Advent of Code 2025 in GO - Day 1 - Part #2

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In my previous blog post part 1 I showed one way to solve the first part of day 1. Now let us continue with part two of day 1. As always with Advent of Code challenges the second part of the challenge is much harder to solve. Sometimes I even had to rethink my challenge one solution.

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In case you prefer learning with a video, checkout my YouTube video 📺

Day 1: Secret Entrance Part 2

Okay now the big difference is we always also count when 0 is passed and also when the dial stops at 0. I had to rethink my whole solution for part one here is what I came up with:

func countDialPointsAndPassedZero(rotations []Rotator) int {
	dialPos := 50
	counter := 0
	for _, rotator := range rotations {
		if rotator.direction == "R" {
			counter += (dialPos + rotator.steps) / allHundredPos
			dialPos = (dialPos + rotator.steps) % allHundredPos
		} else {
			// If we start at 0, we only hit 0 again after a full 100 steps
			startPos := dialPos
			if dialPos == zeroPos {
				startPos = allHundredPos
			}
			// Check if we move far enough to reach or pass the '0' mark.
			// Note: if we start at 0, 'startPos' is 100, meaning we only hit zero again if we complete a full rotation.
			if rotator.steps >= startPos {
				// We count 1 for the first time we hit or pass zero.
				// Then add 1 for every full 100-step rotation thereafter.
				counter += moveOneStep + (rotator.steps-startPos)/allHundredPos
			}

			// Update the dial position:
			// Use (steps % 100) to find the relative movement, subtract it from the current position.
			// Add 100 before taking the modulo again to ensure the result is always a positive index between 0 and 99.
			dialPos = (dialPos - (rotator.steps % allHundredPos) + allHundredPos) % allHundredPos
		}
	}
	return counter
}

Initialization: The dial starts at position 50.
Right Rotations ("R"): For each right rotation, it calculates how many full 100-step rotations occur (adding to the counter) and updates the dial position using modulo arithmetic.
Left Rotations (not "R"): For left rotations, it checks if the movement is enough to reach or pass "0" from the current position. If so, it increments the counter for each full rotation and updates the dial position, ensuring it stays within 0–99.
Return Value: The total count of times the dial passed or landed on "0" during all rotations.

Unit Testing

Now, open again secret_entrance_test.go and let us again start with a table-driven test for the core logic:

func TestCountDialPointsAndPassedZero(t *testing.T) {
	tests := []struct {
		name  string
		input []Rotator
		want  int
	}{
		{
			name: "multiple instructions",
			input: []Rotator{
				{"L", 68},
				{"L", 30},
				{"R", 48},
				{"L", 5},
				{"R", 60},
				{"L", 55},
				{"L", 1},
				{"L", 99},
				{"R", 14},
				{"L", 82},
			},
			want: 6,
		},
		{
			name:  "zero instructions",
			input: []Rotator{},
			want:  0,
		},
		{
			name: "single instruction",
			input: []Rotator{
				{"R", 1000},
			},
			want: 10,
		},
	}

	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			got := countDialPointsAndPassedZero(test.input)
			if got != test.want {
				t.Errorf("%s: countDialPointsAndPassedZero() = %d; want %d", test.name, got, test.want)
			}
		})
	}
}

For this test I simply added the sample input of rotations which was given from the instructions. Now, if you run go test ./... from your Terminal in the root directory you should see: ok advent-of-code-2025-blog/internal/util 0.502s

Solution part two

Open again main.go add these lines:

package main

import (
	"advent-of-code-2025-blog/internal/util"
	"fmt"
	"log"
)

func main() {
	input, err := util.ReadInput("../advent-of-code-2025/cmd/day01/input1.txt")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println("The password for Part1 is: ", countDialPointsZero(generateInstructions(input)))
	fmt.Println("The password for Part2 is: ", countDialPointsAndPassedZero(generateInstructions(input)))
}

Then run the code and see copy the output.

Conclusion

This Go implementation for day 1 part 2 should solve the challenge efficiently as possible. If you had problems to follow along for part one and two just checkout my: GitHub Repo

Here is the link to Part 1

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