(fix) unsafe array access in skipf and skipb

skipf was accessed at index 81 (len only 81)
skipb was accessed at index -1 (u64::MAX)

I still don't know, why the skipb case didn't crash.

.cargo/config.toml

@@ -0,0 +1,2 @@
+[build]
+rustflags = ["-C", "target-cpu=native", "-C", "target-feature=+crt-static"]

.gitignore

@@ -1 +1,2 @@
 /target
+/c_version/sudk_c

Cargo.toml

@@ -9,5 +9,6 @@ edition = "2021"
 [profile.release]
 lto = true
 debug = false
-codegen-units = 1
+strip = true
 panic = "abort"
+codegen-units = 1

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

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

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);*/
+}

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);
+}

c_version/sudk_stack.c

@@ -0,0 +1,266 @@
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.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 {
+  uint8_t *field;
+  bool *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
+  uint8_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;
+  uint8_t *field = calloc(size * size, sizeof(uint8_t));
+  bool *fixed = calloc(size * size, sizeof(bool));
+
+  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] = true;
+    }
+  }
+
+  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("%hhu", 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]; }
+
+static inline uint8_t Sudoku_GetFieldAtPos(Sudoku *s, size_t pos) {
+  return s->field[pos];
+}
+
+static inline void Sudoku_Set(Sudoku *s, uint8_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(uint8_t *arr, size_t len, uint8_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, uint8_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, uint8_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, uint8_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, uint8_t nr) {
+  bool ok;
+  uint8_t block[9] = {0};
+  Sudoku_GetBlock(s, block);
+  ok = !Array_Contains(block, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_RowOk(Sudoku *s, uint8_t nr) {
+  bool ok;
+  uint8_t row[9] = {0};
+  Sudoku_GetRow(s, row);
+  ok = !Array_Contains(row, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_ColOk(Sudoku *s, uint8_t nr) {
+  bool ok;
+  uint8_t col[9] = {0};
+  Sudoku_GetCol(s, col);
+  ok = !Array_Contains(col, s->size, nr);
+  return ok;
+}
+
+static inline bool Sudoku_Ok(Sudoku *s, uint8_t nr) {
+  return Sudoku_BlockOk(s, nr) && Sudoku_ColOk(s, nr) && Sudoku_RowOk(s, nr);
+}
+
+static inline bool Sudoku_PutValidNr(Sudoku *s) {
+  uint8_t current_nr = Sudoku_GetFieldAtPos(s, s->pos);
+  for (uint8_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);
+}

flamegraph.svg

@@ -0,0 +1,414 @@
+<?xml version="1.0" standalone="no"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd"><svg version="1.1" width="1200" height="470" onload="init(evt)" viewBox="0 0 1200 470" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:fg="http://github.com/jonhoo/inferno"><!--Flame graph stack visualization. See https://github.com/brendangregg/FlameGraph for latest version, and http://www.brendangregg.com/flamegraphs.html for examples.--><!--NOTES: --><defs><linearGradient id="background" y1="0" y2="1" x1="0" x2="0"><stop stop-color="#eeeeee" offset="5%"/><stop stop-color="#eeeeb0" offset="95%"/></linearGradient></defs><style type="text/css">
+text { font-family:"Verdana"; font-size:12px; fill:rgb(0,0,0); }
+#title { text-anchor:middle; font-size:17px; }
+#search { opacity:0.1; cursor:pointer; }
+#search:hover, #search.show { opacity:1; }
+#subtitle { text-anchor:middle; font-color:rgb(160,160,160); }
+#unzoom { cursor:pointer; }
+#frames > *:hover { stroke:black; stroke-width:0.5; cursor:pointer; }
+.hide { display:none; }
+.parent { opacity:0.5; }
+</style><script type="text/ecmascript"><![CDATA[
+        var nametype = 'Function:';
+        var fontsize = 12;
+        var fontwidth = 0.59;
+        var xpad = 10;
+        var inverted = false;
+        var searchcolor = 'rgb(230,0,230)';
+        var fluiddrawing = true;
+        var truncate_text_right = false;
+    ]]><![CDATA["use strict";
+var details, searchbtn, unzoombtn, matchedtxt, svg, searching, frames;
+function init(evt) {
+    details = document.getElementById("details").firstChild;
+    searchbtn = document.getElementById("search");
+    unzoombtn = document.getElementById("unzoom");
+    matchedtxt = document.getElementById("matched");
+    svg = document.getElementsByTagName("svg")[0];
+    frames = document.getElementById("frames");
+    total_samples = parseInt(frames.attributes.total_samples.value);
+    searching = 0;
+
+    // Use GET parameters to restore a flamegraph's state.
+    var restore_state = function() {
+        var params = get_params();
+        if (params.x && params.y)
+            zoom(find_group(document.querySelector('[*|x="' + params.x + '"][y="' + params.y + '"]')));
+        if (params.s)
+            search(params.s);
+    };
+
+    if (fluiddrawing) {
+        // Make width dynamic so the SVG fits its parent's width.
+        svg.removeAttribute("width");
+        // Edge requires us to have a viewBox that gets updated with size changes.
+        var isEdge = /Edge\/\d./i.test(navigator.userAgent);
+        var update_for_width_change = function() {
+            if (isEdge) {
+                svg.attributes.viewBox.value = "0 0 " + svg.width.baseVal.value + " " + svg.height.baseVal.value;
+            }
+
+            // Keep consistent padding on left and right of frames container.
+            frames.attributes.width.value = svg.width.baseVal.value - xpad * 2;
+
+            // Text truncation needs to be adjusted for the current width.
+            var el = frames.children;
+            for(var i = 0; i < el.length; i++) {
+                update_text(el[i]);
+            }
+
+            // Keep search elements at a fixed distance from right edge.
+            var svgWidth = svg.width.baseVal.value;
+            searchbtn.attributes.x.value = svgWidth - xpad - 100;
+            matchedtxt.attributes.x.value = svgWidth - xpad - 100;
+        };
+        window.addEventListener('resize', function() {
+            update_for_width_change();
+        });
+        // This needs to be done asynchronously for Safari to work.
+        setTimeout(function() {
+            unzoom();
+            update_for_width_change();
+            restore_state();
+            if (!isEdge) {
+                svg.removeAttribute("viewBox");
+            }
+        }, 0);
+    } else {
+        restore_state();
+    }
+}
+// event listeners
+window.addEventListener("click", function(e) {
+    var target = find_group(e.target);
+    if (target) {
+        if (target.nodeName == "a") {
+            if (e.ctrlKey === false) return;
+            e.preventDefault();
+        }
+        if (target.classList.contains("parent")) unzoom();
+        zoom(target);
+
+        // set parameters for zoom state
+        var el = target.querySelector("rect");
+        if (el && el.attributes && el.attributes.y && el.attributes["fg:x"]) {
+            var params = get_params()
+            params.x = el.attributes["fg:x"].value;
+            params.y = el.attributes.y.value;
+            history.replaceState(null, null, parse_params(params));
+        }
+    }
+    else if (e.target.id == "unzoom") {
+        unzoom();
+
+        // remove zoom state
+        var params = get_params();
+        if (params.x) delete params.x;
+        if (params.y) delete params.y;
+        history.replaceState(null, null, parse_params(params));
+    }
+    else if (e.target.id == "search") search_prompt();
+}, false)
+// mouse-over for info
+// show
+window.addEventListener("mouseover", function(e) {
+    var target = find_group(e.target);
+    if (target) details.nodeValue = nametype + " " + g_to_text(target);
+}, false)
+// clear
+window.addEventListener("mouseout", function(e) {
+    var target = find_group(e.target);
+    if (target) details.nodeValue = ' ';
+}, false)
+// ctrl-F for search
+window.addEventListener("keydown",function (e) {
+    if (e.keyCode === 114 || (e.ctrlKey && e.keyCode === 70)) {
+        e.preventDefault();
+        search_prompt();
+    }
+}, false)
+// functions
+function get_params() {
+    var params = {};
+    var paramsarr = window.location.search.substr(1).split('&');
+    for (var i = 0; i < paramsarr.length; ++i) {
+        var tmp = paramsarr[i].split("=");
+        if (!tmp[0] || !tmp[1]) continue;
+        params[tmp[0]]  = decodeURIComponent(tmp[1]);
+    }
+    return params;
+}
+function parse_params(params) {
+    var uri = "?";
+    for (var key in params) {
+        uri += key + '=' + encodeURIComponent(params[key]) + '&';
+    }
+    if (uri.slice(-1) == "&")
+        uri = uri.substring(0, uri.length - 1);
+    if (uri == '?')
+        uri = window.location.href.split('?')[0];
+    return uri;
+}
+function find_child(node, selector) {
+    var children = node.querySelectorAll(selector);
+    if (children.length) return children[0];
+    return;
+}
+function find_group(node) {
+    var parent = node.parentElement;
+    if (!parent) return;
+    if (parent.id == "frames") return node;
+    return find_group(parent);
+}
+function orig_save(e, attr, val) {
+    if (e.attributes["fg:orig_" + attr] != undefined) return;
+    if (e.attributes[attr] == undefined) return;
+    if (val == undefined) val = e.attributes[attr].value;
+    e.setAttribute("fg:orig_" + attr, val);
+}
+function orig_load(e, attr) {
+    if (e.attributes["fg:orig_"+attr] == undefined) return;
+    e.attributes[attr].value = e.attributes["fg:orig_" + attr].value;
+    e.removeAttribute("fg:orig_" + attr);
+}
+function g_to_text(e) {
+    var text = find_child(e, "title").firstChild.nodeValue;
+    return (text)
+}
+function g_to_func(e) {
+    var func = g_to_text(e);
+    // if there's any manipulation we want to do to the function
+    // name before it's searched, do it here before returning.
+    return (func);
+}
+function update_text(e) {
+    var r = find_child(e, "rect");
+    var t = find_child(e, "text");
+    var w = parseFloat(r.attributes.width.value) * frames.attributes.width.value / 100 - 3;
+    var txt = find_child(e, "title").textContent.replace(/\([^(]*\)$/,"");
+    t.attributes.x.value = format_percent((parseFloat(r.attributes.x.value) + (100 * 3 / frames.attributes.width.value)));
+    // Smaller than this size won't fit anything
+    if (w < 2 * fontsize * fontwidth) {
+        t.textContent = "";
+        return;
+    }
+    t.textContent = txt;
+    // Fit in full text width
+    if (/^ *\$/.test(txt) || t.getComputedTextLength() < w)
+        return;
+    if (truncate_text_right) {
+        // Truncate the right side of the text.
+        for (var x = txt.length - 2; x > 0; x--) {
+            if (t.getSubStringLength(0, x + 2) <= w) {
+                t.textContent = txt.substring(0, x) + "..";
+                return;
+            }
+        }
+    } else {
+        // Truncate the left side of the text.
+        for (var x = 2; x < txt.length; x++) {
+            if (t.getSubStringLength(x - 2, txt.length) <= w) {
+                t.textContent = ".." + txt.substring(x, txt.length);
+                return;
+            }
+        }
+    }
+    t.textContent = "";
+}
+// zoom
+function zoom_reset(e) {
+    if (e.tagName == "rect") {
+        e.attributes.x.value = format_percent(100 * parseInt(e.attributes["fg:x"].value) / total_samples);
+        e.attributes.width.value = format_percent(100 * parseInt(e.attributes["fg:w"].value) / total_samples);
+    }
+    if (e.childNodes == undefined) return;
+    for(var i = 0, c = e.childNodes; i < c.length; i++) {
+        zoom_reset(c[i]);
+    }
+}
+function zoom_child(e, x, zoomed_width_samples) {
+    if (e.tagName == "text") {
+        var parent_x = parseFloat(find_child(e.parentNode, "rect[x]").attributes.x.value);
+        e.attributes.x.value = format_percent(parent_x + (100 * 3 / frames.attributes.width.value));
+    } else if (e.tagName == "rect") {
+        e.attributes.x.value = format_percent(100 * (parseInt(e.attributes["fg:x"].value) - x) / zoomed_width_samples);
+        e.attributes.width.value = format_percent(100 * parseInt(e.attributes["fg:w"].value) / zoomed_width_samples);
+    }
+    if (e.childNodes == undefined) return;
+    for(var i = 0, c = e.childNodes; i < c.length; i++) {
+        zoom_child(c[i], x, zoomed_width_samples);
+    }
+}
+function zoom_parent(e) {
+    if (e.attributes) {
+        if (e.attributes.x != undefined) {
+            e.attributes.x.value = "0.0%";
+        }
+        if (e.attributes.width != undefined) {
+            e.attributes.width.value = "100.0%";
+        }
+    }
+    if (e.childNodes == undefined) return;
+    for(var i = 0, c = e.childNodes; i < c.length; i++) {
+        zoom_parent(c[i]);
+    }
+}
+function zoom(node) {
+    var attr = find_child(node, "rect").attributes;
+    var width = parseInt(attr["fg:w"].value);
+    var xmin = parseInt(attr["fg:x"].value);
+    var xmax = xmin + width;
+    var ymin = parseFloat(attr.y.value);
+    unzoombtn.classList.remove("hide");
+    var el = frames.children;
+    for (var i = 0; i < el.length; i++) {
+        var e = el[i];
+        var a = find_child(e, "rect").attributes;
+        var ex = parseInt(a["fg:x"].value);
+        var ew = parseInt(a["fg:w"].value);
+        // Is it an ancestor
+        if (!inverted) {
+            var upstack = parseFloat(a.y.value) > ymin;
+        } else {
+            var upstack = parseFloat(a.y.value) < ymin;
+        }
+        if (upstack) {
+            // Direct ancestor
+            if (ex <= xmin && (ex+ew) >= xmax) {
+                e.classList.add("parent");
+                zoom_parent(e);
+                update_text(e);
+            }
+            // not in current path
+            else
+                e.classList.add("hide");
+        }
+        // Children maybe
+        else {
+            // no common path
+            if (ex < xmin || ex >= xmax) {
+                e.classList.add("hide");
+            }
+            else {
+                zoom_child(e, xmin, width);
+                update_text(e);
+            }
+        }
+    }
+}
+function unzoom() {
+    unzoombtn.classList.add("hide");
+    var el = frames.children;
+    for(var i = 0; i < el.length; i++) {
+        el[i].classList.remove("parent");
+        el[i].classList.remove("hide");
+        zoom_reset(el[i]);
+        update_text(el[i]);
+    }
+}
+// search
+function reset_search() {
+    var el = document.querySelectorAll("#frames rect");
+    for (var i = 0; i < el.length; i++) {
+        orig_load(el[i], "fill")
+    }
+    var params = get_params();
+    delete params.s;
+    history.replaceState(null, null, parse_params(params));
+}
+function search_prompt() {
+    if (!searching) {
+        var term = prompt("Enter a search term (regexp " +
+            "allowed, eg: ^ext4_)", "");
+        if (term != null) {
+            search(term)
+        }
+    } else {
+        reset_search();
+        searching = 0;
+        searchbtn.classList.remove("show");
+        searchbtn.firstChild.nodeValue = "Search"
+        matchedtxt.classList.add("hide");
+        matchedtxt.firstChild.nodeValue = ""
+    }
+}
+function search(term) {
+    var re = new RegExp(term);
+    var el = frames.children;
+    var matches = new Object();
+    var maxwidth = 0;
+    for (var i = 0; i < el.length; i++) {
+        var e = el[i];
+        // Skip over frames which are either not visible, or below the zoomed-to frame
+        if (e.classList.contains("hide") || e.classList.contains("parent")) {
+            continue;
+        }
+        var func = g_to_func(e);
+        var rect = find_child(e, "rect");
+        if (func == null || rect == null)
+            continue;
+        // Save max width. Only works as we have a root frame
+        var w = parseInt(rect.attributes["fg:w"].value);
+        if (w > maxwidth)
+            maxwidth = w;
+        if (func.match(re)) {
+            // highlight
+            var x = parseInt(rect.attributes["fg:x"].value);
+            orig_save(rect, "fill");
+            rect.attributes.fill.value = searchcolor;
+            // remember matches
+            if (matches[x] == undefined) {
+                matches[x] = w;
+            } else {
+                if (w > matches[x]) {
+                    // overwrite with parent
+                    matches[x] = w;
+                }
+            }
+            searching = 1;
+        }
+    }
+    if (!searching)
+        return;
+    var params = get_params();
+    params.s = term;
+    history.replaceState(null, null, parse_params(params));
+
+    searchbtn.classList.add("show");
+    searchbtn.firstChild.nodeValue = "Reset Search";
+    // calculate percent matched, excluding vertical overlap
+    var count = 0;
+    var lastx = -1;
+    var lastw = 0;
+    var keys = Array();
+    for (k in matches) {
+        if (matches.hasOwnProperty(k))
+            keys.push(k);
+    }
+    // sort the matched frames by their x location
+    // ascending, then width descending
+    keys.sort(function(a, b){
+        return a - b;
+    });
+    // Step through frames saving only the biggest bottom-up frames
+    // thanks to the sort order. This relies on the tree property
+    // where children are always smaller than their parents.
+    for (var k in keys) {
+        var x = parseInt(keys[k]);
+        var w = matches[keys[k]];
+        if (x >= lastx + lastw) {
+            count += w;
+            lastx = x;
+            lastw = w;
+        }
+    }
+    // display matched percent
+    matchedtxt.classList.remove("hide");
+    var pct = 100 * count / maxwidth;
+    if (pct != 100) pct = pct.toFixed(1);
+    matchedtxt.firstChild.nodeValue = "Matched: " + pct + "%";
+}
+function format_percent(n) {
+    return n.toFixed(4) + "%";
+}
+]]></script><rect x="0" y="0" width="100%" height="470" fill="url(#background)"/><text id="title" x="50.0000%" y="24.00">Flame Graph</text><text id="details" x="10" y="453.00"> </text><text id="unzoom" class="hide" x="10" y="24.00">Reset Zoom</text><text id="search" x="1090" y="24.00">Search</text><text id="matched" x="1090" y="453.00"> </text><svg id="frames" x="10" width="1180" total_samples="4176"><g><title>[unknown] (1 samples, 0.02%)</title><rect x="0.0000%" y="373" width="0.0239%" height="15" fill="rgb(227,0,7)" fg:x="0" fg:w="1"/><text x="0.2500%" y="383.50"></text></g><g><title>[unknown] (1 samples, 0.02%)</title><rect x="0.0000%" y="357" width="0.0239%" height="15" fill="rgb(217,0,24)" fg:x="0" fg:w="1"/><text x="0.2500%" y="367.50"></text></g><g><title>[unknown] (1 samples, 0.02%)</title><rect x="0.0000%" y="341" width="0.0239%" height="15" fill="rgb(221,193,54)" fg:x="0" fg:w="1"/><text x="0.2500%" y="351.50"></text></g><g><title>[unknown] (1 samples, 0.02%)</title><rect 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src/main.rs

@@ -1,49 +1,23 @@
 use std::time::Instant;
 
+const BLOCK_SIZE: usize = 3;
+const SIZE: usize = BLOCK_SIZE * BLOCK_SIZE;
+const NUM_FIELDS: usize = SIZE * SIZE;
+
 struct SField {
-    field: Vec<usize>,
-    fixed: Vec<usize>,
-    size: usize,
-    block_size: usize,
+    field: Vec<u8>,
+    skipf: Vec<u8>,
+    skipb: Vec<u8>,
     pos: usize,
-    pos_last_placed: usize,
-    num_fields: usize,
-    possible_values: Vec<Vec<usize>>,
+    possible_values: Vec<Vec<u8>>,
 }
 
 impl SField {
-    pub fn new(block_size: usize) -> SField {
-        let size = block_size * block_size;
-
-        // EMPTY FIELD
-        // let field = vec![0; size * size];
-
-        // BLOCK_SIZE 4
-        // #[rustfmt::skip]
-        // let field = vec![
-        //     0,15,0,0,0,0,11,4,0,5,0,0,0,0,12,8,
-        //     12,0,0,9,0,1,0,5,0,0,8,15,0,0,0,13,
-        //     0,0,3,0,0,10,13,0,0,11,4,0,15,0,6,0,
-        //     13,10,0,11,0,0,0,14,0,3,2,0,0,9,0,0,
-        //     0,0,15,10,8,0,0,0,0,0,14,0,0,6,2,0,
-        //     0,0,14,0,0,0,6,0,0,0,0,0,1,0,0,0,
-        //     0,11,0,8,3,0,15,1,6,0,0,0,0,0,0,7,
-        //     0,6,0,7,0,0,0,0,8,13,0,0,10,0,0,0,
-        //     0,14,0,2,0,0,0,0,3,0,5,0,11,15,9,0,
-        //     0,0,0,12,11,0,2,0,0,0,0,0,0,0,0,0,
-        //     0,8,0,0,6,14,1,0,13,15,0,0,0,0,0,10,
-        //     10,0,0,3,9,0,7,0,0,1,0,0,13,12,8,0,
-        //     7,0,0,0,0,0,14,0,0,0,0,0,0,0,0,9,
-        //     0,3,0,14,0,0,9,6,0,0,0,0,0,4,0,0,
-        //     0,4,6,0,0,7,0,0,0,0,0,0,0,0,0,0,
-        //     5,2,0,0,0,4,10,15,1,0,3,0,0,0,7,0,
-        // ];
-
-        // MANY SOLUTIONS
+    pub fn new() -> SField {
         #[rustfmt::skip]
         let field = vec![
-            0,0,0,0,0,0,0,0,9,
-            0,0,0,0,8,9,0,2,0,
+            0,0,0,0,0,0,0,0,0,
+            0,0,0,0,8,9,0,0,0,
             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,
@@ -53,64 +27,64 @@ impl SField {
             0,0,8,2,0,0,0,6,0,
         ];
 
-        // HARD
-        // #[rustfmt::skip]
-        // let field = vec![
-        //     8,4,0,0,6,0,5,0,1,
-        //     0,0,0,0,0,3,0,4,0,
-        //     0,0,6,9,0,0,0,0,7,
-        //     0,2,0,7,1,0,0,0,6,
-        //     0,0,0,6,3,0,0,0,0,
-        //     9,0,0,0,0,0,0,5,0,
-        //     0,0,0,0,4,0,0,6,0,
-        //     2,0,0,0,0,0,1,8,0,
-        //     0,0,5,0,0,0,3,0,0,
-        // ];
+        fn find_fixed_streak_forward(mut idx: usize, field: &[u8]) -> u8 {
+            let mut fixed_count = 1;
+            idx += 1;
 
-        // EASY
-        // #[rustfmt::skip]
-        // let field = vec![
-        //     5,9,0,6,1,3,0,0,0,
-        //     0,0,0,9,0,0,5,0,0,
-        //     8,0,3,0,5,7,6,4,0,
-        //     0,7,5,0,0,0,4,0,6,
-        //     0,6,0,7,4,0,2,0,8,
-        //     2,0,8,0,0,0,7,5,3,
-        //     0,0,0,5,6,1,0,7,0,
-        //     0,0,1,0,7,0,9,0,0,
-        //     7,3,6,0,2,0,0,0,0,
-        // ];
-
-        let mut fixed = Vec::with_capacity(size * size);
-        field.iter().for_each(|e| {
-            if *e == 0 {
-                fixed.push(0);
-            } else {
-                fixed.push(1);
+            while idx < NUM_FIELDS && field[idx] > 0 {
+                fixed_count += 1;
+                idx += 1;
             }
-        });
+
+            fixed_count
+        }
+
+        fn find_fixed_streak_backward(mut idx: usize, field: &[u8]) -> u8 {
+            let mut fixed_count = 1;
+            idx -= 1;
+
+            while idx < NUM_FIELDS && field[idx] > 0 {
+                fixed_count += 1;
+                idx -= 1;
+            }
+
+            fixed_count
+        }
+
+        let mut skipf = vec![0; NUM_FIELDS + 1];
+        for (idx, nr) in field.iter().enumerate() {
+            match nr {
+                0 => skipf[idx] = *nr,
+                _ => skipf[idx] = find_fixed_streak_forward(idx, &field),
+            }
+        }
+
+        let mut skipb = vec![0; NUM_FIELDS + 1];
+        for (idx, nr) in field.iter().enumerate().rev() {
+            match nr {
+                0 => skipb[idx + 1] = *nr,
+                _ => skipb[idx + 1] = find_fixed_streak_backward(idx, &field),
+            }
+        }
 
         SField {
             field,
-            fixed,
-            size,
-            block_size,
+            skipf,
+            skipb,
             pos: 0,
-            pos_last_placed: 0,
-            num_fields: size * size,
-            possible_values: vec![vec![]; size * size],
+            possible_values: vec![vec![]; SIZE * SIZE],
         }
     }
 
     fn build_possible_values_db(&mut self) {
-        for idx in 0..self.num_fields {
-            if self.fixed[idx] == 1 {
+        for idx in 0..NUM_FIELDS {
+            if self.field[idx] != 0 {
                 continue;
             }
             self.pos = idx;
             // try all values between 1 and =self.size and remember the good ones
-            let mut good_ones = Vec::with_capacity(self.size);
-            for nr in 1..=self.size {
+            let mut good_ones = Vec::with_capacity(SIZE);
+            for nr in 1..=(SIZE as u8) {
                 if self.ok(nr) {
                     good_ones.push(nr);
                 }
@@ -121,99 +95,59 @@ impl SField {
     }
 
     pub fn solve_backtracking(&mut self) -> bool {
-        let mut found_solution = false;
         let mut num_solutions = 0;
 
-        //let mut loop_count = 0;
         loop {
-            //std::thread::sleep_ms(30);
-            //self.print_clear();
-            //self.print();
-            //loop_count += 1;
-            //if loop_count % 1000000 == 0 {
-            //self.print_clear();
-            //self.print();
-            //}
-            if self.is_end() {
-                found_solution = true;
-                num_solutions += 1;
-                if num_solutions % 100 == 0 {
-                    self.print_clear();
-                    println!("Solutions: {}", num_solutions);
-                    self.print();
-                }
-
-                if self.is_fixed() {
-                    continue;
-                }
+            if !self.put_valid_nr() {
                 self.clear_current_field();
-                self.prev();
-                self.goto_prev_free_field();
-            }
-
-            if self.is_fixed() {
-                self.next();
+                if !self.prev() {
+                    self.print_clear();
+                    self.print();
+                    println!("Number of solutions: {}", num_solutions);
+                    break;
+                }
                 continue;
             }
 
-            if self.goto_next_free_field() {
-                if self.put_valid_nr() {
-                    self.next();
-                    continue;
-                } else {
-                    //println!("put_valid_nr failed for pos {}", self.pos);
-                    //std::thread::sleep_ms(300);
-                    self.clear_current_field();
-                    if !self.prev() {
-                        println!("Number of solutions: {}", num_solutions);
-                        break;
-                    }
-                    if !self.goto_prev_free_field() {
-                        println!("Number of solutions: {}", num_solutions);
-                        break;
-                    }
+            if !self.next() {
+                num_solutions += 1;
+                if num_solutions % 10_000 == 0 {
+                    self.print_clear();
+                    self.print();
+                    println!("Number of solutions: {}", num_solutions);
                 }
+
+                self.clear_current_field();
+                self.prev();
             }
         }
 
-        found_solution
+        num_solutions > 0
     }
 
-    #[inline(always)]
     fn print_gray(&self) {
         print!("\x1b\x5b\x31\x3b\x33\x30\x6d");
     }
 
-    #[inline(always)]
     fn print_green(&self) {
         print!("\x1b\x5b\x31\x3b\x33\x32\x6d");
     }
 
-    #[inline(always)]
-    fn print_red(&self) {
-        print!("\x1b\x5b\x31\x3b\x33\x31\x6d");
-    }
-
-    #[inline(always)]
     fn print_neutral(&self) {
         print!("\x1b\x5b\x31\x3b\x30\x6d");
     }
 
-    #[inline(always)]
     fn print_clear(&self) {
         print!("\x1b\x5b\x48\x1b\x5b\x32\x4a");
     }
 
-    #[inline(always)]
     fn print(&self) {
-        for i in 0..self.num_fields {
-            if i != 0 && i % self.size == 0 {
+        for i in 0..NUM_FIELDS {
+            if i != 0 && i % SIZE == 0 {
                 println!();
             }
 
-            if i == self.pos_last_placed {
-                self.print_red();
-            } else if i == self.pos {
+            if i == self.pos {
                 self.print_green();
             } else if self.get_field_at_pos(i) == 0 {
                 self.print_gray();
@@ -221,57 +155,17 @@ impl SField {
 
             print!("{:2} ", self.get_field_at_pos(i));
 
-            if i == self.pos || i == self.pos_last_placed || self.get_field_at_pos(i) == 0 {
+            if i == self.pos || self.get_field_at_pos(i) == 0 {
                 self.print_neutral();
             }
         }
         println!();
     }
 
-    #[inline(always)]
     fn clear_current_field(&mut self) {
         self.set(0);
     }
 
-    #[inline(always)]
-    fn goto_prev_free_field(&mut self) -> bool {
-        while {
-            if !self.is_fixed() {
-                return true;
-            }
-            self.prev()
-        } {}
-
-        false
-    }
-
-    #[inline(always)]
-    fn goto_next_free_field(&mut self) -> bool {
-        while {
-            if !self.is_fixed() {
-                return true;
-            }
-            self.next()
-        } {}
-
-        false
-    }
-
-    #[inline(always)]
-    fn _put_valid_nr(&mut self) -> bool {
-        let current_nr = self.get_field_at_pos(self.pos);
-
-        for nr in current_nr..self.size + 1 {
-            if self.ok(nr) {
-                self.set(nr);
-                return true;
-            }
-        }
-
-        false
-    }
-
-    #[inline(always)]
     fn put_valid_nr(&mut self) -> bool {
         let current_nr = self.get_field_at_pos(self.pos);
 
@@ -293,33 +187,11 @@ impl SField {
         false
     }
 
-    #[inline(always)]
-    fn is_end(&self) -> bool {
-        !self.field.contains(&0)
-    }
-
-    #[inline(always)]
-    fn ok(&self, nr: usize) -> bool {
+    fn ok(&self, nr: u8) -> bool {
         self.block_ok(nr) && self.row_ok(nr) && self.col_ok(nr)
     }
 
-    #[inline(always)]
-    fn _pos_to_xy(&self) -> (usize, usize) {
-        let y = self.pos / self.size;
-        let x = self.pos % self.size;
-        (x + 1, y + 1)
-    }
-
-    #[inline(always)]
-    fn is_fixed(&self) -> bool {
-        // safety: self.pos can be used to index the field unchecked
-        //         since the only methods modifying self.pos are
-        //         `next()` and `prev()` and they do bounds checking
-        unsafe { *self.fixed.get_unchecked(self.pos) == 1 }
-    }
-
-    #[inline(always)]
-    fn get_field_at_pos(&self, pos: usize) -> usize {
+    fn get_field_at_pos(&self, pos: usize) -> u8 {
         // safety:
         //     TODO
         //     would need to mathematically explain that the calculations in
@@ -327,92 +199,85 @@ impl SField {
         unsafe { *self.field.get_unchecked(pos) }
     }
 
-    #[inline(always)]
-    fn set(&mut self, nr: usize) {
+    fn set(&mut self, nr: u8) {
         self.field[self.pos] = nr;
-        self.pos_last_placed = self.pos;
     }
 
-    #[inline(always)]
-    fn get_row(&self, row: &mut [usize; 9]) {
+    fn get_row(&self, row: &mut [u8; SIZE]) {
         for (idx, row_elem) in row.iter_mut().enumerate() {
-            *row_elem = self.get_field_at_pos((self.pos / self.size) * self.size + idx);
+            *row_elem = self.get_field_at_pos((self.pos / SIZE) * SIZE + idx);
         }
     }
 
-    #[inline(always)]
-    fn get_col(&self, col: &mut [usize; 9]) {
+    fn get_col(&self, col: &mut [u8; SIZE]) {
         for (idx, col_elem) in col.iter_mut().enumerate() {
-            *col_elem = self.get_field_at_pos(idx * self.size + self.pos % self.size);
+            *col_elem = self.get_field_at_pos(idx * SIZE + self.pos % SIZE);
         }
     }
 
-    #[inline(always)]
-    fn get_block(&self, block: &mut [usize; 9]) {
-        let block_start_row = self.pos / self.size / self.block_size * self.block_size;
-        let block_start_col = self.pos % self.size / self.block_size * self.block_size;
+    fn get_block(&self, block: &mut [u8; SIZE]) {
+        let block_start_row = self.pos / SIZE / BLOCK_SIZE * BLOCK_SIZE;
+        let block_start_col = self.pos % SIZE / BLOCK_SIZE * BLOCK_SIZE;
 
-        for r in 0..self.block_size {
-            for c in 0..self.block_size {
-                block[r * self.block_size + c] =
-                    self.get_field_at_pos(self.size * (block_start_row + r) + block_start_col + c);
+        for r in 0..BLOCK_SIZE {
+            for c in 0..BLOCK_SIZE {
+                block[r * BLOCK_SIZE + c] =
+                    self.get_field_at_pos(SIZE * (block_start_row + r) + block_start_col + c);
             }
         }
     }
 
-    #[inline(always)]
-    fn block_ok(&self, nr: usize) -> bool {
-        let mut block = [0; 9];
+    fn block_ok(&self, nr: u8) -> bool {
+        let mut block = [0; SIZE];
         self.get_block(&mut block);
         !block.contains(&nr)
     }
 
-    #[inline(always)]
-    fn row_ok(&self, nr: usize) -> bool {
-        let mut row = [0; 9];
+    fn row_ok(&self, nr: u8) -> bool {
+        let mut row = [0; SIZE];
         self.get_row(&mut row);
         !row.contains(&nr)
     }
 
-    #[inline(always)]
-    fn col_ok(&self, nr: usize) -> bool {
-        let mut col = [0; 9];
+    fn col_ok(&self, nr: u8) -> bool {
+        let mut col = [0; SIZE];
         self.get_col(&mut col);
         !col.contains(&nr)
     }
 
-    #[inline(always)]
     fn next(&mut self) -> bool {
-        if self.pos == self.num_fields - 1 {
+        let new_pos = self.pos + 1 + unsafe { *self.skipf.get_unchecked(self.pos + 1) as usize };
+
+        if new_pos >= NUM_FIELDS {
             return false;
         }
 
-        self.pos += 1;
+        self.pos = new_pos;
         true
     }
 
-    #[inline(always)]
     fn prev(&mut self) -> bool {
-        if self.pos == 0 {
+        let new_pos = self.pos - 1 - unsafe { *self.skipb.get_unchecked(self.pos) as usize };
+
+        if new_pos >= NUM_FIELDS {
             return false;
         }
 
-        self.pos -= 1;
+        self.pos = new_pos;
         true
     }
 
-    #[inline(always)]
     fn solve(&mut self) {
         let now = Instant::now();
         if !self.solve_backtracking() {
             println!("there is no solution.");
         }
-        println!("took {:.3}s", now.elapsed().as_secs_f64());
+        eprintln!("took {:.3}s", now.elapsed().as_secs_f64());
     }
 }
 
 fn run() -> Result<(), String> {
-    let mut field = SField::new(3);
+    let mut field = SField::new();
 
     field.build_possible_values_db();