From b3b47eb8664a6f530fb33df6a1a5ffbe6c86cb32 Mon Sep 17 00:00:00 2001
From: Martin Ashby <martin@ashbysoft.com>
Date: Sun, 3 Dec 2023 22:18:37 +0000
Subject: Add comments to day3

---
 day3.zig | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/day3.zig b/day3.zig
index 4fe95b5..04c432f 100644
--- a/day3.zig
+++ b/day3.zig
@@ -55,6 +55,11 @@ const Grid = struct {
     }
 };
 
+// iterate over the map
+// tracking if we're in a 'span' of digits
+// at the end of the 'span', parse the number and check all the surrounding
+// cells. If any are symbols other than . or a digit, then this span is a 
+// part number and add it to the sum.
 fn solve(a: std.mem.Allocator, input: []const u8) !u32 {
     var spanBuf = std.ArrayList(u8).init(a);
     var spanStart: usize = 0; // easier than backtracking
@@ -103,6 +108,13 @@ fn solve(a: std.mem.Allocator, input: []const u8) !u32 {
     return sum;
 }
 
+// iterate over the map, finding 'spans' like before. 
+// Except this time, store the span's number in a arraylist
+// and record the number's index in the cells (in a 'shadow' grid) 
+// where the span covered the cell
+// Then make a second pass to find gears, and check the 'shadow' grid 
+// for adjescent spans. If there are exactly 2 adjescent spans, 
+// lookup their values from the arraylist and multiple them together!
 fn solve_pt2(a: std.mem.Allocator, input: []const u8) !u32 {
     var spanBuf = std.ArrayList(u8).init(a);
     var spanStart: usize = 0; // easier than backtracking
@@ -141,6 +153,7 @@ fn solve_pt2(a: std.mem.Allocator, input: []const u8) !u32 {
     var sum: u32 = 0;
     var spixes = std.AutoHashMap(usize, bool).init(a);
     defer spixes.deinit();
+    // find gears
     for (0..grid.height) |y| {
         for (0..grid.width) |x| {
             const ch = grid.get(x, y) orelse @panic("borken");
@@ -149,6 +162,7 @@ fn solve_pt2(a: std.mem.Allocator, input: []const u8) !u32 {
                 const mx = std.math.sub(usize, x, 1) catch x;
                 const my = std.math.sub(usize, y, 1) catch y;
 
+                // Find adjescent spans
                 spixes.clearRetainingCapacity();
                 for (mx..(x+2)) |x_p| {
                     for (my..(y+2)) |y_p| {
-- 
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