1 files changed, 83 insertions, 0 deletions
diff --git a/src/09/main.zig b/src/09/main.zig
new file mode 100644
index 0000000..1187e99
--- /dev/null
+++ b/src/09/main.zig
@@ -0,0 +1,83 @@
+const std = @import("std");
+const aoc = @import("aoc");
+
+const preamble_size = 25;
+
+fn preambleHasSum(preamble: []u64, target: u64) bool {
+    for (preamble) |v1| {
+        for (preamble) |v2| {
+            if (v1 != v2 and v1 + v2 == target)
+                return true;
+        }
+    }
+    return false;
+}
+
+fn getInvalid(allocator: std.mem.Allocator, input: []u8) !u64 {
+    var preamble = std.ArrayList(u64).init(allocator);
+    defer preamble.deinit();
+
+    var i: u64 = 0;
+    var numit = std.mem.tokenize(u8, input, "\n");
+    while (numit.next()) |numstr| : (i += 1) {
+        const num = try std.fmt.parseInt(u64, numstr, 10);
+        if (i > preamble_size) {
+            if (!preambleHasSum(preamble.items, num)) {
+                return num;
+            }
+            preamble.items[(i - 1) % preamble_size] = num;
+        } else {
+            try preamble.append(num);
+        }
+    }
+
+    return aoc.Error.InvalidInput;
+}
+
+fn listSum(items: []u64) u64 {
+    var sum: u64 = 0;
+    for (items) |i|
+        sum += i;
+    return sum;
+}
+
+fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    const answer = try getInvalid(allocator, input);
+
+    return try std.fmt.allocPrint(allocator, "{d}", .{answer});
+}
+
+fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    const invalid = try getInvalid(allocator, input);
+
+    var numset = std.ArrayList(u64).init(allocator);
+    defer numset.deinit();
+
+    var numit = std.mem.tokenize(u8, input, "\n");
+    while (true) {
+        const sum = listSum(numset.items);
+        if (sum < invalid) {
+            const numstr = numit.next();
+            if (numstr == null) break;
+            const num = try std.fmt.parseInt(u64, numstr.?, 10);
+            try numset.append(num);
+        } else if (sum > invalid) {
+            _ = numset.orderedRemove(0);
+        } else {
+            break;
+        }
+    }
+
+    if (numset.items.len > 0) {
+        const answer = std.mem.min(u64, numset.items) + std.mem.max(u64, numset.items);
+        return try std.fmt.allocPrint(allocator, "{d}", .{answer});
+    }
+
+    return null;
+}
+
+pub const main = aoc.main(part1, part2, .{ "466456641", "55732936" });