bitsets for fast checking if a number fits (slower)

.gitignore

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

Cargo.lock

@@ -4,4 +4,4 @@ version = 3
 
 [[package]]
 name = "sudk"
-version = "0.1.1"
+version = "0.1.0"

Cargo.toml

@@ -1,19 +1,13 @@
 [package]
 name = "sudk"
-version = "0.1.1"
+version = "0.1.0"
 authors = ["ddidderr <ddidderr@paul.network>"]
 edition = "2021"
 
-[lints.clippy]
-pedantic = { level = "warn", priority = -1 }
-todo = "warn"
-unwrap_used = "warn"
-
 [dependencies]
 
 [profile.release]
 lto = true
 debug = false
-strip = true
-panic = "abort"
 codegen-units = 1
+panic = "abort"

c_version/clangtest

@@ -1,6 +0,0 @@
-#!/bin/bash
-set -e
-
-clang -Weverything -Wno-unsafe-buffer-usage -Werror -O3 -march=native -flto -std=c17 -fstack-protector-all -s -o sudk_c "$1"
-
-time ./sudk_c

c_version/gcctest

@@ -1,7 +0,0 @@
-#!/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

@@ -1,287 +0,0 @@
-#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

@@ -1,278 +0,0 @@
-#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

@@ -1,266 +0,0 @@
-#include <stdbool.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/types.h>
-
-static inline void Print_Clear(void) { 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(void) {
-  Sudoku *s = Sudoku_New(3);
-  Sudoku_Print(s);
-  Sudoku_SolveBacktracking(s);
-  Sudoku_Free(s);
-}

rustfmt.toml

@@ -1,3 +0,0 @@
-group_imports = "StdExternalCrate"
-imports_granularity = "Crate"
-imports_layout = "HorizontalVertical"

src/main.rs

@@ -1,114 +1,249 @@
 use std::time::Instant;
 
-const BLOCK_SIZE: usize = 3;
-const SIZE: usize = BLOCK_SIZE * BLOCK_SIZE;
-const NUM_FIELDS: usize = SIZE * SIZE;
-
-fn print_gray() {
-    print!("\x1b\x5b\x31\x3b\x33\x30\x6d");
+#[derive(Copy, Clone, Debug)]
+struct Bitset {
+    set: u16,
 }
 
-fn print_green() {
-    print!("\x1b\x5b\x31\x3b\x33\x32\x6d");
+impl Bitset {
+    pub fn new() -> Self {
+        Self { set: 0 }
+    }
+
+    #[inline(always)]
+    fn get(&self, nr: usize) -> bool {
+        self.set & (1 << nr) != 0
+    }
+
+    #[inline(always)]
+    fn set(&mut self, nr: usize) {
+        self.set |= 1 << nr;
+    }
+
+    #[inline(always)]
+    fn unset(&mut self, nr: usize) {
+        self.set &= !(1 << nr);
+    }
 }
 
-fn print_neutral() {
-    print!("\x1b\x5b\x31\x3b\x30\x6d");
-}
-
-fn print_clear() {
-    print!("\x1b\x5b\x48\x1b\x5b\x32\x4a");
-}
+const BLOCK_SIZE: usize = 4;
+//const MAX_BLOCK_SIZE: usize = 10;
 
 struct SField {
-    field: Vec<u8>,
-    skip_forward: Vec<u8>,
-    skip_backward: Vec<u8>,
+    field: Vec<usize>,
+    fixed: Vec<usize>,
+    size: usize,
     pos: usize,
-    possible_values: Vec<Vec<u8>>,
+    pos_last_placed: usize,
+    num_fields: usize,
+    possible_values: Vec<Vec<usize>>,
+
+    // For each row/col/block we create a bitset that represents the numbers
+    // that are already in that row, col or block.
+    // This way we can check very fast if a nr fits into a specific field or not.
+    rows: [Bitset; BLOCK_SIZE * BLOCK_SIZE],
+    cols: [Bitset; BLOCK_SIZE * BLOCK_SIZE],
+    blocks: [Bitset; BLOCK_SIZE * BLOCK_SIZE],
+}
+
+#[inline(always)]
+fn xy_to_pos(x: usize, y: usize, size: usize) -> usize {
+    y * size + x
+}
+
+#[inline(always)]
+fn pos_to_xy_blocknr(pos: usize, size: usize) -> (usize, usize, usize) {
+    let x = pos % size;
+    let y = pos / size;
+
+    let block_x = x / BLOCK_SIZE;
+    let block_y = y / BLOCK_SIZE;
+
+    let block_nr = block_y * BLOCK_SIZE + block_x;
+
+    (x, y, block_nr)
 }
 
 impl SField {
     pub fn new() -> SField {
-        #[rustfmt::skip]
-        let field = vec![
-            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,
-            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,
-        ];
+        const SIZE: usize = BLOCK_SIZE * BLOCK_SIZE;
 
-        let mut sudoku = SField {
-            field,
-            skip_forward: vec![0; NUM_FIELDS + 1],
-            skip_backward: vec![0; NUM_FIELDS + 1],
-            pos: 0,
-            possible_values: vec![vec![]; NUM_FIELDS],
+        // EMPTY FIELD
+        // let field = vec![0; size * size];
+
+        #[rustfmt::skip]
+        let field = match BLOCK_SIZE {
+            4 => vec![
+                0usize,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,
+            ],
+            3 => vec![
+                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,
+            ],
+            _ => panic!("Unsupported block size. Only 3 and 4 is ok for now.")
         };
 
-        sudoku.build_possible_values_db();
+        // MANY SOLUTIONS
+        // #[rustfmt::skip]
+        // let field = vec![
+        //     0,0,0,0,0,0,0,0,9,
+        //     0,0,0,0,8,9,0,2,0,
+        //     0,0,0,0,2,0,4,0,0,
+        //     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,
+        // ];
 
-        sudoku.build_skip_dbs();
+        // 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,
+        // ];
 
-        sudoku
-    }
+        // 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,
+        // ];
 
-    fn build_skip_dbs(&mut self) {
-        fn find_fixed_streak_forward(mut idx: usize, field: &[u8]) -> u8 {
-            let mut fixed_count = 1;
-            idx += 1;
+        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;
+        // create bitsets
+        // rows
+        let mut bit_rows = [Bitset::new(); SIZE];
+        for row_nr in 0..SIZE {
+            let mut row = [0; SIZE];
+            for count in 0..SIZE {
+                row[count] = field[xy_to_pos(count, row_nr, SIZE)];
             }
 
-            fixed_count
+            row.into_iter()
+                .filter(|val| *val != 0)
+                .for_each(|val| bit_rows[row_nr].set(val - 1));
         }
 
-        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;
+        // cols
+        let mut bit_cols = [Bitset::new(); SIZE];
+        for col_nr in 0..SIZE {
+            let mut col = [0; SIZE];
+            for count in 0..SIZE {
+                col[count] = field[xy_to_pos(col_nr, count, SIZE)];
             }
 
-            fixed_count
+            col.into_iter()
+                .filter(|val| *val != 0)
+                .for_each(|val| bit_cols[col_nr].set(val - 1));
         }
 
-        let fwd = &mut self.skip_forward;
-        for (idx, nr) in self.field.iter().enumerate() {
-            match nr {
-                0 => fwd[idx] = *nr,
-                _ => fwd[idx] = find_fixed_streak_forward(idx, &self.field),
+        // blocks
+        let mut bit_blocks = [Bitset::new(); SIZE];
+
+        for block_nr in 0..SIZE {
+            let block_start_x = block_nr * BLOCK_SIZE % SIZE;
+            let block_start_y = block_nr * BLOCK_SIZE / SIZE * BLOCK_SIZE;
+
+            let mut block = [0; SIZE];
+            for count in 0..SIZE {
+                let block_offset_x = count % BLOCK_SIZE;
+                let block_offset_y = count / BLOCK_SIZE;
+
+                let field_x = block_start_x + block_offset_x;
+                let field_y = block_start_y + block_offset_y;
+
+                //dbg!(
+                //block_nr,
+                //block_start_x,
+                //block_start_y,
+                //block_offset_x,
+                //block_offset_y,
+                //field_x,
+                //field_y,
+                //);
+
+                block[count] = field[xy_to_pos(field_x, field_y, SIZE)];
             }
+
+            block
+                .into_iter()
+                .filter(|x| *x != 0)
+                .for_each(|x| bit_blocks[block_nr].set(x - 1));
         }
 
-        let bwd = &mut self.skip_backward;
-        for (idx, nr) in self.field.iter().enumerate().rev() {
-            match nr {
-                0 => bwd[idx + 1] = *nr,
-                _ => bwd[idx + 1] = find_fixed_streak_backward(idx, &self.field),
-            }
+        //dbg!(bit_rows);
+        //dbg!(bit_cols);
+        //dbg!(bit_blocks);
+
+        SField {
+            field,
+            fixed,
+            size: SIZE,
+            pos: 0,
+            pos_last_placed: 0,
+            num_fields: SIZE * SIZE,
+            possible_values: vec![vec![]; SIZE * SIZE],
+            rows: bit_rows,
+            cols: bit_cols,
+            blocks: bit_blocks,
         }
     }
 
     fn build_possible_values_db(&mut self) {
-        for idx in 0..NUM_FIELDS {
-            if self.field[idx] != 0 {
+        for idx in 0..self.num_fields {
+            if self.fixed[idx] == 1 {
                 continue;
             }
             self.pos = idx;
             // try all values between 1 and =self.size and remember the good ones
-            let mut good_ones = Vec::with_capacity(SIZE);
-            for nr in 1..=(u8::try_from(SIZE).expect("SIZE is too big")) {
+            let mut good_ones = Vec::with_capacity(self.size);
+            for nr in 1..=self.size {
                 if self.ok(nr) {
                     good_ones.push(nr);
                 }
@@ -119,87 +254,208 @@ impl SField {
     }
 
     pub fn solve_backtracking(&mut self) -> bool {
+        let mut found_solution = false;
         let mut num_solutions = 0;
 
+        let mut loop_count = 0;
+        let mut last_loop_count = 0;
+        let mut now = Instant::now();
         loop {
-            if !self.put_valid_nr() {
-                self.clear_current_field();
-                if !self.prev() {
-                    print_clear();
-                    self.print();
-                    println!("Number of solutions: {num_solutions}");
-                    break;
+            //std::thread::sleep_ms(30);
+            //self.print_clear();
+            //self.print();
+            loop_count += 1;
+            if now.elapsed().as_millis() >= 1000 {
+                now = Instant::now();
+                self.print_clear();
+                self.print();
+                println!("{} loops/s", loop_count - last_loop_count);
+                last_loop_count = loop_count;
+            }
+            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;
                 }
+                self.clear_current_field();
+                self.prev();
+                self.goto_prev_free_field();
+            }
+
+            if self.is_fixed() {
+                self.next();
                 continue;
             }
 
-            if !self.next() {
-                num_solutions += 1;
-                if num_solutions % 10_000 == 0 {
-                    print_clear();
-                    self.print();
-                    println!("Number of solutions: {num_solutions}");
+            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;
+                    }
                 }
-
-                self.clear_current_field();
-                self.prev();
             }
         }
 
-        num_solutions > 0
+        found_solution
     }
 
+    #[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..NUM_FIELDS {
-            if i != 0 && i % SIZE == 0 {
+        for i in 0..self.num_fields {
+            if i != 0 && i % self.size == 0 {
                 println!();
             }
 
-            if i == self.pos {
-                print_green();
+            if i == self.pos_last_placed {
+                self.print_red();
+            } else if i == self.pos {
+                self.print_green();
             } else if self.get_field_at_pos(i) == 0 {
-                print_gray();
+                self.print_gray();
             }
 
             print!("{:2} ", self.get_field_at_pos(i));
 
-            if i == self.pos || self.get_field_at_pos(i) == 0 {
-                print_neutral();
+            if i == self.pos || i == self.pos_last_placed || 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);
 
         // safety:
         //     self.possible_vals is initialized with self.size * self.size
         //     so self.pos can safely be used to index here
-        let possible_vals = &self.possible_values[self.pos];
+        let possible_vals = unsafe { self.possible_values.get_unchecked(self.pos) };
 
-        for nr in possible_vals {
+        for nr in possible_vals.iter() {
             if *nr <= current_nr {
                 continue;
             }
 
-            if self.ok(*nr) {
-                self.set(*nr);
+            let nr = *nr;
+            if self.ok(nr) {
+                self.set(nr);
                 return true;
             }
         }
         false
     }
 
-    fn ok(&self, nr: u8) -> bool {
-        self.block_ok(nr) && self.row_ok(nr) && self.col_ok(nr)
+    #[inline(always)]
+    fn is_end(&self) -> bool {
+        !self.field.contains(&0)
     }
 
-    fn get_field_at_pos(&self, pos: usize) -> u8 {
+    #[inline(always)]
+    fn ok(&self, nr: usize) -> bool {
+        //self.block_ok(nr) && self.row_ok(nr) && self.col_ok(nr)
+
+        let (x, y, block_nr) = pos_to_xy_blocknr(self.pos, self.size);
+
+        !self.rows[y].get(nr - 1) && !self.cols[x].get(nr - 1) && !self.blocks[block_nr].get(nr - 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 {
         // safety:
         //     TODO
         //     would need to mathematically explain that the calculations in
@@ -207,84 +463,125 @@ impl SField {
         unsafe { *self.field.get_unchecked(pos) }
     }
 
-    fn set(&mut self, nr: u8) {
-        self.field[self.pos] = nr;
+    #[inline(always)]
+    fn set(&mut self, nr: usize) {
+        let old = unsafe { self.field.get_unchecked_mut(self.pos) };
+
+        // if another number > 0 was in that cell, we need to remove that number from our bitsets
+        if *old > 0 {
+            let (x, y, block_nr) = pos_to_xy_blocknr(self.pos, self.size);
+            self.rows[y].unset(*old - 1);
+            self.cols[x].unset(*old - 1);
+            self.blocks[block_nr].unset(*old - 1);
+        }
+
+        // add new nr to our bitsets if it is a meaningful (> 0) number
+        if nr > 0 {
+            let (x, y, block_nr) = pos_to_xy_blocknr(self.pos, self.size);
+            self.rows[y].set(nr - 1);
+            self.cols[x].set(nr - 1);
+            self.blocks[block_nr].set(nr - 1);
+        }
+
+        // write the new number into the field
+        *old = nr;
+
+        // remember the last placed position for debugging purposes
+        self.pos_last_placed = self.pos;
     }
 
-    fn get_row(&self, row: &mut [u8; SIZE]) {
+    /*
+    #[inline(always)]
+    fn get_row(&self, row: &mut [usize]) {
         for (idx, row_elem) in row.iter_mut().enumerate() {
-            *row_elem = self.get_field_at_pos((self.pos / SIZE) * SIZE + idx);
+            *row_elem = self.get_field_at_pos((self.pos / self.size) * self.size + idx);
         }
     }
 
-    fn get_col(&self, col: &mut [u8; SIZE]) {
+    #[inline(always)]
+    fn get_col(&self, col: &mut [usize]) {
         for (idx, col_elem) in col.iter_mut().enumerate() {
-            *col_elem = self.get_field_at_pos(idx * SIZE + self.pos % SIZE);
+            *col_elem = self.get_field_at_pos(idx * self.size + self.pos % self.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;
+    #[inline(always)]
+    fn get_block(&self, block: &mut [usize]) {
+        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;
 
-        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);
+        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);
             }
         }
     }
 
-    fn block_ok(&self, nr: u8) -> bool {
-        let mut block = [0; SIZE];
-        self.get_block(&mut block);
+    #[inline(always)]
+    fn block_ok(&self, nr: usize) -> bool {
+        let mut block = [0; MAX_BLOCK_SIZE * MAX_BLOCK_SIZE];
+        self.get_block(&mut block[0..self.size]);
         !block.contains(&nr)
     }
 
-    fn row_ok(&self, nr: u8) -> bool {
-        let mut row = [0; SIZE];
-        self.get_row(&mut row);
+    #[inline(always)]
+    fn row_ok(&self, nr: usize) -> bool {
+        let mut row = [0; MAX_BLOCK_SIZE * MAX_BLOCK_SIZE];
+        self.get_row(&mut row[0..self.size]);
         !row.contains(&nr)
     }
 
-    fn col_ok(&self, nr: u8) -> bool {
-        let mut col = [0; SIZE];
-        self.get_col(&mut col);
+    #[inline(always)]
+    fn col_ok(&self, nr: usize) -> bool {
+        let mut col = [0; MAX_BLOCK_SIZE * MAX_BLOCK_SIZE];
+        self.get_col(&mut col[0..self.size]);
         !col.contains(&nr)
     }
+    */
 
+    #[inline(always)]
     fn next(&mut self) -> bool {
-        let new_pos = self.pos + 1 + self.skip_forward[self.pos + 1] as usize;
-
-        if new_pos >= NUM_FIELDS {
+        if self.pos == self.num_fields - 1 {
             return false;
         }
 
-        self.pos = new_pos;
+        //println!("next {} -> {}", self.pos, self.pos + 1);
+        self.pos += 1;
         true
     }
 
+    #[inline(always)]
     fn prev(&mut self) -> bool {
-        let new_pos = self.pos - 1 - self.skip_backward[self.pos] as usize;
-
-        if new_pos >= NUM_FIELDS {
+        if self.pos == 0 {
             return false;
         }
 
-        self.pos = new_pos;
+        self.pos -= 1;
         true
     }
 
+    #[inline(always)]
     fn solve(&mut self) {
         let now = Instant::now();
         if !self.solve_backtracking() {
             println!("there is no solution.");
         }
-        eprintln!("took {:.3}s", now.elapsed().as_secs_f64());
+        println!("took {:.3}s", now.elapsed().as_secs_f64());
     }
 }
 
-fn main() {
+fn run() -> Result<(), String> {
     let mut field = SField::new();
+
+    field.build_possible_values_db();
+
     field.solve();
+    Ok(())
+}
+
+fn main() {
+    if let Err(e) = run() {
+        println!("{}", e)
+    }
 }