diff --git a/2015/day13/main.go b/2015/day13/main.go
new file mode 100644
index 0000000..5cd424f
--- /dev/null
+++ b/2015/day13/main.go
@@ -0,0 +1,80 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/algos"
+	"github.com/alexchao26/advent-of-code-go/mathutil"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := knightsOfTheDinnerTable(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func knightsOfTheDinnerTable(input string, part int) int {
+	graph := makeHappinessGraph(input)
+	var allPeople []string
+	for name := range graph {
+		allPeople = append(allPeople, name)
+	}
+
+	// for part 2 add "me" to the list of people, and an empty map in the grid
+	// for happiness diffs off of me. hashmap misses will be populated with the
+	// zero value (0) which is correct for this problem.
+	if part == 2 {
+		allPeople = append(allPeople, "me")
+		graph["me"] = map[string]int{}
+	}
+
+	perms := algos.PermuteStringSlice(allPeople)
+
+	maxDiff := math.MinInt32
+	for _, p := range perms {
+		maxDiff = mathutil.MaxInt(maxDiff, calcHappinessDiff(graph, p))
+	}
+
+	return maxDiff
+}
+
+type mapGraph map[string]map[string]int
+
+func makeHappinessGraph(input string) mapGraph {
+	graph := make(mapGraph)
+	for _, line := range strings.Split(input, "\n") {
+		var person1, gainLoss, person2 string
+		var amount int
+		fmt.Sscanf(strings.Trim(line, "."), "%s would %s %d happiness units by sitting next to %s",
+			&person1, &gainLoss, &amount, &person2)
+
+		// ensure nested map exists
+		if graph[person1] == nil {
+			graph[person1] = make(map[string]int)
+		}
+
+		graph[person1][person2] = amount
+		if gainLoss == "lose" {
+			graph[person1][person2] = -amount
+		}
+	}
+	return graph
+}
+
+func calcHappinessDiff(graph mapGraph, seating []string) int {
+	var diffs int
+	for i, person := range seating {
+		indexToLeft := (i - 1 + len(seating)) % len(seating)
+		indexToRight := (i + 1) % len(seating)
+		diffs += graph[person][seating[indexToLeft]] + graph[person][seating[indexToRight]]
+	}
+	return diffs
+}
diff --git a/2015/day13/main_test.go b/2015/day13/main_test.go
new file mode 100644
index 0000000..33f9ef1
--- /dev/null
+++ b/2015/day13/main_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_knightsOfTheDinnerTable(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"part1 actual", util.ReadFile("input.txt"), 1, 709},
+		{"part2 actual", util.ReadFile("input.txt"), 2, 668},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := knightsOfTheDinnerTable(tt.input, tt.part); got != tt.want {
+				t.Errorf("knightsOfTheDinnerTable() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}