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const std = @import("std");
const aoc = @import("aoc");
const Parts = struct { a: u32, b: u32 };
fn findparts(intlist: *const std.ArrayList(u32), sum: u32) ?Parts {
var start: usize = 0;
const items = intlist.items;
var end: usize = items.len - 1;
while (start != end) {
const csum = items[start] + items[end];
if (csum == sum) {
return Parts{ .a = items[start], .b = items[end] };
} else if (csum > sum) {
end -= 1;
} else {
start += 1;
}
}
return null;
}
fn parse(allocator: std.mem.Allocator, input: []u8) !std.ArrayList(u32) {
var intlist = std.ArrayList(u32).init(allocator);
errdefer intlist.deinit();
var it = std.mem.tokenize(u8, input, "\n");
while (it.next()) |line| {
try intlist.append(try std.fmt.parseInt(u32, line, 10));
}
return intlist;
}
fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
const intlist = try parse(allocator, input);
defer intlist.deinit();
std.sort.sort(u32, intlist.items, {}, comptime std.sort.asc(u32));
if (findparts(&intlist, 2020)) |parts| {
return try std.fmt.allocPrint(allocator, "{d}", .{parts.a * parts.b});
}
return null;
}
fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = args;
const intlist = try parse(allocator, input);
defer intlist.deinit();
std.sort.sort(u32, intlist.items, {}, comptime std.sort.asc(u32));
const target: u16 = 2020;
var third: u32 = 0;
while (third < intlist.items.len) : (third += 1) {
const tmp = intlist.items[third];
intlist.items[third] = target + 1;
if (findparts(&intlist, target - tmp)) |parts| {
return try std.fmt.allocPrint(allocator, "{d}", .{parts.a * parts.b * tmp});
}
intlist.items[third] = tmp;
}
return null;
}
pub const main = aoc.main(part1, part2, .{ "658899", "155806250" });
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