advent2025

commit 47634b6c8a003dbee3444f5521e3a40dc433ae53
parent 3056359427c68f4690a0b8848a468c2b4593715c
Author: bsandro <email@bsandro.tech>
Date:   Tue,  2 Dec 2025 07:59:40 +0200

day02 p1

Diffstat:
M
Makefile
 | 
2
+-
A
day02.c
 | 
60
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

2 files changed, 61 insertions(+), 1 deletion(-)
diff --git a/Makefile b/Makefile
@@ -1,7 +1,7 @@
 .SUFFIXES:
 MAKEFLAGS+=-rR
 CFLAGS=-O3 -std=gnu23 -Werror -Wall -Wextra -ffast-math -march=native -mtune=native -I.
-LDFLAGS=-lc
+LDFLAGS=-lc -lm
 CC=cc
 
 .PHONY: clean
diff --git a/day02.c b/day02.c
@@ -0,0 +1,60 @@
+#include <stdio.h>
+#include <inttypes.h>
+#include <math.h>
+#include "cputime.h"
+
+typedef struct Range {
+    uint64_t first;
+    uint64_t last;
+} Range;
+
+inline uint64_t btoi(uint64_t buf[32], int bufs) {
+    uint64_t num = 0;
+    for (int i=0;i<bufs;++i) num = num*10+buf[i];
+    return num;
+}
+
+inline int digits(uint64_t num) {
+    return log10l(num)+1;
+}
+
+inline bool is_silly(uint64_t num) {
+    int d = digits(num);
+    if (d%2==0) {
+        long shift = powl(10, d/2);
+        uint64_t top = num/shift;
+        uint64_t bottom = num-(top*shift);
+        return top==bottom;
+    }
+    return false;
+}
+
+inline uint64_t sum_silly(Range range) {
+    uint64_t sum = 0;
+    for (uint64_t i=range.first;i<=range.last;++i) {
+        if (is_silly(i)) sum+=i;
+    }
+    return sum;
+}
+
+int main(void) {
+    uint64_t buf[32] = {0};
+    int bufs = 0;
+    Range range = {0};
+    uint64_t p1 = 0;
+    for (int c=getchar();c!=EOF;c=getchar()) {
+        if (c>='0'&&c<='9') {
+            buf[bufs++] = c-'0';
+        } else if (c=='-') {
+            range.first = btoi(buf, bufs);
+            bufs = 0;
+        } else if (c==','||c=='\n') {
+            range.last = btoi(buf, bufs);
+            bufs = 0;
+            //printf("%"PRIu64"->%"PRIu64"\n", range.first, range.last);
+            p1 += sum_silly(range);
+        }
+    }
+    printf("p1: %" PRIu64 "\n", p1);
+    return 0;
+}