day09 part2

src/day09.zig

@@ -31,7 +31,9 @@ fn zeroed(seq: []const isize) bool {
 
 fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
     var it = util.splitLines(input);
-    var sum: isize = 0;
+    var prev_sum: isize = 0;
+    var next_sum: isize = 0;
+
     while (it.next()) |line| {
         if (line.len == 0) continue;
         var seqs = std.ArrayList([]const isize).init(alloc);
@@ -41,17 +43,24 @@ fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
         while (!zeroed(seqs.items[seqs.items.len - 1])) {
             try seqs.append(try drv(alloc, seqs.items[seqs.items.len - 1]));
         }
-        // Solve next sequence
-        var predicted: isize = 0;
+        // Extrapolate
+        var next: isize = 0;
+        var prev: isize = 0;
+        for (seqs.items) |seq| {
+            next += seq[seq.len - 1];
+        }
+        for (0..seqs.items.len) |i| {
+            prev = seqs.items[seqs.items.len - i - 1][0] - prev;
+        }
         for (seqs.items) |seq| {
-            predicted += seq[seq.len - 1];
             // Clean up
             alloc.free(seq);
         }
-        sum += predicted;
+        prev_sum += prev;
+        next_sum += next;
     }
 
-    return .{ .a = sum, .b = 0 };
+    return .{ .a = next_sum, .b = prev_sum };
 }
 
 test "silver" {
@@ -66,8 +75,10 @@ test "silver" {
 
 test "gold" {
     const input =
-        \\
+        \\0 3 6 9 12 15
+        \\1 3 6 10 15 21
+        \\10 13 16 21 30 45
     ;
     const sln = try solve(std.testing.allocator, input);
-    try std.testing.expectEqual(@as(isize, 0), sln.b);
+    try std.testing.expectEqual(@as(isize, 2), sln.b);
 }