From 269bb657a61278d16cc87afdd35724807066e8b2 Mon Sep 17 00:00:00 2001
From: Manish <manish@radii.dev>
Date: Mon, 27 Feb 2023 09:19:13 +1100
Subject: [PATCH] Design Pattern: 2. Islands (Matrix Traversal) | Problem:
 Number of Islands

---
 2_num_of_islands.cpp | 98 ++++++++++++++++++++++++++++++++++++++++++++
 CMakeLists.txt       |  4 +-
 2 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 2_num_of_islands.cpp

diff --git a/2_num_of_islands.cpp b/2_num_of_islands.cpp
new file mode 100644
index 0000000..f4c0220
--- /dev/null
+++ b/2_num_of_islands.cpp
@@ -0,0 +1,98 @@
+/* Problem  source: https://leetcode.com/problems/number-of-islands/
+ */
+
+#include <iostream>
+#include <limits>
+#include <stack>
+#include <unordered_set>
+#include <vector>
+
+struct matrix_location {
+  unsigned c; // column
+  unsigned r; // row
+  bool operator==(const matrix_location &other) const {
+    return c == other.c && r == other.r;
+  };
+};
+
+class matrix_location_hash {
+private:
+  unsigned n_cols;
+
+public:
+  matrix_location_hash(const unsigned num_of_cols, const unsigned num_of_rows)
+      : n_cols(num_of_cols) {
+    if (num_of_rows >
+        std::numeric_limits<unsigned long long>::max() / num_of_cols)
+      throw "Grid too big, cannot represent all cells with unique number for "
+            "efficient hashing";
+  };
+  std::size_t operator()(const matrix_location &m) const {
+    unsigned long long cell = m.c + m.r * n_cols;
+    return std::hash<unsigned long long>()(cell);
+  }
+};
+
+unsigned num_of_islands(const std::vector<std::vector<char>> &grid) {
+  unsigned n_islands = 0;
+  matrix_location_hash ml_hash(grid.size(),
+                               grid[0].size()); // FIXME: potential seg fault
+  std::unordered_set<matrix_location, matrix_location_hash> unexplored_land(
+      10, ml_hash);
+  for (unsigned i = 0, columns = grid.size(); i < columns; i++) {
+    for (unsigned j = 0, rows = grid[i].size(); j < rows; j++) {
+      if (grid[i][j] == '0') {
+      } else if (grid[i][j] == '1') {
+        unexplored_land.insert({i, j});
+      } else {
+        throw "Input grid cannot have elements other than '0' or '1'";
+      }
+    }
+  }
+  while (unexplored_land.size()) {
+    for (auto &loc : unexplored_land) {
+      std::stack<matrix_location> sub_explore({loc});
+      bool island_added = false;
+      while (!sub_explore.empty()) {
+        matrix_location curr_loc = sub_explore.top();
+        matrix_location above = {curr_loc.c - 1, curr_loc.r},
+                        left = {curr_loc.c, curr_loc.r - 1},
+                        right = {curr_loc.c, curr_loc.r + 1},
+                        below = {curr_loc.c + 1, curr_loc.r};
+        sub_explore.pop();
+        if (!island_added) {
+          n_islands++;
+          island_added = true;
+        }
+        if (unexplored_land.count(above)) {
+          sub_explore.push(above);
+        }
+        if (unexplored_land.count(left)) {
+          sub_explore.push(left);
+        }
+        if (unexplored_land.count(right)) {
+          sub_explore.push(right);
+        }
+        if (unexplored_land.count(below)) {
+          sub_explore.push(below);
+        }
+        unexplored_land.erase(curr_loc);
+      }
+      break;
+    }
+  }
+  return n_islands;
+}
+
+int main() {
+  const std::vector<std::vector<char>> grid = {{'1', '1', '1', '1', '0'},
+                                               {'1', '1', '0', '1', '0'},
+                                               {'1', '1', '0', '0', '0'},
+                                               {'0', '0', '0', '0', '0'}},
+                                       grid2 = {{'1', '1', '0', '0', '0'},
+                                                {'1', '1', '0', '0', '0'},
+                                                {'0', '0', '1', '0', '0'},
+                                                {'0', '0', '0', '1', '1'}};
+  std::cout << num_of_islands(grid) << '\n' << num_of_islands(grid2) << "\n";
+  return 0;
+}
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 03c52f2..d06b453 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -7,4 +7,6 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall  -Wextra -Werror -g")
 
-add_executable(1_longest_substring_with_k_distinct_characters 1_longest_substring_with_k_distinct_characters.cpp)
\ No newline at end of file
+add_executable(1_longest_substring_with_k_distinct_characters 1_longest_substring_with_k_distinct_characters.cpp)
+
+add_executable(2_num_of_islands 2_num_of_islands.cpp)