"use strict";

const Promise = require("bluebird");
const debug = require("debug")("srap:backend:postgresql:query:get-task-stream");
const buffer = require("@promistream/buffer");
const pipe = require("@promistream/pipe");
const simpleSource = require("@promistream/simple-source");

const fetchQuery = `
	SELECT
		srap_items.*
	FROM
		srap_queue
	LEFT JOIN srap_items
		ON srap_items.id = srap_queue.item_id
	WHERE
		srap_queue.task = :task
		AND srap_queue.started = FALSE
	LIMIT :resultLimit
`;

function makeFillQuery(withDependencies) {
	return `
		WITH
			${withDependencies ? `
				dependencies AS (
					SELECT * FROM json_to_recordset(:dependencyTaskDefinitions) AS x(task text, task_version text)
				),
				satisfied AS (
					SELECT results.* FROM dependencies
					LEFT JOIN srap_task_results AS results
						ON dependencies.task = results.task
						AND dependencies.task_version = results.task_version
					WHERE
						results.is_successful = TRUE
						AND results.is_invalidated = FALSE
						AND results.expires_at > NOW()
				),
				counts AS (
					SELECT item_id, COUNT(task) AS count FROM satisfied GROUP BY item_id
				),
				dependency_candidates AS (
					SELECT item_id FROM counts WHERE count = :dependencyCount
				),
			` : "" }
			tag_candidates AS (
				SELECT item_id FROM srap_tags WHERE name = ANY(:tags)
			),
			full_candidates AS MATERIALIZED (
				${withDependencies
					? `
						SELECT tag_candidates.item_id FROM dependency_candidates
						INNER JOIN tag_candidates
							ON dependency_candidates.item_id = tag_candidates.item_id
					`
					: `
						SELECT item_id FROM tag_candidates
					`
				}
			)
		SELECT
			:task AS task,
			item_id
		FROM full_candidates
		WHERE NOT EXISTS (
			SELECT item_id FROM srap_task_results AS results
			WHERE
				item_id = full_candidates.item_id
				AND results.task = :task
				AND results.task_version = :taskVersion
				AND results.is_successful = TRUE
				AND results.is_invalidated = FALSE
				AND results.expires_at > NOW()
		)
	`;
}

const fillQueryWithDependencies = makeFillQuery(true);
const fillQueryWithoutDependencies = makeFillQuery(false);

module.exports = function ({ metrics, backendSettings, knex }) {
	return function (tx, { task }) {
		let hasDependencies = (task.dependencies.length > 0);

		let refillParameters = {
			tags: task.tags,
			task: task.name,
			taskVersion: task.version,
			... hasDependencies
				? {
					dependencyCount: task.dependencies.length,
					dependencyTaskDefinitions: JSON.stringify(task.dependencies.map((dependency) => {
						// Case-mapping for SQL compatibility
						return { task_version: dependency.version, task: dependency.name };
					}))
				}
				: {}
		};

		let fetchParameters = {
			task: task.name,
			resultLimit: backendSettings.taskBatchSize
			// resultLimit: 1 // For tracking down race conditions
		};

		function refillQueue() {
			let startTime = Date.now();

			return Promise.try(() => {
				let fillQuery = (hasDependencies)
					? fillQueryWithDependencies
					: fillQueryWithoutDependencies;

				// NOTE: We are deliberately bypassing the transaction here! Also deliberately not using VALUES, since we're inserting from the results of another query instead
				return knex.raw(`
					INSERT INTO srap_queue (task, item_id)
					(${fillQuery})
					ON CONFLICT (task, item_id) DO NOTHING;
				`, refillParameters);
			}).then((response) => {
				let timeElapsed = Date.now() - startTime;
		
				metrics.taskRefillTime.labels({ task: task.name }).set(timeElapsed / 1000);
				metrics.taskRefillResults.labels({ task: task.name }).set(response.rowCount);

				debug(`Queue for '${task.name}' was refilled with ${response.rowCount} items in ${timeElapsed}ms`);
				return response.rowCount;
			});
		}

		return pipe([
			simpleSource(() => {
				let startTime = Date.now();
				
				return Promise.try(() => {
					return tx.raw(fetchQuery, fetchParameters);
				}).then((result) => {
					let timeElapsed = Date.now() - startTime;
		
					metrics.taskFetchTime.labels({ task: task.name }).set(timeElapsed / 1000);
					metrics.taskFetchResults.labels({ task: task.name }).set(result.rowCount);
		
					debug(`Task retrieval query for '${task.name}' took ${timeElapsed}ms and produced ${result.rowCount} results`);
		
					if (result.rowCount > 0) {
						return result.rows;
					} else {
						return Promise.try(() => {
							return refillQueue();
						}).then((newItems) => {
							if (newItems === 0) {
								// TODO: Consider using LISTEN/NOTIFY instead? Worth the added complexity?
								let randomization = Math.random() * backendSettings.delayRandomization * backendSettings.taskBatchDelay; // To prevent stampeding by low-throughput tasks
								return Promise.resolve([]).delay(backendSettings.taskBatchDelay + randomization);
							} else {
								// Have another go right away
								return [];
							}
						});
					}
				});
			}),
			buffer()
		]);
	};
};