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Advent of Code 2021 - Day 9
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‎README.md

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -25,4 +25,5 @@ Last year I ran a [cruise log for my 25 days competing in the Advent of Code](ht
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- [The Advent of Code 2021 Day 5 log, pointless points](https://www.codingnagger.com/2021/12/05/the-advent-of-code-2021-day-5-log-pointless-points/)
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- [The Advent of Code 2021 Day 6 log, plenty of fish](https://www.codingnagger.com/2021/12/06/the-advent-of-code-2021-day-6-log-plenty-of-fish/)
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- [The Advent of Code 2021 Day 7 log, return of the crab](https://www.codingnagger.com/2021/12/07/the-advent-of-code-2021-day-7-log-return-of-the-crab/)
28-
- [The Advent of Code 2021 Day 8, Bruteforce for the win](https://www.codingnagger.com/2021/12/08/the-advent-of-code-2021-day-8-bruteforce-for-the-win/)
28+
- [The Advent of Code 2021 Day 8, Bruteforce for the win](https://www.codingnagger.com/2021/12/08/the-advent-of-code-2021-day-8-bruteforce-for-the-win/)
29+
- [The Advent of Code 2021 Day 9, quiet before the storm?](https://www.codingnagger.com/2021/12/09/the-advent-of-code-2021-day-9-quiet-before-the-storm/)

‎src/main/java/com/codingnagger/App.java

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1,7 +1,7 @@
11
package com.codingnagger;
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33
import com.codingnagger.days.Day;
4-
import com.codingnagger.days.Day8;
4+
import com.codingnagger.days.Day9;
55
import com.codingnagger.utils.InputLoader;
66

77
import java.io.IOException;
@@ -14,9 +14,9 @@ public class App {
1414
public static void main(String[] args) throws IOException {
1515
System.out.println("Advent of Code 2021");
1616

17-
List<String> input = InputLoader.Load("day8.txt");
17+
List<String> input = InputLoader.Load("day9.txt");
1818

19-
Day day = new Day8();
19+
Day day = new Day9();
2020

2121
System.out.println("Part 1:");
2222
System.out.println(day.partOne(input));

‎src/main/java/com/codingnagger/days/Day9.java

Lines changed: 150 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,150 @@
1+
package com.codingnagger.days;
2+
3+
import java.util.ArrayList;
4+
import java.util.Arrays;
5+
import java.util.Collections;
6+
import java.util.List;
7+
import java.util.Objects;
8+
import java.util.stream.Collectors;
9+
10+
public class Day9 implements Day {
11+
@Override
12+
public String partOne(List<String> input) {
13+
Integer[][] locations = parseLocations(input);
14+
List<Integer> lowestPoints = findLowestPoints(locations);
15+
return String.valueOf(lowestPoints.size() + lowestPoints.stream().mapToInt(i -> i).sum());
16+
}
17+
18+
private List<Integer> findLowestPoints(Integer[][] locations) {
19+
List<Integer> lowestPoints = new ArrayList<>();
20+
21+
for (int y = 0; y < locations.length; y++) {
22+
for (int x = 0; x < locations[y].length; x++) {
23+
if (isLowestPoint(x, y, locations)) {
24+
lowestPoints.add(locations[y][x]);
25+
}
26+
}
27+
}
28+
29+
return lowestPoints;
30+
}
31+
32+
private boolean isLowestPoint(int x, int y, Integer[][] locations) {
33+
int minX = Math.max(0, x - 1);
34+
int maxX = Math.min(locations[y].length - 1, x + 1);
35+
int minY = Math.max(0, y - 1);
36+
int maxY = Math.min(locations.length - 1, y + 1);
37+
38+
int location = locations[y][x];
39+
40+
for (int cy = minY; cy <= maxY; cy++) {
41+
for (int cx = minX; cx <= maxX; cx++) {
42+
if (cx == x && cy == y) {
43+
continue;
44+
}
45+
46+
if (locations[cy][cx] <= location) {
47+
return false;
48+
}
49+
}
50+
}
51+
52+
return true;
53+
}
54+
55+
private Integer[][] parseLocations(List<String> input) {
56+
int maxY = input.size();
57+
58+
Integer[][] locations = new Integer[maxY][];
59+
60+
for (int i = 0; i < maxY; i++) {
61+
locations[i] = Arrays.stream(input.get(i).split("")).map(Integer::parseInt).toArray(Integer[]::new);
62+
}
63+
64+
return locations;
65+
}
66+
67+
@Override
68+
public String partTwo(List<String> input) {
69+
List<List<Position>> basins = findBasins(input);
70+
basins.sort((b1, b2) -> b2.size() - b1.size());
71+
return String.valueOf(basins.get(0).size() * basins.get(1).size() * basins.get(2).size());
72+
}
73+
74+
private List<List<Position>> findBasins(List<String> input) {
75+
Integer[][] locations = parseLocations(input);
76+
List<Position> lowestPointPositions = findLowestPointPositions(locations);
77+
return lowestPointPositions.stream().map(p -> findBasin(p, locations, Collections.emptyList())).collect(Collectors.toList());
78+
}
79+
80+
private List<Position> findBasin(Position current, Integer[][] locations, List<Position> explored) {
81+
if (explored.contains(current) || locations[current.y][current.x] == 9) {
82+
return Collections.emptyList();
83+
}
84+
85+
int x = current.x;
86+
int y = current.y;
87+
int minX = Math.max(0, x - 1);
88+
int maxX = Math.min(locations[y].length - 1, x + 1);
89+
int minY = Math.max(0, y - 1);
90+
int maxY = Math.min(locations.length - 1, y + 1);
91+
92+
List<Position> basin = new ArrayList<>(explored);
93+
94+
basin.add(current);
95+
96+
if (y != minY) {
97+
basin.addAll(findBasin(new Position(x, minY), locations, basin));
98+
}
99+
100+
if (y != maxY) {
101+
basin.addAll(findBasin(new Position(x, maxY), locations, basin));
102+
}
103+
104+
if (x != minX) {
105+
basin.addAll(findBasin(new Position(minX, y), locations, basin));
106+
}
107+
108+
if (x != maxX) {
109+
basin.addAll(findBasin(new Position(maxX, y), locations, basin));
110+
}
111+
112+
return basin.stream().distinct().collect(Collectors.toList());
113+
}
114+
115+
private List<Position> findLowestPointPositions(Integer[][] locations) {
116+
List<Position> lowestPointsPositions = new ArrayList<>();
117+
118+
for (int y = 0; y < locations.length; y++) {
119+
for (int x = 0; x < locations[y].length; x++) {
120+
if (isLowestPoint(x, y, locations)) {
121+
lowestPointsPositions.add(new Position(x, y));
122+
}
123+
}
124+
}
125+
126+
return lowestPointsPositions;
127+
}
128+
129+
public class Position {
130+
private final int x, y;
131+
132+
public Position(int x, int y) {
133+
this.x = x;
134+
this.y = y;
135+
}
136+
137+
@Override
138+
public boolean equals(Object o) {
139+
if (this == o) return true;
140+
if (o == null || getClass() != o.getClass()) return false;
141+
Position position = (Position) o;
142+
return x == position.x && y == position.y;
143+
}
144+
145+
@Override
146+
public int hashCode() {
147+
return Objects.hash(x, y);
148+
}
149+
}
150+
}

‎src/main/resources/day9.txt

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,100 @@
1+
3235678976543234569109899875456987676387679999878953234987656895457998743212347892123999912987632489
2+
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4+
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8+
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64+
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‎src/test/java/com/codingnagger/days/Day9Test.java

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package com.codingnagger.days;
2+
3+
import com.codingnagger.utils.InputLoader;
4+
import org.junit.jupiter.api.Test;
5+
6+
import java.util.List;
7+
8+
import static org.assertj.core.api.Assertions.assertThat;
9+
10+
public class Day9Test {
11+
private static final List<String> INPUT = InputLoader.LoadTest("day9.txt");
12+
private static final Day DAY = new Day9();
13+
14+
@Test
15+
public void partOne_shouldReturnIncreaseCount() {
16+
String result = DAY.partOne(INPUT);
17+
18+
assertThat(result).isEqualTo("15");
19+
}
20+
21+
@Test
22+
public void partTwo_shouldReturnSlidingIncreaseCount() {
23+
String result = DAY.partTwo(INPUT);
24+
25+
assertThat(result).isEqualTo("1134");
26+
}
27+
}

‎src/test/resources/day9.txt

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@@ -0,0 +1,5 @@
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2199943210
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3987894921
3+
9856789892
4+
8767896789
5+
9899965678

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