Day 15

Solution to part 2 is awful brute force. Surely there is a way to avoid
looping over all 4000000 rows...

15.input

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+Sensor at x=193758, y=2220950: closest beacon is at x=652350, y=2000000
+Sensor at x=3395706, y=3633894: closest beacon is at x=3404471, y=3632467
+Sensor at x=3896022, y=3773818: closest beacon is at x=3404471, y=3632467
+Sensor at x=1442554, y=1608100: closest beacon is at x=652350, y=2000000
+Sensor at x=803094, y=813675: closest beacon is at x=571163, y=397470
+Sensor at x=3491072, y=3408908: closest beacon is at x=3404471, y=3632467
+Sensor at x=1405010, y=486446: closest beacon is at x=571163, y=397470
+Sensor at x=3369963, y=3641076: closest beacon is at x=3404471, y=3632467
+Sensor at x=3778742, y=2914974: closest beacon is at x=4229371, y=3237483
+Sensor at x=1024246, y=3626229: closest beacon is at x=2645627, y=3363491
+Sensor at x=3937091, y=2143160: closest beacon is at x=4229371, y=3237483
+Sensor at x=2546325, y=2012887: closest beacon is at x=2645627, y=3363491
+Sensor at x=3505386, y=3962087: closest beacon is at x=3404471, y=3632467
+Sensor at x=819467, y=239010: closest beacon is at x=571163, y=397470
+Sensor at x=2650614, y=595151: closest beacon is at x=3367919, y=-1258
+Sensor at x=3502942, y=6438: closest beacon is at x=3367919, y=-1258
+Sensor at x=3924022, y=634379: closest beacon is at x=3367919, y=-1258
+Sensor at x=2935977, y=2838245: closest beacon is at x=2645627, y=3363491
+Sensor at x=1897626, y=7510: closest beacon is at x=3367919, y=-1258
+Sensor at x=151527, y=640680: closest beacon is at x=571163, y=397470
+Sensor at x=433246, y=1337298: closest beacon is at x=652350, y=2000000
+Sensor at x=2852855, y=3976978: closest beacon is at x=3282750, y=3686146
+Sensor at x=3328398, y=3645875: closest beacon is at x=3282750, y=3686146
+Sensor at x=3138934, y=3439134: closest beacon is at x=3282750, y=3686146
+Sensor at x=178, y=2765639: closest beacon is at x=652350, y=2000000
+Sensor at x=3386231, y=3635056: closest beacon is at x=3404471, y=3632467
+Sensor at x=3328074, y=1273456: closest beacon is at x=3367919, y=-1258
+Sensor at x=268657, y=162438: closest beacon is at x=571163, y=397470

15.py

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+from itertools import combinations
+
+lines = [[chunk.split("=") for chunk in line.split()] for line in open("15.input")]
+
+lines = [(int(line[2][1][:-1]), int(line[3][1][:-1]), int(line[8][1][:-1]), int(line[9][1])) for line in lines]
+
+def dist(a, b):
+    return abs(a[0] - b[0]) + abs(a[1] - b[1])
+
+sensors = set()
+beacons = set()
+ranges = {}
+
+for sx, sy, bx, by in lines:
+    sensors.add((sx, sy))
+    beacons.add((bx, by))
+    ranges[(sx, sy)] = dist((sx, sy), (bx, by))
+
+def row_coverage(sensor, y):
+    dy = abs(sensor[1] - y)
+    if ranges[sensor] < dy:
+        return None
+    dx = abs(ranges[sensor] - dy)
+    return (sensor[0] - dx, sensor[0] + dx + 1)
+
+def col_coverage(sensor, x):
+    dx = abs(sensor[0] - x)
+    if ranges[sensor] < dx:
+        return None
+    dy = abs(ranges[sensor] - dx)
+    return (sensor[1] - dy, sensor[1] + dy + 1)
+
+def join_segments(a):
+    b = []
+    for begin,end in sorted(a):
+        if b and b[-1][1] >= begin:
+            b[-1] = (b[-1][0], max(b[-1][1], end))
+        else:
+            b.append((begin, end))
+
+        # print((begin, end), b)
+    return b
+
+def part1(y):
+    coverage = [row_coverage(sensor, y) for sensor in sensors]
+    coverage = [c for c in coverage if c is not None]
+    joined = join_segments(coverage)
+
+    print("Part 1:", sum(max(0, seg[1] - seg[0] - 1) for seg in joined))
+
+
+
+#part1(y=10)
+part1(y=2000000)
+
+def possible_positions(covered):
+    result = []
+    current = 0
+
+    for start, end in covered:
+        for x in range(current, start):
+            result.append(x)
+        if end > 4000000:
+            break
+        current = end
+
+    return result
+
+def part2(max_y):
+    # Brute force let's gooooooo
+    for y in range(0, max_y):
+        # This takes a few minutes, so better log the progress
+        if y % (max_y / 100) == 0:
+            print(y / max_y * 100, "%")
+
+        y_coverage = [row_coverage(sensor, y) for sensor in sensors]
+        y_coverage = [c for c in y_coverage if c is not None]
+        y_joined = join_segments(y_coverage)
+
+        # Here we can actually be a bit smart, only check the columns
+        # which are not already covered
+        for x in possible_positions(y_joined):
+            x_coverage = [col_coverage(sensor, x) for sensor in sensors]
+            x_coverage = [c for c in x_coverage if c is not None]
+            x_joined = join_segments(x_coverage)
+            if y in possible_positions(x_joined):
+                return (x, y)
+
+#x, y = part2(20)
+x, y = part2(4000000 )
+print("Part 2:", x * 4000000 + y)