    if trimmed.eq_ignore_ascii_case("show rules") || trimmed.eq_ignore_ascii_case("show rules.") {
        return Ok(Command::ShowRules);
    }
    if trimmed.eq_ignore_ascii_case("reset") || trimmed.eq_ignore_ascii_case("reset.") {
        return Ok(Command::Reset);
    }
    if trimmed.eq_ignore_ascii_case("help") || trimmed.eq_ignore_ascii_case("help.") {
        return Ok(Command::Help);
    }

    if let Some(rest) = strip_keyword(trimmed, "sql") {
        let select = parse_select(trim_suffix(rest, ';')?).map_err(|err| err.to_string())?;
        return Ok(Command::Sql(select));
    }

    if let Some(rest) = strip_keyword(trimmed, "fact") {
        let atom = parse_atom(trim_suffix(rest, '.')?)?;
        if !atom.is_ground() {
            return Err("facts must be ground atoms".to_string());
        }
        return Ok(Command::Fact(atom));
    }

    if let Some(rest) = strip_keyword(trimmed, "rule") {
        let rule_text = trim_suffix(rest, '.')?;
        let arrow = find_top_level_arrow(rule_text)
            .ok_or_else(|| "rule must contain a top-level `->`".to_string())?;
        let body_text = rule_text[..arrow].trim();
        let head_text = rule_text[arrow + 2..].trim();
        if body_text.is_empty() || head_text.is_empty() {
            return Err("rule body and head must both be non-empty".to_string());
        }

        let body = parse_atom_list(body_text)?;
        let head = parse_atom_list(head_text)?;
        let mut builder = RuleBuilder::new();
        for atom in body {
            builder = builder.when(&atom.predicate, atom.terms);
        }
        for atom in head {
            builder = builder.then(&atom.predicate, atom.terms);
        }
        return Ok(Command::Rule(builder.build()));
    }

    if let Some(rest) = strip_keyword(trimmed, "query") {
        let atoms = parse_atom_list(trim_suffix(rest, '?')?)?;
        return Ok(Command::Query(atoms));
    }

    if let Some(rest) = strip_keyword(trimmed, "explain") {
        let atoms = parse_atom_list(trim_suffix(rest, '?')?)?;
        return Ok(Command::Explain(atoms));
    }

    Err("unknown command; try `help`".to_string())
}

fn strip_keyword<'a>(input: &'a str, keyword: &str) -> Option<&'a str> {
    let prefix = input.get(..keyword.len())?;
    if !prefix.eq_ignore_ascii_case(keyword) {
        return None;
    }

    let rest = input.get(keyword.len()..)?;
    if rest.is_empty() {
        return Some(rest);
    }

    let mut chars = rest.chars();
    let first = chars.next()?;
    if first.is_whitespace() {
        Some(rest.trim_start())
    } else {
        None
    }
}

fn trim_suffix(input: &str, suffix: char) -> Result<&str, String> {
    let trimmed = input.trim();
    if let Some(stripped) = trimmed.strip_suffix(suffix) {
        Ok(stripped.trim_end())
    } else {
        Err(format!("command must end with `{}`", suffix))
    }
}

fn parse_atom_list(input: &str) -> Result<Vec<Atom>, String> {
    split_top_level(input, ',')?
        .into_iter()
        .map(parse_atom)
        .collect()
}

fn parse_atom(input: &str) -> Result<Atom, String> {
    let trimmed = input.trim();
    let open = trimmed
        .find('(')
        .ok_or_else(|| format!("expected `(` in atom `{}`", trimmed))?;
    let close = trimmed
        .rfind(')')
        .ok_or_else(|| format!("expected `)` in atom `{}`", trimmed))?;
    if close <= open {
        return Err(format!("malformed atom `{}`", trimmed));
    }
    if close != trimmed.len() - 1 {
        return Err(format!("unexpected content after atom `{}`", trimmed));
    }

    let predicate = trimmed[..open].trim();
    validate_identifier(predicate, "predicate")?;

    let args = trimmed[open + 1..close].trim();
    let terms = if args.is_empty() {
        Vec::new()
    } else {
        split_top_level(args, ',')?
            .into_iter()
            .map(parse_term)
            .collect::<Result<Vec<_>, _>>()?
    };

    Ok(Atom::new(predicate, terms))
}

fn parse_term(input: &str) -> Result<Term, String> {
    let trimmed = input.trim();
    if trimmed.is_empty() {
        return Err("empty term".to_string());
    }

    if let Some(var) = trimmed.strip_prefix('?') {
        validate_identifier(var, "variable")?;
        return Ok(Term::var(var));
    }

    if trimmed.starts_with('"') {
        return parse_string_literal(trimmed).map(Term::constant);
    }

    if trimmed.chars().any(char::is_whitespace) {
        return Err(format!(
            "constants with spaces must be quoted: `{}`",
            trimmed
        ));
    }

    validate_identifier(trimmed, "constant")?;
    Ok(Term::constant(trimmed))
}

fn parse_string_literal(input: &str) -> Result<String, String> {
    if !input.ends_with('"') || input.len() < 2 {
        return Err(format!("unterminated string literal `{}`", input));
    }

    let inner = &input[1..input.len() - 1];
    let mut value = String::new();
    let mut escaped = false;

    for ch in inner.chars() {
        if escaped {
            let translated = match ch {
                '\\' => '\\',
                '"' => '"',
                'n' => '\n',
                't' => '\t',
                other => {
                    return Err(format!("unsupported escape sequence `\\{}`", other));
                }
            };
            value.push(translated);
            escaped = false;
            continue;
        }

        if ch == '\\' {
            escaped = true;
        } else {
            value.push(ch);
        }
    }

    if escaped {
        return Err("string literal ends with a trailing escape".to_string());
    }

    Ok(value)
}

fn validate_identifier(value: &str, label: &str) -> Result<(), String> {
    if value.is_empty() {
        return Err(format!("{} cannot be empty", label));
    }

    if value.chars().all(is_identifier_char) {
        Ok(())
    } else {
        Err(format!("invalid {} `{}`", label, value))
    }
}

fn is_identifier_char(ch: char) -> bool {
    ch.is_ascii_alphanumeric() || matches!(ch, '_' | '-' | ':')
}

fn find_top_level_arrow(input: &str) -> Option<usize> {
    let bytes = input.as_bytes();
    let mut depth = 0usize;
    let mut in_string = false;
    let mut escaped = false;
    let mut index = 0usize;

    while index < bytes.len() {
        let ch = bytes[index] as char;
        if in_string {
            if escaped {
                escaped = false;
            } else if ch == '\\' {
                escaped = true;
            } else if ch == '"' {
                in_string = false;
            }
            index += 1;
            continue;
        }

        match ch {
            '"' => in_string = true,
            '(' => depth += 1,
            ')' => depth = depth.saturating_sub(1),
            '-' if depth == 0 && bytes.get(index + 1).copied() == Some(b'>') => {
                return Some(index);
            }
            _ => {}
        }
        index += 1;
    }

    None
}

fn split_top_level(input: &str, separator: char) -> Result<Vec<&str>, String> {
    let mut parts = Vec::new();
    let mut depth = 0usize;
    let mut in_string = false;
    let mut escaped = false;
    let mut start = 0usize;

    for (index, ch) in input.char_indices() {
        if in_string {
            if escaped {
                escaped = false;
            } else if ch == '\\' {
                escaped = true;
            } else if ch == '"' {
                in_string = false;
            }
            continue;
        }

        match ch {
            '"' => in_string = true,
            '(' => depth += 1,
            ')' => {
                if depth == 0 {
                    return Err(format!("unexpected `)` in `{}`", input));
                }
                depth -= 1;
            }
            ch if ch == separator && depth == 0 => {
                let part = input[start..index].trim();
                if part.is_empty() {
                    return Err(format!("empty element in `{}`", input));
                }
                parts.push(part);
                start = index + ch.len_utf8();
            }
            _ => {}
        }
    }

    if in_string {
        return Err(format!("unterminated string literal in `{}`", input));
    }
    if depth != 0 {
        return Err(format!("unbalanced parentheses in `{}`", input));
    }

    let tail = input[start..].trim();
    if tail.is_empty() {
        return Err(format!("empty element in `{}`", input));
    }
    parts.push(tail);
    Ok(parts)
}

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

    #[test]
    fn parse_fact_command() {
        let command = parse_command(r#"fact Parent(alice, "bob smith")."#).unwrap();
        match command {
            Command::Fact(atom) => {
                assert_eq!(atom.predicate, "Parent");
                assert_eq!(atom.terms.len(), 2);
            }
            other => panic!("unexpected command: {:?}", other),
        }
    }

    #[test]
    fn parse_fact_command_rejects_variables() {
        let error = parse_command("fact Parent(?X, bob).").unwrap_err();
        assert_eq!(error, "facts must be ground atoms");
    }

    #[test]
    fn parse_rule_command() {
        let command = parse_command("rule P(?X), Q(?X, a) -> R(?X).").unwrap();
        match command {
            Command::Rule(rule) => {
                assert_eq!(rule.body.len(), 2);
                assert_eq!(rule.head.len(), 1);
            }
            other => panic!("unexpected command: {:?}", other),
        }
    }

    #[test]
    fn parse_sql_command() {
        let command = parse_command("sql SELECT c0 FROM Parent WHERE c1 = 'bob';").unwrap();
        match command {
            Command::Sql(select) => {
                assert_eq!(select.from, "Parent");
                assert!(select.selection.is_some());
            }
            other => panic!("unexpected command: {:?}", other),
        }
    }

    #[test]
    fn parse_query_command() {
        let command = parse_command("query Ancestor(?X, ?Y), Parent(?Y, ?Z)?").unwrap();
        match command {
            Command::Query(atoms) => assert_eq!(atoms.len(), 2),
            other => panic!("unexpected command: {:?}", other),
        }
    }

    #[test]
    fn parse_explain_command() {
        let command = parse_command("explain Ancestor(alice, carol)?").unwrap();
        match command {
            Command::Explain(atoms) => assert_eq!(atoms.len(), 1),
            other => panic!("unexpected command: {:?}", other),
        }
    }

    #[test]
    fn parse_script_reports_line_numbers() {
        let error = parse_script("help\nbogus\nrun.").unwrap_err();
        assert!(error.contains("line 2"));
    }
}