day 20

2021/20.input

@@ -0,0 +1,102 @@
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2021/20.py

@@ -0,0 +1,186 @@
+import itertools
+
+# TODO: Should FixedGrid allow offset domains (say -1->4 for x and 3->8 for y)?
+class FixedGrid:
+    '''Fixed size grid utility class to simplify grid operations and minimize
+    bugs.
+    '''
+    def __init__(self, grid):
+        self._grid = grid
+        self._width = len(self._grid)
+        self._height = len(self._grid[0])
+
+    @staticmethod
+    def parse(s, linesplit_fn=None, line_separator='\n', value_fn=None):
+        grid = []
+        for line in s.split(line_separator):
+            if linesplit_fn is not None:
+                line = linesplit_fn(line)
+            if value_fn is None:
+                grid.append(list(line))
+            else:
+                grid.append(list(map(value_fn, line)))
+        return FixedGrid(grid).transpose()
+
+    @staticmethod
+    def from_dict(d, missing=None):
+        '''Converts a coordinate->value dictionary to a FixedGrid.
+        Expects that minimum x and y coordinates in the grid are both 0.
+
+        Arguments:
+        missing -- Value to use for any coordinates not present in the dictionary
+        '''
+        low_x, high_x = None, None
+        low_y, high_y = None, None
+        for x, y in d.keys():
+            if low_x is None:
+                low_x, high_x = x, x
+                low_y, high_y = y, y
+            else:
+                low_x = min(low_x, x)
+                high_x = max(high_x, x)
+                low_y = min(low_y, y)
+                high_y = max(high_y, y)
+
+        assert(low_x == low_y == 0)
+
+        return FixedGrid([[d.get((x, y), missing)
+                           for y in range(low_y, high_y+1)]
+                          for x in range(low_x, high_x+1)])
+
+    def to_dict(self):
+        return {(x,y): val
+                for x,col in enumerate(self._grid)
+                for y,val in enumerate(col)}
+
+    def transpose(self):
+        return FixedGrid([[self._grid[x][y]
+                           for x in range(self._width)]
+                          for y in range(self._height)])
+
+    @property
+    def width(self):
+        return self._width
+
+    @property
+    def height(self):
+        return self._height
+
+    @property
+    def area(self):
+        return self._width * self._height
+
+    def __getitem__(self, c):
+        x, y = c
+        assert(0 <= x < self._width and 0 <= y < self._height)
+        return self._grid[x][y]
+
+    def __setitem__(self, c, val):
+        x, y = c
+        assert(0 <= x < self._width and 0 <= y < self._height)
+        self._grid[x][y] = val
+
+    def items(self, column_first = False):
+        '''Generates all coordinate,value pairs in the grid for iteration.
+
+        Arguments:
+        column_first -- Iterate by column first rather than by row first
+        '''
+        if column_first:
+            for y in range(self._height):
+                for x, col in enumerate(self._grid):
+                    yield (x, y), col[y]
+        else:
+            for x, col in enumerate(self._grid):
+                for y, val in enumerate(col):
+                    yield (x, y), val
+
+    def neighbors(self, x, y, diagonals=False):
+        assert(0 <= x < self._width and 0 <= y < self._height)
+        if diagonals:
+            for nx, ny in itertools.product((x-1, x, x+1),
+                                            (y-1, y, y+1)):
+                if x == nx and y == ny:
+                    continue
+                if 0 <= nx < self._width and 0 <= ny < self._height:
+                    yield nx, ny
+        else:
+            if 0 < x:
+                yield x-1, y
+            if x+1 < self._width:
+                yield x+1, y
+            if 0 < y:
+                yield x, y-1
+            if y+1 < self._height:
+                yield x, y+1
+
+    def print(self, line_spacing=' '):
+        print('\n'.join(line_spacing.join(str(self._grid[x][y])
+                                          for x in range(self._width))
+                        for y in range(self._height)))
+
+# TODO: ExpandingGrid (probably dict-based)
+
+def parse_input(s):
+    algo, image = s.split('\n\n')
+    image = FixedGrid.parse(image)
+    return algo, image.to_dict(), image.width, image.height
+
+def neighbors(x, y):
+    for ay in (-1, 0, 1):
+        for ax in (-1, 0, 1):
+            yield (x+ax, y+ay)
+
+def step(algo, image, xrange, yrange, default):
+    new_im = {}
+    xrange = range(xrange[0]-1, xrange[-1]+2)
+    yrange = range(yrange[0]-1, yrange[-1]+2)
+    for x in xrange:
+        for y in yrange:
+            section = [image.get(n, default)
+                       for n in neighbors(x,y)]
+            section = ''.join(section)
+            section = section.replace('.', '0')
+            section = section.replace('#', '1')
+            idx = int(section, 2)
+            new_im[x,y] = algo[idx]
+
+    # Figure out new out of bounds default
+    section = (default*9).replace('.', '0').replace('#', '1')
+    default = algo[int(section, 2)]
+
+    return new_im, xrange, yrange, default
+
+def part1(s):
+    algo, image, width, height = parse_input(s)
+    xrange = range(width)
+    yrange = range(height)
+
+    default = '.'
+
+    for _ in range(2):
+        image, xrange, yrange, default = step(algo, image, xrange, yrange, default)
+
+    answer = sum(1 for c in image.values() if c == '#')
+
+    print(f'The answer to part one is {answer}')
+
+def part2(s):
+    algo, image, width, height = parse_input(s)
+    xrange = range(width)
+    yrange = range(height)
+
+    default = '.'
+
+    for _ in range(50):
+        image, xrange, yrange, default = step(algo, image, xrange, yrange, default)
+
+    answer = sum(1 for c in image.values() if c == '#')
+
+    print(f'The answer to part two is {answer}')
+
+with open('./2021/20.input') as input_file:
+
+    INPUT = input_file.read()
+part1(INPUT)
+part2(INPUT)

2021/20.test

@@ -0,0 +1,7 @@
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+
+#..#.
+#....
+##..#
+..#..
+..###