Day 9 pt2

1 files changed, 31 insertions, 0 deletions

diff --git a/day9.zig b/day9.zig
index 15863af..d2a2dd6 100644
--- a/day9.zig
+++ b/day9.zig
@@ -2,6 +2,7 @@ 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)});
+    try std.fmt.format(std.io.getStdOut().writer(), "day 9 pt2: {}\n", .{try solve_pt2(std.heap.page_allocator, puzzle_input)});
 }
 
 fn solve_pt1(a: std.mem.Allocator, input: []const u8) !i64 {
@@ -44,6 +45,36 @@ test "pt1" {
     try std.testing.expectEqual(@as(i64, 114), try solve_pt1(std.testing.allocator, test_input));
 }
 
+fn solve_pt2(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 prv = nums.items[0] - try predictPrev(a, nums);
+        // std.log.warn("prv {}", .{prv});
+        sum += prv;
+    }
+    return sum;
+}
+
+// note uses recursion!
+fn predictPrev(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]);
+    }
+    return sublist.items[0] - try predictPrev(a, sublist);
+}
+
+test "pt2" {
+    try std.testing.expectEqual(@as(i64, 2), try solve_pt2(std.testing.allocator, test_input));
+}
+
 const test_input =
     \\0 3 6 9 12 15
     \\1 3 6 10 15 21