AdventOfCode2020/15/main.go

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package main
import (
"bufio"
"fmt"
"os"
"strconv"
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"strings"
"time"
)
func main() {
start := time.Now()
partOne()
duration := time.Since(start)
partTwo()
duration2 := time.Since(start)
fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
}
func partOne() {
f, _ := os.Open("input")
reader := bufio.NewReader(f)
scanner := bufio.NewScanner(reader)
// naive implementation
scanner.Scan()
numbers := strings.Split(scanner.Text(), ",")
for {
i := len(numbers) - 1
lastNumber := numbers[i]
// walk backward through the list, looking for a match
for j := range numbers {
if len(numbers) == 2020 {
fmt.Println(numbers[2019])
return
}
if i-j == 0 {
// made it to the head of the list, no matches; append 0
numbers = append(numbers, "0")
break
} else if numbers[i-j-1] == lastNumber {
// a number matched; insert the difference between the last number and when we last saw it
numbers = append(numbers, fmt.Sprintf("%d", len(numbers)-1-(i-j-1)))
break
}
}
}
}
func partTwo() {
f, _ := os.Open("input")
reader := bufio.NewReader(f)
scanner := bufio.NewScanner(reader)
// caching implementation. It's still slow, but it at least finishes...!
scanner.Scan()
numbers := strings.Split(scanner.Text(), ",")
seen := make(map[int]int)
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// load in numbers
for i, v := range numbers[:len(numbers)-1] {
j, _ := strconv.Atoi(v)
seen[j] = i
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}
i := len(numbers) - 1
lastNumber, _ := strconv.Atoi(numbers[i])
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for {
if seen[lastNumber] == 30000000-1 {
fmt.Println(lastNumber)
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return
}
if v, ok := seen[lastNumber]; ok {
// a number matched; insert the difference between the last number and when we last saw it
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seen[lastNumber] = i
lastNumber = i - v
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} else {
// never seen that before
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seen[lastNumber] = i
lastNumber = 0
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}
i = i + 1
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}
}