... by: osiguy

BGH, i will stop posting the long ones, thank you for informing me that i've reached the level of getting noticed :-) I appreciate the feedback and will stop being annoying. I think you are awesome and amazing :-)

For thomas, here is a last long example to respect BGH's feedback of my code to solve this puzzle there are many presets, i hope you enjoy. now that i am noticed, i will keep my post to a minumum and honor BGH's request! Thank you for letting me know that BGH your posts are very informative and i have learned a lot from them and you are very well respected by me!

Also For Thomas, i think Andrew based on our emails, will be updating his Forcing chains to accomplish what mine do but in a way more compatible with his solver so that is on the horizon from what i hear from Andrew! So i have a public github called wsrf-sudoku-solved-series under my name oppressionslayer! and python code will be posted here soon, i'm just polishing it up:

python3 cli.py "000004006000201090001070800060000020350000008000000370009080500040302000700100000" --steps --gf2 --board --detail --preset expert

Solving: 00000400600020109000...
──────────────────────────────────────────────────
──────────────────────────────────────────────────

✦ Sudoku Expert Approved Techniques ✦

Status: SOLVED
Steps: 58
Time: 702.3ms
Verify: All techniques are Sudoku Expert Approved ✓

Techniques:
crossHatch 24 ( 32.4%) L1 ██████████
lastRemaining 19 ( 25.7%) L1 ████████
ALS_XZ 10 ( 13.5%) L5 ████
nakedSingle 9 ( 12.2%) L1 ████
ALS_XYWing 5 ( 6.8%) L5 ██
ForcingChain 3 ( 4.1%) L5 █
fullHouse 3 ( 4.1%) L1 █
KrakenFish 1 ( 1.4%) L6 █

════════════════════════════════════════════════════════════
DETAILED SOLVE LOG (20 rounds)
════════════════════════════════════════════════════════════
Round 1
ALS_XZ 1 elimination
R6C5: 9
ALS-XZ: 1 eliminations
lastRemaining R1C4 = 8✓
Notes before: 8 → placed 8
8 can only go here in Box 2 — peers block all other spots
Round 2
ForcingChain R3C8 = 5✓
Notes before: 3 4 5 → placed 5
Forcing Chain: all branches from bivalue cell lead to 5@R3C8
Round 3
ALS_XZ 1 elimination
R1C2: 4
ALS-XZ: 1 eliminations
crossHatch R9C6 = 5✓
Notes before: 5 → placed 5
5 can only go here in Box 8 — peers block all other spots
Round 4
ALS_XZ 1 elimination
R7C8: 2
ALS-XZ: 1 eliminations
Round 5
ALS_XYWing 1 elimination
R6C2: 9
ALS-XY Wing: 1 eliminations
Round 6
ALS_XYWing 1 elimination
R9C9: 4
ALS-XY Wing: 1 eliminations
Round 7
KrakenFish 1 elimination
R6C6: 9
Kraken Fish: 1 eliminations
Round 8
ForcingChain R1C8 = 1✓
Notes before: 1 3 → placed 1
Forcing Chain: all branches from bivalue cell lead to 1@R1C8
Round 9
ForcingChain R7C8 = 3✓
Notes before: 3 4 6 → placed 3
Forcing Chain: all branches from bivalue cell lead to 3@R7C8
Round 10
ALS_XZ 1 elimination
R3C1: 7
ALS-XZ: 1 eliminations
Round 11
ALS_XZ 1 elimination
R6C4: 9
ALS-XZ: 1 eliminations
crossHatch R4C6 = 3✓
Notes before: 3 → placed 3
3 can only go here in Box 5 — peers block all other spots
crossHatch R6C6 = 8✓
Notes before: 8 → placed 8
8 can only go here in Box 5 — peers block all other spots
Round 12
ALS_XZ 1 elimination
R6C9: 2
ALS-XZ: 1 eliminations
Round 13
ALS_XZ 1 elimination
R4C1: 2
ALS-XZ: 1 eliminations
Round 14
ALS_XZ 1 elimination
R6C1: 5
ALS-XZ: 1 eliminations
Round 15
ALS_XZ 1 elimination
R5C7: 9
ALS-XZ: 1 eliminations
Round 16
ALS_XYWing 1 elimination
R9C3: 3
ALS-XY Wing: 1 eliminations
Round 17
ALS_XYWing 1 elimination
R8C1: 9
ALS-XY Wing: 1 eliminations
Round 18
ALS_XZ 1 elimination
R2C3: 5
ALS-XZ: 1 eliminations

... by: BGH

To osiguy,
Everyone doing "unsolvable Sudoku" is a "serious student" of the the art form.
We don't need a blow-by-blow of ALL the basic deterministic steps,
only the crucial last steps that solve the puzzle.

If it pleases you, continue the llllooooonnnngggg version step-by-step.
I am only interested in the nondeterministic last step(s) & **how you selected them**

... by: Tomas

osiguy, can you elaborate:

Round 4
forcingChain 1 elimination
R1C1: 5
Forcing Chain JIT from R1C1: eliminate contradicted | Removed: R1C1(5)

Can you show chains used, and logic of elimination using these chains?

... by: osiguy

Here is the solve up to move 12 where it solves itself!

17x lastRemaining
11x pointingPair (elim)
9x alsXZ (elim)
5x forcingChain (elim)
7x nakedPair (elim)
13x nakedSingle
23x hiddenSingle
2x claiming (elim)
1x emptyRectangle (elim)
1x alsXYWing (elim)
1x finnedXWing (elim)
5x fullHouse
63 total eliminations 58/58 placed in 36 rounds
✓ FULLY SOLVED —
Round 1
lastRemaining R1C4 = 8✓
Notes before: 5 8 9 → placed 8
8 can only go here in Box 2 — peers block all other spots
Round 2
pointingPair 2 eliminations
R1C3: 7 R2C3: 7
7 in Box 4 points along C3 — eliminates 2 | Removed: R1C3(7), R2C3(7)
pointingPair 2 eliminations
R2C9: 5 R3C9: 5
5 in Box 6 points along C9 — eliminates 2 | Removed: R2C9(5), R3C9(5)
pointingPair 3 eliminations
R5C4: 6 R5C5: 6 R5C6: 6
6 in Box 6 points along R5 — eliminates 3 | Removed: R5C4(6), R5C5(6), R5C6(6)
pointingPair 1 elimination
R7C9: 7
7 in Box 8 points along R7 — eliminates 1 | Removed: R7C9(7)
Round 3
alsXZ 1 elimination
R6C5: 9
ALS-XZ: RCC=2, eliminates 9 | Removed: R6C5(9)
Round 4
forcingChain 1 elimination
R1C1: 5
Forcing Chain JIT from R1C1: eliminate contradicted | Removed: R1C1(5)
Round 5
forcingChain 1 elimination
R7C2: 3
Forcing Chain from R1C1: eliminate from R7C2 | Removed: R7C2(3)
Round 6
pointingPair 2 eliminations
R9C8: 3 R9C9: 3
3 in Box 7 points along R9 — eliminates 2 | Removed: R9C8(3), R9C9(3)
Round 7
forcingChain 2 eliminations
R6C2: 2,8
Forcing Chain from R1C1: eliminate from R6C2 | Removed: R6C2(2), R6C2(8)
Round 8
forcingChain 1 elimination
R1C3: 2
Forcing Chain from R1C3: eliminate contradicted | Removed: R1C3(2)
Round 9
forcingChain 1 elimination
R1C5: 9
Forcing Chain JIT from R1C5: eliminate contradicted | Removed: R1C5(9)
Round 10
nakedPair 3 eliminations
R1C2: 3 R1C8: 3,5
Naked Pair {3,5} in R1 at R1C3,R1C5 — eliminates 3 candidates | Removed: R1C2(3), R1C8(3), R1C8(5)
pointingPair 2 eliminations
R3C1: 9 R3C2: 9
9 in Box 2 points along R3 — eliminates 2 | Removed: R3C1(9), R3C2(9)
nakedSingle R1C8 = 1✓
Notes before: 1 → placed 1
Elimination left only {1} → place 1
Round 11
lastRemaining R3C8 = 5✓
Notes before: 3 4 5 → placed 5
5 can only go here in Box 3 — peers block all other spots
hiddenSingle R7C8 = 3✓
Notes before: 3 4 6 → placed 3
3 has only one spot in C8 (cell had candidates {3,4,6})
Round 12
pointingPair 4 eliminations
R4C5: 5 R6C5: 5 R8C5: 5 R9C5: 5
5 in Box 2 points along C5 — eliminates 4 | Removed: R4C5(5), R6C5(5), R8C5(5), R9C5(5)
claiming 1 elimination
R9C3: 5
5 in R8 confined to Box 7 — eliminates 1 | Removed: R9C3(5)
claiming 2 eliminations
R4C6: 5 R6C6: 5
5 in C4 confined to Box 5 — eliminates 2 | Removed:

... by: BGH

All your solutions confirmed.
Most interesting:
2 @ J2 Solves for 2 in Blocks R3C1, R3C3, R1C3 & R1C1

... by: BGH

After basics 29.2
2 Step Solution (Pretty interesting double swordfish in 7’s)
7 @ B7 (Swordfish #1, solves for all 7’s in swordfish #1) 33.2
7 @ E4 (Swordfish #2, solves for all 7’s in swordfish #2) Solution

... by: Frans Goosens

#689

Combination 2-digits cells

B7=4 No solution, Undo calculation
B7=7 ( G6=7 ) Solved,

Total solving time is : 8 sec.
Number of logical steps is : 856

... by: Dieter

#689 - initially populated cells: 23 = 28,4 %
No solver used, solver = brain.
basics: %

Strategy: numbers 1,2,3,7,8 occur most frequently, combine two cells.
combinations of G2 = (1,2,3) and J2 = (2,3,8)
G2=1 & J2=2 solves.

... by: Yves

Two one cell solutions
J2=2 or J7=4