diff --git a/src/day04.zig b/src/day04.zig
new file mode 100644
index 0000000..2e495a6
--- /dev/null
+++ b/src/day04.zig
@@ -0,0 +1,99 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day04.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.value});
+    std.debug.print("{d}\n", .{sln.copies});
+}
+
+const Solution = struct {
+    value: usize,
+    copies: usize,
+};
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var value: usize = 0;
+    var card_counts = std.AutoHashMap(usize, usize).init(alloc);
+    defer card_counts.deinit();
+
+    var card: usize = 0;
+    var it = util.splitLines(input);
+    while (it.next()) |line| : (card += 1) {
+        if (line.len == 0) continue;
+
+        const sep = mem.indexOfScalar(u8, line, ':').?;
+        var line_it = util.split(line[sep + 1 ..], '|');
+        const winning = try util.parseIntsScalar(usize, alloc, line_it.next().?, .{});
+        defer alloc.free(winning);
+        const mine = try util.parseIntsScalar(usize, alloc, line_it.next().?, .{});
+        defer alloc.free(mine);
+
+        var matches: usize = 0;
+        for (mine) |n| {
+            if (mem.containsAtLeast(usize, winning, 1, &[_]usize{n})) {
+                matches += 1;
+            }
+        }
+        if (matches > 0) {
+            value += std.math.pow(usize, 2, matches - 1);
+        }
+
+        // Initialise with 1 copy of current card
+        const copies = blk: {
+            const entry = try card_counts.getOrPut(card);
+            if (entry.found_existing) {
+                entry.value_ptr.* += 1;
+            } else {
+                entry.value_ptr.* = 1;
+            }
+            break :blk entry.value_ptr.*;
+        };
+
+        // 1 additional card for each match, multiplied by copies of this card
+        for (1..matches + 1) |offset| {
+            const winning_card_entry = try card_counts.getOrPut(card + offset);
+            if (winning_card_entry.found_existing) {
+                winning_card_entry.value_ptr.* += copies;
+            } else {
+                winning_card_entry.value_ptr.* = copies;
+            }
+        }
+    }
+
+    var copy_count: usize = 0;
+    var copy_it = card_counts.valueIterator();
+    while (copy_it.next()) |v| {
+        copy_count += v.*;
+    }
+
+    return .{ .value = value, .copies = copy_count };
+}
+
+test "silver" {
+    const input =
+        \\Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+        \\Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+        \\Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+        \\Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+        \\Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+        \\Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 13), sln.value);
+}
+
+test "gold" {
+    const input =
+        \\Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+        \\Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+        \\Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+        \\Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+        \\Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+        \\Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 30), sln.copies);
+}
diff --git a/src/util.zig b/src/util.zig
index b32ab82..d262933 100644
--- a/src/util.zig
+++ b/src/util.zig
@@ -1,4 +1,37 @@
 const std = @import("std");
+const mem = std.mem;
 
 var gpa_impl = std.heap.GeneralPurposeAllocator(.{}){};
+/// General purpose allocator.
 pub const gpa = gpa_impl.allocator();
+
+pub const ParseIntsOptions = struct {
+    base: u8 = 10,
+    sep: u8 = ' ',
+};
+
+/// Parses separated integers. Sequential separators are treated as one. Caller owns memory.
+pub fn parseIntsScalar(comptime T: type, alloc: std.mem.Allocator, input: []const u8, opts: ParseIntsOptions) ![]T {
+    var items = std.ArrayList(T).init(alloc);
+    errdefer items.deinit();
+
+    var it = split(input, opts.sep);
+    while (it.next()) |i| {
+        if (i.len == 0) continue;
+        try items.append(try std.fmt.parseInt(T, i, opts.base));
+    }
+    return try items.toOwnedSlice();
+}
+
+/// Shortcut to splitScalar with a byte buffer.
+pub fn split(buffer: []const u8, sep: u8) mem.SplitIterator(u8, .scalar) {
+    return mem.splitScalar(u8, buffer, sep);
+}
+
+pub fn splitLines(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
+    return split(buffer, '\n');
+}
+
+pub fn splitSpace(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
+    return split(buffer, ' ');
+}