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day14.go
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day14.go
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// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package day14 solves AoC 2018 day 14.
package day14
import (
"fmt"
"github.com/fis/aoc/glue"
)
func init() {
glue.RegisterSolver(2018, 14, glue.IntSolver(solve))
}
func solve(input []int) ([]string, error) {
if len(input) != 1 {
return nil, fmt.Errorf("expected one int, got %d", len(input))
}
part1 := asInt(evolve(input[0], 10))
part2 := find(asBytes(input[0]))
return glue.Ints(part1, part2), nil
}
func evolve(skip, keep int) []byte {
scoreboard := []byte{3, 7}
ep, fp := 0, 1
for len(scoreboard) < skip+keep {
er, fr := scoreboard[ep], scoreboard[fp]
nr := er + fr
if nr >= 10 {
scoreboard = append(scoreboard, nr/10, nr%10)
} else {
scoreboard = append(scoreboard, nr)
}
ep = (ep + 1 + int(er)) % len(scoreboard)
fp = (fp + 1 + int(fr)) % len(scoreboard)
}
return scoreboard[skip : skip+keep]
}
func find(scores []byte) int {
scoreboard := []byte{3, 7}
ep, fp := 0, 1
for {
er, fr := scoreboard[ep], scoreboard[fp]
nr := er + fr
i := len(scoreboard)
if nr >= 10 {
scoreboard = append(scoreboard, nr/10, nr%10)
} else {
scoreboard = append(scoreboard, nr)
}
for i < len(scoreboard) {
if j := i - len(scores); j >= 0 && bytesEqual(scoreboard[j:i], scores) {
return j
}
i++
}
ep = (ep + 1 + int(er)) % len(scoreboard)
fp = (fp + 1 + int(fr)) % len(scoreboard)
}
}
func asInt(scores []byte) (out int) {
for _, s := range scores {
out = 10*out + int(s)
}
return out
}
func asBytes(in int) (out []byte) {
var (
buf [16]byte
at int
)
for at = 15; at >= 0 && in > 0; at-- {
buf[at] = byte(in % 10)
in /= 10
}
return buf[at+1 : 16]
}
func bytesEqual(a, b []byte) bool {
if len(a) != len(b) {
return false
}
for i := range a {
if a[i] != b[i] {
return false
}
}
return true
}