Add day 2 solution.

2018/day02-1.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/util"
+)
+
+func main() {
+	lines := util.ReadInput()
+
+	twos := 0
+	threes := 0
+	for _, id := range lines {
+		// here we build up a map of letters and counts
+		letters := make(map[byte]int)
+		for i := 0; i < len(id); i++ {
+			letters[id[i]] += 1
+		}
+
+		// these are the values to add to the 'twos' and 'threes'
+		// count, respectively. They will only ever be 0 or 1.
+		add2 := 0
+		add3 := 0
+
+		// now we iterate over the map of letter->count, and set
+		// add2 and add3 if we find any values equal to 2 or 3
+		for _, v := range letters {
+			if v == 2 {
+				add2 = 1
+			}
+			if v == 3 {
+				add3 = 1
+			}
+		}
+
+		twos += add2
+		threes += add3
+	}
+
+	fmt.Println(twos * threes)
+}

2018/day02-2.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/util"
+)
+
+func main() {
+	ids := util.ReadInput()
+
+	for _, box1 := range ids {
+		for _, box2 := range ids {
+			diffIndex := -1 // diffIndex doubles as the letter to remove and a 'found' flag
+			for i := range box1 {
+				if box1[i] != box2[i] {
+					if diffIndex != -1 {
+						// If we reach here, we've detected more than one error, so we set
+						// diffIndex back to -1 here to avoid the conditional below this loop.
+						diffIndex = -1
+						break
+					}
+					diffIndex = i
+				}
+			}
+			if diffIndex != -1 {
+				fmt.Printf("%s%s\n", box1[0:diffIndex], box1[diffIndex+1:])
+				return
+			}
+		}
+	}
+}

2018/internal/util/input.go

@@ -0,0 +1,24 @@
+package util
+
+import (
+	"bufio"
+	"os"
+)
+
+// ReadInput isn't here to make friends. It is highly specific to this domain.
+// It assumes the first argument on the command-line is a file with input. It
+// returns an array of strings containing the lines of that file.
+func ReadInput() []string {
+	file, err := os.Open(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+
+	lines := []string{}
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+
+	return lines
+}