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-rw-r--r--2022/15-beacon-exclusion-zone/second.zig87
1 files changed, 87 insertions, 0 deletions
diff --git a/2022/15-beacon-exclusion-zone/second.zig b/2022/15-beacon-exclusion-zone/second.zig
new file mode 100644
index 0000000..d70e6d8
--- /dev/null
+++ b/2022/15-beacon-exclusion-zone/second.zig
@@ -0,0 +1,87 @@
+const std = @import("std");
+
+const example = @embedFile("example");
+const input = @embedFile("input");
+
+var allocator: std.mem.Allocator = undefined;
+
+pub fn main() anyerror!void {
+ var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+ defer arena.deinit();
+ allocator = arena.allocator();
+
+ try std.testing.expectEqual(try solve(example, 20), 56000011);
+ const result = try solve(input, 4000000);
+ try std.io.getStdOut().writer().print("{}\n", .{result});
+}
+
+const Point = struct {
+ x: i64,
+ y: i64,
+};
+
+const Line = struct {
+ a: Point,
+ b: Point,
+};
+
+fn solve(puzzle: []const u8, size: i64) !i64 {
+ var it = std.mem.tokenize(u8, puzzle, "\n");
+ var descendingLines = std.ArrayList(Line).init(allocator);
+ var ascendingLines = std.ArrayList(Line).init(allocator);
+ // process input
+ while (it.next()) |line| {
+ var coords = std.mem.tokenize(u8, line, "Sensor at x=, y=: closest beacon is at x=, y=");
+ const sx = try std.fmt.parseInt(i64, coords.next() orelse continue, 10);
+ const sy = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const bx = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const by = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const d = try std.math.absInt(sx - bx) + try std.math.absInt(sy - by) + 1; // plus one!
+ var i = try descendingLines.addOne();
+ i.* = Line{ .a = Point{ .x = sx, .y = sy - d }, .b = Point{ .x = sx + d, .y = sy } };
+ i = try ascendingLines.addOne();
+ i.* = Line{ .a = Point{ .x = sx, .y = sy + d }, .b = Point{ .x = sx + d, .y = sy } };
+ i = try descendingLines.addOne();
+ i.* = Line{ .a = Point{ .x = sx - d, .y = sy }, .b = Point{ .x = sx, .y = sy + d } };
+ i = try ascendingLines.addOne();
+ i.* = Line{ .a = Point{ .x = sx - d, .y = sy }, .b = Point{ .x = sx, .y = sy - d } };
+ }
+ // We look for intersections
+ var intersections = std.ArrayList(Point).init(allocator);
+ for (ascendingLines.items) |al| { // for each ascending line
+ var n = al.a.y - al.a.x; // y = x + n
+ for (descendingLines.items) |dl| { // for each descending line
+ var p = dl.a.y + dl.a.x; // y = -x + p
+ if (@mod(p + n, 2) == 1) {
+ continue;
+ }
+ var ix = @divExact(p - n, 2);
+ var iy = @divExact(p + n, 2);
+ if (ix < 0 or ix > size or iy < 0 or iy > size) {
+ continue;
+ }
+ //if (ix >= al.a.x and ix <= al.b.x and iy >= al.b.y and iy <= al.a.y) { // this is missing some + 1
+ // if (ix >= dl.a.x and ix <= dl.b.x and iy >= dl.a.y and iy <= dl.b.y) {
+ try intersections.append(Point{ .x = ix, .y = iy });
+ // }
+ //}
+ }
+ }
+ // we look into those one by one
+ outer: for (intersections.items) |i| {
+ var it2 = std.mem.tokenize(u8, puzzle, "\n"); // should cache this parsing result
+ while (it2.next()) |line| {
+ var coords = std.mem.tokenize(u8, line, "Sensor at x=, y=: closest beacon is at x=, y=");
+ const sx = try std.fmt.parseInt(i64, coords.next() orelse continue, 10);
+ const sy = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const bx = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const by = try std.fmt.parseInt(i64, coords.next() orelse unreachable, 10);
+ const d = try std.math.absInt(sx - bx) + try std.math.absInt(sy - by);
+ if (try std.math.absInt(sx - i.x) + try std.math.absInt(sy - i.y) <= d) {
+ continue :outer;
+ }
+ }
+ return i.x * 4000000 + i.y;
+ }
+ return 0;
+}