Add 2021 day 8 puzzles

Part two needs a tidy-up which will come in a bit.

But this is how I got the correct answer.

2021/puzzle-08-01.cc

@@ -0,0 +1,32 @@
+//
+// Created by Matthew Gretton-Dann on 08/12/2021.
+//
+
+#include <iostream>
+#include <string>
+
+auto main() -> int
+{
+  std::string line;
+  unsigned count{0};
+  while (std::getline(std::cin, line)) {
+    line = line.substr(line.find('|') + 1);
+    unsigned this_digit{0};
+    for (auto c : line) {
+      if (c == ' ') {
+        if (this_digit == 2 || this_digit == 3 || this_digit == 4 || this_digit == 7) {
+          ++count;
+        }
+        this_digit = 0;
+      }
+      else {
+        ++this_digit;
+      }
+    }
+    if (this_digit == 2 || this_digit == 3 || this_digit == 4 || this_digit == 7) {
+      ++count;
+    }
+  }
+  std::cout << "Simple digits: " << count << '\n';
+  return 0;
+}

2021/puzzle-08-02.cc

@@ -0,0 +1,150 @@
+//
+// Created by Matthew Gretton-Dann on 08/12/2021.
+//
+
+#include <array>
+#include <iostream>
+#include <string>
+#include <vector>
+
+auto main() -> int
+{
+  std::string line;
+  unsigned total{0};
+
+  while (std::getline(std::cin, line)) {
+    std::vector<std::string> six_segments;
+    std::string one;
+    std::string four;
+    std::string seven;
+    std::string eight;
+    auto it{line.begin()};
+    for (; *it != '|'; ++it) {
+      if (*it == ' ') {
+        continue;
+      }
+      std::string num;
+      while (*it != ' ') {
+        num += *it++;
+      }
+      std::sort(num.begin(), num.end());
+      switch (num.size()) {
+      case 2:
+        one = num;
+        break;
+      case 3:
+        seven = num;
+        break;
+      case 4:
+        four = num;
+        break;
+      case 6:
+        six_segments.push_back(num);
+        break;
+      case 7:
+        eight = num;
+        break;
+      default:
+        break;
+      }
+    }
+
+    /* Now decode: */
+    char a{'.'}, b{'.'}, c{'.'}, d{'.'}, e{'.'}, f{'.'}, g{'.'};
+    std::string temp;
+    std::set_difference(seven.begin(), seven.end(), one.begin(), one.end(),
+                        std::back_inserter(temp));
+    a = temp[0];
+    std::string union47;
+    std::set_union(seven.begin(), seven.end(), four.begin(), four.end(),
+                   std::back_inserter(union47));
+    for (auto segment : six_segments) {
+      temp.clear();
+      std::set_difference(segment.begin(), segment.end(), union47.begin(), union47.end(),
+                          std::back_inserter(temp));
+      if (temp.size() == 1) {
+        g = temp[0];
+        temp.clear();
+        std::set_difference(eight.begin(), eight.end(), segment.begin(), segment.end(),
+                            std::back_inserter(temp));
+        e = temp[0];
+        break;
+      }
+    }
+    std::string union7ge{seven + g + e};
+    std::sort(union7ge.begin(), union7ge.end());
+    for (auto segment : six_segments) {
+      temp.clear();
+      std::set_difference(segment.begin(), segment.end(), union7ge.begin(), union7ge.end(),
+                          std::back_inserter(temp));
+      if (temp.size() == 1) {
+        b = temp[0];
+      }
+    }
+
+    std::string union1b{one + b};
+    std::sort(union1b.begin(), union1b.end());
+    temp.clear();
+    std::set_difference(four.begin(), four.end(), union1b.begin(), union1b.end(),
+                        std::back_inserter(temp));
+    d = temp[0];
+
+    std::string unionabdeg{a, b, d, e, g};
+    std::sort(unionabdeg.begin(), unionabdeg.end());
+    for (auto segment : six_segments) {
+      temp.clear();
+      std::set_difference(segment.begin(), segment.end(), unionabdeg.begin(), unionabdeg.end(),
+                          std::back_inserter(temp));
+      if (temp.size() == 1) {
+        f = temp[0];
+      }
+    }
+
+    temp.clear();
+    std::string unionf{f};
+    std::set_difference(one.begin(), one.end(), unionf.begin(), unionf.end(),
+                        std::back_inserter(temp));
+    c = temp[0];
+
+    std::array<std::string, 10> nums{
+      std::string{a, b, c, e, f, g},     // 0
+      std::string{c, f},                 // 1
+      std::string{a, c, d, e, g},        // 2
+      std::string{a, c, d, f, g},        // 3
+      std::string{b, c, d, f},           // 4
+      std::string{a, b, d, f, g},        // 5
+      std::string{a, b, d, e, f, g},     // 6
+      std::string{a, c, f},              // 7
+      std::string{a, b, c, d, e, f, g},  // 8
+      std::string{a, b, c, d, f, g}      // 9
+    };
+
+    for (auto& num : nums) {
+      std::sort(num.begin(), num.end());
+    }
+
+    for (auto num : nums) {
+      std::cout << num << '\n';
+    }
+
+    unsigned value{0};
+    for (++it; it != line.end(); it = (it == line.end() ? it : it + 1)) {
+      if (*it == ' ') {
+        continue;
+      }
+      std::string s;
+      while (it != line.end() && *it != ' ') {
+        s += *it++;
+      }
+      std::sort(s.begin(), s.end());
+      auto num{std::find(nums.begin(), nums.end(), s)};
+      assert(num != nums.end());
+      value *= 10;
+      value += num - nums.begin();
+    }
+    total += value;
+  }
+
+  std::cout << "Total: " << total << '\n';
+  return 0;
+}