1 files changed, 105 insertions, 0 deletions
diff --git a/aoc24/src/day2.zig b/aoc24/src/day2.zig
new file mode 100644
index 0000000..656fd41
--- /dev/null
+++ b/aoc24/src/day2.zig
@@ -0,0 +1,105 @@
+const std = @import("std");
+
+test "part 1" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+    const input = try parse(allocator,
+        \\7 6 4 2 1
+        \\1 2 7 8 9
+        \\9 7 6 2 1
+        \\1 3 2 4 5
+        \\8 6 4 4 1
+        \\1 3 6 7 9
+    );
+    const value = try part1(allocator, input);
+    const ok = value == 2;
+    if (!ok) std.debug.print("got {}\n", .{value});
+    std.debug.assert(ok);
+}
+
+test "part 2" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+    const input = try parse(allocator,
+        \\7 6 4 2 1
+        \\1 2 7 8 9
+        \\9 7 6 2 1
+        \\1 3 2 4 5
+        \\8 6 4 4 1
+        \\1 3 6 7 9
+    );
+    const value = try part2(allocator, input);
+    const ok = value == 4;
+    if (!ok) std.debug.print("got {}\n", .{value});
+    std.debug.assert(ok);
+}
+
+pub fn parse(
+    allocator: std.mem.Allocator,
+    input: []const u8,
+) !std.ArrayList(std.ArrayList(u32)) {
+    var lines = std.mem.split(u8, input, "\n");
+
+    var reports = std.ArrayList(std.ArrayList(u32)).init(allocator);
+
+    while (lines.next()) |line| {
+        if (line.len == 0) continue;
+        var report = std.ArrayList(u32).init(allocator);
+        var words = std.mem.split(u8, line, " ");
+        while (words.next()) |word| {
+            try report.append(try std.fmt.parseInt(u32, word, 10));
+        }
+        try reports.append(report);
+    }
+
+    return reports;
+}
+
+pub fn part1(
+    _: std.mem.Allocator,
+    reports: std.ArrayList(std.ArrayList(u32)),
+) !u32 {
+    var safe: u32 = 0;
+    for (reports.items) |report| {
+        const increasing = report.items[0] < report.items[1];
+        for (report.items[0 .. report.items.len - 1], report.items[1..]) |prev, curr| {
+            if ((prev < curr) != increasing) break;
+            if (prev == curr) break;
+            if (@max(prev, curr) - @min(prev, curr) > 3) break;
+        } else {
+            safe += 1;
+        }
+    }
+
+    return safe;
+}
+
+pub fn part2(
+    _: std.mem.Allocator,
+    reports: std.ArrayList(std.ArrayList(u32)),
+) !u32 {
+    var safe: u32 = 0;
+    for (reports.items) |report| {
+        var is_safe = false;
+        for ([_]bool{ false, true }) |increasing| for (0..report.items.len) |skipped| {
+            const first: u32 = if (skipped == 0) 1 else 0;
+            var prev = report.items[first];
+            for (first + 1..report.items.len) |i| {
+                if (i == skipped) continue;
+                const curr = report.items[i];
+                if ((prev < curr) != increasing or prev == curr or @max(prev, curr) - @min(prev, curr) > 3) {
+                    break;
+                }
+                prev = curr;
+            } else {
+                is_safe = true;
+            }
+        };
+        if (is_safe) safe += 1;
+        // std.debug.print("{any} is {s}safe\n", .{ report.items, if (is_safe) "" else "un" });
+    }
+
+    return safe;
+}