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1. Outies c34567 -> r19c28 = +10
Max r9c3467 = +30 -> Min r9c28 = +7
-> Max r1c28 = +3 -> r1c28 = {12}
-> r9c28 = +7
-> r9c3467 = +30 = {6789}
-> r9c28 = {25} or {34}
-> r9c159 = {134} or {125}

2. Outies r2345 = +4 -> r16c5 = [31]
-> Innies r1 -> r1c19 = +9 = {45}
-> r1c3467 = {6789}
Also 1 in r9 in r9c1 or r9c9
Also r9c5 from (45)

3. Innies r67 = +10
-> r7c5 = 9

4. 15/3@r1c6 = [{68}1] or [{67}2]
-> r1c7 from {678}
-> 26/4@r2c8 cannot be {5678}
-> 26/4@r2c8 contains a 9.
Since r1c9 from (45) -> 26/4@r2c8 from {9872}, {9863}, or {9764}
-> (6789) locked in n3 in r1c7 and r23c89

5. 18/3@r1c2 contains a 9 in r1c34
Also 33/7@r1c5 must contain a 9 in r2c346
-> 9 locked in n1 and c3 in r12c3
-> HS 9 in r9 -> r9c7 = 9
-> HS 9 in D\ -> r6c6 = 9
-> 9 in n2 in r12c4
-> 9 in n3 in r3c89
-> HS 9 in D/ -> r8c2 = 9
-> 9 in n4 in r45c1
Also 9 in n6 in r45c89

6. Innies r123 -> r3c3467 = +13
-> r4c3467 = +12. Must be from {1236} or {1245}
Since 1 already in r6c5 -> 1 in r4 in c3 or c7

7. 33/7@r1c5 is missing either {48} or {57}
-> At least one of (45) must be in r3c46
-> HG 13/4@r3c3467 = {1345} or {1246}

8. Max r1c1+r1c3+r3c3 = 5+9+6 = +20
-> Min r1c2+r2c3 = +4
Since r1c2 from (12) and 3 cannot go in r2c3 -> Min r2c3 = 4
Since 21/4@r2c1 does not contain a 9 -> it contains at most one of (123)
-> limited number of places for (123) in n1 -> r3c3 is max 3

9. 33/7@r1c5 contains a 1 in r2. HG 13/4@r3c3467 contains a 1 in r3
-> 1 not in 21/4@r2c1
-> 1 in n1 in r1c2 or r3c3

Also 1 in n3 cannot go in 26/4@r2c8
-> 1 in n3 in r1c8 or r23c7

-> (Remembering that 1 in r4 is in c37) Either:
a) 1 in r1c2 -> 1 in r23c7 -> 1 in r4c3, or
b) 1 in r1c8 -> 1 in r3c3

-> 1 in c3 locked in r34c3

10. Considering the following:
From Step 7, HG 13/4@r3c3467 = {1345} or {1246}
From Step 8, r3c3 is max 3
From Step 9, 1 in r34c3

-> putting a 3 in r3c3 would require a 2 or a 6 in r3c4 (12/4@r3c3) which contradicts Step 7.
-> r3c3 is max 2.
-> (12) in n1 locked in n1 in r1c2 and r3c3
-> (Since 9 in n1 locked in r12c3) -> (36) in n1 locked in the 21/4@r2c1

11. 33/7@r1c5 contains a 6 and since it cannot go in r2c3 or r2c7 -> 6 is in the 33/7 in n2
-> r1c7 = 6
-> 26/4@r2c8 = {2789}
-> r1c8 = 1
-> 15/3@r1c6 = [861]
-> r3c7 = 3
Also 18/3@r1c2 = [2{79}]
-> r3c3 = 1
-> r3c46 = {45}
-> r4c7 = 1
-> r4c346 = {245} (Since r3c4 from (45))
Also 1 in r5 in r5c12
-> 25/4@r4c3 = {1789}

Easier from here

Prelims

a) 19(3) cage at R9C2 = {289/379/469/478/568}, no 1
b) 26(4) cage at R2C8 = {2789/3689/4589/4679/5678}, no 1
c) 12(4) cage at R3C3 = {1236/1245}, no 7,8,9
d) 13(4) cage at R3C6 = {1237/1246/1345}, no 8,9

1. Caged X-Wing for 1 in 12(4) cage at R3C3 and 13(4) cage at R3C6, no other 1 in R34

2. 45 rule on R9 3 innies R9C159 = 8 = {125/134}, 1 locked for R9
2a. 1 in R9 only in R9C159, CPE no 1 in R5C5 using the diagonals

3. 45 rule on R67 2 innies R67C5 = 10 = {19/28/37/46}, no 5

[I first saw 45 rule on R2345678, but this one is more powerful …]
4. 45 rule on R2345 2 outies R16C5 = 4 = {13}, locked for C5, R7C5 (step 3) = {79}
4a. 32(7) cage at R7C5 must contain 1,3, locked for R8

5. 1 in R9 only in R9C19, CPE no 1 in R1C19 using the diagonals
5a. R9C159 (step 2) = {125/134}
5b. 4 of {134} must be in R9C5 -> no 4 in R9C19

6. 45 rule on C34567 4(2+2) outies R19C28 = 10
6a. Min R19C2 = 3 -> max R19C8 = 7, no 6,7,8,9 in R19C8, no 5 in R1C8 (cannot be [16] because R19C2 = 3 uses the 1 in R1, cannot be [52] because R19C2 = 3 uses the 2 in R9)
6b. Min R19C8 = 3 -> max R19C2 = 7, no 6,7,8,9 in R19C2, no 5 in R1C2 (cannot be [16] because R19C8 = 3 uses the 1 in R1, cannot be [52] because R19C8 = 3 uses the 2 in R9)
[Steps 6a and 6b were extended later, and step 6c added then; it probably explains why I didn’t spot step 12 sooner.]
6c. Naked quint {12345} in R9C12589, locked for R9
[Now for a human solvable step, which I assume that SudokuSolver won’t be able to find …]
6d. R19C28 cannot be {23}{23} which clashes with R9C159 (step 2) = {125/134} -> there must be 1 in one of R1C28, locked for R1 -> R1C5 = 3, R6C5 = 1, R7C5 = 9 (step 3)
6e. 33(7) cage at R1C5 must contain 1, locked for R2
6f. 33(7) cage at R1C5 contains 3 so must contain 9, locked for R2
6g. 31(7) cage at R4C5 must contain 3, locked for R5

7. 45 rule on R1 2 remaining innies R1C19 = 9 = {27/45}, no 6,8,9 in R1C19
7a. Killer pair 2,4 in R1C19 and R1C28, locked for R1

8. Caged Swordfish for 1 in 33(7) cage at R1C5, 12(4) cage at R3C3 and 13(4) at R3C6 for N2, C3 and C7, no other 1 in C3 and C7 (there are no other 1s in N2)
[I think Swordfish is a 3-way X-Wing.]
8a. 32(7) cage at R7C5 must contain 1, locked for N8

9. 45 rule on R23 4 remaining outies R4C3467 = 12 = {1236/1245}, no 7, 2 locked for R4
9a. 31(7) cage at R4C5 must contain 2, locked for R5

10. 9 on D\ only in R6C6 + R8C8, 9 on D/ only in R6C4 + R8C2 -> grouped X-wing for 9 in R6C46 + R8C28, no other 9 in R68
10a. 9 in R6 only in R6C46, locked for N5

11. 1 on D\ only in R3C3 + R9C9, 1 on D/ only in R3C7 + R9C1 -> grouped X-Wing for 1 in R3C37 + R9C19, no other 1 in R3

[Just spotted …]
12. R19C28 = 10 (step 6)
12a. Naked quad {6789} in R9C3467 = 30, 19(3) cage at R9C2 + 18(3) cage at R9C6 = 37 -> R9C28 = 7 -> R1C28 = 3 = {12}, locked for R1, clean-up: no 7 in R1C19 (step 7)

13. Naked pair {45} in R1C19, locked for R1
13a. R1C19 = {45}, CPE no 4,5 in R5C5 + R9C19, using the diagonals
13b. R9C159 (step 2) = {125/134}
13c. 4,5 only in R9C5 -> R9C5 = {45}

14. 18(3) cage at R1C2 = {189/279}, no 6, 9 locked for R1
14a. Consider the placement for 9 in R1
R1C3 = 9
or R1C4 = 9 => R8C2 = 9 (hidden single on D/)
-> no 9 in R9C3
14b. R9C7 = 9 (hidden single in R9)
14c. R6C6 = 9 (hidden single on D\)
14d. R8C2 = 9 (hidden single on D/)

15. 19(3) cage at R9C2 = {478/568}, no 2,3, 8 locked for R9
15a. Naked pair {45} in R9C25, locked for R9
15b. Naked pair {45} in R9C25, CPE no 4,5 in R8C3

16. 45 rule on N3 5 innies R1C789 + R23C7 = 19 = {12358/12457/13456} (cannot be {12367} because R1C8 only contains 4,5), 5 locked for N3
16a. R1C7 = {678} -> no 6,7,8 in R23C7

17. R1C789 + R23C7 (step 16) = {12358/12457/13456}
17a. 18(3) cage at R9C6 = {279/369}
17b. Consider placements for 6 in R1
R1C6 = 6 => 18(3) cage = [792]
or R1C7 = 6 => R1C789 + R23C7 = {13456}, no 2
-> R1C8 = 1, R1C67 = 14 = {68}, locked for R1, R1C2 = 2
[Cracked, but there are a lot more steps before this puzzle is finished.]

18. R3C3 = 1 (hidden single in R3), placed for D\
18a. R4C7 = 1 (hidden single in R4)
18b. Naked pair {23} in R9C89, locked for R9 and N9 -> R9C1 = 1
18c. R7C9 = 1 (hidden single in R7)

19. 16(4) cage at R6C1 = {2347/2356}, no 8, 2 locked for C1

20. 21(4) cage at R2C1 = {3468/3567} (cannot be {3459} which clashes with R1C1), no 9
20a. Killer pair 4,5 in R1C1 and 21(4) cage, locked for N1
20b. 9 in N1 only in R12C3, locked for C3

21. 31(7) cage at R4C5 = {1234678} (only remaining combination), no 5

22. R5C2 = 1 (hidden single in N4) -> 25(4) cage at R4C1 = {1789}, locked for N4
22a. Caged X-Wing for 8 in 25(4) cage and 31(7) cage at R4C5, no other 8 in R45
22b. Deleted

23. 9 in N1 only in R12C3
23a. 45 rule on N1 3 remaining innies R1C1 + R12C3 = 21 = {489/579}, no 6

24. 33(7) cage at R1C5 must contain 6, locked for N2 -> R1C67 = [86]
24a. R1C789 + R23C7 (step 16) = {13456} (only remaining combination) -> R3C7 = 3, placed for D/, R1C9 + R2C7 = {45}, locked for N3
24b. 2 in C7 only in R56C7, locked for N6

25. R34C7 = [31] = 4 -> R34C6 = 9 = [45/54/72], no 2 in R3C6, no 6 in R4C6

26. R9C89 = {23} = 5
26a. 45 rule on C89 3(1+2) remaining innies R1C9 + R8C89 = 15 = 4{47}/4{56}/5{46}, no 8 in R8C89

27. 33(7) cage at R1C5 = {1234689/1235679}
27a. R2C7 = {45} -> no 4,5 in R2C456 + R3C5
27b. R3C46 = {45} (hidden pair in N2), locked for R3
27c. R34C6 (step 25) = {45}, locked for C6
27d. R3C4 = {45} -> 12(4) cage at R3C3 = {1245} (only remaining combination), no 3,6
27e. Naked pair {245} in R4C346, locked for R4
27f. Naked pair {45} in R1C9 and R4C6, locked for D/

28. 3 in R4 only in R4C89, locked for N6
28a. 5 in R5 only in R5C89, locked for N6
28b. 23(4) cage at R4C8 contains 3,5 = {3569} (only possible combination), locked for N6

29. R7C9 = 1, R6C89 = {478} -> 20(4) cage at R6C8 = {1478} (only possible combination)

30. 6 in N9 only in R8C89, locked for R8
30a. R1C9 + R8C89 (step 26a) = 4{56}/5{46}, no 7 in R8C89

31. 32(7) cage at R7C5 = {1234589}, no 7

32. R8C1 = 7 (hidden single in R8)
32a. Naked pair {89} in R45C1, locked for C1 and N4 -> R3C1 = 6, R4C2 = 7, R3C2 = 8
32b. R3C12 = [68] = 14 -> R2C12 = 7 = {34}, locked for R2 and N1 -> R1C1 = 5, placed for D\, R1C9 = 4, placed for D/, R2C7 = 5, R34C6 = [45], R3C4 = 5
32c. Deleted

33. Naked triple {478} in R7C78 + R8C7, locked for N9, 7 also locked for R7 -> R8C8 = 6, placed for D\, R8C9 = 5

34. 6 on D/ only in R6C4 + R7C3, locked for 21(4) cage at R6C3, no 6 in R6C3 + R7C4
34a. 21(4) cage at R6C3 contains 6 = {2568/3468/3567}
34b. 5 of {2568} must be in R6C3 -> no 2 in R6C3

35. 16(4) cage at R6C1 (step 19) = {2356} (only remaining combination), no 4
35a. R2C1 = 4 (hidden single in C1), R2C2 = 3, placed for D\, R9C9 = 2, placed for D\, R4C4 = 4, placed for D\
35b. R9C8 = 3, R9C6 = 6 (cage sum), R9C34 = [87], R9C2 = 4 (cage sum)

36. Naked pair {23} in R7C16, locked for R7 -> R7C34 = [68], R6C4 = 2, placed for D/, R7C7 = 8, placed for D\, R5C5 = 8, placed for D/

and the rest is naked singles, without using the diagonals.

I'll rate my walkthrough for HS 18 at Easy 1.5. As well as the HS step, I used two short forcing chains; I'm not sure what rating I'd give to my HS step, but definitely not higher than short forcing chains.

1. R1239+C5 !
a) Outies R2345 = 4(2) = {13} locked for C5
b) Innies R9 = 8(3) = 1{25/34} -> 1 locked for R9
c) ! Outies C34567 = 10(2+2): R1C28 = 1{2/3} since R9C28 >= 6 because R9C28 <> 1
and {23} blocked by Killer pair (23) of Innies R9 -> 1 locked for R1
d) R1C5 = 3, R6C5 = 1
e) Innie R67 = R7C5 = 9
f) Naked pair (12) locked in R1C28 for R1
g) Innies R1 = 9(2) = {45} locked for R1
h) 18(3) @ R1 = 9{18/27} -> 9 locked for R1
i) 33(7) = 12369{48/57} -> 1,9 locked for R2
j) 26(4) = 9{278/368/467} <> 5 since {4589} blocked by R1C9 = (45) and {5678}
blocked by R1C7 = (678) -> 9 locked for R3+N3
k) Outies C34567 = 10(2+2) = {12}+{25/34}

2. N1379+D\/ !
a) 9 locked in R12C3 @ N1 for C3
b) ! Generalized X-Wing: 9 locked in R6C46+R8C28 @ D\/ for R68
c) Hidden Singles: R8C2 = 9 @ N7, R9C7 = 9 @ N9, R6C6 = 9 @ D\
d) 19(3) = 8{47/56} -> 8 locked for R9
e) Outies C34567 = 10(2+2) = {12}+[43/52]
f) 18(3) @ R9 = 9{27/36} since R9C8 = (23) -> R9C6 = (67)
g) Killer triple (678) locked in R1C7+26(4) for N3
h) ! Caged Jellyfish: 1 locked in 33(7)+12(4)+13(4)+32(7) for R2348+C3467
i) Generalized X-Wing: 1 locked in R3C37+R9C19 @ D\/ for R39
j) 31(7) = {1234678}

3. R123+C456 !
a) 1 locked in R2C46 @ N2 for R2
b) Innies N1 = 24(5) = 9{1248/1257/1347/1356/2346}: R3C3 <> 6 since 3 only possible there
c) Innies R23 = 13(4) = 1{237/246/345}: R3C46 can have at most one of (24) since 6 only possible there
-> Hidden Killer pair (24) in 33(7) @ N2
d) ! 33(7) = 12369{48/57}: R2C456+R3C5 <> 5 because of R2C7 = (245) and Hidden Killer pair (24) (step 3c)
since 33(7) can only have two of (245)
e) 5 locked in 32(7) @ C5 for N8+32(7) -> 32(7) = {1234589} -> 8 locked for R8
f) 5 locked in R3C46 @ N3 for R3 -> Innies R23 = 13(4) = {1345} -> 4,5 locked for R3+N2, 3 locked for R3
h) 12(4) = {1245} since R3C4 = (45) -> R3C3 = 1; 2 locked for R4
i) 13(4) = {1345} -> R3C7 = 3, R4C7 = 1; 4 locked for C6

4. R123+N79
a) 18(3) @ N1 = {279} -> R1C2 = 2; 7 locked for R1
b) Killer pair (45) locked in 21(4) + 33(7) for R2
c) 26(4) = {2789} locked for N3
d) Hidden Single: R7C9 = 1 @ C9
e) 20(4) = 18{47/56}

5. R4567
a) Naked triple (245) locked in R4C346 for R4
b) 31(7) = {1234678} -> 2,3,4 locked for R5
c) 9 locked in 23(4) @ N6 @ R4C8 -> 23(4) = {3569} locked for N6; 5 locked for R5
d) 20(4) = {1478} since (56) only possible @ R7C8
e) 23(4) @ R6 = 9{248/257/347} <> 6 since R6C7 <> 3,5,6
f) 6 locked in 16(4) + 21(4) for R67 -> 16(4) = {2356}

6. C347+N7+D/
a) Killer pair (45) locked in R2C7+23(4) for C7
b) Naked pair (45) locked in R1C9+R4C6 for D/
c) 4 locked in 12(4) + 21(4) for C34
d) 21(4) = 56{28/37} -> R6C3 = 5
e) 16(4) = {2356} -> 5 locked for R7+N7; 2 locked for C1
f) 19(3) = {478} locked for R9; R9C2 = 4
g) R9C6 = 6 -> R9C8 = 3
h) Hidden Single: R2C7 = 5 @ C7
i) R1C9 = 4, R1C1 = 5, R9C9 = 2, R4C4 = 4, R8C7 = 8, R7C7 = 7, R8C8 = 6, R5C5 = 8
j) 23(4) = {3479} -> R6C7 = 4, R7C6 = 3
k) 21(4) = {2568} -> R7C4 = 8 -> 2 locked for D/

7. Rest is Naked Singles.

Hard 1.25. I used Killer subsets combined with combo analysis and some Generalized X-Wings.