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day03.go
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day03.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 day03 solves AoC 2021 day 3.
package day03
import (
"strconv"
"github.com/fis/aoc/glue"
)
func init() {
glue.RegisterSolver(2021, 3, glue.LineSolver(solve))
}
func solve(lines []string) ([]string, error) {
gamma, epsilon := gammaEpsilon(lines)
oxygen := filterBits(lines, 0)
co2 := filterBits(lines, 1)
return glue.Ints(gamma*epsilon, oxygen*co2), nil
}
func gammaEpsilon(lines []string) (gamma, epsilon int) {
N, K := len(lines), len(lines[0])
ones := make([]int, K)
for _, line := range lines {
for i := 0; i < K; i++ {
if line[i]&1 != 0 {
ones[i]++
}
}
}
for i := 0; i < K; i++ {
gamma <<= 1
if ones[i] > N/2 {
gamma |= 1
}
}
epsilon = gamma ^ ((1 << K) - 1)
return gamma, epsilon
}
func filterBits(lines []string, keepLCB int) int {
var surviving []string
K := len(lines[0])
for i := 0; i < K; i++ {
ones := 0
for _, line := range lines {
if line[i]&1 != 0 {
ones++
}
}
keep := (2 * ones / len(lines)) ^ keepLCB
for _, line := range lines {
if int(line[i]&1) == keep {
surviving = append(surviving, line)
}
}
lines = surviving
if len(lines) == 1 {
n, _ := strconv.ParseInt(lines[0], 2, 64)
return int(n)
}
surviving = lines[:0]
}
panic("invalid input: no unique survivor")
}