Add solution for day 8

day8/input.txt

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day8/part1.py

@@ -0,0 +1,113 @@
+"""
+--- Day 8: Treetop Tree House ---
+
+The expedition comes across a peculiar patch of tall trees all planted
+carefully in a grid. The Elves explain that a previous expedition
+planted these trees as a reforestation effort. Now, they're curious if
+this would be a good location for a tree house.
+
+First, determine whether there is enough tree cover here to keep a tree
+house hidden. To do this, you need to count the number of trees that are
+visible from outside the grid when looking directly along a row or
+column.
+
+The Elves have already launched a quadcopter to generate a map with the
+height of each tree (your puzzle input). For example:
+
+    30373
+    25512
+    65332
+    33549
+    35390
+
+Each tree is represented as a single digit whose value is its height,
+where 0 is the shortest and 9 is the tallest.
+
+A tree is visible if all of the other trees between it and an edge of
+the grid are shorter than it. Only consider trees in the same row or
+column; that is, only look up, down, left, or right from any given tree.
+
+All of the trees around the edge of the grid are visible - since they
+are already on the edge, there are no trees to block the view. In this
+example, that only leaves the interior nine trees to consider:
+
+- The top-left 5 is visible from the left and top. (It isn't visible
+  from the right or bottom since other trees of height 5 are in the
+  way.)
+- The top-middle 5 is visible from the top and right.
+- The top-right 1 is not visible from any direction; for it to be
+  visible, there would need to only be trees of height 0 between it and
+  an edge.
+- The left-middle 5 is visible, but only from the right.
+- The center 3 is not visible from any direction; for it to be visible,
+  there would need to be only trees of at most height 2 between it and
+  an edge.
+- The right-middle 3 is visible from the right.
+- In the bottom row, the middle 5 is visible, but the 3 and 4 are not.
+
+With 16 trees visible on the edge and another 5 visible in the interior,
+a total of 21 trees are visible in this arrangement.
+
+Consider your map; how many trees are visible from outside the grid?
+"""
+from utils import load_trees
+
+
+def tree_is_visible(trees, pos_x, pos_y):
+    tree_size = trees[pos_y][pos_x]
+    num_rows_of_trees = len(trees)
+    num_trees_per_row = len(trees[0])
+
+    visible_from_north = True
+    visible_from_east = True
+    visible_from_south = True
+    visible_from_west = True
+
+    # See if any trees north of our tree are taller,
+    for y in range(0, pos_y):
+        if trees[y][pos_x] >= tree_size:
+            visible_from_north = False
+            break
+
+    # See if any trees east of our tree are taller,
+    for x in range(pos_x + 1, num_trees_per_row):
+        if trees[pos_y][x] >= tree_size:
+            visible_from_east = False
+            break
+
+    # See if any trees south of our tree are taller,
+    for y in range(pos_y + 1, num_rows_of_trees):
+        if trees[y][pos_x] >= tree_size:
+            visible_from_south = False
+            break
+
+    # See if any trees west of our tree are taller,
+    for x in range(0, pos_x):
+        if trees[pos_y][x] >= tree_size:
+            visible_from_west = False
+            break
+
+    return visible_from_north or visible_from_east or visible_from_south or visible_from_west
+
+
+def main():
+    trees = load_trees("input.txt")
+
+    # The left and right outer edges are always visible.
+    visible_tree_count = 2 * len(trees)
+
+    # The top and bottom edges are also visible, but we must subtract 4
+    # to account for the trees in the corners, which we already counted
+    # as part of the left and right outer edges.
+    visible_tree_count += (2 * len(trees[0])) - 4
+
+    for y, row_of_trees in enumerate(trees[1:-1], 1):
+        for x, tree in enumerate(row_of_trees[1:-1], 1):
+            if tree_is_visible(trees, x, y):
+                visible_tree_count += 1
+
+    print(f"There are {visible_tree_count} visible trees")
+
+
+if __name__ == "__main__":
+    main()

day8/part2.py

@@ -0,0 +1,121 @@
+"""
+--- Part Two ---
+
+Content with the amount of tree cover available, the Elves just need to
+know the best spot to build their tree house: they would like to be able
+to see a lot of trees.
+
+To measure the viewing distance from a given tree, look up, down, left,
+and right from that tree; stop if you reach an edge or at the first tree
+that is the same height or taller than the tree under consideration. (If
+a tree is right on the edge, at least one of its viewing distances will
+be zero.)
+
+The Elves don't care about distant trees taller than those found by the
+rules above; the proposed tree house has large eaves to keep it dry, so
+they wouldn't be able to see higher than the tree house anyway.
+
+In the example above, consider the middle 5 in the second row:
+
+    30373
+    25512
+    65332
+    33549
+    35390
+
+- Looking up, its view is not blocked; it can see 1 tree (of height 3).
+- Looking left, its view is blocked immediately; it can see only 1 tree
+  (of height 5, right next to it).
+- Looking right, its view is not blocked; it can see 2 trees.
+- Looking down, its view is blocked eventually; it can see 2 trees (one
+  of height 3, then the tree of height 5 that blocks its view).
+
+A tree's scenic score is found by multiplying together its viewing
+distance in each of the four directions. For this tree, this is 4 (found
+by multiplying 1 * 1 * 2 * 2).
+
+However, you can do even better: consider the tree of height 5 in the
+middle of the fourth row:
+
+    30373
+    25512
+    65332
+    33549
+    35390
+
+- Looking up, its view is blocked at 2 trees (by another tree with a
+  height of 5).
+- Looking left, its view is not blocked; it can see 2 trees.
+- Looking down, its view is also not blocked; it can see 1 tree.
+- Looking right, its view is blocked at 2 trees (by a massive tree of
+  height 9).
+
+This tree's scenic score is 8 (2 * 2 * 1 * 2); this is the ideal spot
+for the tree house.
+
+Consider each tree on your map. What is the highest scenic score
+possible for any tree?
+"""
+from utils import load_trees
+
+
+def calculate_scenic_score(trees, x_pos, y_pos):
+    tree_size = trees[y_pos][x_pos]
+    num_rows_of_trees = len(trees)
+    num_trees_in_row = len(trees[0])
+
+    view_distance_north = 0
+    view_distance_east = 0
+    view_distance_south = 0
+    view_distance_west = 0
+
+    # See how far we can see to the north.
+    y = y_pos - 1
+    while y >= 0:
+        view_distance_north += 1
+        if trees[y][x_pos] >= tree_size:
+            break
+        y -= 1
+
+    # See how far we can see to the east.
+    x = x_pos + 1
+    while x < num_trees_in_row:
+        view_distance_east += 1
+        if trees[y_pos][x] >= tree_size:
+            break
+        x += 1
+
+    # See how far we can see to the south.
+    y = y_pos + 1
+    while y < num_rows_of_trees:
+        view_distance_south += 1
+        if trees[y][x_pos] >= tree_size:
+            break
+        y += 1
+
+    # See how far we can see to the west.
+    x = x_pos - 1
+    while x >= 0:
+        view_distance_west += 1
+        if trees[y_pos][x] >= tree_size:
+            break
+        x -= 1
+
+    return view_distance_north * view_distance_east * view_distance_south * view_distance_west
+
+
+def main():
+    trees = load_trees("input.txt")
+
+    best_scenic_score = 0
+
+    for y, row_of_trees in enumerate(trees):
+        for x, tree in enumerate(row_of_trees):
+            scenic_score = calculate_scenic_score(trees, x, y)
+            best_scenic_score = max([best_scenic_score, scenic_score])
+
+    print(f"The best possible scenic score is {best_scenic_score}")
+
+
+if __name__ == "__main__":
+    main()

day8/testinput.txt

@@ -0,0 +1,5 @@
+30373
+25512
+65332
+33549
+35390

day8/utils.py

@@ -0,0 +1,18 @@
+def load_trees(filename):
+    trees = []
+
+    with open(filename, "r", encoding="utf-8") as f:
+        for line in f:
+            contents = line.strip()
+
+            if not contents:
+                continue
+
+            row_of_trees = []
+
+            for char in contents:
+                row_of_trees.append(int(char))
+
+            trees.append(row_of_trees)
+
+    return trees