commit 600b41ba7a31440343b32dc9b3e0bf3cd4c163a2
Author: Mindaugas Bernotas <mindaugas@bernotas.lt>
Date:   Wed Dec 3 22:38:57 2025 +0200

    Init commit

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c7e3278
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
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+debug*.txt
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..e69de29
diff --git a/cmd/day1/example.txt b/cmd/day1/example.txt
new file mode 100644
index 0000000..53287c7
--- /dev/null
+++ b/cmd/day1/example.txt
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diff --git a/cmd/day1/input.txt b/cmd/day1/input.txt
new file mode 100644
index 0000000..4e124bc
--- /dev/null
+++ b/cmd/day1/input.txt
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diff --git a/cmd/day1/main.go b/cmd/day1/main.go
new file mode 100644
index 0000000..49b0385
--- /dev/null
+++ b/cmd/day1/main.go
@@ -0,0 +1,147 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug flag.")
+	flag.Parse()
+
+	d1 := dial{n: 50}
+	d2 := dial{n: 50}
+	if *debug {
+		fmt.Println("LINE d1 d2")
+		fmt.Println("     50 50")
+	}
+
+	sc := bufio.NewScanner(os.Stdin)
+	for sc.Scan() {
+		if sc.Text() == "" {
+			log.Fatal("empty line")
+		}
+		n, err := strconv.ParseUint(sc.Text()[1:], 10, 0)
+		if err != nil {
+			log.Fatal(err)
+		}
+		i := int(n)
+		if sc.Text()[0] == 'L' {
+			i = -int(n)
+		}
+
+		// if err := handleLine(d1.Add1, sc.Text()); err != nil {
+		// 	log.Fatal(err)
+		// }
+		if err := handleLine(d1.Add1, sc.Text()); err != nil {
+			log.Fatal(err)
+		}
+		if err := handleLine(d2.Add2, sc.Text()); err != nil {
+			log.Fatal(err)
+		}
+		if *debug {
+			fmt.Printf("%+4d %2d %2d (%+3d %+3d)\n", i, d1.n, d2.n, d1.lastClicks, d2.lastClicks)
+		}
+	}
+	if err := sc.Err(); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println("zeros:", d1.zeros, d2.zeros)
+	fmt.Println("zero passes:", d1.zeroClicks, d2.zeroClicks)
+}
+
+func handleLine(add func(int), line string) error {
+	if line == "" {
+		return errors.New("empty line")
+	}
+	n, err := strconv.ParseUint(line[1:], 10, 0)
+	if err != nil {
+		return err
+	}
+	switch line[0] {
+	case 'L':
+		add(-int(n))
+		return nil
+	case 'R':
+		add(int(n))
+		return nil
+	default:
+		return fmt.Errorf("unexpected line: %q", line)
+	}
+}
+
+type dial struct {
+	n          int
+	lastClicks int
+	zeros      int
+	zeroClicks int
+}
+
+func (d *dial) Add1(n int) {
+	wasAtZero := d.n == 0
+	d.n += n
+	step := +100
+	if d.n < 0 {
+		step = -100
+	}
+	d.lastClicks = 0
+	for d.n < 0 || d.n > 99 {
+		d.lastClicks++
+		d.n -= step
+	}
+	if d.lastClicks > 0 && wasAtZero {
+		d.lastClicks--
+	}
+	if d.n == 0 {
+		d.zeros++
+		if d.lastClicks == 0 {
+			d.lastClicks++
+		}
+	}
+	d.zeroClicks += d.lastClicks
+}
+
+func (d *dial) Add2(diff int) {
+	n := d.n + diff
+
+	var clicks int
+	switch {
+	case n < 0:
+		wasAtZero := d.n == 0
+		for n < 0 {
+			if wasAtZero {
+				wasAtZero = false
+			} else {
+				clicks++
+			}
+			n += 100
+		}
+		if n == 0 && !wasAtZero {
+			clicks++
+		}
+
+	case n > 99:
+		for n > 99 {
+			clicks++
+			n -= 100
+		}
+
+	default:
+		if n == 0 && d.n != 0 {
+			clicks++
+		}
+	}
+
+	if n == 0 {
+		d.zeros++
+	}
+	d.n = n
+	d.lastClicks = clicks
+	d.zeroClicks += clicks
+}
diff --git a/cmd/day1/main_test.go b/cmd/day1/main_test.go
new file mode 100644
index 0000000..6ec28bc
--- /dev/null
+++ b/cmd/day1/main_test.go
@@ -0,0 +1,80 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_dial_Add2(t *testing.T) {
+	tests := []struct {
+		start      int
+		turn       int
+		want       int
+		wantClicks int
+	}{
+		{
+			start:      50,
+			turn:       300,
+			want:       50,
+			wantClicks: 3,
+		},
+		{
+			start:      50,
+			turn:       -300,
+			want:       50,
+			wantClicks: 3,
+		},
+
+		{
+			start:      0,
+			turn:       -100,
+			want:       0,
+			wantClicks: 1,
+		},
+		{
+			start:      1,
+			turn:       -101,
+			want:       0,
+			wantClicks: 2,
+		},
+		{
+			start:      99,
+			turn:       -199,
+			want:       0,
+			wantClicks: 2,
+		},
+
+		{
+			start:      0,
+			turn:       100,
+			want:       0,
+			wantClicks: 1,
+		},
+		{
+			start:      99,
+			turn:       101,
+			want:       0,
+			wantClicks: 2,
+		},
+		{
+			start:      1,
+			turn:       199,
+			want:       0,
+			wantClicks: 2,
+		},
+	}
+	for _, tt := range tests {
+		name := fmt.Sprintf("R%d", tt.turn)
+		if tt.turn < 0 {
+			name = fmt.Sprintf("L%d", -tt.turn)
+		}
+		t.Run(name, func(t *testing.T) {
+			d := dial{n: tt.start}
+			d.Add2(tt.turn)
+			assert.Equal(t, tt.want, d.n, "dial")
+			assert.Equal(t, tt.wantClicks, d.zeroClicks, "clicks")
+		})
+	}
+}
diff --git a/cmd/day2/digits.go b/cmd/day2/digits.go
new file mode 100644
index 0000000..3b55ba2
--- /dev/null
+++ b/cmd/day2/digits.go
@@ -0,0 +1,22 @@
+package main
+
+func digits(n uint) []uint8 {
+	if n == 0 {
+		return []uint8{0}
+	}
+	var digits []uint8
+	for ; n != 0; n /= 10 {
+		digits = append(digits, uint8(n%10))
+	}
+	return digits
+}
+
+func number(digits []uint8) uint {
+	var n uint
+	base := uint(1)
+	for _, d := range digits {
+		n += uint(d) * base
+		base *= 10
+	}
+	return n
+}
diff --git a/cmd/day2/digits_test.go b/cmd/day2/digits_test.go
new file mode 100644
index 0000000..3f36657
--- /dev/null
+++ b/cmd/day2/digits_test.go
@@ -0,0 +1,30 @@
+package main
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_digits_number(t *testing.T) {
+	tests := []struct {
+		n uint
+		d []uint8
+	}{
+		{n: 0, d: []uint8{0}},
+		{n: 1234, d: []uint8{4, 3, 2, 1}},
+		{n: 12345, d: []uint8{5, 4, 3, 2, 1}},
+	}
+	for _, tt := range tests {
+		name := strconv.FormatUint(uint64(tt.n), 10)
+		t.Run(name, func(t *testing.T) {
+			t.Run("digits", func(t *testing.T) {
+				assert.Equal(t, tt.d, digits(tt.n))
+			})
+			t.Run("number", func(t *testing.T) {
+				assert.Equal(t, tt.n, number(tt.d))
+			})
+		})
+	}
+}
diff --git a/cmd/day2/example.txt b/cmd/day2/example.txt
new file mode 100644
index 0000000..a3f22ef
--- /dev/null
+++ b/cmd/day2/example.txt
@@ -0,0 +1 @@
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124
diff --git a/cmd/day2/input.txt b/cmd/day2/input.txt
new file mode 100644
index 0000000..f3832db
--- /dev/null
+++ b/cmd/day2/input.txt
@@ -0,0 +1 @@
+959516-995437,389276443-389465477,683-1336,15687-26722,91613-136893,4-18,6736-12582,92850684-93066214,65-101,6868676926-6868700146,535033-570760,826141-957696,365650-534331,1502-2812,309789-352254,79110404-79172400,18286593-18485520,34376-65398,26-63,3333208697-3333457635,202007-307147,1859689-1936942,9959142-10053234,2318919-2420944,5142771457-5142940464,1036065-1206184,46314118-46413048,3367-6093,237-481,591751-793578
diff --git a/cmd/day2/main.go b/cmd/day2/main.go
new file mode 100644
index 0000000..d077e27
--- /dev/null
+++ b/cmd/day2/main.go
@@ -0,0 +1,97 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"math"
+	"os"
+	"slices"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug.")
+	flag.Parse()
+
+	ranges, err := readRanges(os.Stdin)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var sum1, sum2 uint
+	for _, r := range ranges {
+		invalid1 := handleRange(r, scanPart1)
+		invalid2 := handleRange(r, scanPart2)
+
+		if *debug {
+			fmt.Printf("range: %d-%d\n", r.from, r.to)
+		}
+		if *debug && len(invalid1) > 0 {
+			fmt.Println("invalid (part 1):")
+			for _, id := range invalid1 {
+				fmt.Printf("  %d\n", id)
+			}
+		}
+		if *debug && len(invalid2) > 0 {
+			fmt.Println("invalid (part 2):")
+			for _, id := range invalid2 {
+				fmt.Printf("  %d\n", id)
+			}
+		}
+
+		for _, id := range invalid1 {
+			sum1 += id
+		}
+		for _, id := range invalid2 {
+			sum2 += id
+		}
+	}
+
+	fmt.Println("sum (part 1):", sum1)
+	fmt.Println("sum (part 2):", sum2)
+}
+
+func handleRange(r idRange, scan func(id uint) (advance uint, ok bool)) []uint {
+	var invalid []uint
+	for id := r.from; id <= r.to; {
+		advance, ok := scan(id)
+		if !ok {
+			invalid = append(invalid, id)
+		}
+		id += advance
+	}
+	return invalid
+}
+
+func scanPart1(id uint) (advance uint, ok bool) {
+	digits := digits(id)
+	if len(digits)%2 == 1 { // odd is always valid
+		nextSmallestEven := uint(math.Pow10(len(digits)))
+		return nextSmallestEven - id, true
+	}
+	var (
+		x     = uint(math.Pow10(len(digits) / 2))
+		left  = id / x
+		right = id % x
+	)
+	return 1, left != right
+}
+
+func scanPart2(id uint) (advance uint, ok bool) {
+	digits := digits(id)
+	if len(digits) <= 1 {
+		return 1, true
+	}
+
+loop:
+	for i := 1; i <= len(digits)/2; i++ {
+		ref := digits[:i]
+		for chunk := range slices.Chunk(digits, i) {
+			if !slices.Equal(chunk, ref) {
+				continue loop
+			}
+		}
+		return 1, false
+	}
+	return 1, true
+}
diff --git a/cmd/day2/main_test.go b/cmd/day2/main_test.go
new file mode 100644
index 0000000..4bf2d16
--- /dev/null
+++ b/cmd/day2/main_test.go
@@ -0,0 +1,77 @@
+package main
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_scanPart1(t *testing.T) {
+	tests := []struct {
+		id      uint
+		advance uint
+		ok      bool
+	}{
+		// {
+		// 	id:      9,
+		// 	advance: 1,
+		// 	ok:      true,
+		// },
+		// {
+		// 	id:      10,
+		// 	advance: 1,
+		// 	ok:      true,
+		// },
+		// {
+		// 	id:      11,
+		// 	advance: 1,
+		// 	ok:      false,
+		// },
+		// {
+		// 	id:      99,
+		// 	advance: 1,
+		// 	ok:      false,
+		// },
+		// {
+		// 	id:      100,
+		// 	advance: 900,
+		// 	ok:      true,
+		// },
+		{
+			id:      1234,
+			advance: 1,
+			ok:      true,
+		},
+	}
+	for _, tt := range tests {
+		name := strconv.FormatUint(uint64(tt.id), 10)
+		t.Run(name, func(t *testing.T) {
+			advance, ok := scanPart1(tt.id)
+			assert.Equal(t, tt.advance, advance, "advance")
+			assert.Equal(t, tt.ok, ok, "ok")
+		})
+	}
+}
+
+func Test_scanPart2(t *testing.T) {
+	tests := []struct {
+		id      uint
+		advance uint
+		ok      bool
+	}{
+		{
+			id:      111,
+			advance: 1,
+			ok:      false,
+		},
+	}
+	for _, tt := range tests {
+		name := strconv.FormatUint(uint64(tt.id), 10)
+		t.Run(name, func(t *testing.T) {
+			advance, ok := scanPart2(tt.id)
+			assert.Equal(t, tt.advance, advance, "advance")
+			assert.Equal(t, tt.ok, ok, "ok")
+		})
+	}
+}
diff --git a/cmd/day2/range.go b/cmd/day2/range.go
new file mode 100644
index 0000000..f25d409
--- /dev/null
+++ b/cmd/day2/range.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+type idRange struct {
+	from, to uint
+}
+
+func readRanges(r io.Reader) ([]idRange, error) {
+	sc := bufio.NewScanner(r)
+	sc.Split(scanSimpleCSV)
+
+	var ranges []idRange
+	for sc.Scan() {
+		before, after, ok := strings.Cut(sc.Text(), "-")
+		if !ok {
+			return nil, fmt.Errorf("range %q: missing dash", sc.Text())
+		}
+		from, err := strconv.ParseUint(before, 10, 0)
+		if err != nil {
+			return nil, fmt.Errorf("range %q: %w", sc.Text(), err)
+		}
+		to, err := strconv.ParseUint(after, 10, 0)
+		if err != nil {
+			return nil, fmt.Errorf("range %q: %w", sc.Text(), err)
+		}
+		if from > to {
+			return nil, fmt.Errorf("range %q: refers from higher to lower", sc.Text())
+		}
+		ranges = append(ranges, idRange{
+			from: uint(from),
+			to:   uint(to),
+		})
+	}
+	return ranges, sc.Err()
+}
+
+func scanSimpleCSV(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+	if i := bytes.IndexByte(data, ','); i >= 0 {
+		return i + 1, data[:i], nil
+	}
+	if atEOF {
+		if trimmed, ok := bytes.CutSuffix(data, []byte("\r\n")); ok {
+			return len(data), trimmed, nil
+		}
+		if trimmed, ok := bytes.CutSuffix(data, []byte{'\n'}); ok {
+			return len(data), trimmed, nil
+		}
+		return len(data), data, nil
+	}
+	return 0, nil, nil
+}
diff --git a/cmd/day3/battery.go b/cmd/day3/battery.go
new file mode 100644
index 0000000..2cb89ca
--- /dev/null
+++ b/cmd/day3/battery.go
@@ -0,0 +1,22 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+)
+
+func readBatteries(r io.Reader) (banks [][]uint8, err error) {
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		var bank []uint8
+		for _, b := range sc.Bytes() {
+			if b < '0' || b > '9' {
+				return nil, fmt.Errorf("bank %q: invalid joltage %q", sc.Text(), b)
+			}
+			bank = append(bank, b-'0')
+		}
+		banks = append(banks, bank)
+	}
+	return banks, sc.Err()
+}
diff --git a/cmd/day3/example.txt b/cmd/day3/example.txt
new file mode 100644
index 0000000..7255fca
--- /dev/null
+++ b/cmd/day3/example.txt
@@ -0,0 +1,4 @@
+987654321111111
+811111111111119
+234234234234278
+818181911112111
diff --git a/cmd/day3/input.txt b/cmd/day3/input.txt
new file mode 100644
index 0000000..fc7c422
--- /dev/null
+++ b/cmd/day3/input.txt
@@ -0,0 +1,200 @@
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diff --git a/cmd/day3/main.go b/cmd/day3/main.go
new file mode 100644
index 0000000..368a1c9
--- /dev/null
+++ b/cmd/day3/main.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"cmp"
+	"flag"
+	"fmt"
+	"log"
+	"math"
+	"os"
+)
+
+func main() {
+	debug := flag.Bool("debug", false, "Debug.")
+	flag.Parse()
+
+	f := os.Stdin
+	if flag.NArg() > 0 {
+		var err error
+		f, err = os.Open(flag.Arg(0))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+	}
+
+	banks, err := readBatteries(f)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var sum1, sum2 uint
+	for _, bank := range banks {
+		power1 := bestPowerPart1(bank)
+		power2 := bestPowerPart2(bank)
+		if *debug {
+			for _, b := range bank {
+				fmt.Print(b)
+			}
+			fmt.Println(":", power1, power2)
+		}
+		sum1 += power1
+		sum2 += power2
+	}
+
+	fmt.Println("sum (part 1):", sum1)
+	fmt.Println("sum (part 2):", sum2)
+}
+
+func bestPowerPart1(bank []uint8) uint {
+	// i := maxIndex(bank[:len(bank)-1])
+	// j := maxIndex(bank[i+1:]) + i + 1
+	// return uint(bank[i])*10 + uint(bank[j])
+	return bestPower(bank, 2)
+}
+
+func bestPowerPart2(bank []uint8) uint {
+	return bestPower(bank, 12)
+}
+
+func bestPower(bank []uint8, batteries int) uint {
+	if len(bank) < batteries {
+		panic(fmt.Sprintf("bank (%d) is smaller than requested batteries (%d)", len(bank), batteries))
+	}
+	var joltage uint
+	var i int
+	for j := batteries - 1; j >= 0; j-- {
+		k := maxIndex(bank[i : len(bank)-j])
+		k += i
+		i = k + 1
+		joltage += uint(bank[k]) * uint(math.Pow10(j))
+	}
+	return joltage
+}
+
+func maxIndex[S ~[]E, E cmp.Ordered](x S) int {
+	if len(x) < 1 {
+		panic("maxIndex: empty list")
+	}
+	i := 0
+	for j := 1; j < len(x); j++ {
+		if x[j] > x[i] {
+			i = j
+		}
+	}
+	return i
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..3314e41
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,11 @@
+module git.bernotas.lt/sewiti/advent-of-code-2025
+
+go 1.25.5
+
+require github.com/stretchr/testify v1.11.1
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..c4c1710
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=