aoc-2024/cmd/day_2/main.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
	"strconv"
	"strings"
)

var (
	MAX_INT int = int(^uint(0) >> 1)
	INT_SZ  int = 0
)

func init() {
	tmp := MAX_INT
	for tmp > 0 {
		tmp >>= 1
		INT_SZ++
	}
}

func main() {
	reports := make([][]int, 0, 128)
	scanner := bufio.NewScanner(os.Stdin)
	reportLen := 0

	var lineCnt int
	for scanner.Scan() {
		if scanner.Err() != nil {
			log.Fatal("Unexpected error while reading input: ", scanner.Err())
		}
		lineCnt++
		fs := strings.Fields(scanner.Text())
		if reportLen != 0 && len(fs) != reportLen {
			log.Fatal(fmt.Sprintf("Unexpected number of fields at line %d of input", lineCnt))
		}
		report := make([]int, 0, 128)
		for i, f := range fs {
			level, err := strconv.ParseInt(f, 10, INT_SZ)
			if err != nil {
				log.Fatal(fmt.Sprintf("Unexpected level at line %d, level %d", lineCnt, i+1))
			}
			report = append(report, int(level))
		}
		reports = append(reports, report)
	}

	var safeCnt int
	for _, report := range reports {
		if isSafe(report) {
			safeCnt++
		}
	}
	fmt.Printf("safe = %d\n", safeCnt)
}

func isSafe(report []int) bool {
	if len(report) < 2 {
		return true
	}
	var lastDelta int
	for i := 1; i < len(report); i++ {
		delta := report[i] - report[i-1]

		switch {
		case delta < -3 || 3 < delta: // Not locally bounded
			return false

		case delta == 0: // Not strict monotonic
			return false

		case lastDelta*delta < 0 || 9 < lastDelta*delta: // Not monotonic
			return false
		}
		lastDelta = delta
	}
	return true
}