A prime number generator using an incremental sieve of Eratosthenes. It's fast but memory bound.

main.go

package main

import (
	"fmt"
)

func main() {
	// The number of primes to display before stopping.
	maxPrimes := 1000000

	// The number of primes collected so far.
	primesGot := 0

	// 2 is the first prime number
	possiblePrime := 2

	// A mapping of composite numbers to their factors.
	composites := map[int][]int{}

	deleted := 0

	for {
		// check if number is prime
		primesOfComposite, isComposite := composites[possiblePrime]
		if isComposite {
			// the number is not prime. Move its factors to future composites.
			for primeFactorIndex := range primesOfComposite {
				primeFactor := primesOfComposite[primeFactorIndex]
				composites[possiblePrime+primeFactor] = append(
					composites[possiblePrime+primeFactor], primeFactor)
			}

			deleted++
			delete(composites, possiblePrime)
		} else {
			// the number is prime

			// increment counter
			primesGot++

			// stop if done
			if primesGot >= maxPrimes {
				fmt.Println(possiblePrime)
				break
			}

			// add possiblePrime * possiblePrime as composite
			composites[possiblePrime*possiblePrime] = append(
				composites[possiblePrime*possiblePrime], possiblePrime)
		}

		possiblePrime++
	}
}