Merge pull request #213 from vaskoz/day101

Day101

README.md

@@ -110,3 +110,4 @@ problems from
 * [Day 98](https://github.com/vaskoz/dailycodingproblem-go/issues/204)
 * [Day 99](https://github.com/vaskoz/dailycodingproblem-go/issues/205)
 * [Day 100](https://github.com/vaskoz/dailycodingproblem-go/issues/209)
+* [Day 101](https://github.com/vaskoz/dailycodingproblem-go/issues/212)

day101/problem.go

@@ -0,0 +1,47 @@
+package day101
+
+import (
+	"math"
+)
+
+// Sieve of prime numbers up to n.
+// Runs in O(N log log N)
+func Sieve(n int) []int {
+	data := make([]bool, n+1)
+	for i := 2; i < int(math.Sqrt(float64(n)))+1; i++ {
+		if !data[i] {
+			for j := i * i; j <= n; j += i {
+				data[j] = true
+			}
+		}
+	}
+	var results []int
+	for i := 2; i <= n; i++ {
+		if !data[i] {
+			results = append(results, i)
+		}
+	}
+	return results
+}
+
+// GoldbachsConjecture returns two prime numbers whose sum
+// is equal to the given number.
+// If even param is less than 4 or an odd number, nil is returned.
+func GoldbachsConjecture(even int) []int {
+	if even < 4 || even%2 != 0 {
+		return nil
+	}
+	primes := Sieve(even)
+	set := make(map[int]struct{}, len(primes))
+	for _, prime := range primes {
+		set[prime] = struct{}{}
+	}
+	result := make([]int, 2)
+	for _, prime := range primes {
+		if _, found := set[even-prime]; found {
+			result[0], result[1] = prime, even-prime
+			break
+		}
+	}
+	return result
+}

day101/problem_test.go

@@ -0,0 +1,62 @@
+package day101
+
+import (
+	"reflect"
+	"testing"
+)
+
+var testcases = []struct {
+	even     int
+	expected []int
+}{
+	{4, []int{2, 2}},
+	{6, []int{3, 3}},
+	{8, []int{3, 5}},
+	{10, []int{3, 7}},
+	{10, []int{3, 7}},
+	{18, []int{5, 13}},
+}
+
+func TestSieve(t *testing.T) {
+	t.Parallel()
+	expected := []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}
+
+	if result := Sieve(100); !reflect.DeepEqual(expected, result) {
+		t.Errorf("Expected %v got %v", expected, result)
+	}
+}
+
+func BenchmarkSieve(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Sieve(100)
+	}
+}
+
+func TestGoldbachsConjecture(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := GoldbachsConjecture(tc.even); !reflect.DeepEqual(tc.expected, result) {
+			t.Errorf("Expected %v got %v", tc.expected, result)
+		}
+	}
+}
+
+func BenchmarkGoldbachsConjecture(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			GoldbachsConjecture(tc.even)
+		}
+	}
+}
+
+func TestGoldbachsConjectureBadInput(t *testing.T) {
+	t.Parallel()
+	if result := GoldbachsConjecture(3); result != nil {
+		t.Errorf("Parameters less than 4 should result in nil return")
+	}
+	if result := GoldbachsConjecture(9); result != nil {
+		t.Errorf("Odd parameters should result in nil return")
+	}
+
+}