From 794dddf854820854c9086cb23974230dfd11a1df Mon Sep 17 00:00:00 2001
From: xeals <dev@xeal.me>
Date: Wed, 13 Dec 2023 16:51:23 +1100
Subject: [PATCH] day13 part1

---
 src/day13.zig | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 105 insertions(+)
 create mode 100644 src/day13.zig

diff --git a/src/day13.zig b/src/day13.zig
new file mode 100644
index 0000000..62d358a
--- /dev/null
+++ b/src/day13.zig
@@ -0,0 +1,105 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day13.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.a});
+    std.debug.print("{d}\n", .{sln.b});
+}
+
+const Solution = struct {
+    a: usize,
+    b: usize,
+};
+
+fn findReflection(input: []const []const u8) usize {
+    if (input.len == 0) return 0;
+    std.debug.print("{s}\n", .{input});
+
+    const h = input.len;
+    const w = input[0].len;
+
+    // Scan vertical reflectivity
+    for (0..w - 1) |scan| {
+        var offset: usize = 0;
+        var row: usize = 0;
+        while (input[row][scan - offset] == input[row][scan + offset + 1]) {
+            // std.debug.print("{} {}±{}\n", .{ row, scan, offset });
+            row = (row + 1) % h;
+            if (row == 0) offset += 1;
+            if (scan < offset or scan + offset + 1 >= w) {
+                return scan + 1;
+            }
+        }
+    }
+
+    // Scan horizontal reflectivity
+    for (0..h - 1) |scan| {
+        var offset: usize = 0;
+        var col: usize = 0;
+        while (input[scan - offset][col] == input[scan + offset + 1][col]) {
+            // std.debug.print("{}±{} {}\n", .{ scan, offset, col });
+            col = (col + 1) % w;
+            if (col == 0) offset += 1;
+            if (scan < offset or scan + offset + 1 >= h) {
+                return (scan + 1) * 100;
+            }
+        }
+    }
+
+    unreachable;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var curr_pattern = std.ArrayList([]const u8).init(alloc);
+    defer curr_pattern.deinit();
+
+    var it = util.splitLines(input);
+    var sum: usize = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) {
+            const pattern = try curr_pattern.toOwnedSlice();
+            defer alloc.free(pattern);
+            const score = findReflection(pattern);
+            // std.debug.print("{}\n", .{score});
+            sum += score;
+        } else {
+            try curr_pattern.append(line);
+        }
+    }
+
+    return .{ .a = sum, .b = 0 };
+}
+
+test "silver" {
+    const input =
+        \\#.##..##.
+        \\..#.##.#.
+        \\##......#
+        \\##......#
+        \\..#.##.#.
+        \\..##..##.
+        \\#.#.##.#.
+        \\
+        \\#...##..#
+        \\#....#..#
+        \\..##..###
+        \\#####.##.
+        \\#####.##.
+        \\..##..###
+        \\#....#..#
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 405), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 0), sln.b);
+}