1 files changed, 53 insertions, 0 deletions
diff --git a/day9.zig b/day9.zig
new file mode 100644
index 0000000..15863af
--- /dev/null
+++ b/day9.zig
@@ -0,0 +1,53 @@
+const std = @import("std");
+
+pub fn main() !void {
+    try std.fmt.format(std.io.getStdOut().writer(), "day 9 pt1: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input)});
+}
+
+fn solve_pt1(a: std.mem.Allocator, input: []const u8) !i64 {
+    var sum: i64 = 0;
+    var spl = std.mem.split(u8, input, "\n");
+    while (spl.next()) |line| {
+        const nums = try readnums(a, line);
+        defer nums.deinit();
+        const nxt = nums.getLast() + try predictNext(a, nums);
+        // std.log.warn("nxt {}", .{nxt});
+        sum += nxt;
+    }
+    return sum;
+}
+
+// note uses recursion!
+fn predictNext(a: std.mem.Allocator, list: std.ArrayList(i64)) !i64 {
+    if (std.mem.allEqual(i64, list.items, 0)) {
+        return 0;
+    }
+    var sublist = std.ArrayList(i64).init(a);
+    defer sublist.deinit();
+    for (1..list.items.len) |ix| {
+        try sublist.append(list.items[ix] - list.items[ix - 1]);
+    }
+    const nxt_diff = try predictNext(a, sublist);
+    return sublist.getLast() + nxt_diff;
+}
+
+fn readnums(a: std.mem.Allocator, input: []const u8) !std.ArrayList(i64) {
+    var toks = std.mem.tokenize(u8, input, " ");
+    var res = std.ArrayList(i64).init(a);
+    while (toks.next()) |tok| {
+        try res.append(try std.fmt.parseInt(i64, tok, 10));
+    }
+    return res;
+}
+
+test "pt1" {
+    try std.testing.expectEqual(@as(i64, 114), try solve_pt1(std.testing.allocator, test_input));
+}
+
+const test_input =
+    \\0 3 6 9 12 15
+    \\1 3 6 10 15 21
+    \\10 13 16 21 30 45
+;
+
+const puzzle_input = @embedFile("day9.in");