Day 15: Part 1

Signed-off-by: James Griffin <james@unsupervised.ca>

fifteen/day_fifteen.go

@@ -0,0 +1,83 @@
+package fifteen
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type game struct {
+	starting  []int
+	numbers   []int
+	spoken    map[int]int
+	lastIndex map[int]int
+}
+
+func (g *game) load(filename string) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		input := strings.Split(scanner.Text(), ",")
+
+		for _, i := range input {
+			v, err := strconv.Atoi(i)
+			if err != nil {
+				return err
+			}
+
+			g.starting = append(g.starting, v)
+		}
+	}
+
+	return nil
+}
+
+func (g *game) valueAt(index int) int {
+	g.spoken = make(map[int]int, index)
+	g.lastIndex = make(map[int]int, index)
+	for i, n := range g.starting {
+		if i != len(g.starting)-1 {
+			g.spoken[n]++
+		}
+		g.lastIndex[n] = i
+		g.numbers = append(g.numbers, n)
+	}
+
+	for i := len(g.starting); i < index; i++ {
+		last := g.numbers[i-1]
+		g.spoken[last]++
+		lastIndex := g.lastIndex[last]
+		g.lastIndex[last] = i - 1
+
+		next := 0
+
+		if g.spoken[last] > 1 {
+			next = i - 1 - lastIndex
+		}
+
+		g.numbers = append(g.numbers, next)
+	}
+
+	if len(g.numbers) != index {
+		return -1
+	}
+
+	return g.numbers[index-1]
+}
+
+// PartOne What is the 2020th number given my input
+func PartOne() string {
+	g := game{}
+	if err := g.load("fifteen/input.txt"); err != nil {
+		return err.Error()
+	}
+
+	return fmt.Sprintf("The 2020th number is %d", g.valueAt(2020))
+}