Challenge 1: A Flow with Operators — Solution

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun numberStream(): Flow<Int> = flow {
    for (i in 1..5) {
        delay(200)
        emit(i)
    }
}

fun main() = runBlocking {
    numberStream()
        .filter { it % 2 == 0 }
        .map { it * it }
        .collect { println(it) }
}

/*
Output (roughly one per 200ms as the underlying flow emits):
4
16

Notes:
- numberStream() emits 1, 2, 3, 4, 5 one at a time, each after a 200ms
  delay, since flow { } only runs when collect() is called.
- filter { it % 2 == 0 } keeps only 2 and 4 as they come through.
- map { it * it } squares each of those: 2 -> 4, 4 -> 16.
- Because Flow is cold and processes values one at a time as they're
  emitted, the operators apply to each value individually as it flows
  through, rather than waiting for the whole stream to finish first.
*/