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day06.go
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day06.go
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// Copyright 2021 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 day06 solves AoC 2017 day 6.
package day06
import "github.com/fis/aoc/glue"
func init() {
glue.RegisterSolver(2017, 6, glue.IntSolver(solve))
}
func solve(input []int) ([]string, error) {
banks := make([]byte, len(input))
for i, n := range input {
banks[i] = byte(n)
}
p1 := loopLength(banks)
p2 := loopLength(banks)
return glue.Ints(p1, p2), nil
}
func loopLength(banks []byte) int {
seen := map[string]struct{}{string(banks): {}}
steps := 0
for {
balance(banks)
steps++
state := string(banks)
if _, found := seen[state]; found {
return steps
}
seen[state] = struct{}{}
}
}
func balance(banks []byte) {
N := byte(len(banks))
src, blocks := argmax(banks)
banks[src] = 0
if blocks >= N {
all := blocks / N
blocks = blocks % N
for i := range banks {
banks[i] += all
}
}
for i := byte(1); i <= blocks; i++ {
banks[(byte(src)+i)%N]++
}
}
func argmax(xs []byte) (maxI int, maxX byte) {
maxI, maxX = -1, 0
for i, x := range xs {
if maxI == -1 || x > maxX {
maxI, maxX = i, x
}
}
return maxI, maxX
}