2020 day6 part1

4 files changed, 97 insertions, 1 deletions

diff --git a/src/2020/day6/README.md b/src/2020/day6/README.md
index c3e09dc..3f124ec 100644
--- a/src/2020/day6/README.md
+++ b/src/2020/day6/README.md
@@ -38,4 +38,39 @@ In this example, the sum of these counts is 3 + 3 + 3 + 1 + 1 = 11.
 
 For each group, count the number of questions to which anyone answered "yes". What is the sum of those counts?
 
+--- Part Two ---
+
+As you finish the last group's customs declaration, you notice that you misread one word in the instructions:
+
+You don't need to identify the questions to which anyone answered "yes"; you need to identify the questions to which everyone answered "yes"!
+
+Using the same example as above:
+
+abc
+
+a
+b
+c
+
+ab
+ac
+
+a
+a
+a
+a
+
+b
+
+This list represents answers from five groups:
+
+    In the first group, everyone (all 1 person) answered "yes" to 3 questions: a, b, and c.
+    In the second group, there is no question to which everyone answered "yes".
+    In the third group, everyone answered yes to only 1 question, a. Since some people did not answer "yes" to b or c, they don't count.
+    In the fourth group, everyone answered yes to only 1 question, a.
+    In the fifth group, everyone (all 1 person) answered "yes" to 1 question, b.
+
+In this example, the sum of these counts is 3 + 0 + 1 + 1 + 1 = 6.
+
+For each group, count the number of questions to which everyone answered "yes". What is the sum of those counts?
 
diff --git a/src/2020/day6/aoc.cpp b/src/2020/day6/aoc.cpp
index 849bad9..d64262e 100644
--- a/src/2020/day6/aoc.cpp
+++ b/src/2020/day6/aoc.cpp
@@ -1,5 +1,26 @@
 #include "aoc.h"
+#include <algorithm>
+#include <vector>
 
 namespace aoc2020 {
 
+int day6(line_view file) {
+  std::vector<question_group> gs;
+
+  const char* p1 = file.line;
+  const char* p = p1;
+  while (p < file.line + file.length) {
+    if (*p == '\n' && *(p + 1) == '\n') {
+      gs.emplace_back(line_view{p1, p});
+      p1 = p + 2;
+    }
+    p++;
+  }
+  gs.emplace_back(line_view{p1, p});
+
+  int total{0};
+  std::for_each(gs.begin(), gs.end(), [&total](const question_group& g) { total += g.summary(); });
+  return total;
 }
+
+} // namespace aoc2020
diff --git a/src/2020/day6/aoc.h b/src/2020/day6/aoc.h
index c5ad3d1..dc2989d 100644
--- a/src/2020/day6/aoc.h
+++ b/src/2020/day6/aoc.h
@@ -3,4 +3,29 @@
 
 namespace aoc2020 {
 
-}
+struct question_group {
+  line_view people;
+  int count[26] = {0};
+
+  int summary() const noexcept {
+    int total{0};
+    for (int x : count) {
+      total += int(x > 0);
+    }
+    return total;
+  }
+
+  question_group(line_view lv) : people(lv) {
+    const char* p = lv.line;
+    while (p < lv.line + lv.length) {
+      if (*p >= 'a' && *p <= 'z') {
+        count[*p - 'a'] += 1;
+      }
+      p++;
+    }
+  }
+};
+
+int day6(line_view);
+
+} // namespace aoc2020
diff --git a/src/2020/day6/input0 b/src/2020/day6/input0
new file mode 100644
index 0000000..0f5b3bc
--- /dev/null
+++ b/src/2020/day6/input0
@@ -0,0 +1,15 @@
+abc
+
+a
+b
+c
+
+ab
+ac
+
+a
+a
+a
+a
+
+b