feat: Solving day1 part two

Author: truggeri

docs/day1.md

@@ -44,4 +44,38 @@ In the example list above, the pairs and distances would be as follows:
 
 To find the total distance between the left list and the right list, add up the distances between all of the pairs you found. In the example above, this is `2 + 1 + 0 + 1 + 2 + 5`, a total distance of `11`!
 
-Your actual left and right lists contain many location IDs. What is the total distance between your lists?
+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:
+
+```txt
+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?

src/day1/day1.go

@@ -13,6 +13,19 @@ func Solve(input string) uint {
 	return solve(parseInput(input))
 }
 
+func PartTwo(input string) uint {
+	left, right := parseInput(input)
+	occurances := make(map[uint]uint)
+	for _, r := range right {
+		occurances[r] += 1
+	}
+	var result uint = 0
+	for _, l := range left {
+		result += l * occurances[l]
+	}
+	return result
+}
+
 func parseInput(input string) ([]uint, []uint) {
 	var a, b []uint
 	for _, line := range strings.Split(input, "\n") {
@@ -22,9 +35,9 @@ func parseInput(input string) ([]uint, []uint) {
 		}
 
 		x, _ := strconv.Atoi(values[0])
-		a = slices.Concat(a, []uint{uint(x)})
+		a = append(a, uint(x))
 		y, _ := strconv.Atoi(values[1])
-		b = slices.Concat(b, []uint{uint(y)})
+		b = append(b, uint(y))
 	}
 	return a, b
 }
@@ -34,7 +47,7 @@ func solve(a, b []uint) uint {
 	slices.Sort(b)
 	var sum uint = 0
 	for i := range a {
-		sum = sum + absDiffUint(a[i], b[i])
+		sum += absDiffUint(a[i], b[i])
 	}
 	return sum
 }

src/day1/day1_test.go

@@ -16,3 +16,16 @@ func TestSample(t *testing.T) {
 		t.Errorf("Calculated solution was not expected")
 	}
 }
+
+func TestSamplePartTwo(t *testing.T) {
+	input := `3   4
+4   3
+2   5
+1   3
+3   9
+3   3`
+	result := PartTwo(input)
+	if result != 31 {
+		t.Errorf("Calculated solution was not expected")
+	}
+}