/**
 * Tests that the $limit stage is pushed before $lookup stages, except when there is an $unwind.
 */
(function() {
"use strict";

const coll = db.lookup_with_limit;
const other = db.lookup_with_limit_other;
coll.drop();
other.drop();

// Checks that the order of the pipeline stages matches the expected ordering depending on whether
// the pipeline is optimized or not.
function checkResults(pipeline, isOptimized, expected) {
    assert.commandWorked(db.adminCommand({
        "configureFailPoint": 'disablePipelineOptimization',
        "mode": isOptimized ? 'off' : 'alwaysOn'
    }));
    const explain = coll.explain().aggregate(pipeline);
    if (expected.length > 0) {
        assert.eq(explain.stages[0].$cursor.queryPlanner.winningPlan.stage, expected[0], explain);
    }
    for (let i = 1; i < expected.length; i++) {
        assert.eq(Object.keys(explain.stages[i]), expected[i], explain);
    }
}

// Insert ten documents into coll: {x: 0}, {x: 1}, ..., {x: 9}.
const bulk = coll.initializeOrderedBulkOp();
Array.from({length: 10}, (_, i) => ({x: i})).forEach(doc => bulk.insert(doc));
assert.commandWorked(bulk.execute());

// Insert twenty documents into other: {x: 0, y: 0}, {x: 0, y: 1}, ..., {x: 9, y: 0}, {x: 9, y: 1}.
const bulk_other = other.initializeOrderedBulkOp();
Array.from({length: 10}, (_, i) => ({x: i, y: 0})).forEach(doc => bulk_other.insert(doc));
Array.from({length: 10}, (_, i) => ({x: i, y: 1})).forEach(doc => bulk_other.insert(doc));
assert.commandWorked(bulk_other.execute());

// Check that lookup->limit is reordered to limit->lookup, with the limit stage pushed down to query
// system.
var pipeline = [
    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
    {$limit: 5}
];
checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$limit"]);
checkResults(pipeline, true, ["LIMIT", "$lookup"]);

// Check that lookup->addFields->lookup->limit is reordered to limit->lookup->addFields->lookup,
// with the limit stage pushed down to query system.
pipeline = [
    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
    {$addFields: {z: 0}},
    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "additional"}},
    {$limit: 5}
];
checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$addFields", "$lookup", "$limit"]);
checkResults(pipeline, true, ["LIMIT", "$lookup", "$addFields", "$lookup"]);

// Check that lookup->unwind->limit is reordered to lookup->limit, with the unwind stage being
// absorbed into the lookup stage and preventing the limit from swapping before it.
pipeline = [
    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
    {$unwind: "$from_other"},
    {$limit: 5}
];
checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$unwind", "$limit"]);
checkResults(pipeline, true, ["COLLSCAN", "$lookup", "$limit"]);

// Check that lookup->unwind->sort->limit is reordered to lookup->sort, with the unwind stage being
// absorbed into the lookup stage and preventing the limit from swapping before it, and the limit
// stage being absorbed into the sort stage.
pipeline = [
    {$lookup: {from: other.getName(), localField: "x", foreignField: "x", as: "from_other"}},
    {$unwind: "$from_other"},
    {$sort: {x: 1}},
    {$limit: 5}
];
checkResults(pipeline, false, ["COLLSCAN", "$lookup", "$unwind", "$sort", "$limit"]);
checkResults(pipeline, true, ["COLLSCAN", "$lookup", "$sort"]);
}());