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y2022/p12.py

@@ -0,0 +1,48 @@
+def parse_grid(text):
+    grid = {}
+    for y, row in enumerate(text):
+        for x, v in enumerate(row):
+            grid[complex(x, y)] = v if v in 'SE' else ord(v)
+    return grid
+
+
+def print_grid(mapping):
+    xmin, *_, xmax = {int(p.real) for p in mapping}
+    ymin, *_, ymax = {int(p.imag) for p in mapping}
+    for y in range(ymin, ymax + 1):
+        for x in range(xmin, xmax + 1):
+            print(mapping.get(complex(x, y), ' '), end=' ')
+        print()
+
+
+def shortest_path(starts, ends, evolve):
+    starts = {k: [k] for k in starts}
+    known = starts.copy()
+    while starts:
+        old = list(starts)[0]
+        path = starts.pop(old)
+        starts |= {new: path + [new] for new in evolve(old) if new not in known}
+        known |= starts
+        for final in set(starts) & set(ends):
+            # print_grid({k: '.' for k in known[final]})
+            return known[final]
+
+
+def main():
+    text = open(0).read().splitlines()
+    grid = parse_grid(text)
+
+    rev = {v: k for k, v in grid.items()}
+    start, end = rev['S'], rev['E']
+    grid[start], grid[end] = ord('a'), ord('z')
+
+    dof = [1, -1, 1j, -1j]
+    evolve = lambda old: {old + step for step in dof if is_valid(old, step)}
+    is_valid = lambda old, step: old + step in grid and grid[old + step] - grid[old] <= 1
+
+    print(len(shortest_path({start}, {end}, evolve)) - 1)
+    starts = {k for k, v in grid.items() if v == ord('a')}
+    print(len(shortest_path(starts, {end}, evolve)) - 1)
+
+
+main()