diff --git a/.gitignore b/.gitignore
index ea8c4bf..e3318bb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /target
+/c_version/sudk_c
diff --git a/c_version/clangtest b/c_version/clangtest
new file mode 100755
index 0000000..b7ce4e8
--- /dev/null
+++ b/c_version/clangtest
@@ -0,0 +1,6 @@
+#!/bin/bash
+set -e
+
+clang -Wall -Wextra -Weverything -Werror -O3 -march=native -flto -std=c17 -fstack-protector-all -s -o sudk_c "$1"
+
+time ./sudk_c
diff --git a/c_version/gcctest b/c_version/gcctest
new file mode 100755
index 0000000..6b0eeb7
--- /dev/null
+++ b/c_version/gcctest
@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -e
+
+gcc -Wall -Wextra -Werror --std=c17 -march=native -O3 -fstack-protector-all -flto -s -o sudk_c "$1"
+
+time ./sudk_c
diff --git a/c_version/sudk_lots_of_allocs_for_tmp_rows_cols_blocks.c b/c_version/sudk_lots_of_allocs_for_tmp_rows_cols_blocks.c
new file mode 100644
index 0000000..e05caa8
--- /dev/null
+++ b/c_version/sudk_lots_of_allocs_for_tmp_rows_cols_blocks.c
@@ -0,0 +1,287 @@
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+#define PFSALLOC(bytes) malloc(bytes)
+
+static inline void Print_Clear() { printf("\x1b\x5b\x48\x1b\x5b\x32\x4a"); }
+
+typedef struct Sudoku {
+  size_t *field;
+  size_t *fixed;
+  size_t *known;
+  size_t size;
+  size_t block_size;
+  size_t pos;
+  size_t pos_last_placed;
+  size_t num_fields;
+} Sudoku;
+
+static Sudoku *Sudoku_New(size_t block_size) {
+
+  Sudoku *s = calloc(1, sizeof(Sudoku));
+
+  size_t many_solutions_field[81] = {
+      0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 8, 9, 0, 2, 0, 0, 0, 0,
+      0, 2, 0, 4, 0, 0, 0, 0, 4, 0, 6, 0, 0, 0, 8, 0, 0, 0, 5, 0, 0,
+      0, 0, 0, 0, 6, 5, 0, 0, 2, 0, 7, 4, 0, 3, 0, 0, 0, 5, 0, 4, 0,
+      0, 0, 1, 8, 0, 0, 0, 0, 0, 0, 0, 8, 2, 0, 0, 0, 6, 0,
+  };
+
+  size_t size = block_size * block_size;
+  size_t *field = calloc(size * size, sizeof(size_t));
+  size_t *fixed = calloc(size * size, sizeof(size_t));
+  size_t *known = calloc(size * size, sizeof(size_t));
+
+  printf("num_fields: %zu\n", size * size);
+
+  s->field = field;
+  s->fixed = fixed;
+  s->known = known;
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    s->field[idx] = many_solutions_field[idx];
+  }
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    if (s->field[idx] != 0) {
+      s->fixed[idx] = 1;
+      s->known[idx] = 1;
+    }
+  }
+
+  s->size = size;
+  s->block_size = block_size;
+  s->pos = 0;
+  s->pos_last_placed = 0;
+  s->num_fields = size * size;
+
+  return s;
+}
+
+static void Sudoku_Print(Sudoku *s) {
+  for (size_t idx = 0; idx < s->size * s->size; idx++) {
+
+    // newline
+    if (idx != 0 && idx % s->size == 0)
+      printf("\n");
+
+    // nr
+    printf("%zu", s->field[idx]);
+
+    // space
+    if ((idx + 1) % s->size != 0)
+      printf(" ");
+  }
+  printf("\n");
+}
+
+static inline bool Sudoku_Next(Sudoku *s) {
+  if (s->pos == s->num_fields - 1) {
+    /*printf("there is no next\n");*/
+    return false;
+  }
+
+  s->pos++;
+  return true;
+}
+
+static inline bool Sudoku_Prev(Sudoku *s) {
+  if (s->pos == 0)
+    return false;
+
+  s->pos--;
+  return true;
+}
+
+static inline bool Sudoku_IsFixed(Sudoku *s) { return s->fixed[s->pos] == 1; }
+
+static inline size_t Sudoku_GetFieldAtPos(Sudoku *s, size_t pos) {
+  return s->field[pos];
+}
+
+static inline void Sudoku_Set(Sudoku *s, size_t nr) {
+  s->field[s->pos] = nr;
+  s->pos_last_placed = s->pos;
+}
+
+static inline void Sudoku_ClearCurrentField(Sudoku *s) { Sudoku_Set(s, 0); }
+
+/*static void Array_Print(char *name, size_t *arr, size_t len) {*/
+/*printf("%s: ", name);*/
+/*for (size_t idx = 0; idx < len; idx++) {*/
+/*if (idx < len - 1)*/
+/*printf("%zu ", arr[idx]);*/
+/*else*/
+/*printf("%zu", arr[idx]);*/
+/*}*/
+/*printf("\n");*/
+/*}*/
+
+static inline bool Array_Contains(size_t *arr, size_t len, size_t nr) {
+  /*printf("ARRAY_CONTAINS_LEN: %zu\n", len);*/
+  for (size_t idx = 0; idx < len; idx++) {
+    if (arr[idx] == nr) {
+      /*printf("SEARCH FOR %zu --- ", nr);*/
+      /*Array_Print("ARRAY_CONTAINS_TRUE", arr, len);*/
+      return true;
+    }
+  }
+  /*printf("SEARCH FOR %zu --- ", nr);*/
+  /*Array_Print("ARRAY_CONTAINS_FALSE", arr, len);*/
+  return false;
+}
+
+static inline bool Sudoku_IsEnd(Sudoku *s) {
+  return !Array_Contains(s->field, s->num_fields, 0);
+}
+
+static inline bool Sudoku_GotoPrevFreeField(Sudoku *s) {
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Prev(s));
+
+  return false;
+}
+
+static inline bool Sudoku_GotoNextFreeField(Sudoku *s) {
+  /*printf("Sudoku_GotoNextFreeField\n");*/
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Next(s));
+
+  return false;
+}
+
+static inline size_t *Sudoku_GetRow(Sudoku *s) {
+  size_t *tmp = PFSALLOC(s->size * sizeof(size_t));
+  for (size_t pos = 0; pos < s->size; pos++) {
+    tmp[pos] = Sudoku_GetFieldAtPos(s, (s->pos / s->size) * s->size + pos);
+  }
+  return tmp;
+}
+
+static inline size_t *Sudoku_GetCol(Sudoku *s) {
+  size_t *tmp = PFSALLOC(s->size * sizeof(size_t));
+  for (size_t pos = 0; pos < s->size; pos++) {
+    tmp[pos] = Sudoku_GetFieldAtPos(s, pos * s->size + s->pos % s->size);
+  }
+  return tmp;
+}
+
+static inline size_t *Sudoku_GetBlock(Sudoku *s) {
+  size_t *tmp = PFSALLOC(s->size * sizeof(size_t));
+
+  size_t block_start_row = s->pos / s->size / s->block_size * s->block_size;
+  size_t block_start_col = s->pos % s->size / s->block_size * s->block_size;
+
+  for (size_t r = 0; r < s->block_size; r++) {
+    for (size_t c = 0; c < s->block_size; c++) {
+      tmp[r * s->block_size + c] = Sudoku_GetFieldAtPos(
+          s, s->size * (block_start_row + r) + block_start_col + c);
+    }
+  }
+  return tmp;
+}
+
+static inline bool Sudoku_BlockOk(Sudoku *s, size_t nr) {
+  size_t *block = Sudoku_GetBlock(s);
+  /*Array_Print("BLOCK", block, s->size);*/
+  bool ok = !Array_Contains(block, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_RowOk(Sudoku *s, size_t nr) {
+  size_t *row = Sudoku_GetRow(s);
+  bool ok = !Array_Contains(row, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_ColOk(Sudoku *s, size_t nr) {
+  size_t *col = Sudoku_GetCol(s);
+  bool ok = !Array_Contains(col, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_Ok(Sudoku *s, size_t nr) {
+  return Sudoku_BlockOk(s, nr) && Sudoku_ColOk(s, nr) && Sudoku_RowOk(s, nr);
+}
+
+static inline bool Sudoku_PutValidNr(Sudoku *s) {
+  size_t current_nr = Sudoku_GetFieldAtPos(s, s->pos);
+
+  /*printf("%s() pos=%zu current_nr=%zu\n", __func__, s->pos, current_nr);*/
+
+  for (size_t nr = current_nr; nr < s->size + 1; nr++) {
+    if (Sudoku_Ok(s, nr)) {
+      /*printf("%s() pos=%zu current_nr=%zu nr=%zu\n", __func__, s->pos,*/
+      /*current_nr, nr);*/
+      Sudoku_Set(s, nr);
+      return true;
+    }
+  }
+  return false;
+}
+
+static bool Sudoku_SolveBacktracking(Sudoku *s) {
+  bool found_solution = false;
+  size_t num_solutions = 0;
+  while (1) {
+    /*printf("LOOP: pos=%zu\n", s->pos);*/
+    /*Sudoku_Print(s);*/
+    if (Sudoku_IsEnd(s)) {
+      /*printf("END\n");*/
+      found_solution = true;
+      num_solutions++;
+      if (num_solutions % 100 == 0) {
+        Print_Clear();
+        printf("Solutions: %zu\n", num_solutions);
+        Sudoku_Print(s);
+      }
+
+      if (Sudoku_IsFixed(s)) {
+        /*printf("fixed\n");*/
+        continue;
+      }
+      Sudoku_ClearCurrentField(s);
+      Sudoku_Prev(s);
+      Sudoku_GotoPrevFreeField(s);
+    }
+
+    if (Sudoku_IsFixed(s)) {
+      /*printf("fixed -> goto next\n");*/
+      Sudoku_Next(s);
+      continue;
+    }
+
+    if (Sudoku_GotoNextFreeField(s)) {
+      if (Sudoku_PutValidNr(s)) {
+        Sudoku_Next(s);
+        continue;
+      } else {
+        Sudoku_ClearCurrentField(s);
+        if (!Sudoku_Prev(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+        if (!Sudoku_GotoPrevFreeField(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+      }
+    }
+  }
+
+  return found_solution;
+}
+
+int main() {
+  Sudoku *s = Sudoku_New(3);
+  Sudoku_Print(s);
+  Sudoku_SolveBacktracking(s);
+  /*Sudoku_Print(s);*/
+}
diff --git a/c_version/sudk_no_alloc_instead_use_global_mut_mem_once_initialized_on_start.c b/c_version/sudk_no_alloc_instead_use_global_mut_mem_once_initialized_on_start.c
new file mode 100644
index 0000000..4f8b508
--- /dev/null
+++ b/c_version/sudk_no_alloc_instead_use_global_mut_mem_once_initialized_on_start.c
@@ -0,0 +1,278 @@
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+static size_t *tmpArr;
+
+static inline void Print_Clear() { printf("\x1b\x5b\x48\x1b\x5b\x32\x4a"); }
+
+typedef struct Sudoku {
+  size_t *field;
+  size_t *fixed;
+  size_t size;
+  size_t block_size;
+  size_t pos;
+  size_t pos_last_placed;
+  size_t num_fields;
+} Sudoku;
+
+static Sudoku *Sudoku_New(size_t block_size) {
+
+  Sudoku *s = calloc(1, sizeof(Sudoku));
+
+  size_t many_solutions_field[81] = {
+      0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 8, 9, 0, 2, 0, 0, 0, 0,
+      0, 2, 0, 4, 0, 0, 0, 0, 4, 0, 6, 0, 0, 0, 8, 0, 0, 0, 5, 0, 0,
+      0, 0, 0, 0, 6, 5, 0, 0, 2, 0, 7, 4, 0, 3, 0, 0, 0, 5, 0, 4, 0,
+      0, 0, 1, 8, 0, 0, 0, 0, 0, 0, 0, 8, 2, 0, 0, 0, 6, 0,
+  };
+
+  size_t size = block_size * block_size;
+  size_t *field = calloc(size * size, sizeof(size_t));
+  size_t *fixed = calloc(size * size, sizeof(size_t));
+
+  printf("num_fields: %zu\n", size * size);
+
+  s->field = field;
+  s->fixed = fixed;
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    s->field[idx] = many_solutions_field[idx];
+  }
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    if (s->field[idx] != 0) {
+      s->fixed[idx] = 1;
+    }
+  }
+
+  s->size = size;
+  s->block_size = block_size;
+  s->pos = 0;
+  s->pos_last_placed = 0;
+  s->num_fields = size * size;
+
+  tmpArr = calloc(s->size, sizeof(size_t));
+
+  return s;
+}
+
+static void Sudoku_Print(Sudoku *s) {
+  for (size_t idx = 0; idx < s->size * s->size; idx++) {
+
+    // newline
+    if (idx != 0 && idx % s->size == 0)
+      printf("\n");
+
+    // nr
+    printf("%zu", s->field[idx]);
+
+    // space
+    if ((idx + 1) % s->size != 0)
+      printf(" ");
+  }
+  printf("\n");
+}
+
+static inline bool Sudoku_Next(Sudoku *s) {
+  if (s->pos == s->num_fields - 1) {
+    /*printf("there is no next\n");*/
+    return false;
+  }
+
+  s->pos++;
+  return true;
+}
+
+static inline bool Sudoku_Prev(Sudoku *s) {
+  if (s->pos == 0)
+    return false;
+
+  s->pos--;
+  return true;
+}
+
+static inline bool Sudoku_IsFixed(Sudoku *s) { return s->fixed[s->pos] == 1; }
+
+static inline size_t Sudoku_GetFieldAtPos(Sudoku *s, size_t pos) {
+  return s->field[pos];
+}
+
+static inline void Sudoku_Set(Sudoku *s, size_t nr) {
+  s->field[s->pos] = nr;
+  s->pos_last_placed = s->pos;
+}
+
+static inline void Sudoku_ClearCurrentField(Sudoku *s) { Sudoku_Set(s, 0); }
+
+static inline bool Array_Contains(size_t *arr, size_t len, size_t nr) {
+  /*printf("ARRAY_CONTAINS_LEN: %zu\n", len);*/
+  for (size_t idx = 0; idx < len; idx++) {
+    if (arr[idx] == nr) {
+      /*printf("SEARCH FOR %zu --- ", nr);*/
+      /*Array_Print("ARRAY_CONTAINS_TRUE", arr, len);*/
+      return true;
+    }
+  }
+  /*printf("SEARCH FOR %zu --- ", nr);*/
+  /*Array_Print("ARRAY_CONTAINS_FALSE", arr, len);*/
+  return false;
+}
+
+static inline bool Sudoku_IsEnd(Sudoku *s) {
+  return !Array_Contains(s->field, s->num_fields, 0);
+}
+
+static inline bool Sudoku_GotoPrevFreeField(Sudoku *s) {
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Prev(s));
+
+  return false;
+}
+
+static inline bool Sudoku_GotoNextFreeField(Sudoku *s) {
+  /*printf("Sudoku_GotoNextFreeField\n");*/
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Next(s));
+
+  return false;
+}
+
+static inline size_t *Sudoku_GetRow(Sudoku *s) {
+  for (size_t pos = 0; pos < s->size; pos++) {
+    tmpArr[pos] = Sudoku_GetFieldAtPos(s, (s->pos / s->size) * s->size + pos);
+  }
+  return tmpArr;
+}
+
+static inline size_t *Sudoku_GetCol(Sudoku *s) {
+  for (size_t pos = 0; pos < s->size; pos++) {
+    tmpArr[pos] = Sudoku_GetFieldAtPos(s, pos * s->size + s->pos % s->size);
+  }
+  return tmpArr;
+}
+
+static inline size_t *Sudoku_GetBlock(Sudoku *s) {
+  size_t block_start_row = s->pos / s->size / s->block_size * s->block_size;
+  size_t block_start_col = s->pos % s->size / s->block_size * s->block_size;
+
+  for (size_t r = 0; r < s->block_size; r++) {
+    for (size_t c = 0; c < s->block_size; c++) {
+      tmpArr[r * s->block_size + c] = Sudoku_GetFieldAtPos(
+          s, s->size * (block_start_row + r) + block_start_col + c);
+    }
+  }
+  return tmpArr;
+}
+
+static inline bool Sudoku_BlockOk(Sudoku *s, size_t nr) {
+  size_t *block = Sudoku_GetBlock(s);
+  /*Array_Print("BLOCK", block, s->size);*/
+  bool ok = !Array_Contains(block, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_RowOk(Sudoku *s, size_t nr) {
+  size_t *row = Sudoku_GetRow(s);
+  bool ok = !Array_Contains(row, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_ColOk(Sudoku *s, size_t nr) {
+  size_t *col = Sudoku_GetCol(s);
+  bool ok = !Array_Contains(col, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_Ok(Sudoku *s, size_t nr) {
+  return Sudoku_BlockOk(s, nr) && Sudoku_ColOk(s, nr) && Sudoku_RowOk(s, nr);
+}
+
+static inline bool Sudoku_PutValidNr(Sudoku *s) {
+  size_t current_nr = Sudoku_GetFieldAtPos(s, s->pos);
+
+  /*printf("%s() pos=%zu current_nr=%zu\n", __func__, s->pos, current_nr);*/
+
+  for (size_t nr = current_nr; nr < s->size + 1; nr++) {
+    if (Sudoku_Ok(s, nr)) {
+      /*printf("%s() pos=%zu current_nr=%zu nr=%zu\n", __func__, s->pos,*/
+      /*current_nr, nr);*/
+      Sudoku_Set(s, nr);
+      return true;
+    }
+  }
+  return false;
+}
+
+static bool Sudoku_SolveBacktracking(Sudoku *s) {
+  bool found_solution = false;
+  size_t num_solutions = 0;
+  while (1) {
+    /*printf("LOOP: pos=%zu\n", s->pos);*/
+    /*Sudoku_Print(s);*/
+    if (Sudoku_IsEnd(s)) {
+      /*printf("END\n");*/
+      found_solution = true;
+      num_solutions++;
+      if (num_solutions % 100 == 0) {
+        Print_Clear();
+        printf("Solutions: %zu\n", num_solutions);
+        Sudoku_Print(s);
+      }
+
+      if (Sudoku_IsFixed(s)) {
+        /*printf("fixed\n");*/
+        continue;
+      }
+      Sudoku_ClearCurrentField(s);
+      Sudoku_Prev(s);
+      Sudoku_GotoPrevFreeField(s);
+    }
+
+    if (Sudoku_IsFixed(s)) {
+      /*printf("fixed -> goto next\n");*/
+      Sudoku_Next(s);
+      continue;
+    }
+
+    if (Sudoku_GotoNextFreeField(s)) {
+      if (Sudoku_PutValidNr(s)) {
+        Sudoku_Next(s);
+        continue;
+      } else {
+        Sudoku_ClearCurrentField(s);
+        if (!Sudoku_Prev(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+        if (!Sudoku_GotoPrevFreeField(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+      }
+    }
+  }
+
+  return found_solution;
+}
+
+static void Sudoku_Free(Sudoku *s) {
+  free(s->field);
+  free(s->fixed);
+  free(s);
+  s = NULL;
+}
+
+int main() {
+  Sudoku *s = Sudoku_New(3);
+  Sudoku_Print(s);
+  Sudoku_SolveBacktracking(s);
+  Sudoku_Free(s);
+  free(tmpArr);
+}
diff --git a/c_version/sudk_stack.c b/c_version/sudk_stack.c
new file mode 100644
index 0000000..ae97249
--- /dev/null
+++ b/c_version/sudk_stack.c
@@ -0,0 +1,265 @@
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+static inline void Print_Clear() { printf("\x1b\x5b\x48\x1b\x5b\x32\x4a"); }
+
+typedef struct Sudoku {
+  size_t *field;
+  size_t *fixed;
+  size_t size;
+  size_t block_size;
+  size_t pos;
+  size_t pos_last_placed;
+  size_t num_fields;
+} Sudoku;
+
+static Sudoku *Sudoku_New(size_t block_size) {
+
+  Sudoku *s = calloc(1, sizeof(Sudoku));
+
+  // clang-format off
+  size_t many_solutions_field[81] = {
+      0, 0, 0, 0, 0, 0, 0, 0, 9,
+      0, 0, 0, 0, 8, 9, 0, 2, 0,
+      0, 0, 0, 0, 2, 0, 4, 0, 0,
+      0, 0, 4, 0, 6, 0, 0, 0, 8,
+      0, 0, 0, 5, 0, 0, 0, 0, 0,
+      0, 6, 5, 0, 0, 2, 0, 7, 4,
+      0, 3, 0, 0, 0, 5, 0, 4, 0,
+      0, 0, 1, 8, 0, 0, 0, 0, 0,
+      0, 0, 8, 2, 0, 0, 0, 6, 0,
+  };
+  // clang-format on
+
+  size_t size = block_size * block_size;
+  size_t *field = calloc(size * size, sizeof(size_t));
+  size_t *fixed = calloc(size * size, sizeof(size_t));
+
+  printf("num_fields: %zu\n", size * size);
+
+  s->field = field;
+  s->fixed = fixed;
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    s->field[idx] = many_solutions_field[idx];
+  }
+
+  for (size_t idx = 0; idx < 81; idx++) {
+    if (s->field[idx] != 0) {
+      s->fixed[idx] = 1;
+    }
+  }
+
+  s->size = size;
+  s->block_size = block_size;
+  s->pos = 0;
+  s->pos_last_placed = 0;
+  s->num_fields = size * size;
+
+  return s;
+}
+
+static inline void Sudoku_Print(Sudoku *s) {
+  for (size_t idx = 0; idx < s->num_fields; idx++) {
+
+    // newline
+    if (idx != 0 && idx % s->size == 0)
+      printf("\n");
+
+    // nr
+    printf("%zu", s->field[idx]);
+
+    // space
+    if ((idx + 1) % s->size != 0)
+      printf(" ");
+  }
+  printf("\n");
+}
+
+static inline bool Sudoku_Next(Sudoku *s) {
+  if (s->pos == s->num_fields - 1) {
+    return false;
+  }
+
+  s->pos++;
+  return true;
+}
+
+static inline bool Sudoku_Prev(Sudoku *s) {
+  if (s->pos == 0)
+    return false;
+
+  s->pos--;
+  return true;
+}
+
+static inline bool Sudoku_IsFixed(Sudoku *s) { return s->fixed[s->pos] == 1; }
+
+static inline size_t Sudoku_GetFieldAtPos(Sudoku *s, size_t pos) {
+  return s->field[pos];
+}
+
+static inline void Sudoku_Set(Sudoku *s, size_t nr) {
+  s->field[s->pos] = nr;
+  s->pos_last_placed = s->pos;
+}
+
+static inline void Sudoku_ClearCurrentField(Sudoku *s) { Sudoku_Set(s, 0); }
+
+static inline bool Array_Contains(size_t *arr, size_t len, size_t nr) {
+  for (size_t idx = 0; idx < len; idx++) {
+    if (arr[idx] == nr) {
+      return true;
+    }
+  }
+  return false;
+}
+
+static inline bool Sudoku_IsEnd(Sudoku *s) {
+  return !Array_Contains(s->field, s->num_fields, 0);
+}
+
+static inline bool Sudoku_GotoPrevFreeField(Sudoku *s) {
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Prev(s));
+
+  return false;
+}
+
+static inline bool Sudoku_GotoNextFreeField(Sudoku *s) {
+  do {
+    if (!Sudoku_IsFixed(s))
+      return true;
+  } while (Sudoku_Next(s));
+
+  return false;
+}
+
+static inline void Sudoku_GetRow(Sudoku *s, size_t *row) {
+  for (size_t pos = 0; pos < s->size; pos++) {
+    row[pos] = Sudoku_GetFieldAtPos(s, (s->pos / s->size) * s->size + pos);
+  }
+}
+
+static inline void Sudoku_GetCol(Sudoku *s, size_t *col) {
+  for (size_t pos = 0; pos < s->size; pos++) {
+    col[pos] = Sudoku_GetFieldAtPos(s, pos * s->size + s->pos % s->size);
+  }
+}
+
+static inline void Sudoku_GetBlock(Sudoku *s, size_t *block) {
+  size_t block_start_row = s->pos / s->size / s->block_size * s->block_size;
+  size_t block_start_col = s->pos % s->size / s->block_size * s->block_size;
+
+  for (size_t r = 0; r < s->block_size; r++) {
+    for (size_t c = 0; c < s->block_size; c++) {
+      block[r * s->block_size + c] = Sudoku_GetFieldAtPos(
+          s, s->size * (block_start_row + r) + block_start_col + c);
+    }
+  }
+}
+
+static inline bool Sudoku_BlockOk(Sudoku *s, size_t nr) {
+  bool ok;
+  size_t block[9] = {0};
+  Sudoku_GetBlock(s, block);
+  ok = !Array_Contains(block, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_RowOk(Sudoku *s, size_t nr) {
+  bool ok;
+  size_t row[9] = {0};
+  Sudoku_GetRow(s, row);
+  ok = !Array_Contains(row, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_ColOk(Sudoku *s, size_t nr) {
+  bool ok;
+  size_t col[9] = {0};
+  Sudoku_GetCol(s, col);
+  ok = !Array_Contains(col, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_Ok(Sudoku *s, size_t nr) {
+  return Sudoku_BlockOk(s, nr) && Sudoku_ColOk(s, nr) && Sudoku_RowOk(s, nr);
+}
+
+static inline bool Sudoku_PutValidNr(Sudoku *s) {
+  size_t current_nr = Sudoku_GetFieldAtPos(s, s->pos);
+  for (size_t nr = current_nr; nr < s->size + 1; nr++) {
+    if (Sudoku_Ok(s, nr)) {
+      Sudoku_Set(s, nr);
+      return true;
+    }
+  }
+  return false;
+}
+
+static bool Sudoku_SolveBacktracking(Sudoku *s) {
+  bool found_solution = false;
+  size_t num_solutions = 0;
+  while (1) {
+    if (Sudoku_IsEnd(s)) {
+      found_solution = true;
+      num_solutions++;
+      if (num_solutions % 100 == 0) {
+        Print_Clear();
+        printf("Solutions: %zu\n", num_solutions);
+        Sudoku_Print(s);
+      }
+
+      if (Sudoku_IsFixed(s)) {
+        continue;
+      }
+      Sudoku_ClearCurrentField(s);
+      Sudoku_Prev(s);
+      Sudoku_GotoPrevFreeField(s);
+    }
+
+    if (Sudoku_IsFixed(s)) {
+      Sudoku_Next(s);
+      continue;
+    }
+
+    if (Sudoku_GotoNextFreeField(s)) {
+      if (Sudoku_PutValidNr(s)) {
+        Sudoku_Next(s);
+        continue;
+      } else {
+        Sudoku_ClearCurrentField(s);
+        if (!Sudoku_Prev(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+        if (!Sudoku_GotoPrevFreeField(s)) {
+          printf("Number of solutions: %zu\n", num_solutions);
+          break;
+        }
+      }
+    }
+  }
+
+  return found_solution;
+}
+
+static void Sudoku_Free(Sudoku *s) {
+  free(s->field);
+  free(s->fixed);
+  free(s);
+  s = NULL;
+}
+
+int main() {
+  Sudoku *s = Sudoku_New(3);
+  Sudoku_Print(s);
+  Sudoku_SolveBacktracking(s);
+  Sudoku_Free(s);
+}