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day16.go
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day16.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 day16 solves AoC 2021 day 16.
package day16
import (
"encoding/hex"
"fmt"
"github.com/fis/aoc/glue"
)
func init() {
glue.RegisterSolver(2021, 16, glue.LineSolver(solve))
}
func solve(lines []string) ([]string, error) {
if len(lines) != 1 {
return nil, fmt.Errorf("expected one line, got %d", len(lines))
}
p, _ := parsePacket(bitReaderHex(lines[0]))
p1 := p.versionSum()
p2 := p.eval()
return glue.Ints(p1, p2), nil
}
type opCode int
const (
opSum opCode = 0
opProd opCode = 1
opMin opCode = 2
opMax opCode = 3
opLit opCode = 4
opGt opCode = 5
opLt opCode = 6
opEq opCode = 7
)
type packet struct {
ver int
op opCode
lit int
sub []packet
}
func (p packet) versionSum() (vs int) {
vs = p.ver
for _, sub := range p.sub {
vs += sub.versionSum()
}
return vs
}
func (p packet) eval() (v int) {
switch p.op {
case opSum:
for _, sub := range p.sub {
v += sub.eval()
}
return v
case opProd:
v = 1
for _, sub := range p.sub {
v *= sub.eval()
}
return v
case opMin:
v = p.sub[0].eval()
for _, sub := range p.sub[1:] {
if sv := sub.eval(); sv < v {
v = sv
}
}
return v
case opMax:
v = p.sub[0].eval()
for _, sub := range p.sub[1:] {
if sv := sub.eval(); sv > v {
v = sv
}
}
return v
case opLit:
return p.lit
case opGt:
if p.sub[0].eval() > p.sub[1].eval() {
return 1
} else {
return 0
}
case opLt:
if p.sub[0].eval() < p.sub[1].eval() {
return 1
} else {
return 0
}
case opEq:
if p.sub[0].eval() == p.sub[1].eval() {
return 1
} else {
return 0
}
}
return -1
}
func parsePacket(br *bitReader) (p packet, totalLength int) {
p.ver = br.take(3)
p.op = opCode(br.take(3))
totalLength += 6
if p.op == opLit {
for {
chunk := br.take(5)
totalLength += 5
p.lit <<= 4
p.lit |= chunk & 0b1111
if chunk&0b10000 == 0 {
break
}
}
return p, totalLength
}
var bodyLength int
inPackets := br.take(1) == 1
totalLength++
if inPackets {
bodyLength = br.take(11)
totalLength += 11
} else {
bodyLength = br.take(15)
totalLength += 15
}
for bodyLength > 0 {
sub, subLength := parsePacket(br)
totalLength += subLength
p.sub = append(p.sub, sub)
if inPackets {
bodyLength--
} else {
bodyLength -= subLength
}
}
return p, totalLength
}
type bitReader struct {
data []byte
at int
}
func bitReaderHex(str string) (br *bitReader) {
src := []byte(str)
if len(src)&1 == 1 {
src = append(src, '0')
}
dst := make([]byte, hex.DecodedLen(len(src)))
hex.Decode(dst, src)
return &bitReader{data: dst, at: 0}
}
func (br *bitReader) take(bits int) (val int) {
for br.at+bits >= 8 {
n := 8 - br.at
val <<= n
val |= int(br.data[0]) & ((1 << n) - 1)
bits -= n
br.data, br.at = br.data[1:], 0
}
if bits == 0 {
return val
}
val <<= bits
val |= (int(br.data[0]) >> (8 - br.at - bits)) & ((1 << bits) - 1)
br.at += bits
return val
}