feat: Day 22: Mode Maze (part 1)

Author: mariotacke

README.md

@@ -32,6 +32,7 @@ Collect stars by solving puzzles. Two puzzles will be made available on each day
 - [Day 16: Chronal Classification](day-16-chronal-classification/)
 - [Day 17: Reservoir Research](day-17-reservoir-research/)
 - [Day 18: Settlers of The North Pole](day-18-settlers-of-the-north-pole/)
+- [Day 22: Mode Maze](day-22-mode-maze/)
 - [Day 23: Experimental Emergency Teleportation](day-23-experimental-emergency-teleportation/)
 - [Day 25: Four-Dimensional Adventure](day-25-four-dimensional-adventure/)
 

day-22-mode-maze/README.md

@@ -0,0 +1,63 @@
+## Day 22: Mode Maze
+
+This is it, your final stop: the year -483. It's snowing and dark outside; the only light you can see is coming from a small cottage in the distance. You make your way there and knock on the door.
+
+A portly man with a large, white beard answers the door and invites you inside. For someone living near the North Pole in -483, he must not get many visitors, but he doesn't act surprised to see you. Instead, he offers you some milk and cookies.
+
+After talking for a while, he asks a favor of you. His friend hasn't come back in a few hours, and he's not sure where he is. Scanning the region briefly, you discover one life signal in a cave system nearby; his friend must have taken shelter there. The man asks if you can go there to retrieve his friend.
+
+The cave is divided into square **regions** which are either dominantly **rocky**, **narrow**, or **wet** (called its **type**). Each region occupies exactly one **coordinate** in `X,Y` format where `X` and `Y` are integers and zero or greater. (Adjacent regions can be the same type.)
+
+The scan (your puzzle input) is not very detailed: it only reveals the **depth** of the cave system and the **coordinates of the target**. However, it does not reveal the type of each region. The mouth of the cave is at `0,0`.
+
+The man explains that due to the unusual geology in the area, there is a method to determine any region's type based on its **erosion level**. The erosion level of a region can be determined from its **geologic index**. The geologic index can be determined using the first rule that applies from the list below:
+
+- The region at `0,0` (the mouth of the cave) has a geologic index of `0`.
+- The region at the coordinates of the target has a geologic index of `0`.
+- If the region's `Y` coordinate is `0`, the geologic index is its `X` coordinate times `16807`.
+- If the region's `X` coordinate is `0`, the geologic index is its `Y` coordinate times `48271`.
+- Otherwise, the region's geologic index is the result of multiplying the erosion **levels** of the regions at `X-1,Y` and `X,Y-1`.
+
+A region's **erosion level** is its **geologic index** plus the cave system's **depth**, all [modulo](https://en.wikipedia.org/wiki/Modulo_operation) `20183`. Then:
+
+- If the **erosion level modulo `3`** is `0`, the region's type is **rocky**.
+- If the **erosion level modulo `3`** is `1`, the region's type is **wet**.
+- If the **erosion level modulo `3`** is `2`, the region's type is **narrow**.
+
+For example, suppose the cave system's depth is `510` and the target's coordinates are `10,10`. Using `%` to represent the modulo operator, the cavern would look as follows:
+
+- At `0,0`, the geologic index is `0`. The erosion level is `(0 + 510) % 20183 = 510`. The type is `510 % 3 = 0`, **rocky**.
+- At `1,0`, because the `Y` coordinate is `0`, the geologic index is `1 * 16807 = 16807`. The erosion level is `(16807 + 510) % 20183 = 17317`. The type is `17317 % 3 = 1`, **wet**.
+- At `0,1`, because the `X` coordinate is `0`, the geologic index is `1 * 48271 = 48271`. The erosion level is `(48271 + 510) % 20183 = 8415`. The type is `8415 % 3 = 0`, **rocky**.
+- At `1,1`, neither coordinate is `0` and it is not the coordinate of the target, so the geologic index is the erosion level of `0,1` (`8415`) times the erosion level of `1,0` (`17317`), `8415 * 17317 = 145722555`. The erosion level is `(145722555 + 510) % 20183 = 1805`. The type is `1805 % 3 = 2`, **narrow**.
+- At `10,10`, because they are the target's coordinates, the geologic index is `0`. The erosion level is `(0 + 510) % 20183 = 510`. The type is `510 % 3 = 0`, **rocky**.
+
+Drawing this same cave system with rocky as `.`, wet as `=`, narrow as `|`, the mouth as `M`, the target as `T`, with `0,0` in the top-left corner, `X` increasing to the right, and `Y` increasing downward, the top-left corner of the map looks like this:
+
+```
+M=.|=.|.|=.|=|=.
+.|=|=|||..|.=...
+.==|....||=..|==
+=.|....|.==.|==.
+=|..==...=.|==..
+=||.=.=||=|=..|=
+|.=.===|||..=..|
+|..==||=.|==|===
+.=..===..=|.|||.
+.======|||=|=.|=
+.===|=|===T===||
+=|||...|==..|=.|
+=.=|=.=..=.||==|
+||=|=...|==.=|==
+|=.=||===.|||===
+||.|==.|.|.||=||
+```
+
+Before you go in, you should determine the **risk level** of the area. For the rectangle that has a top-left corner of region `0,0` and a bottom-right corner of the region containing the target, add up the risk level of each individual region: `0` for rocky regions, `1` for wet regions, and `2` for narrow regions.
+
+In the cave system above, because the mouth is at `0,0` and the target is at `10,10`, adding up the risk level of all regions with an `X` coordinate from `0` to `10` and a `Y` coordinate from `0` to `10`, this total is **`114`**.
+
+**What is the total risk level for the smallest rectangle that includes `0,0` and the target's coordinates?**
+
+## References
+- https://adventofcode.com/2018/day/22

day-22-mode-maze/maze.js

@@ -0,0 +1,31 @@
+module.exports = (input) => {
+  const depth = +/depth: (\d+)/.exec(input)[1];
+  const target = /target: (\d+),(\d+)/.exec(input).slice(1).map((c) => +c);
+
+  const erosionLevelMap = Array
+    .from({ length: target[1] + 1 })
+    .map(() => Array.from({ length: target[0] + 1 }));
+
+  const erosionLevel = (geologicIndex) => (geologicIndex + depth) % 20183;
+
+  let riskLevel = 0;
+
+  for (let y = 0; y <= target[1]; y++) {
+    for (let x = 0; x <= target[0]; x++) {
+      if ((x === 0 && y === 0) || (x === target[0] && y === target[1])) {
+        erosionLevelMap[y][x] = 0;
+      } else if (y === 0) {
+        erosionLevelMap[y][x] = erosionLevel(x * 16807);
+      } else if (x === 0) {
+        erosionLevelMap[y][x] = erosionLevel(y * 48271);
+      } else {
+        const geologicIndex = erosionLevelMap[y][x - 1] * erosionLevelMap[y - 1][x];
+        erosionLevelMap[y][x] = erosionLevel(geologicIndex);
+      }
+
+      riskLevel += erosionLevelMap[y][x] % 3;
+    }
+  }
+
+  return riskLevel;
+};

day-22-mode-maze/test.js

@@ -0,0 +1,13 @@
+const assert = require('assert');
+
+const maze = require('./maze');
+
+describe('Day 22: Mode Maze', () => {
+  it('should calculate total risk level', () => {
+    const scan =
+      `depth: 510
+       target: 10,10`;
+
+    assert.strictEqual(maze(scan), 114);
+  });
+});