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const std = @import("std");
const Result = @import("util/aoc.zig").Result;
const GRID_SIZE = [_]usize{ 500, 500 };
const Rope = struct {
grid: [GRID_SIZE[0]][GRID_SIZE[1]]i16,
head_position: *[2]i16 = undefined,
tail_position: *[2]i16 = undefined,
knots: [][2]i16,
tail_visits: [GRID_SIZE[0]][GRID_SIZE[1]]u2,
allocator: std.mem.Allocator,
const start_position = [2]i16{ GRID_SIZE[0] / 2 - 1, GRID_SIZE[1] / 2 - 1 };
// in theory? knots can be comptime
fn init(allocator: std.mem.Allocator, knots: usize) Rope {
var k = allocator.alloc([2]i16, knots + 2) catch unreachable;
for (k) |_, i| k[i] = Rope.start_position;
var rope = Rope{
.knots = k,
.grid = std.mem.zeroes([GRID_SIZE[0]][GRID_SIZE[1]]i16),
.tail_visits = std.mem.zeroes([GRID_SIZE[0]][GRID_SIZE[1]]u2),
.allocator = allocator,
};
rope.tail_position = &rope.knots[k.len - 1];
rope.head_position = &rope.knots[0];
return rope;
}
fn deinit(self: *Rope) void {
self.allocator.free(self.knots);
}
fn sync(self: *Rope) void {
for (self.knots[0..self.knots.len]) |*knot, knot_idx| {
if (knot_idx == 0) continue;
const previous_knot = self.knots[knot_idx - 1];
const delta = [2]i16{ previous_knot[0] - knot[0], previous_knot[1] - knot[1] };
if (std.math.absCast(delta[0]) == 2 and std.math.absCast(delta[1]) == 0) {
knot[0] += std.math.divExact(i16, delta[0], 2) catch unreachable;
} else if (std.math.absCast(delta[0]) == 0 and std.math.absCast(delta[1]) == 2) {
knot[1] += std.math.divExact(i16, delta[1], 2) catch unreachable;
} else if (std.math.absCast(delta[0]) == 2 and std.math.absCast(delta[1]) == 2) {
knot[0] += std.math.divExact(i16, delta[0], 2) catch unreachable;
knot[1] += std.math.divExact(i16, delta[1], 2) catch unreachable;
} else if (std.math.absCast(delta[0]) == 1 and std.math.absCast(delta[1]) == 2) {
knot[0] += delta[0];
knot[1] += std.math.divExact(i16, delta[1], 2) catch unreachable;
} else if (std.math.absCast(delta[0]) == 2 and std.math.absCast(delta[1]) == 1) {
knot[0] += std.math.divExact(i16, delta[0], 2) catch unreachable;
knot[1] += delta[1];
}
}
self.tail_visits[@intCast(usize, self.tail_position.*[0])][@intCast(usize, self.tail_position.*[1])] = 1;
return;
}
fn up(self: *Rope) void {
self.head_position.*[0] -= 1;
self.sync();
}
fn down(self: *Rope) void {
self.head_position.*[0] += 1;
self.sync();
}
fn left(self: *Rope) void {
self.head_position.*[1] -= 1;
self.sync();
}
fn right(self: *Rope) void {
self.head_position.*[1] += 1;
self.sync();
}
};
pub fn puzzle_1(allocator: std.mem.Allocator, input: []const u8) Result {
var rope = Rope.init(allocator, 0);
defer rope.deinit();
var iter = std.mem.split(u8, input, "\n");
while (iter.next()) |line| {
const n = std.fmt.parseInt(u8, line[2..], 0) catch unreachable;
var i: usize = 0;
switch (line[0]) {
'R' => while (i < n) : (i += 1) rope.right(),
'L' => while (i < n) : (i += 1) rope.left(),
'U' => while (i < n) : (i += 1) rope.up(),
'D' => while (i < n) : (i += 1) rope.down(),
else => unreachable,
}
}
var i: u16 = 0;
for (rope.tail_visits) |_, x| {
for (rope.tail_visits[x]) |n| {
if (n == 1) i += 1;
}
}
return .{ .int = i };
}
pub fn puzzle_2(allocator: std.mem.Allocator, input: []const u8) Result {
var rope = Rope.init(allocator, 8);
defer rope.deinit();
var iter = std.mem.split(u8, input, "\n");
while (iter.next()) |line| {
const n = std.fmt.parseInt(u8, line[2..], 0) catch unreachable;
var i: usize = 0;
switch (line[0]) {
'R' => while (i < n) : (i += 1) rope.right(),
'L' => while (i < n) : (i += 1) rope.left(),
'U' => while (i < n) : (i += 1) rope.up(),
'D' => while (i < n) : (i += 1) rope.down(),
else => unreachable,
}
}
var i: u16 = 0;
for (rope.tail_visits) |_, x| {
for (rope.tail_visits[x]) |n| {
if (n == 1) i += 1;
}
}
return .{ .int = i };
}
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