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