1 files changed, 119 insertions, 0 deletions
diff --git a/src/10/main.zig b/src/10/main.zig
new file mode 100644
index 0000000..993b2cf
--- /dev/null
+++ b/src/10/main.zig
@@ -0,0 +1,119 @@
+const std = @import("std");
+const aoc = @import("aoc");
+
+fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    var target: u32 = 0;
+
+    var intlist = std.ArrayList(u32).init(allocator);
+    defer intlist.deinit();
+
+    var lineit = std.mem.tokenize(u8, input, "\n");
+    while (lineit.next()) |line| {
+        var val = try std.fmt.parseInt(u32, line, 10);
+        if (val > target) {
+            target = val;
+        }
+        try intlist.append(val);
+    }
+
+    target += 3;
+    try intlist.append(target);
+
+    if (intlist.items.len == 0) return null;
+
+    std.sort.sort(u32, intlist.items, {}, comptime std.sort.asc(u32));
+
+    var diffs = [_]u32{ 0, 0, 0, 0 };
+    var last: u32 = 0;
+    for (intlist.items) |ov| {
+        if (ov - last > 3 or ov == last) {
+            aoc.debugfmt("Unable to find a suitable adapter for {} output jolts (next: {} jolts)\n", .{ last, ov });
+            return aoc.Error.InvalidInput;
+        }
+        diffs[ov - last] += 1;
+        last = ov;
+    }
+
+    return try std.fmt.allocPrint(allocator, "{d}", .{diffs[1] * diffs[3]});
+}
+
+fn countArrangements(arrangement: []u32, adapters: []const u32, adaptermask: []u1, target: u32, fillcount: u32, searchstart: u32, count: *u32) void {
+    var last: u32 = 0;
+    if (fillcount > 0) {
+        last = arrangement[fillcount - 1];
+    }
+
+    if (last == target) {
+        count.* += 1;
+        return;
+    }
+
+    for (adapters[searchstart..]) |adap, i| {
+        if (adaptermask[searchstart + i] == 1) continue;
+        if (adap > last + 3) break;
+
+        adaptermask[searchstart + i] = 1;
+        arrangement[fillcount] = adap;
+        countArrangements(arrangement, adapters, adaptermask, target, fillcount + 1, searchstart + @intCast(u32, i) + 1, count);
+        adaptermask[searchstart + i] = 0;
+    }
+}
+
+fn groupPermutations(adapters: []const u32) u64 {
+    if (adapters.len == 1) return 1;
+    var count: u64 = 0;
+    for (adapters) |_, i| {
+        if (i == 0) continue;
+        if (adapters[i] <= adapters[0] + 3) {
+            count += groupPermutations(adapters[i..]);
+        }
+    }
+    return count;
+}
+
+fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    var target: u32 = 0;
+
+    var intlist = std.ArrayList(u32).init(allocator);
+    defer intlist.deinit();
+
+    try intlist.append(0);
+
+    var lineit = std.mem.tokenize(u8, input, "\n");
+    while (lineit.next()) |line| {
+        var val = try std.fmt.parseInt(u32, line, 10);
+        if (val > target) {
+            target = val;
+        }
+        try intlist.append(val);
+    }
+
+    target += 3;
+    try intlist.append(target);
+
+    if (intlist.items.len == 0) return aoc.Error.InvalidInput;
+
+    std.sort.sort(u32, intlist.items, {}, comptime std.sort.asc(u32));
+
+    var gapstart: u32 = 0;
+    var multiplier: u128 = 1;
+
+    var i: u32 = 0;
+    while (i < intlist.items.len - 1) : (i += 1) {
+        // whenever we encounter a 3 jolt gap, we can calc permutations of group before
+        if (i + 1 < intlist.items.len and intlist.items[i + 1] == intlist.items[i] + 3) {
+            multiplier *= groupPermutations(intlist.items[gapstart .. i + 1]);
+            aoc.debugfmt("gap from {} to {} => {}\n", .{ intlist.items[i], intlist.items[i + 1], multiplier });
+            aoc.debugfmt("group size: {}\n", .{i + 1 - gapstart});
+            gapstart = i + 1;
+        }
+    }
+
+    return try std.fmt.allocPrint(allocator, "{d}", .{multiplier});
+}
+
+pub const main = aoc.main(part1, part2, .{ "1998", "347250213298688" });