Implemented the second part of day2

CMakeLists.txt

@@ -14,10 +14,11 @@ endfunction(add_challenge)
 
 add_challenge(day1)
 add_challenge(day1_part2)
-add_challenge(day2_part1)
+add_challenge(day2)
 add_challenge(day3)
 add_challenge(day4)
 add_challenge(day5)
+add_challenge(day6)
 
 ### Day 7
 add_challenge(day7)

src/day2.cpp

@@ -0,0 +1,128 @@
+#include <algorithm>
+#include <iostream>
+#include <fstream>
+#include <charconv>
+#include <vector>
+#include <stdexcept>
+#include <span>
+
+namespace {
+void parseLevels(const std::string& report, std::vector<int>& out)
+{
+    if(report.empty()) return;
+
+    const char *begin = &report[0];
+    const char * const end = &report[report.size()];
+    std::from_chars_result result{};
+    int val{};
+    while(begin != end) {
+        result = std::from_chars(begin, end, val);
+        if(result.ec != std::errc()) {
+            std::cerr << "\nreport: " << report;
+            throw std::logic_error{"Invalid report input string"};
+        }
+        begin = std::min(result.ptr + 1, end);
+        out.push_back(val);
+    }
+}
+
+template<typename S>
+[[nodiscard]] bool safe(std::span<const int> levels, const unsigned tolerate, const S& safer) noexcept
+{
+    unsigned violations = 0;
+    for (size_t i = 1; i < levels.size() && violations <= tolerate; ++i) {
+        if (!safer(levels[i-1], levels[i])) {
+            ++violations;
+
+            if(i + 1 == levels.size()) break;
+
+
+            // We have to figure out which value(i - 1 or i) have to be *removed.
+            // This results in the next index position.
+
+            // Edge cases:
+            // 59 62 65 64 65 <- 65(2nd) element should be removed
+            // 36 34 36 33 30 <- 34(2nd) element should be removed
+
+            if(safer(levels[i - 1], levels[i + 1])) {
+                ++i;
+            }
+            // Prev should be deleted because current and next elements are safe.
+            else if(safer(levels[i], levels[i + 1])) {
+                // However, we must check that current element and prevous elements build safe consequence.
+                if(i > 1) {
+                    // 1 2 3 10 12. 2 and 10 are not safe. This results in increase in violations.
+                    violations += !safer(levels[i - 2], levels[i]);
+                }
+            }
+        }
+    }
+    return violations <= tolerate;
+}
+
+[[nodiscard]] constexpr bool isLevelsSafe(int largelLevel, int smallerLevel) noexcept
+{
+    const auto diff{largelLevel - smallerLevel};
+    return 1 <= diff && diff <= 3;
+}
+
+[[nodiscard]] bool isAscendingSafe(std::span<const int> levels, unsigned tolerate) noexcept
+{
+    return safe(levels, tolerate, [](int smaller, int larger) {
+        return isLevelsSafe(larger, smaller);
+    });
+}
+
+[[nodiscard]] bool isDescendingSafe(std::span<const int> levels, unsigned tolerate) noexcept
+{
+    return safe(levels, tolerate, [](int larger, int smaller) {
+        return isLevelsSafe(larger, smaller);
+    });
+}
+
+[[nodiscard]] bool isSafe(std::span<const int> levels, unsigned tolerateLevel) noexcept
+{
+    return isAscendingSafe(levels, tolerateLevel) || isDescendingSafe(levels, tolerateLevel);
+}
+
+void printHelp()
+{
+    std::cerr << "\nUsage:\n"
+    << "The program requires 2 args: (part1, part2) and the path to the file."
+    << "\nFor example, ./day2 part1 data/day2.txt";
+}
+
+} //< anonymous namespace
+
+int main(int argc, char *argv[]) {
+    if(argc != 3) {
+        printHelp();
+        return 1;
+    }
+    std::string_view task{argv[1]};
+    if(task != "part1" && task != "part2") {
+        std::cerr << "\nfirst arg can be either `part1` or `part2`\n";
+        printHelp();
+        return 1;
+    }
+
+    std::ifstream ifile(argv[2]);
+    if(!ifile) {
+        std::cerr << "\nFile cannot be open";
+        return 1;
+    }
+
+    const unsigned tolerateLevel = task == "part1" ? 0 : 1;
+
+    size_t count{};
+    std::string report;
+    std::vector<int> levels;
+    while(std::getline(ifile, report)) {
+        levels.clear();
+        parseLevels(report, levels);
+        count += isSafe(levels, tolerateLevel);
+    }
+    std::cout << "\nanswer: " << count;
+
+    return 0;
+}