//! Atom operator: scan a [`Table`] under an [`AtomPattern`] and return a
//! binding [`Relation`].
//!
//! An atom pattern specifies, for each table column, either a variable to bind
//! or a literal that the cell must equal. A variable appearing in more than one
//! column forces those cells to be equal (so `Edge(X, X)` keeps only
//! self-loops). The output relation has one column per distinct variable, in
//! first-occurrence order.

use std::collections::HashMap;

use storage::table::Table;
use storage::value::Value;

use crate::relation::Relation;

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Term {
    Var(String),
    Lit(Value),
}

#[derive(Debug, Clone)]
pub struct AtomPattern {
    pub columns: Vec<Term>,
}

/// # Panics
/// Panics if `pattern.columns.len() != table.arity`.
#[must_use]
pub fn scan_atom(table: &Table, pattern: &AtomPattern) -> Relation {
    assert_eq!(
        pattern.columns.len(),
        table.arity,
        "pattern arity mismatch: pattern has {}, table has {}",
        pattern.columns.len(),
        table.arity,
    );

    let mut output_vars: Vec<String> = Vec::new();
    let mut output_positions: Vec<usize> = Vec::new();
    let mut equality_pairs: Vec<(usize, usize)> = Vec::new();
    let mut literal_checks: Vec<(usize, &Value)> = Vec::new();
    let mut first_position: HashMap<&str, usize> = HashMap::new();

    for (i, term) in pattern.columns.iter().enumerate() {
        match term {
            Term::Var(name) => {
                if let Some(&j) = first_position.get(name.as_str()) {
                    equality_pairs.push((j, i));
                } else {
                    first_position.insert(name.as_str(), i);
                    output_vars.push(name.clone());
                    output_positions.push(i);
                }
            }
            Term::Lit(value) => literal_checks.push((i, value)),
        }
    }

    let mut output = Relation::new(output_vars);
    'rows: for row in &table.rows {
        for &(i, lit) in &literal_checks {
            if &row[i] != lit {
                continue 'rows;
            }
        }
        for &(j, i) in &equality_pairs {
            if row[i] != row[j] {
                continue 'rows;
            }
        }
        let projected: Vec<Value> = output_positions.iter().map(|&i| row[i].clone()).collect();
        output.push(projected);
    }
    output
}

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

    fn var(name: &str) -> Term {
        Term::Var(name.to_string())
    }

    fn lit(value: i64) -> Term {
        Term::Lit(Value::Int(value))
    }

    fn int(value: i64) -> Value {
        Value::Int(value)
    }

    #[test]
    fn repeated_variable_keeps_only_self_loops() {
        let edge = Table::from_rows(
            2,
            vec![
                vec![int(1), int(2)],
                vec![int(2), int(2)],
                vec![int(3), int(3)],
                vec![int(1), int(1)],
            ],
        );
        let pattern = AtomPattern {
            columns: vec![var("X"), var("X")],
        };
        let result = scan_atom(&edge, &pattern);
        assert_eq!(result.columns, vec!["X".to_string()]);
        assert_eq!(result.rows, vec![vec![int(2)], vec![int(3)], vec![int(1)]]);
    }

    #[test]
    fn literal_filters_rows_to_match() {
        let edge = Table::from_rows(
            2,
            vec![
                vec![int(1), int(2)],
                vec![int(2), int(3)],
                vec![int(1), int(4)],
            ],
        );
        let pattern = AtomPattern {
            columns: vec![lit(1), var("Y")],
        };
        let result = scan_atom(&edge, &pattern);
        assert_eq!(result.columns, vec!["Y".to_string()]);
        assert_eq!(result.rows, vec![vec![int(2)], vec![int(4)]]);
    }

    #[test]
    fn distinct_variables_project_in_first_occurrence_order() {
        let triples = Table::from_rows(
            3,
            vec![vec![int(1), int(2), int(3)], vec![int(4), int(5), int(6)]],
        );
        let pattern = AtomPattern {
            columns: vec![var("A"), var("B"), var("C")],
        };
        let result = scan_atom(&triples, &pattern);
        assert_eq!(
            result.columns,
            vec!["A".to_string(), "B".to_string(), "C".to_string()],
        );
        assert_eq!(
            result.rows,
            vec![vec![int(1), int(2), int(3)], vec![int(4), int(5), int(6)]],
        );
    }

    #[test]
    fn variable_repeated_three_times_requires_all_equal() {
        let triples = Table::from_rows(
            3,
            vec![
                vec![int(1), int(1), int(1)],
                vec![int(1), int(1), int(2)],
                vec![int(2), int(2), int(2)],
                vec![int(1), int(2), int(1)],
            ],
        );
        let pattern = AtomPattern {
            columns: vec![var("X"), var("X"), var("X")],
        };
        let result = scan_atom(&triples, &pattern);
        assert_eq!(result.columns, vec!["X".to_string()]);
        assert_eq!(result.rows, vec![vec![int(1)], vec![int(2)]]);
    }

    #[test]
    fn literal_filter_repeated_var_and_projection_combine() {
        // Pattern: [Lit(1), Var("X"), Lit(2), Var("X")].
        // Keep rows where col0 == 1, col2 == 2, and col1 == col3.
        // Output is one column [X], bound to col1 (the first occurrence).
        let table = Table::from_rows(
            4,
            vec![
                vec![int(1), int(7), int(2), int(7)],
                vec![int(1), int(7), int(2), int(8)],
                vec![int(0), int(7), int(2), int(7)],
                vec![int(1), int(7), int(3), int(7)],
                vec![int(1), int(9), int(2), int(9)],
            ],
        );
        let pattern = AtomPattern {
            columns: vec![lit(1), var("X"), lit(2), var("X")],
        };
        let result = scan_atom(&table, &pattern);
        assert_eq!(result.columns, vec!["X".to_string()]);
        assert_eq!(result.rows, vec![vec![int(7)], vec![int(9)]]);
    }
}