Windoku Solver - Free Online Hyper Sudoku Puzzle Game

Windoku (also known as Hyper Sudoku, Window Sudoku, or NRC Sudoku) elevates classic Sudoku by adding four extra overlapping 3×3 shaded regions called "windows." These additional constraints mean you must satisfy 31 total constraint units: 9 rows, 9 columns, 9 traditional boxes, AND 4 window regions. The four windows are positioned at coordinates creating maximum strategic complexity—overlapping with traditional boxes to produce hidden interaction constraints that demand advanced solving techniques.

Our interactive Windoku calculator features computer-generated puzzles optimized for the window constraint system, multiple difficulty levels, real-time validation highlighting all four window regions, strategic hints, and comprehensive tutorials to help you master this challenging and intellectually satisfying Sudoku variant.

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The Four Window Regions (Shaded in Blue)

Top-Left Window
Rows 2-4, Cols 2-4

Top-Right Window
Rows 2-4, Cols 6-8

Bottom-Left Window
Rows 6-8, Cols 2-4

Bottom-Right Window
Rows 6-8, Cols 6-8

Time

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Hints

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Constraints

How to Play Windoku (Hyper Sudoku)

Windoku extends standard Sudoku rules by adding four extra 3×3 window regions that must also contain numbers 1-9:

Row Constraint: Each of the nine horizontal rows must contain numbers 1-9 exactly once (standard Sudoku rule)
Column Constraint: Each of the nine vertical columns must contain numbers 1-9 exactly once (standard Sudoku rule)
Box Constraint: Each of the nine 3×3 traditional boxes must contain numbers 1-9 exactly once (standard Sudoku rule)
Window Constraint (NEW): Each of the four shaded 3×3 window regions must also contain numbers 1-9 exactly once
Window Positions: Top-Left (rows 2-4, cols 2-4), Top-Right (rows 2-4, cols 6-8), Bottom-Left (rows 6-8, cols 2-4), Bottom-Right (rows 6-8, cols 6-8)
Overlapping Constraints: Cells within windows must satisfy row, column, traditional box, AND window constraints simultaneously
Total Constraints: 31 constraint units (9 rows + 9 columns + 9 boxes + 4 windows) create significantly more logical leverage than standard Sudoku

Mathematical Structure of Windoku (Hyper Sudoku)

Let \(W_1, W_2, W_3, W_4\) represent the four window regions. For the 9×9 grid with value \(a_{ij}\) at position \((i,j)\), Windoku satisfies all standard Sudoku constraints PLUS:

\(\forall k \in \{1,2,3,4\}: |\{a_{ij} : (i,j) \in W_k\}| = 9\) (Window uniqueness)

Window positions: \(W_1 = \{(i,j) : i \in [2,4], j \in [2,4]\}\), \(W_2 = \{(i,j) : i \in [2,4], j \in [6,8]\}\)

\(W_3 = \{(i,j) : i \in [6,8], j \in [2,4]\}\), \(W_4 = \{(i,j) : i \in [6,8], j \in [6,8]\}\)

Total constraints: \(C_{total} = 9 + 9 + 9 + 4 = 31\) (but creates 5 hidden cross-constraints)

The strategic window placement creates 36 cells (44% of the grid) subject to both traditional box AND window constraints, producing dramatically increased logical leverage.

Essential Solving Strategies for Windoku

Beginner-Level Techniques

1. Naked Singles with Window Awareness

Apply standard naked singles but remember to check FOUR constraints: row, column, traditional box, AND window region (if the cell is in a window). Cells within windows are more constrained and often become naked singles earlier in solving. Always verify window membership before finalizing placements.

2. Hidden Singles in Windows

For each number 1-9, scan not just rows, columns, and boxes, but also the four window regions. If a number can only fit in one location within a window, place it immediately. Windows often reveal hidden singles missed by standard box-only analysis. This is THE signature technique for Windoku.

3. Box-Window Intersection Analysis

Each window overlaps with four different traditional 3×3 boxes. When a candidate in a window appears only in cells that also belong to one traditional box, you can eliminate that candidate from the rest of that box outside the window. This reciprocal elimination works both ways: box-to-window and window-to-box.

4. Window-Priority Scanning

Begin solving by focusing on the four window regions before deeply analyzing traditional boxes. Windows create tighter constraints due to their overlapping positions. A breakthrough in one window often cascades through adjacent windows and boxes, creating rapid progress in early stages.

Intermediate Window-Specific Strategies

Pointing Pairs/Triples in Windows

When a candidate appears exactly twice (or three times) in a window and all occurrences lie in the same row or column, that candidate can be eliminated from the rest of that row or column outside the window. The strategic window positions make pointing pairs more frequent and powerful in Windoku compared to standard Sudoku.

Naked Pairs Across Window-Box Overlaps

Naked pairs can span both window and traditional box constraints. Two cells in a window containing only the same two candidates eliminate those candidates from other cells in both the window AND the traditional box they belong to. Track both constraint types simultaneously to maximize eliminations.

Cross-Window Relationships

The four windows don't overlap with each other, but they interact through shared rows and columns. A placement in the top-left window affects the bottom-left window through columns 2-4, and affects the top-right window through rows 2-4. Map these cross-window connections to trace cascading implications.

Hidden Constraint Exploitation

The four window positions create five "hidden constraints"—additional intersection patterns beyond the explicit 31 constraints. These hidden patterns emerge from the specific geometric arrangement of windows. Advanced solvers learn to recognize and exploit these implicit constraint interactions for breakthrough placements.

Advanced Expert Techniques

Multi-Unit Forcing Chains

In Windoku, forcing chains can span five constraint types: rows, columns, traditional boxes, window regions, and hidden constraints. Start with a candidate assumption in a window cell and trace implications through all five constraint layers. If contradictions appear in any layer, the candidate is FALSE. This powerful technique leverages Windoku's dense constraint network.

X-Wing Patterns Through Windows

X-Wing patterns (four cells forming a rectangle where a candidate appears exactly twice in two rows and two columns) remain valid in Windoku. However, verify that window constraints don't invalidate the pattern. Windows can sometimes prevent X-Wing eliminations if the wing cells share a window region. Check all five constraint types before eliminating.

Window-Based XY-Wing

XY-Wing patterns (three cells with candidates XY, XZ, YZ) gain additional power in Windoku. The pivot cell and wing cells can form Y-shapes through window regions in addition to traditional row/column/box connections. Window membership creates more potential XY-Wing configurations, especially in cells belonging to multiple constraint units.

Swordfish Across Windows

Swordfish patterns (three rows/columns with a candidate appearing in specific columns/rows forming a 3×3 pattern) work in Windoku but require careful verification that windows don't break the pattern. When valid, Swordfish eliminations apply across all constraint types. The increased constraint density makes Swordfish rarer but more impactful in Windoku.

The Unique Power of Window Constraints

What makes Windoku fundamentally different from standard Sudoku is the strategic positioning of the four windows. These aren't randomly placed—their specific coordinates were mathematically chosen to maximize constraint interactions while maintaining puzzle solvability.

Understanding the Window Geometry

Each 3×3 window occupies 9 cells at precisely calculated positions:

Top-Left Window: Rows 2-4, Columns 2-4 (overlaps with boxes 1, 2, 4, 5)
Top-Right Window: Rows 2-4, Columns 6-8 (overlaps with boxes 2, 3, 5, 6)
Bottom-Left Window: Rows 6-8, Columns 2-4 (overlaps with boxes 4, 5, 7, 8)
Bottom-Right Window: Rows 6-8, Columns 6-8 (overlaps with boxes 5, 6, 8, 9)

This arrangement means Box 5 (the center box) overlaps with ALL FOUR windows, creating a "nexus" of constraint interactions. Boxes 1, 3, 7, and 9 (the corners) don't overlap with any windows. Boxes 2, 4, 6, and 8 (the edges) each overlap with two windows. This creates a three-tier constraint hierarchy:

Tier 1 (High Constraint): Box 5 + 4 windows = cells belong to box + row + column + window
Tier 2 (Medium Constraint): Edge boxes 2, 4, 6, 8 = some cells in windows, some not
Tier 3 (Standard Constraint): Corner boxes 1, 3, 7, 9 = standard Sudoku constraints only

Windoku vs Standard Sudoku

Understanding the specific differences helps adapt your solving approach:

Feature	Standard Sudoku	Windoku (Hyper Sudoku)
Constraint Units	27 (9 rows + 9 cols + 9 boxes)	31 (27 standard + 4 windows)
Hidden Constraints	0 additional	5 additional from window interactions
Cells in Extra Regions	0	36 cells (44% of grid)
Box 5 (Center) Importance	Equal to other boxes	Overlaps all 4 windows—critical nexus
Minimum Givens Known	17 clues	11 clues (proven minimum)
Typical Difficulty Increase	Baseline	+20-30% harder at same clue count
Solving Time (Same Clues)	Baseline	+15-25% due to additional constraint checking

Computational Aspects and Generation

Generating valid Windoku puzzles requires modified algorithms compared to standard Sudoku:

Generation Algorithm Modifications

Enhanced backtracking: The solver must check 31 constraints instead of 27 at each recursive step, slightly increasing computation time
Window-aware clue placement: Clues should be strategically distributed across window and non-window regions for balanced difficulty
Uniqueness verification: The additional constraints actually make uniqueness verification faster—more constraints mean fewer alternative solutions to check
Difficulty calibration: Fewer givens are needed for unique solutions due to the extra constraints. A Windoku with 28 clues may be harder than a standard Sudoku with 24 clues

Minimum Givens

The proven minimum number of givens for a unique Windoku solution is 11 clues—significantly fewer than standard Sudoku's 17-clue minimum. This counterintuitive result occurs because the four windows add constraint leverage that reduces the number of givens needed to constrain the solution space to a single possibility.

However, 11-clue Windoku puzzles are extremely rare and may not be humanly solvable without advanced techniques or guessing. Most published Windoku puzzles contain 24-42 givens for practical solvability through logical deduction.

The Five Hidden Constraints

Beyond the explicit 31 constraints, the specific window positioning creates five "hidden" or implicit constraint patterns:

Hidden Constraint 1: The central 3×3 region (rows 4-6, cols 4-6) forms a pseudo-box from window overlap interactions
Hidden Constraint 2: Vertical "channel" (column 5) connects all four windows through Box 5, creating a constraint spine
Hidden Constraint 3: Horizontal "channel" (row 5) connects all four windows through Box 5, creating a constraint spine
Hidden Constraint 4: Diagonal patterns through Box 5 create symmetry relationships between opposite windows
Hidden Constraint 5: The "frame" formed by boxes 1, 3, 7, 9 (corners without windows) creates isolation patterns

Expert Windoku solvers consciously exploit these hidden constraints. When progress stalls using standard techniques, analyzing the hidden constraint patterns often reveals breakthrough placements. The most powerful hidden constraint is the central pseudo-box—while not explicitly a constraint unit, the window overlaps through Box 5 create box-like elimination patterns in this region.

Common Mistakes and How to Avoid Them

Mistake #1: Forgetting Window Constraints

Error: Placing a number that satisfies row, column, and traditional box constraints but violates a window constraint. Prevention: Before every placement, explicitly check: "Is this cell in a window? If yes, does the number already appear in that window?" Use the blue shading as a constant visual reminder of window regions.

Mistake #2: Not Exploiting Window-First Strategy

Error: Treating Windoku like standard Sudoku by focusing on boxes before windows. Prevention: Reverse your priority. Scan the four windows BEFORE analyzing traditional boxes. Windows provide more constraint leverage early in solving. Only after exhausting window-based placements should you shift to corner boxes.

Mistake #3: Missing Box-Window Intersections

Error: Failing to recognize when a candidate in a window is confined to cells that also belong to one traditional box, missing elimination opportunities. Prevention: For each window, systematically check which traditional boxes it overlaps with. When candidates cluster in the overlap zones, apply box-line reduction techniques in BOTH directions.

Mistake #4: Ignoring Hidden Constraints

Error: Only using the explicit 31 constraints and missing the five hidden constraint patterns. Prevention: When stuck, specifically analyze: (1) The central pseudo-box region, (2) Row 5 and Column 5 as constraint spines, (3) Symmetry patterns between opposite windows. These hidden constraints often provide breakthroughs when standard techniques fail.

Windoku Variations and Related Puzzles

The window constraint concept has inspired several creative variations:

Windoku-X (Hyper-X Sudoku): Combines four windows with diagonal constraints on both main diagonals
Super Windoku: Adds more than four windows, sometimes 8-12 overlapping regions
Windoku Killer: Combines window constraints with Killer Sudoku cage-sum clues
NRC Sudoku: Dutch variant name (from Dutch Sudoku Championship) that is identical to Windoku
Double Windoku: Two overlapping 9×9 grids each with their own window constraints (similar to Samurai structure)
Windoku Irregular: Replaces traditional 3×3 boxes with irregular regions while maintaining window constraints
Color Windoku: Adds color constraints where cells of the same color must contain different numbers

Frequently Asked Questions

Is Windoku (Hyper Sudoku) harder than standard Sudoku?

Paradoxically, Windoku can be both easier and harder depending on perspective. With the same number of givens, Windoku is typically 20-30% harder because you must track 31 constraints instead of 27, increasing cognitive load. However, the additional window constraints provide more logical leverage—you can solve Windoku puzzles with fewer givens (11 minimum vs 17 for standard Sudoku). In practice, a medium Windoku feels similar in difficulty to a hard standard Sudoku, but requires different strategic thinking focused on window exploitation.

Where exactly are the four window regions located?

The four 3×3 windows are strategically positioned: Top-Left window occupies rows 2-4 and columns 2-4, Top-Right window is rows 2-4 and columns 6-8, Bottom-Left window is rows 6-8 and columns 2-4, and Bottom-Right window is rows 6-8 and columns 6-8. These positions create maximum overlap with the central box (Box 5) while maintaining symmetry. Each window overlaps with exactly four traditional 3×3 boxes, creating a dense constraint network.

Do I need to learn new solving techniques for Windoku?

Most standard Sudoku techniques (naked singles, hidden singles, pairs, triples, X-Wing, Swordfish) transfer directly to Windoku with one key modification: you must check window constraints in addition to row/column/box constraints. However, Windoku introduces one unique technique called "Box-Window Intersection Analysis" where you eliminate candidates based on the overlap between windows and traditional boxes. This reciprocal elimination technique is specific to Windoku and critical for solving medium-to-hard puzzles efficiently.

Why is the center box so important in Windoku?

Box 5 (the center box) is the "nexus" of Windoku puzzles because it overlaps with ALL FOUR window regions. Every cell in Box 5 belongs to not just a row, column, and box, but also a window—creating four constraints per cell instead of three. This makes Box 5 the most constrained region of the grid. Breakthroughs in Box 5 often cascade through all four windows simultaneously. Many expert solvers start by analyzing Box 5 and its window overlaps before examining other regions.

Can I solve Windoku if I'm still learning standard Sudoku?

It's recommended to master standard Sudoku first. You should consistently solve Medium and Hard standard Sudoku puzzles before attempting Windoku. The additional window constraints add complexity that can be overwhelming for beginners. However, if you're comfortable with standard Sudoku and want a fresh challenge without jumping to extreme variants like Samurai or Killer Sudoku, Windoku is an excellent next step. Start with Easy Windoku puzzles (40+ givens) that have generous clue distributions in the window regions.

How long does it take to solve a Windoku puzzle?

Solve times depend on difficulty and experience. Easy Windoku puzzles take 10-20 minutes for experienced standard Sudoku solvers. Medium puzzles require 20-35 minutes. Hard puzzles can take 40-60 minutes. Expert Windoku with 24-27 givens may require 60-90+ minutes. First-time Windoku solvers should expect to take 50-100% longer than they would on equivalent-difficulty standard Sudoku until they develop window-specific pattern recognition. The cognitive overhead of checking four extra constraints adds 15-25% to solving time even after mastery.

Tips for Your First Windoku Puzzle

Transitioning from standard to Windoku Sudoku requires adjusting your mental model:

Pre-Solving Preparation

Memorize window positions: Before starting, internalize the four window locations. Know instinctively which cells are "in windows" to avoid constant visual checking.
Identify Box 5 priority: Recognize that the center box is your strategic focal point. Start analyzing there first in every puzzle.
Start with Easy puzzles: Don't attempt Medium or Hard Windoku until you've completed 5-10 Easy puzzles. The window-thinking mindset takes practice to develop.

During Solving

Four-way checking routine: For every candidate placement, develop an automatic four-step check: (1) Row? (2) Column? (3) Traditional box? (4) Window (if applicable)? Make this routine habitual.
Scan windows first: When looking for naked or hidden singles, scan the four windows BEFORE scanning traditional boxes. Windows often reveal placements earlier due to their strategic positions.
Exploit Box 5 overlaps: Whenever you place a number in Box 5, immediately check how it affects all four windows. This single placement impacts up to four different window regions simultaneously.
Use color coding: If solving on paper, use a highlighter to shade the four window regions before starting. This visual reminder prevents window-constraint forgetting.

Historical Background and Naming

Windoku emerged in the mid-2000s during the global Sudoku boom when puzzle designers sought to create variants that added complexity without fundamentally changing Sudoku's core mechanics. The variant has multiple names reflecting its independent discovery by different puzzle creators:

Windoku: The most common English name, referencing the four "window" regions
Hyper Sudoku: Alternative English name emphasizing the "hyper-constrained" nature
NRC Sudoku: Dutch variant name from the NRC Handelsblad newspaper's Sudoku championship
Window Sudoku: Descriptive name focusing on the windowed regions

The variant gained popularity because it achieved an optimal balance: significantly harder than standard Sudoku without being as time-consuming as Samurai or as calculation-heavy as Killer Sudoku. Windoku is now a staple in competitive Sudoku championships and puzzle books worldwide.

Conclusion

Windoku (Hyper Sudoku) represents a masterclass in elegant puzzle design—adding just four extra 3×3 window regions transforms the solving experience through 31 total constraints and five hidden interaction patterns. The strategic window positioning creates a dense constraint network where the center box becomes a critical nexus overlapping all four windows. Whether you're a standard Sudoku master seeking the next challenge or a competitive solver training for championships, Windoku offers intellectually rewarding puzzles that demand window-first thinking, box-window intersection analysis, and exploitation of the unique constraint geometry. Master the window regions, prioritize the center box, and prepare for a fascinating evolution of classic Sudoku logic. Use our interactive solver above to experience this exceptional variant and develop the constraint-tracking skills that separate Windoku masters from casual solvers.