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const std = @import("std");
const aoc = @import("aoc");
// Links in this list are arranges in a circle
const RingLink = struct {
data: u32,
next: *RingLink,
const Self = @This();
pub fn insertNext(self: *Self, link: *RingLink) void {
link.next = self.next;
self.next = link;
}
pub fn popNext(self: *Self) !*Self {
const nlink = self.next;
if (nlink == self) return error.EmptyRingList;
self.next = nlink.next;
return nlink;
}
pub fn length(self: *Self) usize {
var count: usize = 1;
var link = self.next;
while (link != self) : (link = link.next) {
count += 1;
}
return count;
}
pub fn advance(self: *Self, count: usize) *RingLink {
var link = self;
var counter: usize = 0;
while (counter < count) : (link = link.next) {
counter += 1;
}
return link;
}
};
fn printLinks(start: *RingLink, end: *RingLink, skipfirst: bool) void {
var link = start;
var b = skipfirst;
while (b or link != end) : (link = link.next) {
b = false;
aoc.debugfmt("{} ", .{link.data});
}
}
fn parseInput(input: []const u8, lookup: []RingLink, list: *?*RingLink) !u32 {
var max: u32 = 0;
var link: *RingLink = undefined;
for (input) |c, i| {
if (c == '\n') break;
const v = try std.fmt.parseInt(u32, input[i .. i + 1], 10);
if (v > max) max = v;
if (list.* == null) {
list.* = &lookup[v];
link = list.*.?;
link.next = link;
} else {
link.insertNext(&lookup[v]);
link = link.next;
}
}
return max;
}
fn doRound(list: *RingLink, len: u32, lookup: []RingLink, current: **RingLink, max: u32, round: usize) !void {
_ = list;
var target = (current.*.data + max - 2) % max + 1;
var k: usize = 0;
var check = current.*.next;
while (k < 9) : (k += 1) {
if (check.data == target)
target = (target + max - 2) % max + 1;
check = if (k % 3 == 0) current.*.next else check.next;
}
var closest = &lookup[target];
if (aoc.debug) {
aoc.debugfmt("\n-- move {} --\ncups:", .{round});
if (aoc.debuglvl == 1) {
var start = current.*.advance(len - ((round - 1) % len));
var link = start;
while (true) {
if (link == current.*) {
aoc.debugfmt(" ({})", .{link.data});
} else {
aoc.debugfmt(" {} ", .{link.data});
}
link = link.next;
if (link == start) break;
}
aoc.debugfmt("\n", .{});
} else {
aoc.debugfmt("\n{} {} {}\n", .{ current.*.data, target, closest.data });
aoc.debugfmt(".. ", .{});
printLinks(current.*, current.*.next.next.next.next, false);
aoc.debugfmt("..\n.. ", .{});
printLinks(closest, closest.next.next.next.next, false);
aoc.debugfmt("..\n", .{});
}
}
var i: usize = 0;
while (i < 3) : (i += 1) {
const poplink = try current.*.popNext();
closest.insertNext(poplink);
closest = poplink;
}
current.* = current.*.next;
}
fn createLookup(allocator: std.mem.Allocator, len: usize) ![]RingLink {
var lookup = try allocator.alloc(RingLink, len + 1);
errdefer allocator.free(lookup);
var i: usize = 1;
while (i <= len) : (i += 1) {
lookup[i] = RingLink{ .data = @intCast(u32, i), .next = undefined };
}
return lookup;
}
fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
var lookup = try createLookup(allocator, 9);
defer allocator.free(lookup);
var list: ?*RingLink = null;
const max = try parseInput(input, lookup, &list);
if (list == null) return aoc.Error.InvalidInput;
const real_len = @intCast(u32, list.?.length());
var round: usize = 0;
var current = list.?;
while (round < 100) : (round += 1) {
try doRound(list.?, real_len, lookup, ¤t, max, round + 1);
}
var start = &lookup[1];
var link = start.next;
var parts = std.ArrayList(u8).init(allocator);
defer parts.deinit();
while (link != start) : (link = link.next) {
try parts.append('0' + @intCast(u8, link.data));
}
return try std.fmt.allocPrint(allocator, "{s}", .{parts.items});
}
fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
var lookup = try createLookup(allocator, 1000000);
defer allocator.free(lookup);
var list: ?*RingLink = null;
_ = try parseInput(input, lookup, &list);
if (list == null) return aoc.Error.InvalidInput;
const real_len = list.?.length();
var end = list.?.advance(real_len - 1);
var i = real_len + 1;
while (i <= 1000000) : (i += 1) {
end.insertNext(&lookup[i]);
end = end.next;
}
var round: usize = 0;
var current = list.?;
while (round < 10000000) : (round += 1) {
if (round % 1000000 == 0) aoc.debugfmt(".", .{});
try doRound(list.?, 1000000, lookup, ¤t, 1000000, round + 1);
}
aoc.debugfmt("\n", .{});
var link = (&lookup[1]).next;
const a = @intCast(u64, link.data);
const b = @intCast(u64, link.next.data);
aoc.debugfmt("{} * {} = {}\n", .{ a, b, a * b });
return try std.fmt.allocPrint(allocator, "{}", .{a * b});
}
pub const main = aoc.main(part1, part2, .{ "97245386", "156180332979" });
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