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const std = @import("std");
const aoc = @import("aoc");
const Occurance = struct { last: u32, beforelast: u32 };
fn getNthSpoken(allocator: std.mem.Allocator, input: []u8, spoken: u32) !u32 {
var occurances = std.AutoHashMap(u32, Occurance).init(allocator);
defer occurances.deinit();
var intlist = std.ArrayList(u32).init(allocator);
defer intlist.deinit();
var numit = std.mem.tokenize(u8, input, "\n,");
var i: u32 = 0;
while (numit.next()) |numstr| : (i += 1) {
const num = try std.fmt.parseInt(u32, numstr, 10);
try intlist.append(num);
try occurances.put(num, Occurance{ .last = i, .beforelast = i });
}
while (i < spoken) : (i += 1) {
if (aoc.debug and i % 100000 == 0)
aoc.debugfmt("\r{}", .{i});
const num = intlist.items[i - 1];
var entry = occurances.getEntry(num);
var diff: u32 = 0;
if (entry) |occ_entry| {
diff = occ_entry.value_ptr.last - occ_entry.value_ptr.beforelast;
}
entry = occurances.getEntry(diff);
if (entry) |occ_entry| {
occ_entry.value_ptr.beforelast = occ_entry.value_ptr.last;
occ_entry.value_ptr.last = i;
} else {
try occurances.put(diff, Occurance{ .last = i, .beforelast = i });
}
try intlist.append(diff);
}
if (aoc.debug)
aoc.debugfmt("\r \r", .{});
return intlist.items[spoken - 1];
}
fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
const answer = try getNthSpoken(allocator, input, 2020);
return try std.fmt.allocPrint(allocator, "{}", .{answer});
}
fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
const answer = try getNthSpoken(allocator, input, 30000000);
return try std.fmt.allocPrint(allocator, "{}", .{answer});
}
pub const main = aoc.main(part1, part2, .{ "1259", "689" });
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