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package main | |
import ( | |
"bytes" | |
"fmt" | |
"io/ioutil" | |
"log" | |
"os" | |
"strconv" | |
"github.com/dhconnelly/advent-of-code-2019/ints" | |
) | |
func parseVec(b []byte) []int { | |
v := make([]int, len(b)) | |
for i, a := range b { | |
v[i] = int(a - '0') | |
} | |
return v | |
} | |
func coef(row, col int) int { | |
switch (col / row) % 4 { | |
case 0: | |
return 0 | |
case 1: | |
return 1 | |
case 2: | |
return 0 | |
default: | |
return -1 | |
} | |
} | |
func copied(data []int) []int { | |
data2 := make([]int, len(data)) | |
copy(data2, data) | |
return data2 | |
} | |
func fft(signal []int, phases int) []int { | |
signal = copied(signal) | |
scratch := make([]int, len(signal)) | |
for ; phases > 0; phases-- { | |
for i := 0; i < len(signal); i++ { | |
sum := 0 | |
for j := 0; j < len(signal); j++ { | |
sum += coef(i+1, j+1) * signal[j] | |
} | |
scratch[i] = ints.Abs(sum) % 10 | |
} | |
signal, scratch = scratch, signal | |
} | |
return signal | |
} | |
func toSignal(signal []int) string { | |
b := make([]byte, len(signal)) | |
for i, v := range signal { | |
b[i] = byte(v + '0') | |
} | |
return string(b) | |
} | |
func offset(b []byte) int { | |
i, err := strconv.Atoi(string(b[:7])) | |
if err != nil { | |
log.Fatal(err) | |
} | |
return i | |
} | |
func sliceSignal(signal []int, from, to int) []int { | |
sliced := make([]int, to-from) | |
for i := range sliced { | |
sliced[i] = signal[(from+i)%len(signal)] | |
} | |
return sliced | |
} | |
func extractMessage(signal []int, reps, phases, offset, digits int) []int { | |
msg := sliceSignal(signal, offset, len(signal)*reps) | |
n := len(msg) | |
// for offset >= len(signal)/2, coef(i) = 0 for i < offset/2 and 1 for | |
// i >= offset/2 | |
if offset < len(signal)*reps/2 { | |
log.Fatal("offset too great:", offset) | |
} | |
for ; phases > 0; phases-- { | |
sum := 0 | |
for i := n - 1; i >= 0; i-- { | |
sum += msg[i] | |
msg[i] = ints.Abs(sum) % 10 | |
} | |
} | |
return msg[:digits] | |
} | |
func main() { | |
txt, err := ioutil.ReadFile(os.Args[1]) | |
if err != nil { | |
log.Fatal(err) | |
} | |
v := parseVec(bytes.TrimSpace(txt)) | |
i := offset(txt) | |
const digits = 8 | |
const phases = 100 | |
const reps = 10000 | |
fmt.Println(toSignal(fft(v, phases)[:digits])) | |
fmt.Println(toSignal(extractMessage(v, reps, phases, i, digits))) | |
} |