2021 day3

1 files changed, 64 insertions, 5 deletions

diff --git a/src/2021/day3/aoc.cpp b/src/2021/day3/aoc.cpp
index dc3d349..fde8a2b 100644
--- a/src/2021/day3/aoc.cpp
+++ b/src/2021/day3/aoc.cpp
@@ -1,4 +1,6 @@
 #include "aoc.h"
+#include <algorithm>
+#include <vector>
 
 namespace aoc2021 {
 
@@ -11,13 +13,67 @@ int decimal(const char* p) {
   return d;
 }
 
-int day3(line_view file) {
+std::vector<line_view> filter(const std::vector<line_view>& vs, int i, char c) {
+  if (vs.size() == 1) {
+    return vs;
+  }
+  std::vector<line_view> n;
+  for (auto& lv : vs) {
+    if (*(lv.line + i) == c) {
+      n.push_back(lv);
+    }
+  }
+  return n;
+}
+
+char most_common(std::vector<line_view>& v, int i, char x) {
+  size_t t0{0};
+  size_t t1{0};
+  for (auto& l : v) {
+    if (*(l.line + i) == '1') {
+      t1++;
+    }
+    else {
+      t0++;
+    }
+  }
+  return t1 > t0 ? '1' : (t1 < t0 ? '0' : x);
+}
+
+char least_common(std::vector<line_view>& v, int i, char x) {
+  size_t t0{0};
+  size_t t1{0};
+  for (auto& l : v) {
+    if (*(l.line + i) == '1') {
+      t1++;
+    }
+    else {
+      t0++;
+    }
+  }
+  return t1 > t0 ? '0' : (t1 < t0 ? '1' : x);
+}
+
+void filter(std::vector<line_view>& v1, std::vector<line_view>& v2, int i) {
+  if (i < 12) {
+    char c1 = most_common(v1, i, '1');
+    char c2 = least_common(v2, i, '0');
+
+    v1 = filter(v1, i, c1);
+    v2 = filter(v2, i, c2);
+    filter(v1, v2, i + 1);
+  }
+}
+
+std::pair<int, int> day3(line_view file) {
   char gamma[13] = {0};
   char epsilon[13] = {0};
 
   int is[12] = {0};
   int total{0};
-  per_line(file, [&is, &total](line_view lv) {
+  std::vector<line_view> vs;
+  per_line(file, [&is, &total, &vs](line_view lv) {
+    vs.push_back(lv);
     for (int i = 0; i < 12; i++) {
       if (*(lv.line + i) == '1') {
         is[i] += 1;
@@ -37,10 +93,13 @@ int day3(line_view file) {
     }
   }
 
-  // printf("%s\n", gamma);
-  // printf("%s\n", epsilon);
+  auto v1{vs};
+  auto v2{vs};
+  filter(v1, v2, 0);
 
-  return decimal(gamma) * decimal(epsilon);
+  int p1 = decimal(gamma) * decimal(epsilon);
+  int p2 = decimal(v1[0].line) * decimal(v2[0].line);
+  return {p1, p2};
 }
 
 } // namespace aoc2021