From a0ee3c44393a8f24153fe48727ea312f93ec6afa Mon Sep 17 00:00:00 2001
From: Mistivia <i@mistivia.com>
Date: Sun, 14 Dec 2025 22:50:15 +0800
Subject: day 2 part 1

---
 02/Makefile |  11 ++++
 02/part1.c  | 166 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 README.md   |   3 ++
 3 files changed, 180 insertions(+)
 create mode 100644 02/Makefile
 create mode 100644 02/part1.c
 create mode 100644 README.md

diff --git a/02/Makefile b/02/Makefile
new file mode 100644
index 0000000..f69d8a5
--- /dev/null
+++ b/02/Makefile
@@ -0,0 +1,11 @@
+part1: part1.c
+	gcc part1.c -o part1 -lalgds
+
+part2: part2.c
+	gcc part2.c -o part2 -lalgds
+
+1: part1
+	cat input | ./part1
+
+2: part2
+	cat input | ./part2
\ No newline at end of file
diff --git a/02/part1.c b/02/part1.c
new file mode 100644
index 0000000..6d08d4b
--- /dev/null
+++ b/02/part1.c
@@ -0,0 +1,166 @@
+#include <ctype.h>
+#include <errno.h>
+#include <stdio.h>
+#include <assert.h>
+
+#include <algds/vec.h>
+#include <algds/str.h>
+
+#define PANIC do { \
+    fprintf(stderr, "panic at %s:%d\n", __FILE__, __LINE__); \
+    abort(); \
+} while (0)
+
+typedef struct {
+    char *first;
+    char *second;
+} StrPair;
+
+void StrPair_show(StrPair self, FILE *fp) {
+    fprintf(fp, "{%s-%s}", self.first, self.second);
+}
+
+VECTOR_DEF(StrPair);
+VECTOR_IMPL(StrPair);
+
+void expect_char(char e) {
+    int c = fpeek(stdin);
+    if (c != e) {
+        PANIC;
+    }
+    fgetc(stdin);
+}
+
+void skip_space() {
+    while (1) {
+        int c = fpeek(stdin);
+        if (c == EOF) return;
+        if (isspace(c)) {
+            fgetc(stdin);
+        } else {
+            return;
+        }
+    }
+}
+
+void sep_or_end() {
+    int c = fpeek(stdin);
+    if (c == ',') {
+        fgetc(stdin);
+        skip_space();
+        return;
+    } else if (c == EOF) {
+        return;
+    }
+    PANIC;
+}
+
+char *parse_integer() {
+    StrBuilder sb;
+    StrBuilder_init(&sb);
+    while (1) {
+        int c = fpeek(stdin);
+        if (isdigit(c)) {
+            fgetc(stdin);
+            StrBuilder_append_char(&sb, c);
+        } else {
+            break;
+        }
+    }
+    if (strlen(sb.buf) == 0) {
+        PANIC;
+    }
+    StrBuilder_append_char(&sb, '\0');
+    return sb.buf;
+}
+
+StrPair parse_product() {
+    StrPair ret;
+    ret.first = parse_integer();
+    expect_char('-');
+    ret.second = parse_integer();
+    sep_or_end();
+    return ret;
+}
+
+StrPairVector parse() {
+    StrPairVector res;
+    StrPairVector_init(&res);
+    while(1) {
+        int c = fpeek(stdin);
+        if (c == EOF) {
+            return res;
+        } else if (isdigit(c)) {
+            StrPair sp = parse_product();
+            StrPairVector_push_back(&res, sp);
+        } else {
+            PANIC;
+        }
+    }
+    return res;
+}
+
+bool string2long(const char *s, long *num) {
+    char *end;
+    errno = 0;
+    *num = strtol(s, &end, 10);
+    if (end == s) {
+        return false;
+    }
+    if (errno != 0) {
+        return false;
+    }
+    return true;
+}
+
+static int numlen(long num) {
+    if (num < 0) return -1;
+    int count = 1;
+    while (num >= 10) {
+        num /= 10;
+        count++;
+    }
+    return count;
+}
+
+static int pow10halflen(long num) {
+    int l = numlen(num);
+    l /= 2;
+    int res = 1;
+    for (int i = 0; i < l; i++) {
+        res *= 10;
+    }
+    return res;
+}
+
+static int is_invalid_id(long num) {
+    long p = pow10halflen(num);
+    if (num % p == num / p) {
+        return 1;
+    }
+    return 0;
+}
+
+int main() {
+    StrPairVector spv = parse();
+    StrPairVector_show(spv, stdout);
+    putchar('\n');
+    StrPair * iter = StrPairVector_begin(&spv);
+    int64_t sum = 0;
+    for (; iter != StrPairVector_end(&spv); iter++) {
+        long start, end;
+        if (!string2long(iter->first, &start)) {
+            PANIC;
+        }
+        if (!string2long(iter->second, &end)) {
+            PANIC;
+        }
+        for (long t = start; t <= end; t++) {
+            if (is_invalid_id(t)) {
+                sum += t;
+            } 
+        }
+    }
+    printf("%ld\n", sum);
+    return 0;
+}
\ No newline at end of file
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..bbec624
--- /dev/null
+++ b/README.md
@@ -0,0 +1,3 @@
+# Advent of Code 2025
+
+Solutions are written in C, with [algds](https://github.com/mistivia/algds).
\ No newline at end of file
-- 
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