aoc2024: make build tool download input & add shared main function

1 files changed, 70 insertions, 0 deletions

diff --git a/aoc24/src/day1.zig b/aoc24/src/day1.zig
new file mode 100644
index 0000000..1f26943
--- /dev/null
+++ b/aoc24/src/day1.zig
@@ -0,0 +1,70 @@
+const std = @import("std");
+
+test {
+    std.debug.assert(try part1(
+        \\3   4
+        \\4   3
+        \\2   5
+        \\1   3
+        \\3   9
+        \\3   3
+    ) == 11);
+}
+
+const Input = struct {
+    lefts: std.ArrayList(u32),
+    rights: std.ArrayList(u32),
+};
+
+pub fn parse(allocator: std.mem.Allocator, input: []const u8) !Input {
+    var lines = std.mem.split(u8, input, "\n");
+
+    var lefts = std.ArrayList(u32).init(allocator);
+    // defer lefts.deinit();
+    var rights = std.ArrayList(u32).init(allocator);
+    // defer rights.deinit();
+
+    while (lines.next()) |line| {
+        if (line.len == 0) continue;
+        var words = std.mem.split(u8, line, "   ");
+        const left = try std.fmt.parseInt(u32, words.next() orelse return error.InvalidInput, 10);
+        const right = try std.fmt.parseInt(u32, words.next() orelse return error.InvalidInput, 10);
+        try lefts.append(left);
+        try rights.append(right);
+    }
+
+    return .{ .lefts = lefts, .rights = rights };
+}
+
+pub fn part1(_: std.mem.Allocator, input: Input) !u32 {
+    std.mem.sort(u32, input.lefts.items, {}, std.sort.asc(u32));
+    std.mem.sort(u32, input.rights.items, {}, std.sort.asc(u32));
+
+    var sum: u32 = 0;
+    for (input.lefts.items, input.rights.items) |l, r| {
+        sum += @max(l, r) - @min(l, r);
+    }
+
+    return sum;
+}
+
+pub fn part2(allocator: std.mem.Allocator, input: Input) !u32 {
+    var right_counts = std.AutoHashMap(u32, u16).init(allocator);
+    defer right_counts.deinit();
+
+    for (input.rights.items) |right| {
+        const entry = try right_counts.getOrPut(right);
+        if (entry.found_existing)
+            entry.value_ptr.* += 1
+        else
+            entry.value_ptr.* = 1;
+    }
+
+    var sum: u32 = 0;
+    for (input.lefts.items) |x| {
+        const count: u32 = @intCast(right_counts.get(x) orelse 0);
+        sum += x * count;
+    }
+
+    return sum;
+}