qsort: stack based implementation

.dir-locals.el

@@ -0,0 +1,5 @@
+;;; Directory Local Variables         -*- no-byte-compile: t; -*-
+;;; For more information see (info "(emacs) Directory Variables")
+
+((c-mode . ((mode . clang-format)))
+ (c++-mode . ((mode . clang-format))))

Makefile

@@ -15,6 +15,9 @@ list.out: impls/list.cpp
 bsearch.out: impls/bsearch.cpp
 	$(CC) $(CFLAGS) $^ -o $@
 
+qsort.out: impls/qsort.cpp
+	$(CC) $(CFLAGS) $^ -o $@
+
 vec.out: impls/vec.c
 	$(CC) $(CFLAGS) $^ -o $@
 

impls/bsearch-gen.el

@@ -1,9 +0,0 @@
-(defconst *LIST-SIZE* 128)
-(defconst *UPPER-BOUND* 1024)
-(with-current-buffer (find-file "bsearch.txt")
-  (mapcar
-   #'(lambda (res) (insert (format "%s\n" res)))
-   (cl-loop
-    for i from 0 to *LIST-SIZE*
-    collect
-    (random *UPPER-BOUND*))))

impls/bsearch.cpp

@@ -4,6 +4,7 @@
  */
 
 #include <algorithm>
+#include <cassert>
 #include <cstdlib>
 #include <fstream>
 #include <iostream>
@@ -17,9 +18,7 @@ using std::ostream;
 using std::string;
 using std::vector;
 
-vector<int> arr;
-
-ostream &print_arr(ostream &os)
+ostream &print_arr(ostream &os, std::vector<int> &arr)
 {
   os << "[";
   for (size_t i = 0; i < arr.size(); ++i)
@@ -27,31 +26,46 @@ ostream &print_arr(ostream &os)
   return os << "]";
 }
 
-int bsearch(int n, int l = 0, int u = arr.size() - 1)
+int bsearch(int n, std::vector<int> arr)
 {
-  int midpoint = ((u + l) / 2);
-  if (l >= u || u <= 0)
-    return -1;
-  int val = arr[midpoint];
-  if (val == n)
-    return midpoint;
-  else if (val > n)
-    return bsearch(n, l, midpoint - 1);
-  else
-    return bsearch(n, midpoint + 1, u);
+  int l = 0;
+  int u = arr.size() - 1;
+  while (l <= u)
+  {
+    int midpoint = l + ((u - l) / 2);
+    int val      = arr[midpoint];
+
+    if (val == n)
+    {
+      return midpoint;
+    }
+    else if (val > n)
+    {
+      u = midpoint - 1;
+    }
+    else
+    {
+      l = midpoint + 1;
+    }
+  }
+  return -1;
 }
 
-int main(void)
+int main(int argc, char *argv[])
 {
-  std::ifstream input("bsearch.txt");
-  string line;
-  while (std::getline(input, line))
-    arr.push_back(std::stoi(line));
+  std::ifstream input(argc > 1 ? argv[1] : "bsearch.txt");
+  std::vector<int> arr;
+
+  for (std::string line; std::getline(input, line);
+       arr.push_back(std::stoi(line)))
+    continue;
+
   std::sort(std::begin(arr), std::end(arr));
-  string inp;
-  cout << "Enter number to search: ";
-  cin >> inp;
-  int to_search = std::stoi(inp);
-  cout << bsearch(to_search) << endl;
+
+  for (size_t i = 0; i < arr.size(); ++i)
+  {
+    int index = bsearch(arr[i], arr);
+    assert(index == (int)i);
+  }
   return 0;
 }

impls/num-gen.el

@@ -0,0 +1,10 @@
+(defconst *LIST-SIZE* 128)
+(defconst *UPPER-BOUND* 1024)
+(with-current-buffer (find-file "nums.txt")
+  (mapcar
+   #'(lambda (res) (insert (format "%s\n" res)))
+   (cl-remove-duplicates
+    (cl-loop
+     for i from 0 to *LIST-SIZE*
+     collect
+     (random *UPPER-BOUND*)))))

impls/pingala.cpp

@@ -4,38 +4,39 @@
  */
 
 #include <cstdio>
+#include <iostream>
+#include <sstream>
 #include <string>
 #include <vector>
 
-void padding(size_t n, size_t depth)
-{
-  for (size_t i = 0; i < ((depth - n) / 2); ++i)
-    printf("\t");
-}
-
-void generate_triangle(const size_t depth)
+std::vector<std::string> generate_triangle(const size_t depth)
 {
+  std::vector<std::string> levels;
   std::vector<size_t> items;
-  items.reserve(depth * depth);
+  std::stringstream ss;
+  levels.reserve(depth);
+  items.resize(depth * depth);
+
 #define AT(i, j) items[((i) * depth) + (j)]
   AT(0, 0) = 1;
-  padding(0, depth);
-  printf("%lu\n", items[0]);
+  levels.push_back("1");
   for (size_t i = 1; i < depth; ++i)
   {
     AT(i, 0) = 1;
-    padding(i, depth);
-    printf("%lu,\t", AT(i, 0));
+    ss << "1 ";
     for (size_t j = 1; j < i; ++j)
     {
-      // Recurrence relation
       AT(i, j) = AT(i - 1, j - 1) + AT(i - 1, j);
-      printf("%lu,\t", AT(i, j));
+      ss << AT(i, j) << " ";
     }
     AT(i, i) = 1;
-    printf("%lu\n", AT(i, i));
+    ss << "1";
+    levels.push_back(ss.str());
+    ss.str(std::string{});
   }
 #undef AT
+
+  return levels;
 }
 
 void usage(FILE *fp)
@@ -46,18 +47,29 @@ void usage(FILE *fp)
 
 int main(int argc, char *argv[])
 {
-  if (argc < 2)
-  {
-    usage(stderr);
-    return 1;
-  }
-  int arg = std::stoi(argv[1]);
-  if (arg <= 0)
-  {
-    usage(stderr);
-    return 1;
-  }
+  // Variable declarations
+  std::vector<std::string> levels;
+  int depth = 0;
 
-  generate_triangle(arg);
+  if (argc < 2)
+    goto error;
+  depth = std::stoi(argv[1]);
+  if (depth <= 0)
+    goto error;
+
+  levels = generate_triangle(depth);
+
+  for (const auto &level : levels)
+  {
+    for (size_t i = 0;
+         i < (levels[levels.size() - 1].size() - level.size()) / 2; ++i)
+    {
+      printf(" ");
+    }
+    printf("%s\n", level.c_str());
+  }
   return 0;
+error:
+  usage(stderr);
+  return 1;
 }

impls/qsort.cpp

@@ -4,8 +4,15 @@
  * Commentary: The inplace O(nlog(n)) sortitng algorithm.  Originally designed
  * in C, hence the lack of iostream.
  */
+#include <cstdint>
 #include <cstdio>
 #include <cstdlib>
+#include <fstream>
+#include <iostream>
+#include <stack>
+#include <vector>
+
+using u64 = uint64_t;
 
 #define ARR_SIZE(XS) (sizeof(XS) / sizeof((XS)[0]))
 #define SWAP(A, B) \
@@ -16,85 +23,90 @@
     (A) ^= (B);    \
   } while (0)
 
-#define TEST_SIZE 20
-
-void print_arr(int *arr, int size)
+void print_arr(u64 *arr, u64 size)
 {
-  printf("{");
-  for (int i = 0; i < size - 1; ++i)
-    printf("%d, ", arr[i]);
-  printf("%d}", arr[size - 1]);
+  printf("{\n");
+  for (u64 i = 0; i < size; ++i)
+    printf("  %lu,\n", arr[i]);
+  printf("}\n");
 }
 
-void quicksort(int *arr, int arr_size)
+struct Partition
 {
-#ifdef TRACE
-  printf("before, arr=");
-  print_arr(arr, arr_size);
-  printf("\n");
-#endif
-  // Optimisation: for small arr_size, a different sort like the
-  // bubble sort may be better, instead of initialising new stack
-  // frames.
-  if (arr_size < 2)
-    return;
-  // Optimisation: Use some heuristics to figure out a better pivot
-  // than some constant choice of element.
-  int pivot       = 0;
-  int pivot_value = arr[0];
-  for (int i = 1; i < arr_size; ++i)
+  u64 *arr;
+  u64 arr_size;
+};
+
+void quicksort(u64 *arr, u64 arr_size)
+{
+  std::stack<Partition> partitions;
+  partitions.push({arr, arr_size});
+
+  while (!partitions.empty())
   {
-    if (arr[i] <= pivot_value)
+    const auto partition = partitions.top();
+    arr                  = partition.arr;
+    arr_size             = partition.arr_size;
+    partitions.pop();
+
+    // TODO: Use a different sort for small enough sizes?
+    if (arr_size > 1)
     {
-      if (i != pivot)
-        SWAP(arr[i], arr[pivot]);
-      ++pivot;
+      // TODO: Choose a better pivot maybe?
+      u64 pivot       = arr[arr_size - 1]; // Choose the last element as pivot
+      u64 pivot_index = -1;
+
+      for (u64 j = 0; j < arr_size; ++j)
+      {
+        if (arr[j] < pivot)
+        {
+          ++pivot_index;
+          std::swap(arr[pivot_index], arr[j]);
+        }
+      }
+
+      std::swap(arr[pivot_index + 1], arr[arr_size - 1]);
+
+      partitions.push({arr, pivot_index + 1});
+      partitions.push({arr + pivot_index + 1, arr_size - pivot_index - 1});
     }
   }
-
-#ifdef TRACE
-  printf("after, arr=");
-  print_arr(arr, arr_size);
-  printf("\n");
-#endif
-  // Optimisation: Use a stack to do this instead of recursion.
-  //
-  // This algorithm is similar to a pre order traversal/dfs, in this
-  // case on an array where left and right partition are children
-  // nodes to the original array node.  Also with recursion there's a
-  // real risk for very large ARR_SIZE of running out of space in the
-  // call stack.
-  if (pivot != 0)
-    quicksort(arr, pivot);
-  if (pivot < arr_size - 1)
-    quicksort(arr + pivot + 1, arr_size - pivot - 1);
 }
 
-int main(void)
+int main(int argc, char *argv[])
 {
-  int arr[TEST_SIZE] = {0};
-  const int size     = TEST_SIZE;
+  (void)argc;
+  (void)argv;
 
-  // Generate a completely reverse ordered list
-  /*for (size_t i = 0; i < size; ++i) */
-  /*  arr[TEST_SIZE - i - 1] = i;*/
+#define TEST_SIZE 10
+  // randomly generate some numbers
+  // u64 *arr     = new u64[TEST_SIZE];
+  // u64 arr_size = TEST_SIZE;
 
-  // Generate a list where the first half is completely sorted and the
-  // other half is in reverse order.
-  for (size_t i = 0; i < size / 2; ++i)
-    arr[i] = i;
-  for (size_t i = size / 2; i < size; ++i)
-    arr[TEST_SIZE - (i - size / 2) - 1] = i;
+  // for (size_t i = 0; i < TEST_SIZE; ++i)
+  //   arr[i] = rand() % (2L << 30);
+
+  // Use nums.txt
+  std::ifstream input(argc > 1 ? argv[1] : "nums.txt");
+  std::vector<u64> num_vec;
+
+  for (std::string line; std::getline(input, line);
+       num_vec.push_back(std::stoi(line)))
+    continue;
+  u64 *arr     = &num_vec[0];
+  u64 arr_size = num_vec.size();
 
   printf("Before: ");
-  print_arr(arr, size);
+  print_arr(arr, arr_size);
   printf("\n");
 
-  quicksort(arr, size);
+  quicksort(arr, arr_size);
 
   printf("After: ");
-  print_arr(arr, size);
+  print_arr(arr, arr_size);
   printf("\n");
 
+  // delete[] arr;
+
   return 0;
 }

impls/vec.c

@@ -16,7 +16,7 @@
    @brief A vector, consisting of a header and some payload (data).
    @details The idea is that the user must never actually interact with this
    structure directly, only the data involved.  Any vector related functions
-   deal with
+   will take the pointer to the data and look behind it to get the header.
  */
 typedef struct
 {