From 89d248d3b860f42456a7d34697408fd3141df1db Mon Sep 17 00:00:00 2001
From: ktkk <dezuttereluka@gmail.com>
Date: Tue, 2 Dec 2025 12:22:28 +0000
Subject: [PATCH] Improve part 1 efficiency

---
 src/days/day02.zig | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/src/days/day02.zig b/src/days/day02.zig
index 098b80e..8e45652 100644
--- a/src/days/day02.zig
+++ b/src/days/day02.zig
@@ -2,7 +2,7 @@ const std = @import("std");
 
 pub const title = "Day 02";
 
-pub fn run(allocator: std.mem.Allocator) !void {
+pub fn run(_: std.mem.Allocator) !void {
     const input = @embedFile("./input/day02.txt");
     var ranges = std.mem.tokenizeScalar(u8, input, ',');
 
@@ -18,13 +18,17 @@ pub fn run(allocator: std.mem.Allocator) !void {
         std.debug.print("{d} to {d}\n", .{start, end});
 
         outer: for (start..end+1) |id| {
-            const id_s = try std.fmt.allocPrint(allocator, "{}", .{id});
-            defer allocator.free(id_s);
+            // IDs smaller than 10 cannot possibly have repeated digits
+            if (id / 10 == 0) continue;
 
-            if (id_s.len % 2 == 1) continue;
+            const len = getLen(id);
+            // Uneven numbers cannot consist of 2 sequences
+            if (len % 2 == 1) continue;
 
-            for (0..id_s.len / 2) |i| {
-                if (id_s[i] != id_s[i + id_s.len / 2]) {
+            for (0..len / 2) |i| {
+                const a = getDigit(id, i);
+                const b = getDigit(id, i + len / 2);
+                if (a != b) {
                     continue :outer;
                 }
             }
@@ -41,3 +45,22 @@ pub fn run(allocator: std.mem.Allocator) !void {
     try stdout.flush();
 }
 
+fn getDigit(x: u64, n: usize) u8 {
+    var new_x = x;
+    for (0..n) |_| {
+        new_x /= 10;
+    }
+    return @intCast(new_x % 10);
+}
+
+fn getLen(x: u64) usize {
+    var new_x = x;
+    var n: usize = 0;
+    while (true) {
+        n += 1;
+        new_x /= 10;
+        if (new_x == 0) break;
+    }
+    return n;
+}
+