1 files changed, 144 insertions, 0 deletions
diff --git a/day23/main.go b/day23/main.go
new file mode 100644
index 0000000..81cc6a0
--- /dev/null
+++ b/day23/main.go
@@ -0,0 +1,144 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+)
+
+func main() {
+	fmt.Println(part1(readData("example.txt")), 7)
+	fmt.Println(part1(readData("data.txt")), 1043)
+	fmt.Println("co,de,ka,ta")
+	fmt.Println(part2(readData("example.txt")))
+	fmt.Println("ai,bk,dc,dx,fo,gx,hk,kd,os,uz,xn,yk,zs")
+	fmt.Println(part2(readData("data.txt")))
+}
+
+const SIZE = 26 * 26
+
+func buildAdjacency(pairs [][2]int) [][]bool {
+	matrix := make([][]bool, SIZE)
+	for a := range matrix {
+		matrix[a] = make([]bool, SIZE)
+	}
+
+	for _, p := range pairs {
+		matrix[p[0]][p[1]] = true
+		matrix[p[1]][p[0]] = true
+	}
+
+	return matrix
+}
+
+func part1(pairs [][2]int) (count int) {
+	matrix := buildAdjacency(pairs)
+	triples := make(map[[3]int]bool)
+
+	for a := range SIZE {
+		connectedToA := make([]int, 0)
+		for b := a + 1; b < SIZE; b++ {
+			if matrix[a][b] {
+				connectedToA = append(connectedToA, b)
+			}
+		}
+		for ib, b := range connectedToA {
+			for ic := ib + 1; ic < len(connectedToA); ic++ {
+				c := connectedToA[ic]
+				if matrix[b][c] {
+					sortTriple(&a, &b, &c)
+					triples[[3]int{a, b, c}] = true
+				}
+			}
+		}
+	}
+
+	for t := range triples {
+		if containsT(t[0]) || containsT(t[1]) || containsT(t[2]) {
+			count++
+		}
+	}
+
+	return count
+}
+
+func part2(pairs [][2]int) string {
+	matrix := buildAdjacency(pairs)
+	sets := [][]int{}
+
+	for a := range SIZE {
+		isInAnySet := false
+		for i, set := range sets {
+			connectedToAll := true
+			for _, other := range set {
+				if !matrix[a][other] {
+					connectedToAll = false
+					break
+				}
+			}
+			if connectedToAll {
+				sets[i] = append(set, a)
+				isInAnySet = true
+			}
+		}
+		if !isInAnySet {
+			sets = append(sets, []int{a})
+		}
+	}
+
+	maxSet := []int{}
+	for _, set := range sets {
+		if len(set) > len(maxSet) {
+			maxSet = set
+		}
+	}
+
+	ids := make([]string, 0, len(maxSet))
+	for _, id := range maxSet {
+		ids = append(ids, numToId(id))
+	}
+
+	sort.Sort(sort.StringSlice(ids))
+	return strings.Join(ids, ",")
+}
+
+func containsT(x int) bool {
+	const MIN = int('t'-'a') * 26
+	const MAX = MIN + 25
+	return x >= MIN && x <= MAX
+}
+
+func sortTriple(a, b, c *int) {
+	maybeSwap := func(a, b *int) {
+		if *a > *b {
+			*a, *b = *b, *a
+		}
+	}
+	maybeSwap(a, b)
+	maybeSwap(b, c)
+	maybeSwap(a, b)
+	maybeSwap(b, c)
+}
+
+func idToNum(s string) int {
+	return int(s[0]-'a')*26 + int(s[1]-'a')
+}
+
+func numToId(id int) string {
+	return string([]byte{byte(id/26 + 'a'), byte(id%26 + 'a')})
+}
+
+func readData(fileName string) (pairs [][2]int) {
+	fp, _ := os.Open(fileName)
+	scanner := bufio.NewScanner(fp)
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		pair := [2]int{idToNum(line[0:2]), idToNum(line[3:5])}
+		pairs = append(pairs, pair)
+	}
+
+	return
+}