From b6213c659319cc92ef9a2e71bded528157eaa547 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Fri, 3 Dec 2021 12:32:15 +0000
Subject: [PATCH] Tidy up 2021 day 3 puzzle 2 code.

This adds some comments and removes code duplication.
---
 2021/puzzle-03-02.cc | 54 ++++++++++++++++++++------------------------
 1 file changed, 24 insertions(+), 30 deletions(-)

diff --git a/2021/puzzle-03-02.cc b/2021/puzzle-03-02.cc
index bc91316..f5bffda 100644
--- a/2021/puzzle-03-02.cc
+++ b/2021/puzzle-03-02.cc
@@ -8,18 +8,35 @@
 #include <string>
 #include <vector>
 
+/** \brief  Convert the string s to a number
+ *  @param s String of 1s and 0s to convert
+ *  @return  Value of string
+ */
 auto to_num(std::string const& s) -> std::uint64_t
 {
   std::uint64_t result{0};
   for (auto c : s) {
+    assert(c == '1' || c == '0');
     result <<= 1;
-    result |= (c == '1' ? 1 : 0);
+    result |= (c == '1') ? 1 : 0;
   }
   return result;
 }
 
-auto find_oxygen_rating(std::vector<std::string> const& lines) -> std::string
+/** \brief Find the rating for a given selector and set of lines.
+ *
+ * @tparam Fn       Selector function type
+ * @param  lines    Lines to select from
+ * @param  selector Takes bool saying whether ones are predominant, should return character to keep.
+ * @return          String of selected number.
+ */
+template<typename Fn>
+auto find_rating(std::vector<std::string> const& lines, Fn selector) -> std::string
 {
+  /* Algorithm description: We copy the given lines and then for each index in the strings in turn
+   * we see how many 1 digits are in that index.  The function \a selector is then called to get the
+   * character to keep, before erasing the strings we do not want to keep.
+   */
   std::vector<std::string> data{lines};
   assert(!data.empty());
   std::size_t len{data[1].size()};
@@ -27,32 +44,7 @@ auto find_oxygen_rating(std::vector<std::string> const& lines) -> std::string
     auto one_bits{std::accumulate(
       data.begin(), data.end(), std::uint64_t{0},
       [i](std::uint64_t a, std::string const& s) { return a + (s[i] == '1' ? 1 : 0); })};
-    char keep{(one_bits * 2 >= data.size()) ? '1' : '0'};
-    std::cout << "Size of data = " << data.size() << " Number of one bits = " << one_bits
-              << " keeping " << keep << '\n';
-    auto it{std::remove_if(data.begin(), data.end(),
-                           [keep, i](std::string const& s) { return s[i] != keep; })};
-    data.erase(it, data.end());
-    if (data.size() <= 1) {
-      break;
-    }
-  }
-  assert(data.size() == 1);
-  return data[0];
-}
-
-auto find_co2_rating(std::vector<std::string> const& lines) -> std::string
-{
-  std::vector<std::string> data{lines};
-  assert(!data.empty());
-  std::size_t len{data[1].size()};
-  for (std::size_t i{0}; i < len; ++i) {
-    auto one_bits{std::accumulate(
-      data.begin(), data.end(), std::uint64_t{0},
-      [i](std::uint64_t a, std::string const& s) { return a + (s[i] == '1' ? 1 : 0); })};
-    char keep{(one_bits * 2 >= data.size()) ? '0' : '1'};
-    std::cout << "Size of data = " << data.size() << " Number of one bits = " << one_bits
-              << " keeping " << keep << '\n';
+    char keep{selector(one_bits * 2 >= data.size())};
     auto it{std::remove_if(data.begin(), data.end(),
                            [keep, i](std::string const& s) { return s[i] != keep; })};
     data.erase(it, data.end());
@@ -72,8 +64,10 @@ auto main() -> int
     lines.push_back(line);
   }
 
-  auto oxygen_rating{to_num(find_oxygen_rating(lines))};
-  auto co2_rating{to_num(find_co2_rating(lines))};
+  auto oxygen_rating{
+    to_num(find_rating(lines, [](bool mostly_ones) { return mostly_ones ? '1' : '0'; }))};
+  auto co2_rating{
+    to_num(find_rating(lines, [](bool mostly_ones) { return mostly_ones ? '0' : '1'; }))};
 
   std::cout << "Oxygen generator rating * CO2 scrubber rating = " << oxygen_rating << " * "
             << co2_rating << " = " << oxygen_rating * co2_rating << '\n';