day9

2 files changed, 60 insertions, 0 deletions

diff --git a/day09/example.txt b/day09/example.txt
new file mode 100644
index 0000000..539a763
--- /dev/null
+++ b/day09/example.txt
@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45
diff --git a/day09/solution.zig b/day09/solution.zig
new file mode 100644
index 0000000..e445c19
--- /dev/null
+++ b/day09/solution.zig
@@ -0,0 +1,57 @@
+const std = @import("std");
+const print = std.debug.print;
+const assert = std.debug.assert;
+const ArrayList = std.ArrayList;
+const HashMap = std.HashMap;
+const mem = std.mem;
+
+const fin = mem.trim(u8, @embedFile("./input.txt"), &std.ascii.whitespace);
+var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+const allocator = gpa.allocator();
+
+fn evaluate(list: []const i64) struct { i64, i64 } {
+    if (mem.allEqual(i64, list, 0)) {
+        return .{ 0, 0 };
+    }
+
+    const n = list.len;
+    var derivative = ArrayList(i64).init(allocator);
+    defer derivative.deinit();
+
+    for (list[0 .. n - 1], list[1..n]) |curr, next| {
+        derivative.append(next - curr) catch unreachable;
+    }
+
+    const e = evaluate(derivative.items);
+    return .{ list[n - 1] + e, list[0] - e };
+}
+
+pub fn solve(listNums: ArrayList(ArrayList(i64))) void {
+    var part1: i64 = 0;
+    var part2: i64 = 0;
+
+    for (listNums.items) |list| {
+        const e = evaluate(list.items);
+        part1 += e[0];
+        part2 += e[1];
+    }
+
+    print("{d}\n{d}\n", .{ part1, part2 });
+}
+
+pub fn main() !void {
+    var splitLines = mem.splitScalar(u8, fin, '\n');
+    var listNums = ArrayList(ArrayList(i64)).init(allocator);
+
+    while (splitLines.next()) |line| {
+        var listText = mem.tokenizeScalar(u8, line, ' ');
+
+        var row = ArrayList(i64).init(allocator);
+        while (listText.next()) |n| {
+            try row.append(try std.fmt.parseInt(i64, n, 10));
+        }
+
+        try listNums.append(row);
+    }
+    solve(listNums);
+}