5 files changed, 136 insertions, 2 deletions
diff --git a/src/2021/day4/aoc.cpp b/src/2021/day4/aoc.cpp
index 6212dcd..e8dbff8 100644
--- a/src/2021/day4/aoc.cpp
+++ b/src/2021/day4/aoc.cpp
@@ -12,7 +12,7 @@ struct bnum {
   bool operator<(const bnum& b) const noexcept { return v < b.v; }
 };
 
-void get_number(const char** pp, int* d) {
+static void get_number(const char** pp, int* d) {
   const char* p = *pp;
   while (*p >= '0' && *p <= '9') {
     *d = *d * 10 + *p - '0';
diff --git a/src/2021/day5/README.md b/src/2021/day5/README.md
index 445b7c2..948263c 100644
--- a/src/2021/day5/README.md
+++ b/src/2021/day5/README.md
@@ -37,4 +37,25 @@ To avoid the most dangerous areas, you need to determine the number of points wh
 
 Consider only horizontal and vertical lines. At how many points do at least two lines overlap?
 
+--- Part Two ---
+Unfortunately, considering only horizontal and vertical lines doesn't give you the full picture; you need to also consider diagonal lines.
+
+Because of the limits of the hydrothermal vent mapping system, the lines in your list will only ever be horizontal, vertical, or a diagonal line at exactly 45 degrees. In other words:
+
+An entry like 1,1 -> 3,3 covers points 1,1, 2,2, and 3,3.
+An entry like 9,7 -> 7,9 covers points 9,7, 8,8, and 7,9.
+Considering all lines from the above example would now produce the following diagram:
+
+1.1....11.
+.111...2..
+..2.1.111.
+...1.2.2..
+.112313211
+...1.2....
+..1...1...
+.1.....1..
+1.......1.
+222111....
+You still need to determine the number of points where at least two lines overlap. In the above example, this is still anywhere in the diagram with a 2 or larger - now a total of 12 points.
 
+Consider all of the lines. At how many points do at least two lines overlap?
diff --git a/src/2021/day5/aoc.cpp b/src/2021/day5/aoc.cpp
index 7a3c895..47cc423 100644
--- a/src/2021/day5/aoc.cpp
+++ b/src/2021/day5/aoc.cpp
@@ -1,5 +1,98 @@
 #include "aoc.h"
+#include <algorithm>
+#include <vector>
 
 namespace aoc2021 {
 
+static void get_number(const char** pp, int* d) {
+  *d = 0;
+  const char* p = *pp;
+  while (*p >= '0' && *p <= '9') {
+    *d = *d * 10 + *p - '0';
+    p++;
+  }
+  *pp = p;
 }
+
+void fill(std::vector<int>& b1, std::vector<int>& b2, const line& l, int width) {
+  int maxx = std::max(l.p1.x, l.p2.x);
+  int minx = std::min(l.p1.x, l.p2.x);
+  int maxy = std::max(l.p1.y, l.p2.y);
+  int miny = std::min(l.p1.y, l.p2.y);
+
+  if (l.p1.x == l.p2.x || l.p1.y == l.p2.y) {
+    for (int i = minx; i <= maxx; i++) {
+      for (int j = miny; j <= maxy; j++) {
+        // printf("[%d, %d]\n", i, j);
+        b1[j * width + i] += 1;
+        b2[j * width + i] += 1;
+        if (b2[j * width + i] > 1) {
+          printf("[%d, %d]\n", i, j);
+        }
+      }
+    }
+  }
+
+  if (l.p1.x == l.p1.y && l.p2.x == l.p2.y) {
+    int j = miny;
+    for (int i = minx; i <= maxx; i++) {
+      b2[j * width + i] += 1;
+      if (b2[j * width + i] > 1) {
+        printf("[%d, %d]\n", i, j);
+      }
+      j++;
+    }
+  }
+
+  if (l.p1.x == l.p2.y && l.p2.x == l.p1.y) {
+    int j = maxy;
+    for (int i = minx; i <= maxx; i++) {
+      b2[j * width + i] += 1;
+      if (b2[j * width + i] > 1) {
+        printf("[%d, %d]\n", i, j);
+      }
+      j--;
+    }
+  }
+}
+
+std::pair<int, int> day5(line_view file) {
+  std::vector<line> vs;
+  int maxx{INT32_MIN};
+  int maxy{INT32_MIN};
+
+  per_line(file, [&vs, &maxx, &maxy](line_view lv) {
+    line l;
+    int* is[] = {&l.p1.x, &l.p1.y, &l.p2.x, &l.p2.y};
+    const char* p = lv.line;
+    int i{0};
+    while (p < lv.line + lv.length) {
+      if (*p >= '0' && *p <= '9') {
+        get_number(&p, is[i++]);
+      }
+      p++;
+    }
+    maxx = std::max(maxx, std::max(l.p1.x, l.p2.x));
+    maxy = std::max(maxy, std::max(l.p1.y, l.p2.y));
+    vs.push_back(l);
+    return true;
+  });
+
+  maxx += 1;
+  maxy += 1;
+
+  printf("%d, %d\n", maxx, maxy);
+  std::vector<int> board1(maxx * maxy, 0);
+  std::vector<int> board2(maxx * maxy, 0);
+  for (auto& l : vs) {
+    fill(board1, board2, l, maxx);
+  }
+
+  int t1{0};
+  int t2{0};
+  std::for_each(board1.begin(), board1.end(), [&t1](int x) { t1 += int(x >= 2); });
+  std::for_each(board2.begin(), board2.end(), [&t2](int x) { t2 += int(x >= 2); });
+  return {t1, t2};
+}
+
+} // namespace aoc2021
diff --git a/src/2021/day5/aoc.h b/src/2021/day5/aoc.h
index 7733f90..c57f439 100644
--- a/src/2021/day5/aoc.h
+++ b/src/2021/day5/aoc.h
@@ -4,4 +4,14 @@
 
 namespace aoc2021 {
 
-}
+struct line {
+  struct point {
+    int x;
+    int y;
+  };
+  point p1;
+  point p2;
+};
+
+std::pair<int, int> day5(line_view);
+} // namespace aoc2021
diff --git a/src/2021/day5/input0 b/src/2021/day5/input0
new file mode 100644
index 0000000..b258f68
--- /dev/null
+++ b/src/2021/day5/input0
@@ -0,0 +1,10 @@
+0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2