2025 day 2: solve part 1

Author: yut23

2025/answer_tests/day02/example1.txt

@@ -0,0 +1,2 @@
+Day 2:
+1227775554

2025/input/day02/example1.txt

@@ -0,0 +1,3 @@
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,
+1698522-1698528,446443-446449,38593856-38593862,565653-565659,
+824824821-824824827,2121212118-2121212124

2025/src/day02.cpp

@@ -0,0 +1,24 @@
+/******************************************************************************
+ * File:        day02.cpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2025-12-02
+ *****************************************************************************/
+
+#include "day02.hpp"
+#include "lib.hpp"
+#include <fstream>  // for ifstream
+#include <iostream> // for cout
+
+int main(int argc, char **argv) {
+    std::ifstream infile = aoc::parse_args(argc, argv).infile;
+
+    std::string line;
+    long part1 = 0;
+    while (std::getline(infile, line, ',')) {
+        part1 += aoc::day02::sum_invalid_ids(line);
+    }
+    std::cout << part1 << "\n";
+
+    return 0;
+}

2025/src/day02.hpp

@@ -0,0 +1,99 @@
+/******************************************************************************
+ * File:        day02.hpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2025-12-02
+ *****************************************************************************/
+
+#ifndef DAY02_HPP_YMBKSAFO
+#define DAY02_HPP_YMBKSAFO
+
+#include "lib.hpp"
+#include "util/math.hpp" // for next_power_of_10, prev_power_of_10
+#include <cassert>       // for assert
+#include <iostream>      // for istream
+#include <string>        // for string, getline
+
+namespace aoc::day02 {
+
+long sum_invalid_ids(const std::string &line) {
+    constexpr auto POWERS_OF_10 = aoc::math::gen_powers_of_10<long>();
+
+    const char *ptr1 = line.c_str();
+    const char *ptr2 = ptr1;
+    // this const_cast is annoying
+    long start = std::strtol(ptr1, const_cast<char **>(&ptr2), 10);
+    std::size_t start_digits = ptr2 - ptr1;
+    assert(*ptr2 == '-');
+    ptr1 = ptr2 + 1;
+    long end = std::strtol(ptr1, const_cast<char **>(&ptr2), 10);
+    std::size_t end_digits = ptr2 - ptr1;
+    if constexpr (aoc::DEBUG)
+        std::cerr << "range " << start << "-" << end << ":";
+
+    if (start_digits % 2 == 1 && end_digits % 2 == 1) {
+        // all values have an odd number of digits
+        if constexpr (aoc::DEBUG)
+            std::cerr << "\n";
+        return 0;
+    }
+
+    if (start_digits % 2 == 1) {
+        start = aoc::math::next_power_of_10(start);
+        ++start_digits;
+        if (start > end) {
+            if constexpr (aoc::DEBUG)
+                std::cerr << "\n";
+            return 0;
+        }
+    }
+    long start_upper = start / POWERS_OF_10[start_digits / 2];
+    long start_lower = start % POWERS_OF_10[start_digits / 2];
+
+    if (end_digits % 2 == 1) {
+        end = aoc::math::prev_power_of_10(end);
+        --end_digits;
+        if (start > end) {
+            if constexpr (aoc::DEBUG)
+                std::cerr << "\n";
+            return 0;
+        }
+    }
+    long end_upper = end / POWERS_OF_10[end_digits / 2];
+    long end_lower = end % POWERS_OF_10[end_digits / 2];
+
+    constexpr auto value_for_upper = [](long x) {
+        return x * aoc::math::next_power_of_10(x) + x;
+    };
+
+    long sum = 0;
+    if (start_lower <= start_upper &&
+        (end_upper > start_upper || end_lower >= start_upper)) {
+        auto x = value_for_upper(start_upper);
+        if constexpr (aoc::DEBUG)
+            std::cerr << " " << x;
+        sum += x;
+    }
+    if (start_upper < end_upper) {
+        for (long upper = start_upper + 1; upper <= end_upper - 1; ++upper) {
+            auto x = value_for_upper(upper);
+            if constexpr (aoc::DEBUG)
+                std::cerr << " " << x;
+            sum += x;
+        }
+        if (end_lower >= end_upper) {
+            auto x = value_for_upper(end_upper);
+            if constexpr (aoc::DEBUG)
+                std::cerr << " " << x;
+            sum += x;
+        }
+    }
+    if constexpr (aoc::DEBUG)
+        std::cerr << std::endl;
+
+    return sum;
+}
+
+} // namespace aoc::day02
+
+#endif /* end of include guard: DAY02_HPP_YMBKSAFO */