From 5d5b41623ab99b7fb34d3fc35a72e40eadc304b7 Mon Sep 17 00:00:00 2001
From: Martin Ashby <martin@ashbysoft.com>
Date: Mon, 4 Dec 2023 21:28:15 +0000
Subject: day4 pt2

---
 day4.zig | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/day4.zig b/day4.zig
index d4d7d0b..03ff475 100644
--- a/day4.zig
+++ b/day4.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 
 pub fn main() !void {
     try std.fmt.format(std.io.getStdOut().writer(), "day4 pt1: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input)});
+    try std.fmt.format(std.io.getStdOut().writer(), "day4 pt2: {}\n", .{try solve_pt2(std.heap.page_allocator, puzzle_input)});
 }
 
 fn solve_pt1(a: std.mem.Allocator, input: []const u8) !u32 {
@@ -26,6 +27,44 @@ fn solve_pt1(a: std.mem.Allocator, input: []const u8) !u32 {
     return sum;
 }
 
+fn solve_pt2(a: std.mem.Allocator, input: []const u8) !u32 {
+    const n_cards = std.mem.count(u8, input, "\n") + 1;
+    const card_counts = try a.alloc(u32, n_cards);
+    defer a.free(card_counts);
+    @memset(card_counts, 1);
+
+    var lines = std.mem.splitScalar(u8, input, '\n');
+    var lineix: usize = 0;
+    while (lines.next()) |line| {
+        defer lineix += 1;
+        var s1 = std.mem.splitScalar(u8, line, ':');
+        const card_num_str = s1.first();
+        _ = card_num_str; // I guess we'll use this next round :)
+        const card_str = s1.next() orelse return error.NoCard;
+        var s2 = std.mem.splitScalar(u8, card_str, '|');
+        const winning_nos = try read_nums(a, s2.first());
+        defer a.free(winning_nos);
+        const your_nos = try read_nums(a, s2.next() orelse return error.NoNos);
+        defer a.free(your_nos);
+        const inter = try intersection(a, winning_nos, your_nos);
+        defer a.free(inter);
+        if (inter.len > 0) {
+            // for however many copies we have of the current card///
+            for (0..card_counts[lineix]) |_| {
+                // add more copies of _subsequent_ cards based on the score of this card
+                for (lineix + 1..(lineix + 1 + inter.len)) |nix| {
+                    card_counts[nix] += 1;
+                }
+            }
+        }
+    }
+    var sum: u32 = 0;
+    for (card_counts) |cc| {
+        sum += cc;
+    }
+    return sum;
+}
+
 fn read_nums(a: std.mem.Allocator, input: []const u8) ![]const u32 {
     var res = std.ArrayList(u32).init(a);
     errdefer res.deinit();
@@ -58,6 +97,9 @@ fn intersection(a: std.mem.Allocator, first: []const u32, second: []const u32) !
 test "pt1" {
     try std.testing.expectEqual(@as(u32, 13), try solve_pt1(std.testing.allocator, test_input));
 }
+test "pt2" {
+    try std.testing.expectEqual(@as(u32, 30), try solve_pt2(std.testing.allocator, test_input));
+}
 const test_input =
     \\Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
     \\Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
-- 
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