You try contacting the Elves using your handheld device, but the river you're following must be too low to get a decent signal.
You ask the device for a heightmap of the surrounding area (your puzzle input). The heightmap shows the local area from above broken into a grid; the elevation of each square of the grid is given by a single lowercase letter, where a is the lowest elevation, b is the next-lowest, and so on up to the highest elevation, z.
Also included on the heightmap are marks for your current position (S) and the location that should get the best signal (E). Your current position (S) has elevation a, and the location that should get the best signal (E) has elevation z.
You'd like to reach E, but to save energy, you should do it in as few steps as possible. During each step, you can move exactly one square up, down, left, or right. To avoid needing to get out your climbing gear, the elevation of the destination square can be at most one higher than the elevation of your current square; that is, if your current elevation is m, you could step to elevation n, but not to elevation o. (This also means that the elevation of the destination square can be much lower than the elevation of your current square.)
For example:
Sabqponm
abcryxxl
accszExk
acctuvwj
abdefghi
Here, you start in the top-left corner; your goal is near the middle. You could start by moving down or right, but eventually you'll need to head toward the e at the bottom. From there, you can spiral around to the goal:
v..v<<<<
>v.vv<<^
.>vv>E^^
..v>>>^^
..>>>>>^
In the above diagram, the symbols indicate whether the path exits each square moving up (^), down (v), left (<), or right (>). The location that should get the best signal is still E, and . marks unvisited squares.
This path reaches the goal in 31 steps, the fewest possible.
What is the fewest steps required to move from your current position to the location that should get the best signal?
Your puzzle answer was 408.
As you walk up the hill, you suspect that the Elves will want to turn this into a hiking trail. The beginning isn't very scenic, though; perhaps you can find a better starting point.
To maximize exercise while hiking, the trail should start as low as possible: elevation a. The goal is still the square marked E. However, the trail should still be direct, taking the fewest steps to reach its goal. So, you'll need to find the shortest path from any square at elevation a to the square marked E.
Again consider the example from above:
Sabqponm
abcryxxl
accszExk
acctuvwj
abdefghi
Now, there are six choices for starting position (five marked a, plus the square marked S that counts as being at elevation a). If you start at the bottom-left square, you can reach the goal most quickly:
...v<<<<
...vv<<^
...v>E^^
.>v>>>^^
>^>>>>>^
This path reaches the goal in only 29 steps, the fewest possible.
What is the fewest steps required to move starting from any square with elevation a to the location that should get the best signal?
Your puzzle answer was 399.
A rather basic pathfinding puzzle that can be solved nicely using BFS. Thanks to the relatively small problem size, even a "simplified" BFS that doesn't keep track of visited nodes is sufficient. (A naive DFS, though, is not, and will explode on the actual input.)
It may not be obvious for part 1 (and, thruth be told, I didn't realize it either), but with hindsight from part 2 it turns out that it's beneficial to perform the DFS "in reverse", i.e. not determine the shortest path from start to any point in the grid, but determine the shortest path from any point to the end. This way, part 2 becomes trivial and is just an additional print statement; that's why I put both parts into the same file. (It's not a huge deal if the BFS is implemented the wrong way round though; it's just that runtime increases from "not even blinking an eye" to 1.5 seconds or so.)
Note that my golf version directly uses the ASCII codes of the input as the height values. It even keeps the linefeeds as one (very low - height 10 instead of 97 to 122) valid column. In terms of the puzzle, these turn into locations that could theoretically be climbed down into, but there's no way up from them, so they don't affect overall pathfinding much.
- Parts 1+2, Python: 312 bytes, <100 ms