1 files changed, 72 insertions, 0 deletions
diff --git a/src/01/main.zig b/src/01/main.zig
new file mode 100644
index 0000000..8111960
--- /dev/null
+++ b/src/01/main.zig
@@ -0,0 +1,72 @@
+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" });