// @flow
import type { QueryCriteria, QueryObj } from './types';

function flatten<A>(arr: Array<Array<A>>): Array<A> {
	return arr.reduce((arr, inner) => [ ...arr, ...inner ], []);
}


function clause(criteria: Array<QueryCriteria>): QueryObj {
	const clauses = criteria.map(criteria => {
		const keys = Object.keys(criteria),
			constraints = keys.map(key => {
				const val = criteria[key];
				if (Array.isArray(val)) {
					const subsontraints = val.map(() => `${ key } = ?`).join(' OR ');
					return {
						sql: subsontraints,
						args: val
					};
				}
				else {
					return {
						sql: `${ key } = ?`,
						args: [ val ]
					};
				}
			});
		return {
			sql: constraints.map(c => c.sql).join(' AND '),
			args: flatten(constraints.map(c => c.args))
		};
	});

	return {
		sql: clauses.map(c => `(${ c.sql })`).join(' OR '),
		args: flatten(clauses.map(c => c.args))
	};
}

/**
 * `whereClause :: { where?: QueryCriteria } -> QueryObj`
 */
export function where(props: {} | { where: Array<QueryCriteria> }): QueryObj {
	if (props.where != null) {
		const where = ((props.where: any): Array<QueryCriteria>);
		if (Array.isArray(where)) {
			const obj = Array.isArray(where) ? clause(where) : {};

			return {
				sql: `WHERE ${ obj.sql }`,
				args: obj.args
			};
		}
	}

	return {
		sql: ``,
		args: []
	};
}