🎾

y2019/p14.py

+import collections
+import math
+import re
+import sys
+
+
+text = sys.stdin.read()
+to_get = {}
+for line in text.strip().splitlines():
+    for i, (qty, name) in enumerate(re.findall(r'(\d+) ([A-Z]+)', line)[::-1]):
+        if i == 0:
+            output = name
+            to_get[output] = {output: -int(qty)}
+        else:
+            to_get[output][name] = int(qty)
+
+
+def fuel_to_ore(wanted):
+    required = collections.Counter({'FUEL': wanted})
+    pending = required.copy()
+    while pending:
+        pending = {k: v for k, v in required.items() if k != 'ORE' and v > 0}
+        for out, out_qty in pending.items():
+            min_qty = -to_get[out][out]
+            n_times = math.ceil(out_qty / min_qty)
+            required.update({k: v * n_times for k, v in to_get[out].items()})
+    return required['ORE']
+
+
+print(fuel_to_ore(1))
+
+
+def bsearch(fn, goal, lo, hi):
+    while hi - lo > 1:
+        mid = lo + (hi - lo) // 2
+        if goal < fn(mid):
+            lo, hi = lo, mid
+        else:
+            lo, hi = mid, hi
+
+    # check
+    a, b = fn(lo), fn(hi)
+    assert a <= goal, 'lower bound too high'
+    assert goal <= b, 'higher bound too low'
+
+    return lo
+
+
+print(bsearch(fuel_to_ore, 1E12, 1, 10_000_000))