advent2025

commit 37fe582c54b04e8e8cd7a2967b8887039d3ab047
parent 8665aa1f238c77ed5b0257987f5ab17de80a81a0
Author: bsandro <email@bsandro.tech>
Date:   Mon,  1 Dec 2025 22:35:47 +0200

Huge optimization to get from 0.00047s to 0.00013s

Diffstat:
M
Makefile
 | 
2
+-
M
cputime.h
 | 
2
+-
M
day01.c
 | 
49
++++++++++++++++++++++++++++++-------------------

3 files changed, 32 insertions(+), 21 deletions(-)
diff --git a/Makefile b/Makefile
@@ -1,6 +1,6 @@
 .SUFFIXES:
 MAKEFLAGS+=-rR
-CFLAGS=-Os -std=gnu23 -Werror -Wall -Wextra -ffast-math -march=native -I.
+CFLAGS=-O3 -std=gnu23 -Werror -Wall -Wextra -ffast-math -march=native -mtune=native -I.
 LDFLAGS=-lc
 CC=cc
 
diff --git a/cputime.h b/cputime.h
@@ -10,5 +10,5 @@ void __attribute((constructor)) begin() {
 void __attribute((destructor)) end() {
     struct timespec ts;
     clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
-    printf("%llu μs\n", ((ts.tv_sec+ts.tv_nsec)-now)/1000);
+    printf("%f s\n", (double)((ts.tv_sec+ts.tv_nsec)-now)/1e9);
 }
diff --git a/day01.c b/day01.c
@@ -1,30 +1,41 @@
 #include <stdio.h>
 #include "cputime.h"
-#define BUF_SIZE 32
 
 int main(void) {
-    char buf[BUF_SIZE];
     int cur = 50;
     int p1 = 0;
     int p2 = 0;
-    while (fgets(buf, sizeof(buf), stdin)) {
-        char dir;
-        int cnt;
-        sscanf(buf, "%c%d", &dir, &cnt);
-        if (cnt>99) p2+=cnt/100;
-        cnt = cnt%100;
-        if (dir=='L') {
-            if (cur&&cnt>cur) p2++;
-            cur-=cnt;
-            if (cur<0) cur=100+cur;
-        } else if (dir=='R') {
-            if (cur+cnt>100) p2++;
-            cur+=cnt;
-            if (cur>99) cur=cur-100;
+    char dir = 0;
+    char buf[4] = {0};
+    int bufs = 0;
+    int cnt = 0;
+    for (char c=getchar();c!=EOF;c=getchar()) {
+        // sscanf() was here but it felt slow
+        if (c=='L'||c=='R') {
+            dir = c;
+            cnt = 0;
+        } else if (c>='0'&&c<='9') {
+            buf[bufs++] = c-'0';
+        } else if (c=='\n') {
+            for (int i=0;i<bufs;++i) {
+                cnt = cnt*10+buf[i];
+            }
+            if (cnt>99) p2+=cnt/100;
+            cnt = cnt%100;
+            if (dir=='L') {
+                if (cur&&cnt>cur) p2++;
+                cur-=cnt;
+                if (cur<0) cur=100+cur;
+            } else if (dir=='R') {
+                if (cur+cnt>100) p2++;
+                cur+=cnt;
+                if (cur>99) cur=cur-100;
+            }
+            if (cur==0) { p1++; p2++; }
+            cnt=0;
+            bufs=0;
         }
-        if (cur==0) { p1++; p2++; }
     }
-    printf("p1: %d\n", p1);
-    printf("p2: %d\n", p2);
+    printf("p1: %d\np2: %d\n", p1, p2);
     return 0;
 }