rust version: backtracking simplified and other improvements

* backtracking: main loop simplified
* instead of the `fixed` Vec, we now use skip fields to calculate
  how many fields to skip in `next()` and `prev()`.
  This is faster because we get rid of the `if` statement
  thus no branching occurs.

.cargo/config.toml

@@ -1,3 +1,2 @@
 [build]
-target = "x86_64-unknown-linux-musl"
+rustflags = ["-C", "target-cpu=native", "-C", "target-feature=+crt-static"]
-rustflags = "-C target-cpu=native -C strip=symbols"

Cargo.toml

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

src/main.rs

@@ -2,120 +2,89 @@ 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<u8>,
-    fixed: Vec<bool>,
+    skipf: Vec<u8>,
-    size: usize,
+    skipb: Vec<u8>,
-    block_size: usize,
     pos: usize,
-    pos_last_placed: usize,
-    num_fields: usize,
     possible_values: Vec<Vec<u8>>,
 }
 
-/// Some test fields:
-///
-/// EMPTY FIELD
-/// let field = vec![0; SIZE * SIZE];
-///
-/// 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,
-/// ];
-///
-/// 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,
-/// ];
-///
-/// 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,
-/// ];
-///
-/// // BLOCK_SIZE 4 MANY SOLUTIONS (DOESN'T FINISH)
-/// #[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,
-/// ];
-
 impl SField {
     pub fn new() -> SField {
-        let field = vec![0; SIZE * SIZE];
+        #[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,
+        ];
 
-        let mut fixed = Vec::with_capacity(SIZE * SIZE);
+        fn find_fixed_streak_forward(mut idx: usize, field: &[u8]) -> u8 {
-        field.iter().for_each(|e| {
+            let mut fixed_count = 1;
-            if *e == 0 {
+            idx += 1;
-                fixed.push(false);
+
-            } else {
+            while idx < NUM_FIELDS && field[idx] > 0 {
-                fixed.push(true);
+                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];
+        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];
+        for (idx, nr) in field.iter().enumerate().rev() {
+            match nr {
+                0 => skipb[idx] = *nr,
+                _ => skipb[idx] = find_fixed_streak_backward(idx, &field),
+            }
+        }
 
         SField {
             field,
-            fixed,
+            skipf,
-            size: SIZE,
+            skipb,
-            block_size: BLOCK_SIZE,
             pos: 0,
-            pos_last_placed: 0,
-            num_fields: SIZE * SIZE,
             possible_values: vec![vec![]; SIZE * SIZE],
         }
     }
 
     fn build_possible_values_db(&mut self) {
-        for idx in 0..self.num_fields {
+        for idx in 0..NUM_FIELDS {
-            if self.fixed[idx] {
+            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);
+            let mut good_ones = Vec::with_capacity(SIZE);
-            for nr in 1..=(self.size as u8) {
+            for nr in 1..=(SIZE as u8) {
                 if self.ok(nr) {
                     good_ones.push(nr);
                 }
@@ -126,121 +95,59 @@ impl SField {
     }
 
     pub fn solve_backtracking(&mut self) -> bool {
-        let mut found_solution = false;
-
         let mut num_solutions = 0;
-        let mut last_num_solutions = num_solutions;
 
-        let mut loop_count = 0;
-        let mut last_loop_count = loop_count;
-
-        let mut now = Instant::now();
         loop {
-            loop_count += 1;
+            if !self.put_valid_nr() {
-            const MILLIS_PER_SEC: u128 = 1_000;
-            const UPDATE_DELAY_MS: u128 = 80;
-            if loop_count % 1000 == 0 {
-                let elapsed_ms = now.elapsed().as_millis();
-                if elapsed_ms <= UPDATE_DELAY_MS {
-                    continue;
-                }
-                self.print_clear();
-                self.print();
-                println!(
-                    "{} loops/sec",
-                    (loop_count - last_loop_count) * (MILLIS_PER_SEC / elapsed_ms)
-                );
-                println!(
-                    "{} solutions/sec",
-                    (num_solutions - last_num_solutions) * (MILLIS_PER_SEC / elapsed_ms)
-                );
-                last_loop_count = loop_count;
-                last_num_solutions = num_solutions;
-                now = Instant::now();
-            }
-            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();
+                if !self.prev() {
-                self.goto_prev_free_field();
+                    self.print_clear();
-            }
+                    self.print();
-
+                    println!("Number of solutions: {}", num_solutions);
-            if self.is_fixed() {
+                    break;
-                self.next();
+                }
                 continue;
             }
 
-            if self.goto_next_free_field() {
+            if !self.next() {
-                if self.put_valid_nr() {
+                num_solutions += 1;
-                    self.next();
+                if num_solutions % 10_000 == 0 {
-                    continue;
+                    self.print_clear();
-                } else {
+                    self.print();
-                    //println!("put_valid_nr failed for pos {}", self.pos);
+                    println!("Number of solutions: {}", num_solutions);
-                    //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();
             }
         }
 
-        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 {
+        for i in 0..NUM_FIELDS {
-            if i != 0 && i % self.size == 0 {
+            if i != 0 && i % SIZE == 0 {
                 println!();
             }
 
-            if i == self.pos_last_placed {
+            if i == self.pos {
-                self.print_red();
-            } else if i == self.pos {
                 self.print_green();
             } else if self.get_field_at_pos(i) == 0 {
                 self.print_gray();
@@ -248,57 +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) 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);
 
@@ -320,32 +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 / 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) }
-    }
-
-    #[inline(always)]
     fn get_field_at_pos(&self, pos: usize) -> u8 {
         // safety:
         //     TODO
@@ -354,81 +199,74 @@ impl SField {
         unsafe { *self.field.get_unchecked(pos) }
     }
 
-    #[inline(always)]
     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 [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 [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 [u8; SIZE]) {
-        let block_start_row = self.pos / self.size / self.block_size * self.block_size;
+        let block_start_row = self.pos / SIZE / BLOCK_SIZE * BLOCK_SIZE;
-        let block_start_col = self.pos % self.size / self.block_size * self.block_size;
+        let block_start_col = self.pos % SIZE / BLOCK_SIZE * BLOCK_SIZE;
 
-        for r in 0..self.block_size {
+        for r in 0..BLOCK_SIZE {
-            for c in 0..self.block_size {
+            for c in 0..BLOCK_SIZE {
-                block[r * self.block_size + c] =
+                block[r * BLOCK_SIZE + c] =
-                    self.get_field_at_pos(self.size * (block_start_row + r) + block_start_col + c);
+                    self.get_field_at_pos(SIZE * (block_start_row + r) + block_start_col + c);
             }
         }
     }
 
-    #[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 == 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 - 1) 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() {