rust version: old method comparable to method in main

src/main.rs

@@ -6,8 +6,7 @@ const NUM_FIELDS: usize = SIZE * SIZE;
 
 struct SField {
     field: Vec<u8>,
-    skipf: Vec<u8>,
+    fixed: Vec<bool>,
-    skipb: Vec<u8>,
     pos: usize,
     possible_values: Vec<Vec<u8>>,
 }
@@ -27,50 +26,18 @@ impl SField {
             0,0,8,2,0,0,0,6,0,
         ];
 
-        fn find_fixed_streak_forward(mut idx: usize, field: &[u8]) -> u8 {
+        let mut fixed = Vec::with_capacity(SIZE * SIZE);
-            let mut fixed_count = 1;
+        field.iter().for_each(|e| {
-            idx += 1;
+            if *e == 0 {
-
+                fixed.push(false);
-            while idx < NUM_FIELDS && field[idx] > 0 {
+            } else {
-                fixed_count += 1;
+                fixed.push(true);
-                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,
-            skipf,
+            fixed,
-            skipb,
             pos: 0,
             possible_values: vec![vec![]; SIZE * SIZE],
         }
@@ -78,7 +45,7 @@ impl SField {
 
     fn build_possible_values_db(&mut self) {
         for idx in 0..NUM_FIELDS {
-            if self.field[idx] != 0 {
+            if self.fixed[idx] {
                 continue;
             }
             self.pos = idx;
@@ -96,51 +63,71 @@ impl SField {
 
     pub fn solve_backtracking(&mut self) -> bool {
         let mut num_solutions = 0;
-
         loop {
-            if !self.put_valid_nr() {
+            if self.is_end() {
-                self.clear_current_field();
-                if !self.prev() {
-                    self.print_clear();
-                    self.print();
-                    println!("Number of solutions: {}", num_solutions);
-                    break;
-                }
-                continue;
-            }
-
-            if !self.next() {
                 num_solutions += 1;
+
                 if num_solutions % 10_000 == 0 {
                     self.print_clear();
                     self.print();
                     println!("Number of solutions: {}", num_solutions);
                 }
 
+                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.goto_next_free_field() {
+                if self.put_valid_nr() {
+                    self.next();
+                    continue;
+                } else {
+                    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;
+                    }
+                }
             }
         }
 
         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_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 {
@@ -162,10 +149,50 @@ impl SField {
         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..(SIZE + 1) as u8 {
+            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);
 
@@ -187,10 +214,32 @@ impl SField {
         false
     }
 
+    #[inline(always)]
+    fn is_end(&self) -> bool {
+        !self.field.contains(&0)
+    }
+
+    #[inline(always)]
     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 / SIZE;
+        let x = self.pos % 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) }
+    }
+
+    #[inline(always)]
     fn get_field_at_pos(&self, pos: usize) -> u8 {
         // safety:
         //     TODO
@@ -199,22 +248,26 @@ impl SField {
         unsafe { *self.field.get_unchecked(pos) }
     }
 
+    #[inline(always)]
     fn set(&mut self, nr: u8) {
         self.field[self.pos] = nr;
     }
 
+    #[inline(always)]
     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 / SIZE) * SIZE + idx);
         }
     }
 
+    #[inline(always)]
     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 * SIZE + self.pos % SIZE);
         }
     }
 
+    #[inline(always)]
     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;
@@ -227,46 +280,48 @@ impl SField {
         }
     }
 
+    #[inline(always)]
     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: u8) -> bool {
         let mut row = [0; SIZE];
         self.get_row(&mut row);
         !row.contains(&nr)
     }
 
+    #[inline(always)]
     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 {
-        let new_pos = self.pos + 1 + unsafe { *self.skipf.get_unchecked(self.pos + 1) as usize };
+        if self.pos == NUM_FIELDS - 1 {
-
-        if new_pos >= NUM_FIELDS {
             return false;
         }
 
-        self.pos = new_pos;
+        self.pos += 1;
         true
     }
 
+    #[inline(always)]
     fn prev(&mut self) -> bool {
-        let new_pos = self.pos - 1 - unsafe { *self.skipb.get_unchecked(self.pos) as usize };
+        if self.pos == 0 {
-
-        if new_pos >= NUM_FIELDS {
             return false;
         }
 
-        self.pos = new_pos;
+        self.pos -= 1;
         true
     }
 
+    #[inline(always)]
     fn solve(&mut self) {
         let now = Instant::now();
         if !self.solve_backtracking() {