2024 Day 1: Historian Hysteria; part 2

Author: jsageryd

2024/day-01/README.md

@@ -80,3 +80,55 @@ distances between all of the pairs you found. In the example above, this is `2 +
 
 Your actual left and right lists contain many location IDs. What is the total
 distance between your lists?
+
+## Part Two
+
+Your analysis only confirmed what everyone feared: the two lists of location IDs
+are indeed very different.
+
+Or are they?
+
+The Historians can't agree on which group made the mistakes or how to read most
+of the Chief's handwriting, but in the commotion you notice an interesting
+detail: a lot of location IDs appear in both lists! Maybe the other numbers
+aren't location IDs at all but rather misinterpreted handwriting.
+
+This time, you'll need to figure out exactly how often each number from the left
+list appears in the right list. Calculate a total similarity score by adding up
+each number in the left list after multiplying it by the number of times that
+number appears in the right list.
+
+Here are the same example lists again:
+
+```
+3   4
+4   3
+2   5
+1   3
+3   9
+3   3
+```
+
+For these example lists, here is the process of finding the similarity score:
+
+- The first number in the left list is `3`. It appears in the right list three
+  times, so the similarity score increases by `3 * 3 = 9`.
+
+- The second number in the left list is `4`. It appears in the right list once,
+  so the similarity score increases by `4 * 1 = 4`.
+
+- The third number in the left list is `2`. It does not appear in the right
+  list, so the similarity score does not increase (`2 * 0 = 0`).
+
+- The fourth number, `1`, also does not appear in the right list.
+
+- The fifth number, `3`, appears in the right list three times; the similarity
+  score increases by `9`.
+
+- The last number, `3`, appears in the right list three times; the similarity
+  score again increases by `9`.
+
+So, for these example lists, the similarity score at the end of this process is
+`31` (`9 + 4 + 0 + 0 + 9 + 9`).
+
+Once again consider your left and right lists. What is their similarity score?

2024/day-01/main.go

@@ -15,6 +15,7 @@ func main() {
 	}
 
 	fmt.Printf("Part 1: %d\n", part1(input))
+	fmt.Printf("Part 2: %d\n", part2(input))
 }
 
 func part1(input []string) int {
@@ -32,6 +33,24 @@ func part1(input []string) int {
 	return sum
 }
 
+func part2(input []string) int {
+	list1, list2 := parse(input)
+
+	freq := make(map[int]int)
+
+	for n := range list2 {
+		freq[list2[n]]++
+	}
+
+	var sum int
+
+	for n := range list1 {
+		sum += list1[n] * freq[list1[n]]
+	}
+
+	return sum
+}
+
 func parse(input []string) (list1, list2 []int) {
 	for _, line := range input {
 		var n1, n2 int

2024/day-01/main_test.go

@@ -2,17 +2,23 @@ package main
 
 import "testing"
 
+var input = []string{
+	"3   4",
+	"4   3",
+	"2   5",
+	"1   3",
+	"3   9",
+	"3   3",
+}
+
 func TestPart1(t *testing.T) {
-	input := []string{
-		"3   4",
-		"4   3",
-		"2   5",
-		"1   3",
-		"3   9",
-		"3   3",
+	if got, want := part1(input), 11; got != want {
+		t.Errorf("got %d, want %d", got, want)
 	}
+}
 
-	if got, want := part1(input), 11; got != want {
+func TestPart2(t *testing.T) {
+	if got, want := part2(input), 31; got != want {
 		t.Errorf("got %d, want %d", got, want)
 	}
 }

2024/day-01/output

@@ -1 +1,2 @@
 Part 1: 2904518
+Part 2: 18650129