(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.

src/main.rs

@@ -6,7 +6,8 @@ const NUM_FIELDS: usize = SIZE * SIZE;
 
 struct SField {
     field: Vec<u8>,
-    fixed: Vec<bool>,
+    skipf: Vec<u8>,
+    skipb: Vec<u8>,
     pos: usize,
     possible_values: Vec<Vec<u8>>,
 }
@@ -26,18 +27,50 @@ impl SField {
             0,0,8,2,0,0,0,6,0,
         ];
 
-        let mut fixed = Vec::with_capacity(SIZE * SIZE);
-        field.iter().for_each(|e| {
-            if *e == 0 {
-                fixed.push(false);
-            } else {
-                fixed.push(true);
+        fn find_fixed_streak_forward(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
+        }
+
+        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,
+            skipf,
+            skipb,
             pos: 0,
             possible_values: vec![vec![]; SIZE * SIZE],
         }
@@ -45,7 +78,7 @@ impl SField {
 
     fn build_possible_values_db(&mut self) {
         for idx in 0..NUM_FIELDS {
-            if self.fixed[idx] {
+            if self.field[idx] != 0 {
                 continue;
             }
             self.pos = idx;
@@ -63,71 +96,51 @@ impl SField {
 
     pub fn solve_backtracking(&mut self) -> bool {
         let mut num_solutions = 0;
-        loop {
-            if self.is_end() {
-                num_solutions += 1;
 
+        loop {
+            if !self.put_valid_nr() {
+                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 {
@@ -149,50 +162,10 @@ 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);
 
@@ -214,32 +187,10 @@ 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
@@ -248,26 +199,22 @@ 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;
@@ -280,48 +227,46 @@ 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 {
-        if self.pos == 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() {