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const std = @import("std");
const data = @embedFile("day09");
var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
const gpa = general_purpose_allocator.allocator();
const Pair = struct { x: i32, y: i32 };
fn signum(n: i32) i32 {
if (n > 0) return 1;
if (n < 0) return -1;
return 0;
}
fn move_after(head: Pair, tail: Pair) Pair {
if (std.math.absInt(head.x - tail.x) catch unreachable <= 1 and
std.math.absInt(head.y - tail.y) catch unreachable <= 1)
return tail;
return .{ .x = tail.x + signum(head.x - tail.x),
.y = tail.y + signum(head.y - tail.y) };
}
fn solve(comptime ropelen: usize) !usize {
var lines = std.mem.tokenize(u8, data, "\n");
var visited = std.AutoHashMap(Pair, void).init(gpa);
defer visited.deinit();
var rope = [_]Pair{.{.x = 0, .y = 0}} ** ropelen;
try visited.put(rope[ropelen-1], {});
while (lines.next()) |line| {
if (line.len == 0)
break;
var parts = std.mem.tokenize(u8, line, " ");
var dir = parts.next().?[0];
var amt = try std.fmt.parseInt(usize, parts.next().?, 10);
var n: usize = 0;
while (n < amt) : (n += 1) {
switch (dir) {
'L' => rope[0].x -= 1,
'R' => rope[0].x += 1,
'U' => rope[0].y -= 1,
'D' => rope[0].y += 1,
else => unreachable,
}
var i: usize = 1;
while (i < ropelen) : (i += 1) {
rope[i] = move_after(rope[i-1], rope[i]);
}
try visited.put(rope[ropelen-1], {});
}
}
return visited.count();
}
pub fn main() !void {
defer std.debug.assert(!general_purpose_allocator.deinit());
std.debug.print("{}\n", .{try solve(2)});
std.debug.print("{}\n", .{try solve(10)});
}
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