day2

day2/main.go

@@ -0,0 +1,201 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func readLines(filename string) []string {
+	raw, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return strings.Split(string(raw), "\n")
+}
+
+type id struct {
+	value  uint64
+	length uint8
+}
+
+func (x id) isSilly() bool {
+	c := uint64(1)
+	for range x.length / 2 {
+		c *= 10
+	}
+	return x.value/c == x.value%c
+}
+
+type pattern struct {
+	length  uint8
+	repeats uint8
+}
+
+var patterns = map[uint8][]pattern{
+	1:  {},
+	2:  {pattern{length: 1, repeats: 2}},
+	3:  {pattern{length: 1, repeats: 3}},
+	4:  {pattern{length: 1, repeats: 4}, pattern{length: 2, repeats: 2}},
+	5:  {pattern{length: 1, repeats: 5}},
+	6:  {pattern{length: 1, repeats: 6}, pattern{length: 2, repeats: 3}, pattern{length: 3, repeats: 2}},
+	7:  {pattern{length: 1, repeats: 7}},
+	8:  {pattern{length: 1, repeats: 8}, pattern{length: 2, repeats: 4}, pattern{length: 4, repeats: 2}},
+	9:  {pattern{length: 1, repeats: 9}, pattern{length: 3, repeats: 3}},
+	10: {pattern{length: 1, repeats: 10}, pattern{length: 5, repeats: 2}, pattern{length: 2, repeats: 5}},
+	11: {pattern{length: 1, repeats: 11}},
+}
+
+func (x id) isVerySilly() bool {
+	ps := patterns[x.length]
+	if ps == nil {
+		panic(x.length)
+	}
+PATTERNS:
+	for _, pattern := range ps {
+		mod := uint64(1)
+		for range pattern.length {
+			mod *= 10
+		}
+		value := x.value
+		candidate := value % mod
+		if candidate == 0 {
+			continue
+		}
+		for range pattern.repeats - 1 {
+			value /= mod
+			if value%mod != candidate {
+				continue PATTERNS
+			}
+		}
+		return true
+	}
+	return false
+}
+
+type seq struct {
+	start id
+	stop  id
+}
+
+func parseLine(line string) ([]seq, error) {
+	parts := strings.Split(line, ",")
+	seqs := make([]seq, len(parts))
+	for i, part := range strings.Split(line, ",") {
+		pieces := strings.Split(part, "-")
+		if len(pieces) != 2 {
+			return seqs, fmt.Errorf("invalid part")
+		}
+		var err error
+		s := seq{
+			start: id{length: uint8(len(pieces[0]))},
+			stop:  id{length: uint8(len(pieces[1]))},
+		}
+		s.start.value, err = strconv.ParseUint(pieces[0], 10, 64)
+		if err != nil {
+			return seqs, err
+		}
+		s.stop.value, err = strconv.ParseUint(pieces[1], 10, 64)
+		if err != nil {
+			return seqs, err
+		}
+		if s.stop.value < s.start.value {
+			return seqs, fmt.Errorf("invalid range")
+		}
+		if !(s.stop.length == s.start.length || s.stop.length == s.start.length+1) {
+			return seqs, fmt.Errorf("unexpectedly large range %v", s)
+		}
+		seqs[i] = s
+	}
+	return seqs, nil
+}
+
+func findSillyPatterns(s seq) []uint64 {
+	sillies := []uint64{}
+	oddStart := s.start.length%2 == 1
+	oddStop := s.stop.length%2 == 1
+	if oddStart && oddStop {
+		return sillies
+	}
+	if oddStart {
+		s.start.value = 1
+		s.start.length++
+		for range s.start.length - 1 {
+			s.start.value *= 10
+		}
+	}
+	if oddStop {
+		s.stop.value = 0
+		s.stop.length--
+		c := 1
+		for range s.stop.length {
+			s.stop.value += uint64(9 * c)
+			c *= 10
+		}
+	}
+	if s.stop.value < s.start.value {
+		return sillies
+	}
+	for {
+		if s.start.value > s.stop.value {
+			break
+		}
+		if s.start.isSilly() {
+			sillies = append(sillies, s.start.value)
+		}
+		s.start.value++
+	}
+	return sillies
+}
+
+func findVerySillyPatterns(s seq) []uint64 {
+	sillies := []uint64{}
+	pivot := uint64(1)
+	for range s.stop.length - 1 {
+		pivot *= 10
+	}
+	for {
+		if s.start.value > s.stop.value {
+			break
+		}
+		if s.start.isVerySilly() {
+			sillies = append(sillies, s.start.value)
+		}
+		s.start.value++
+		if s.start.value == pivot {
+			s.start.length++
+		}
+	}
+	return sillies
+}
+
+func main() {
+	filename := "part0.txt"
+	if len(os.Args) > 1 {
+		filename = os.Args[1]
+	}
+	lines := readLines(filename)
+	if len(lines) != 1 {
+		panic("Invalid lines")
+	}
+	seqs, err := parseLine(lines[0])
+	if err != nil {
+		panic(err)
+	}
+	total := uint64(0)
+	for _, seq := range seqs {
+		for _, value := range findSillyPatterns(seq) {
+			total += value
+		}
+	}
+	fmt.Printf("%v\n", total)
+
+	total = uint64(0)
+	for _, seq := range seqs {
+		for _, value := range findVerySillyPatterns(seq) {
+			total += value
+		}
+	}
+	fmt.Printf("%v\n", total)
+}