From db90e85c8973ec2f25581c223c43ba26f751b5cb Mon Sep 17 00:00:00 2001
From: Vasko Zdravevski <vaskoz@users.noreply.github.com>
Date: Tue, 20 Nov 2018 20:17:31 -0700
Subject: [PATCH 1/2] day 90: randomly generate from missing integers

---
 day90/problem.go      | 41 ++++++++++++++++++++++++++++++++++++
 day90/problem_test.go | 49 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)
 create mode 100644 day90/problem.go
 create mode 100644 day90/problem_test.go

diff --git a/day90/problem.go b/day90/problem.go
new file mode 100644
index 0000000..fb6f868
--- /dev/null
+++ b/day90/problem.go
@@ -0,0 +1,41 @@
+package day90
+
+import (
+	"math/rand"
+	"time"
+)
+
+var random *rand.Rand
+
+// getRandom returns a *math.Rand.
+func getRandom() *rand.Rand {
+	if random == nil {
+		random = rand.New(rand.NewSource(time.Now().UnixNano()))
+	}
+	return random
+}
+
+// RandMissingNumbers randomly generates a number from 0 to n-1 that isn't in l.
+// Runs in O(N) time and O(l) space.
+func RandMissingNumbers(n int, l []int) int {
+	m := make(map[int]struct{})
+	for _, v := range l {
+		m[v] = struct{}{}
+	}
+	size := n - len(m)
+	if size == 0 {
+		return -1
+	}
+	pos := getRandom().Intn(size)
+	val := -1
+	var i int
+	for i = 0; i < n; i++ {
+		if _, found := m[i]; !found {
+			val++
+		}
+		if val == pos {
+			break
+		}
+	}
+	return i
+}
diff --git a/day90/problem_test.go b/day90/problem_test.go
new file mode 100644
index 0000000..fa1b131
--- /dev/null
+++ b/day90/problem_test.go
@@ -0,0 +1,49 @@
+package day90
+
+import "testing"
+
+var testcases = []struct {
+	n            int
+	l            []int
+	distribution float64
+}{
+	{5, []int{0, 1, 2, 2}, 0.5},
+	{5, []int{0, 1, 0, 1}, 0.333},
+	{5, []int{0, 0, 0}, 0.25},
+	{5, []int{0, 1, 2, 3}, 1.0},
+}
+
+func TestRandMissingNumbers(t *testing.T) {
+	//t.Parallel()//don't run in parallel
+	for _, tc := range testcases {
+		results := make(map[int]int)
+		const iterations = 1000
+		for i := 0; i < iterations; i++ {
+			results[RandMissingNumbers(tc.n, tc.l)]++
+		}
+		for _, v := range results {
+			delta := (float64(v) / float64(iterations)) - tc.distribution
+			if delta < 0 {
+				delta = -delta
+			}
+			if delta > 0.1 {
+				t.Errorf("Exceeded 0.1 tolerance for expected %v distribution", tc.distribution)
+			}
+		}
+	}
+}
+
+func TestRandMissingNoNumbers(t *testing.T) {
+	//t.Parallel()//don't run in parallel
+	if result := RandMissingNumbers(5, []int{0, 1, 2, 3, 4}); result != -1 {
+		t.Errorf("Expected -1 for a list that includes all possible numbers")
+	}
+}
+
+func BenchmarkRandMissingNumbers(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			RandMissingNumbers(tc.n, tc.l)
+		}
+	}
+}

From 576b16ae90f514929d3c0096f91fbe505403a8ce Mon Sep 17 00:00:00 2001
From: Vasko Zdravevski <vaskoz@users.noreply.github.com>
Date: Tue, 20 Nov 2018 20:18:04 -0700
Subject: [PATCH 2/2] add day 90 to readme

---
 README.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/README.md b/README.md
index 56aaefc..4b4cd17 100644
--- a/README.md
+++ b/README.md
@@ -99,3 +99,4 @@ problems from
 * [Day 87](https://github.com/vaskoz/dailycodingproblem-go/issues/183)
 * [Day 88](https://github.com/vaskoz/dailycodingproblem-go/issues/185)
 * [Day 89](https://github.com/vaskoz/dailycodingproblem-go/issues/187)
+* [Day 90](https://github.com/vaskoz/dailycodingproblem-go/issues/189)