aargh 20 part 2 is a nightmare
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d2b4bc2921
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244
20/main.go
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244
20/main.go
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package main
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import (
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"bufio"
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"fmt"
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"math"
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"os"
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"strconv"
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"strings"
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"time"
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)
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func main() {
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start := time.Now()
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partOne()
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duration := time.Since(start)
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partTwo()
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duration2 := time.Since(start)
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fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
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}
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type tile struct {
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id int
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neighbors map[int]int // key matches the edge key, so we can skip existing matches if we want
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edges map[int]string
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image map[int]string
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}
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// edges are always defined clockwise starting from 0:north
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// as long as there's only ever one match for an edge, I don't _think_ I need to keep track of piece orientation...?
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func parseTile(header string, body []string) tile {
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id, _ := strconv.Atoi(strings.Trim(header, "Tile :"))
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t := tile{
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id: id,
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neighbors: make(map[int]int, 4),
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edges: make(map[int]string, 4),
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image: make(map[int]string, 8),
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}
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// make edges and extract picture data...
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left := []byte{}
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right := []byte{}
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for i := range body {
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left = append(left, body[i][0])
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right = append(right, body[i][9])
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// image has edges removed; still want 0-indexed for sanity later!
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if i != 0 && i != 9 {
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t.image[i-1] = body[i-1][1:9]
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}
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}
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t.edges[0] = body[0]
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t.edges[1] = string(right)
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t.edges[2] = body[9]
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t.edges[3] = string(left)
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return t
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}
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// return is match, inverted
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func testEdge(one, two string) (bool, bool) {
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if one == two {
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return true, false
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}
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// go doesn't have a built-in "string reversal" method, so we just check it the hard way
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// 9 is our magic number for a 10-character string, which we know they all are.
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// This might not be 100% sound for unicode (byte != rune), but our string is ASCII so it's... fine.
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for i := range one {
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if one[i] != two[9-i] {
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return false, false
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}
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}
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return true, true
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}
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// this is wasteful in that it happily checks edges that we've already determined neighbors for,
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// but it's performant enough that I don't really care to fix it
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func findPartners(id int, tiles map[int]tile) map[int]tile {
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currentTile := tiles[id]
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for tile := range tiles {
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if tile != id {
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for edgeID := range tiles[tile].edges {
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for i := range currentTile.edges {
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if ok, _ := testEdge(tiles[tile].edges[edgeID], currentTile.edges[i]); ok {
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tiles[tile].neighbors[edgeID] = tiles[id].id
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tiles[id].neighbors[i] = tiles[tile].id
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}
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}
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}
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}
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}
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return tiles
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}
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// Starting from a corner, we can "walk" through the image by:
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// stepping to the next tile in a known direction
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// checking where _that_ tile thinks we came from
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// then re-orienting ourselves and stepping again.
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// For example, if we take a corner and go to its 0 neighbor,
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// and that neighbor thinks our previous tile was 3,
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// a "straight line" along the edge would be 1. Repeat as necessary until you hit an edge.
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// Once we hit an edge, return to the start of the row,
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// step to its 90° neighbor, then walk parallel to the initial edge.
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// Repeat until we reach the far corner.
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// Additionally, we need to "line up" the edges, so there's some alignment checking that has to happen.
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func mergeTiles(corner int, tiles map[int]tile) []string {
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// just to make the counting a little easier
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sideLength := int(math.Sqrt(float64(len(tiles))))
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result := make([]string, sideLength*8)
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headOfRow := tiles[corner]
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currentRowDirection := 0
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nextRowDirection := 0
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// what an absurd way to "pick" between two keys in a two-key map
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for i := range headOfRow.neighbors {
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if currentRowDirection == 0 {
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currentRowDirection = i
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} else if nextRowDirection == 0 {
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nextRowDirection = i
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}
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}
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currentTile := tiles[corner]
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image := imageFromEdge(currentTile, 0)
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for row, s := range image {
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result[row] = result[row] + s
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}
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for i := 0; i < sideLength; i = i + 1 {
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nextID := currentTile.neighbors[currentRowDirection]
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for e, id := range tiles[nextID].neighbors {
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if id == currentTile.id {
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_, invert := testEdge(currentTile.edges[currentRowDirection], tiles[nextID].edges[e])
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image := imageFromEdge(tiles[nextID], (e+1)%4)
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if !invert {
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for row, s := range image {
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result[row] = result[row] + s
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}
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} else {
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for row, s := range image {
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result[7-row] = result[7-row] + s
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}
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}
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currentRowDirection = (e + 2) % 4 // flip 180 around the compass
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}
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}
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currentTile = tiles[nextID]
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}
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return result
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}
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// returns the image "rotated" so that the provided edge is on "top" (the 0 position)
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func imageFromEdge(t tile, edge int) map[int]string {
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if edge == 1 {
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newImage := make(map[int]string, 8)
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for i := 0; i < 8; i = i + 1 {
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b := []byte{t.image[0][7-i], t.image[1][7-i], t.image[2][7-i], t.image[3][7-i], t.image[4][7-i], t.image[5][7-i], t.image[6][7-i], t.image[7][7-i]}
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newImage[i] = string(b)
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}
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return newImage
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}
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if edge == 2 {
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newImage := make(map[int]string, 8)
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for i := 0; i < 8; i = i + 1 {
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b := []byte{t.image[7-i][7], t.image[7-i][6], t.image[7-i][5], t.image[7-i][4], t.image[7-i][3], t.image[7-i][2], t.image[7-i][1], t.image[7-i][0]}
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newImage[i] = string(b)
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}
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return newImage
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}
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if edge == 3 {
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newImage := make(map[int]string, 8)
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for i := 0; i < 8; i = i + 1 {
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b := []byte{t.image[7][i], t.image[6][i], t.image[5][i], t.image[4][i], t.image[3][i], t.image[2][i], t.image[1][i], t.image[0][i]}
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newImage[i] = string(b)
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}
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return newImage
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}
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// edge == 0 does nothing
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return t.image
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}
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func partOne() {
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f, _ := os.Open("input")
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reader := bufio.NewReader(f)
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scanner := bufio.NewScanner(reader)
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buffer := []string{}
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tiles := make(map[int]tile)
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for scanner.Scan() {
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line := scanner.Text()
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if line == "" {
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tile := parseTile(buffer[0], buffer[1:])
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tiles[tile.id] = tile
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buffer = []string{}
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} else {
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buffer = append(buffer, line)
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}
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}
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// catch that last tile
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// weirdly, I'd think the trailing newline in the input would have done this for us
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tile := parseTile(buffer[0], buffer[1:])
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tiles[tile.id] = tile
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corners := 1
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for id := range tiles {
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tiles = findPartners(id, tiles)
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if len(tiles[id].neighbors) == 2 {
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corners = corners * id
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}
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}
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fmt.Println(corners)
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}
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func partTwo() {
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f, _ := os.Open("testinput")
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reader := bufio.NewReader(f)
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scanner := bufio.NewScanner(reader)
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buffer := []string{}
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tiles := make(map[int]tile)
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for scanner.Scan() {
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line := scanner.Text()
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if line == "" {
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tile := parseTile(buffer[0], buffer[1:])
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tiles[tile.id] = tile
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buffer = []string{}
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} else {
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buffer = append(buffer, line)
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}
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}
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// catch that last tile
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tile := parseTile(buffer[0], buffer[1:])
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tiles[tile.id] = tile
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corner := 0
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for id := range tiles {
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tiles = findPartners(id, tiles)
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// grab a corner to start the merge from
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if len(tiles[id].neighbors) == 2 && corner == 0 {
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corner = id
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}
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}
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for _, line := range mergeTiles(corner, tiles) {
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fmt.Println(line)
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}
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}
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