From 8c8057fd3a493aaa11aec35602dc429873ccad42 Mon Sep 17 00:00:00 2001
From: Tarik Hijstek <tarikhijstek@florinco.nl>
Date: Fri, 29 Sep 2023 13:17:28 +0200
Subject: [PATCH] non brute force solution.

---
 2020/day13/mainPart2.zig | 48 ++++++++++++----------------------------
 1 file changed, 14 insertions(+), 34 deletions(-)

diff --git a/2020/day13/mainPart2.zig b/2020/day13/mainPart2.zig
index 6c38119..0e493c9 100644
--- a/2020/day13/mainPart2.zig
+++ b/2020/day13/mainPart2.zig
@@ -1,62 +1,42 @@
 const input = @embedFile("inputPart2.txt");
 const std = @import("std");
 
-
 pub fn main() !void {
 
     //Data wrangling
     var it = std.mem.tokenize(u8, input, "\n");
-    var busses:[68]u32 = std.mem.zeroes([68:0]u32);
+    var busses: [68]u32 = std.mem.zeroes([68:0]u32);
     var i: u32 = 0;
 
-    while(it.next())
-        |line|
-        :(i+=1)
-    {
+    while (it.next()) |line| : (i += 1) {
         busses[i] = readNum(line);
     }
 
     //count total active buslines
     var busTotal: u8 = 0;
-    for(busses)|bus|{
-        if(bus > 0)
+    for (busses) |bus| {
+        if (bus > 0)
             busTotal += 1;
     }
 
-    const stepSize: u32 = 1;
+    var stepSize: u64 = 1;
     var n: u64 = 1;
-    var found: u8 = 0;
-
-    //Algorithm
-    while(found != busTotal):(n+=stepSize){
-        
-        for(busses, 0..)|bus, offset|{
-            //skip
-            if(bus == 0){
-                continue;
-            }
-
-            if(((n + offset) % bus) == 0){
-                found +=1;
-            }
-        }
 
-        if(found >= 7){
-            std.debug.print("tick {d} has {d} aligned!\n",.{n, found});
+    for (busses, 0..) |bus, offset| {
+        //skip
+        if (bus == 0) {
+            continue;
         }
 
-        if(found == busTotal){
-            std.debug.print("Final tick {d}\n",.{n});
-            break;
-        }else{
-            found = 0;
-        }
+        while (((n + offset) % bus) != 0) : (n += stepSize) {}
+
+        stepSize = stepSize * bus;
     }
+    std.debug.print("tick: {d}", .{n});
 }
 
 fn readNum(line: []const u8) u32 {
     return std.fmt.parseInt(u32, line, 10) catch {
         return 0;
     };
-
-}
\ No newline at end of file
+}