using Invercargill.Expressions;
using Invercargill.DataStructures;

namespace InvercargillSql.Orm {
    
    /**
     * Represents an ORDER BY clause with expression and direction.
     */
    public class OrderByClause : Object {
        /**
         * The expression to order by.
         */
        public Expression expression { get; construct; }
        
        /**
         * Whether to order in descending order.
         */
        public bool descending { get; construct; }
        
        /**
         * Creates a new OrderByClause.
         * 
         * @param expression The expression to order by
         * @param descending True for descending order, false for ascending
         */
        public OrderByClause(Expression expression, bool descending) {
            Object(expression: expression, descending: descending);
        }
    }
    
    /**
     * Abstract base class for all queries.
     * 
     * Query<T> provides a fluent interface for building database queries.
     * Subclasses implement specific query execution strategies for entities
     * and projections.
     * 
     * Important: Query<T> does NOT extend Enumerable<T>. Instead, call
     * materialise() or materialise_async() to execute the query and get results.
     * 
     * Example usage:
     * {{{
     * var users = session.query<User>()
     *     .where(expr("age > $0", new NativeElement<int?>(18)))
     *     .order_by(expr("name"))
     *     .limit(10)
     *     .materialise();
     * }}}
     */
    public abstract class Query<T> : Object {
        
        // Shared state - protected for subclass access
        protected OrmSession _session;
        protected Vector<OrderByClause> _orderings;
        protected int64? _limit;
        protected int64? _offset;
        protected bool _use_or;
        protected Vector<Expression> _where_expressions;
        
        /**
         * Creates a new Query for the given session.
         * 
         * This constructor is protected - use OrmSession.query<T>() to create queries.
         * 
         * @param session The OrmSession that will execute this query
         */
        protected Query(OrmSession session) {
            _session = session;
            _orderings = new Vector<OrderByClause>();
            _where_expressions = new Vector<Expression>();
            _limit = null;
            _offset = null;
            _use_or = false;
        }
        
        // === Internal accessors for OrmSession compatibility ===
        
        /**
         * The filter expression for this query (for entity queries).
         * Returns null for projection queries which use string-based where clauses.
         */
        internal abstract owned Expression? filter { owned get; }
        
        /**
         * The ORDER BY clauses for this query.
         */
        internal abstract Vector<OrderByClause> orderings { get; }
        
        /**
         * The LIMIT value for this query, or null if not set.
         */
        internal abstract int64? limit_value { get; }
        
        /**
         * The OFFSET value for this query, or null if not set.
         */
        internal abstract int64? offset_value { get; }
        
        // === Fluent query builders ===
        
        /**
         * Adds a WHERE clause using an Expression.
         * 
         * Use the expr() convenience function to create expressions:
         * {{{
         * .where(expr("age > $0", new NativeElement<int?>(18)))
         * }}}
         * 
         * @param expression The filter expression
         * @return This query for method chaining
         */
        public virtual Query<T> where(Expression expression) {
            _where_expressions.add(expression);
            return this;
        }
        
        /**
         * Adds an OR WHERE clause using an Expression.
         * 
         * Multiple where clauses will be combined with OR instead of AND.
         * 
         * @param expression The filter expression
         * @return This query for method chaining
         */
        public virtual Query<T> or_where(Expression expression) {
            _use_or = true;
            _where_expressions.add(expression);
            return this;
        }
        
        /**
         * Adds an ascending ORDER BY clause.
         * 
         * @param expression The expression to order by
         * @return This query for method chaining
         */
        public virtual Query<T> order_by(Expression expression) {
            _orderings.add(new OrderByClause(expression, false));
            return this;
        }
        
        /**
         * Adds a descending ORDER BY clause.
         * 
         * @param expression The expression to order by
         * @return This query for method chaining
         */
        public virtual Query<T> order_by_desc(Expression expression) {
            _orderings.add(new OrderByClause(expression, true));
            return this;
        }
        
        /**
         * Sets the maximum number of results to return.
         * 
         * @param count The maximum number of results
         * @return This query for method chaining
         */
        public virtual Query<T> limit(int64 count) {
            _limit = count;
            return this;
        }
        
        /**
         * Sets the number of results to skip.
         * 
         * @param count The number of results to skip
         * @return This query for method chaining
         */
        public virtual Query<T> offset(int64 count) {
            _offset = count;
            return this;
        }
        
        // === Execution methods ===
        
        /**
         * Executes the query and returns the results as an ImmutableLot.
         * 
         * @return An ImmutableLot<T> containing the query results
         * @throws SqlError if query execution fails
         */
        public abstract Invercargill.ImmutableLot<T> materialise() throws SqlError;
        
        /**
         * Executes the query asynchronously and returns the results.
         * 
         * @return An ImmutableLot<T> containing the query results
         * @throws SqlError if query execution fails
         */
        public abstract async Invercargill.ImmutableLot<T> materialise_async() throws SqlError;
        
        /**
         * Executes the query and returns the first result.
         * 
         * This method automatically applies a limit of 1 before executing.
         * 
         * @return The first result, or null if no results
         * @throws SqlError if query execution fails
         */
        public virtual T? first() throws SqlError {
            _limit = 1;
            var results = materialise();
            if (results.length > 0) {
                return results.first();
            }
            return null;
        }
        
        /**
         * Executes the query asynchronously and returns the first result.
         * 
         * This method automatically applies a limit of 1 before executing.
         * 
         * @return The first result, or null if no results
         * @throws SqlError if query execution fails
         */
        public virtual async T? first_async() throws SqlError {
            _limit = 1;
            var results = yield materialise_async();
            if (results.length > 0) {
                return results.first();
            }
            return null;
        }
        
        /**
         * Returns the SQL for this query.
         * 
         * @return The SQL string for this query
         */
        public abstract string to_sql();
        
        // === Protected helpers ===
        
        /**
         * Combines all where expressions into a single Expression.
         * 
         * If there are no where expressions, returns null.
         * If there is one expression, returns it directly.
         * If there are multiple expressions, combines them with AND or OR
         * based on the _use_or flag.
         * 
         * @return The combined where expression, or null if no expressions
         */
        protected Expression? get_combined_where() {
            if (_where_expressions.length == 0) {
                return null;
            }
            
            if (_where_expressions.length == 1) {
                return _where_expressions.get(0);
            }
            
            // Combine multiple expressions with AND or OR
            BinaryOperator op = _use_or ? BinaryOperator.OR : BinaryOperator.AND;
            Expression? result = _where_expressions.get(0);
            
            for (uint i = 1; i < _where_expressions.length; i++) {
                result = new BinaryExpression(
                    result,
                    _where_expressions.get(i),
                    op
                );
            }
            
            return result;
        }
    }
}