1 files changed, 63 insertions, 0 deletions
diff --git a/src/15/main.zig b/src/15/main.zig
new file mode 100644
index 0000000..3fcb555
--- /dev/null
+++ b/src/15/main.zig
@@ -0,0 +1,63 @@
+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" });