org.litote.kmongo.MongoIterables.kt Maven / Gradle / Ivy

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/*
 * Copyright (C) 2016/2020 Litote
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.litote.kmongo

import com.mongodb.client.MongoIterable

private val NULL = Any()

private class NullIterator(val iterator: Iterator) : Iterator by iterator {
    override fun next(): T? = iterator.next()?.takeUnless { it === NULL }
}

private class NullHandlerSequence(val sequence: Sequence) : Sequence {

    override fun iterator(): Iterator = NullIterator(sequence.iterator())
}

/**
 * Evaluates the current [MongoIterable] given the [expression] of Sequences.
 *
 * The mongo cursor is closed before returning the result.
 *
 * Sample:
 * ```
 *   col.find().evaluate {
 *      //this is a sequence evaluation
 *      //If the first row has a name like "Fred", only one row is loaded in memory!
 *      filter { it.name != "Joe" }.first()
 *   }
 * ```
 */
fun  MongoIterable.evaluate(expression: Sequence.() -> R): R {
    @Suppress("UNCHECKED_CAST")
    return iterator().run {
        use {
            expression(
                NullHandlerSequence(
                    generateSequence {
                        if (hasNext()) {
                            next() ?: NULL
                        } else {
                            null
                        }
                    }
                ) as Sequence
            )
        }
    }
}