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;
+}