aoc-2024/cmd/day_4/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
)

const MAX_LOOP = 1_000_000

func trim(bs []byte) []byte {
	last := len(bs) - 1
	if last >= 0 && bs[last] == '\n' {
		return bs[:last]
	}
	return bs
}

func main() {
	var dim int
	var rows [][]byte

	r := bufio.NewReader(os.Stdin)

	line, err := r.ReadBytes('\n')
	if err != nil {
		log.Fatalf("While reading input: %v", err)
	}
	line = trim(line)
	dim = len(line)
	rows = make([][]byte, dim)
	rows[0] = line

	for i := 1; i < dim; i++ {
		line, err = r.ReadBytes('\n')
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading input: %v", err)
		}
		rows[i] = trim(line)
	}

	diags := dim - 4 + dim - 4 + 1
	ctrs := 1 + dim + diags + diags
	bufs := make([]byte, ctrs*4)
	offset := 0

	horizontal := make([]*Counter, 1)
	horizontal[0] = new(Counter)
	horizontal[0].Init(bufs[offset : offset+4])
	offset += 4

	vertical := make([]*Counter, dim)
	for i := 0; i < dim; i++ {
		vertical[i] = new(Counter)
		vertical[i].Init(bufs[offset : offset+4])
		offset += 4
	}
	swDiag := make([]*Counter, diags)
	for i := 0; i < diags; i++ {
		swDiag[i] = new(Counter)
		swDiag[i].Init(bufs[offset : offset+4])
		offset += 4
	}
	seDiag := make([]*Counter, diags)
	for i := 0; i < diags; i++ {
		seDiag[i] = new(Counter)
		seDiag[i].Init(bufs[offset : offset+4])
		offset += 4
	}

	diagLag := 3
	for i := 0; i < dim; i++ {
		horizontal[0].Reset()
		for j := 0; j < dim; j++ {
			e := rows[i][j]

			horizontal[0].Event(e)
			vertical[j].Event(e)

			swIdx := i + j - diagLag
			if 0 <= swIdx && swIdx < diags {
				swDiag[swIdx].Event(e)
			}
			seIdx := j - i - (diagLag + 1 - dim)
			if 0 <= seIdx && seIdx < diags {
				seDiag[seIdx].Event(e)
			}
		}
	}

	sum := 0
	for _, c := range horizontal {
		sum += c.Count()
	}
	for _, c := range vertical {
		sum += c.Count()
	}
	for _, c := range swDiag {
		sum += c.Count()
	}
	for _, c := range seDiag {
		sum += c.Count()
	}
	fmt.Println(sum)
}

type Counter struct {
	cnt int
	buf []byte
}

func (c *Counter) Init(buf []byte) {
	c.buf = buf
}

func (c *Counter) Reset() {
	c.buf = c.buf[:0]
}

func (c *Counter) Event(e byte) {
	switch {
	case e == 'X' && string(c.buf) == "SAM" ||
		e == 'S' && string(c.buf) == "XMA":
		c.cnt++
		c.Reset()
		c.buf = append(c.buf, e)

	case e == 'M' && string(c.buf) == "SA" ||
		e == 'A' && string(c.buf) == "S" ||
		e == 'S' && string(c.buf) == "" ||
		e == 'A' && string(c.buf) == "XM" ||
		e == 'M' && string(c.buf) == "X" ||
		e == 'X' && string(c.buf) == "":
		c.buf = append(c.buf, e)

	default:
		c.Reset()
		c.buf = append(c.buf, e)
	}
}

func (c *Counter) Count() int {
	return c.cnt
}