Merge ff0828b into e979b4d

README.md

@@ -254,3 +254,4 @@ problems from
 * [Day 240](https://github.com/vaskoz/dailycodingproblem-go/issues/492)
 * [Day 241](https://github.com/vaskoz/dailycodingproblem-go/issues/493)
 * [Day 242](https://github.com/vaskoz/dailycodingproblem-go/issues/498)
+* [Day 244](https://github.com/vaskoz/dailycodingproblem-go/issues/503)

day244/problem.go

@@ -0,0 +1,51 @@
+package day244
+
+import "math"
+
+// SieveOfEratosthenes returns all the primes strictly less than n.
+// If n happens to be a prime, it will NOT be returned in the result.
+// Runtime is O(N log log N)
+func SieveOfEratosthenes(n int) []int {
+	if n < 2 {
+		return nil
+	}
+	isNotPrime := make([]bool, n)
+	isNotPrime[0], isNotPrime[1] = true, true
+	for i := 2; i <= int(math.Sqrt(float64(n))); i++ {
+		if !isNotPrime[i] {
+			for j := i * i; j < n; j += i {
+				isNotPrime[j] = true
+			}
+		}
+	}
+	var primes []int
+	for p, notPrime := range isNotPrime {
+		if !notPrime {
+			primes = append(primes, p)
+		}
+	}
+	return primes
+}
+
+// SieveOfEratosthenesGenerator returns primes indefinitely.
+// Only runs when the channel is read for more primes.
+func SieveOfEratosthenesGenerator() <-chan int {
+	primes := make(chan int)
+	go func() {
+		n := 2
+		composites := make(map[int][]int)
+		for {
+			if _, found := composites[n]; !found {
+				primes <- n
+				composites[n*n] = []int{n}
+			} else {
+				for _, prime := range composites[n] {
+					composites[n+prime] = append(composites[n+prime], prime)
+				}
+				delete(composites, n)
+			}
+			n++
+		}
+	}()
+	return primes
+}

day244/problem_test.go

@@ -0,0 +1,60 @@
+package day244
+
+import (
+	"reflect"
+	"testing"
+)
+
+var testcases = []struct {
+	n      int
+	primes []int
+}{
+	{100, []int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97}},
+	{2, nil},
+	{1, nil},
+	{0, nil},
+}
+
+func TestSieveOfEratosthenes(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if primes := SieveOfEratosthenes(tc.n); !reflect.DeepEqual(primes, tc.primes) {
+			t.Errorf("Expected %v, got %v", tc.primes, primes)
+		}
+	}
+}
+
+func BenchmarkSieveOfEratosthenes(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			SieveOfEratosthenes(tc.n)
+		}
+	}
+}
+
+func TestSieveOfEratosthenesGenerator(t *testing.T) {
+	t.Parallel()
+	primesToThousand := []int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67,
+		71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167,
+		173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271,
+		277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383,
+		389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491,
+		499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613,
+		617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733,
+		739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857,
+		859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983,
+		991, 997}
+	primeGenerator := SieveOfEratosthenesGenerator()
+	for _, prime := range primesToThousand {
+		if p := <-primeGenerator; p != prime {
+			t.Errorf("Expected %v, got %v", prime, p)
+		}
+	}
+}
+
+func BenchmarkSieveOfEratosthenesGenerator(b *testing.B) {
+	primeGenerator := SieveOfEratosthenesGenerator()
+	for i := 0; i < b.N; i++ {
+		<-primeGenerator
+	}
+}