## The N-queens problem, using bit vectors

A smart and efficient code using bit-vector operations to compute the number of solutions of the N-queens problem. This case study is detailed in Inria research report 8821.

**Authors:** Clément Fumex / Claude Marché / Jean-Christophe Filliâtre

**Topics:** Ghost code / Bitwise operations

**Tools:** Why3

**References:** ProofInUse joint laboratory

see also the index (by topic, by tool, by reference, by year)

# N-queens problem

Verification of the following 2-lines C program solving the N-queens problem:

t(a,b,c){int d=0,e=a&~b&~c,f=1;if(a)for(f=0;d=(e-=d)&-e;f+=t(a-d,(b+d)*2,( c+d)/2));return f;}main(q){scanf("%d",&q);printf("%d\n",t(~(~0<## finite sets of integers, with succ and pred operations

theory S use export set.Fsetint function succ (set int) : set int axiom succ_def: forall s: set int, i: int. mem i (succ s) <-> i >= 1 /\ mem (i-1) s function pred (set int) : set int axiom pred_def: forall s: set int, i: int. mem i (pred s) <-> i >= 0 /\ mem (i+1) s end## Formalization of the set of solutions of the N-queens problem

theory Solution use import int.Int use export map.Mapthe number of queens

constant n : int type solution = map int intsolutions

`t`

and`u`

have the same prefix`[0..i[]`

predicate eq_prefix (t u: map int 'a) (i: int) = forall k: int. 0 <= k < i -> t[k] = u[k] predicate eq_sol (t u: solution) = eq_prefix t u n

`s`

stores a partial solution, for the rows`0..k-1`

predicate partial_solution (k: int) (s: solution) = forall i: int. 0 <= i < k -> 0 <= s[i] < n /\ (forall j: int. 0 <= j < i -> s[i] <> s[j] /\ s[i]-s[j] <> i-j /\ s[i]-s[j] <> j-i) predicate solution (s: solution) = partial_solution n s lemma partial_solution_eq_prefix: forall u t: solution, k: int. partial_solution k t -> eq_prefix t u k -> partial_solution k u predicate lt_sol (s1 s2: solution) = exists i: int. 0 <= i < n /\ eq_prefix s1 s2 i /\ s1[i] < s2[i] type solutions = map int solution

`s[a..b[] is sorted for [lt_sol]`

predicate sorted (s: solutions) (a b: int) = forall i j: int. a <= i < j < b -> lt_sol s[i] s[j]a sorted array of solutions contains no duplicate

lemma no_duplicate: forall s: solutions, a b: int. sorted s a b -> forall i j: int. a <= i < j < b -> not (eq_sol s[i] s[j]) end## More realistic code with bitwise operations

module BitsSpec use import S constant size : int = 32 type t = { ghost mdl: set int; } invariant { forall i. mem i self.mdl -> 0 <= i < size } val empty () : t ensures { is_empty result.mdl } val is_empty (x:t) : bool ensures { result <-> is_empty x.mdl } val remove_singleton (a b: t) : t requires { b.mdl = singleton (min_elt b.mdl) } requires { mem (min_elt b.mdl) a.mdl } ensures { result.mdl = remove (min_elt b.mdl) a.mdl } val add_singleton (a b: t) : t requires { b.mdl = singleton (min_elt b.mdl) } (* requires { not (mem (min_elt b.mdl) a.mdl) } *) (* this is not required if the implementation uses or instead of add *) ensures { result.mdl = S.add (min_elt b.mdl) a.mdl } val mul2 (a: t) : t ensures { result.mdl = remove size (succ a.mdl) } val div2 (a: t) : t ensures { result.mdl = pred a.mdl } val diff (a b: t) : t ensures { result.mdl = diff a.mdl b.mdl } use import ref.Ref val rightmost_bit_trick (a: t) (ghost min : ref int) : t requires { not (is_empty a.mdl) } writes { min } ensures { !min = min_elt a.mdl } ensures { result.mdl = singleton !min } use bv.BV32 val below (n: BV32.t) : t requires { BV32.ule n (32:BV32.t) } ensures { result.mdl = interval 0 (BV32.t'int n) } end module Bits "the 1-bits of an integer, as a set of integers" use import S use import bv.BV32 constant size : int = 32 type t = { bv : BV32.t; ghost mdl: set int; } invariant { forall i: int. (0 <= i < size /\ nth self.bv i) <-> mem i self.mdl } let empty () : t ensures { is_empty result.mdl } = { bv = zeros; mdl = empty } let is_empty (x:t) : bool ensures { result <-> is_empty x.mdl } = assert {is_empty x.mdl -> BV32.eq x.bv zeros}; x.bv = zeros let remove_singleton (a b: t) : t requires { b.mdl = singleton (min_elt b.mdl) } requires { mem (min_elt b.mdl) a.mdl } ensures { result.mdl = remove (min_elt b.mdl) a.mdl } = { bv = bw_and a.bv (bw_not b.bv); mdl = remove (min_elt b.mdl) a.mdl } (* { bv = sub a.bv b.bv; mdl = remove (min_elt b.mdl) a.mdl } *) let add_singleton (a b: t) : t requires { b.mdl = singleton (min_elt b.mdl) } (* requires { not (mem (min_elt b.mdl) a.mdl) } *) (* this is not required if the implementation uses or instead of add *) ensures { result.mdl = S.add (min_elt b.mdl) a.mdl } = { bv = bw_or a.bv b.bv; mdl = S.add (min_elt b.mdl) a.mdl } (* { bv = add a.bv b.bv; mdl = add (min_elt b.mdl) a.mdl } *) let mul2 (a: t) : t ensures { result.mdl = remove size (succ a.mdl) } = { bv = lsl_bv a.bv (1:BV32.t); mdl = remove size (succ a.mdl) } let div2 (a: t) : t ensures { result.mdl = pred a.mdl } = { bv = lsr_bv a.bv (1:BV32.t); mdl = pred a.mdl } let diff (a b: t) : t ensures { result.mdl = diff a.mdl b.mdl } = { bv = bw_and a.bv (bw_not b.bv); mdl = diff a.mdl b.mdl } predicate bits_interval_is_zeros_bv (a:BV32.t) (i:BV32.t) (n:BV32.t) = eq_sub_bv a zeros i n predicate bits_interval_is_one_bv (a:BV32.t) (i:BV32.t) (n:BV32.t) = eq_sub_bv a ones i n predicate bits_interval_is_zeros (a:BV32.t) (i : int) (n : int) = eq_sub a zeros i n predicate bits_interval_is_one (a:BV32.t) (i : int) (n : int) = eq_sub a ones i n let rightmost_bit_trick (a: t) : t requires { not (is_empty a.mdl) } ensures { result.mdl = singleton (min_elt a.mdl) } = let ghost n = min_elt a.mdl in let ghost n_bv = of_int n in assert {bits_interval_is_zeros_bv a.bv zeros n_bv}; assert {nth_bv a.bv n_bv}; assert {nth_bv (neg a.bv) n_bv}; let res = bw_and a.bv (neg a.bv) in assert {forall i. 0 <= i < n -> not (nth res i)}; assert {bits_interval_is_zeros_bv res (add n_bv (1:BV32.t)) (sub (31:BV32.t) n_bv )}; assert {bits_interval_is_zeros res (n + 1) (31 - n)}; { bv = res; mdl = singleton n } let below (n: BV32.t) : t requires { BV32.ule n (32:BV32.t) } ensures { result.mdl = interval 0 (t'int n) } = { bv = bw_not (lsl_bv ones n); mdl = interval 0 (t'int n) } end module NQueensBits use import BitsSpec use import ref.Ref use import Solution use import int.Int val ghost col: ref solution (* solution under construction *) (* val ghost k : ref int (\* current row in the current solution *\) *) val ghost sol: ref solutions (* all solutions *) val ghost s : ref int (* next slot for a solution = number of solutions *) let rec t (a b c: BitsSpec.t) (ghost k : int) requires { n <= size } requires { 0 <= k } requires { k + S.cardinal a.mdl = n } requires { !s >= 0 } requires { forall i: int. S.mem i a.mdl <-> ( 0 <= i < n /\ forall j: int. 0 <= j < k -> !col[j] <> i) } requires { forall i: int. 0 <= i < size -> ( not (S.mem i b.mdl) <-> forall j: int. 0 <= j < k -> i - !col[j] <> k - j) } requires { forall i: int. 0 <= i < size -> ( not (S.mem i c.mdl) <-> forall j: int. 0 <= j < k -> i - !col[j] <> j - k ) } requires { partial_solution k !col } variant { S.cardinal a.mdl } ensures { result = !s - old !s >= 0 } ensures { sorted !sol (old !s) !s } ensures { forall i:int. old !s <= i < !s -> solution !sol[i] /\ eq_prefix !col !sol[i] k } ensures { forall u: solution. solution u /\ eq_prefix !col u k -> exists i: int. old !s <= i < !s /\ eq_sol u !sol[i] } (* assigns *) ensures { eq_prefix (old !col) !col k } ensures { eq_prefix (old !sol) !sol (old !s) } = if not (is_empty a) then begin let e = ref (diff (diff a b) c) in (* first, you show that if u is a solution with the same k-prefix as col, then u[k] (the position of the queen on the row k) must belong to e *) assert { forall u:solution. solution u /\ eq_prefix !col u k -> S.mem u[k] !e.mdl }; let f = ref 0 in let ghost min = ref (-1) in 'L: while not (is_empty !e) do variant { S.cardinal !e.mdl } invariant { not (S.is_empty !e.mdl) -> !min < S.min_elt !e.mdl } invariant { !f = !s - at !s 'L >= 0 } invariant { S.subset !e.mdl (S.diff (S.diff a.mdl b.mdl) c.mdl) } invariant { partial_solution k !col } invariant { sorted !sol (at !s 'L) !s } invariant { forall i: int. S.mem i (at !e.mdl 'L) /\ not (S.mem i !e.mdl) -> i <= !min } invariant { forall i:int. at !s 'L <= i < !s -> solution !sol[i] /\ eq_prefix !col !sol[i] k /\ 0 <= !sol[i][k] <= !min } invariant { forall u: solution. (solution u /\ eq_prefix !col u k /\ 0 <= u[k] <= !min) -> S.mem u[k] (at !e.mdl 'L) && not (S.mem u[k] !e.mdl) && exists i: int. (at !s 'L) <= i < !s /\ eq_sol u !sol[i] } (* assigns *) invariant { eq_prefix (at !col 'L) !col k } invariant { eq_prefix (at !sol 'L) !sol (at !s 'L) } 'N: let d = rightmost_bit_trick !e min in ghost col := !col[k <- !min]; assert { 0 <= !col[k] < size }; assert { not (S.mem !col[k] b.mdl) }; assert { not (S.mem !col[k] c.mdl) }; assert { eq_prefix (at !col 'L) !col k }; assert { forall i: int. S.mem i a.mdl -> (forall j: int. 0 <= j < k -> !col[j] <> i) }; assert { forall i: int. S.mem i (S.remove (S.min_elt d.mdl) a.mdl) <-> (0 <= i < n /\ (forall j: int. 0 <= j < k + 1 -> !col[j] <> i)) }; let ghost b' = mul2 (add_singleton b d) in assert { forall i: int. 0 <= i < size -> S.mem i b'.mdl -> (i = !min + 1 \/ S.mem (i - 1) b.mdl) && not (forall j:int. 0 <= j < k + 1 -> i - !col[j] <> k + 1 - j) }; let ghost c' = div2 (add_singleton c d) in assert { forall i: int. 0 <= i < size -> S.mem i c'.mdl -> (i = !min - 1 \/ (i + 1 < size /\ S.mem (i + 1) c.mdl)) && not (forall j:int. 0 <= j < k + 1 -> i - !col[j] <> j - k - 1) }; 'M: f := !f + t (remove_singleton a d) (mul2 (add_singleton b d)) (div2 (add_singleton c d)) (k + 1); assert { forall i j. (at !s 'L) <= i < at !s 'M <= j < !s -> (eq_prefix !sol[i] !sol[j] k /\ !sol[i][k] <= at !min 'N < !min = !sol[j][k]) && lt_sol !sol[i] !sol[j]}; e := remove_singleton !e d done; assert { forall u:solution. solution u /\ eq_prefix !col u k -> S.mem u[k] (at !e.mdl 'L) && 0 <= u[k] <= !min }; !f end else begin ghost sol := !sol[!s <- !col]; ghost s := !s + 1; 1 end let queens (q: BV32.t) requires { BV32.t'int q = n } requires { BV32.ule q BV32.size_bv } requires { !s = 0 } ensures { result = !s } ensures { sorted !sol 0 !s } ensures { forall u: solution. solution u <-> exists i: int. 0 <= i < result /\ eq_sol u !sol[i] } = t (below q) (empty()) (empty()) 0 let test8 () requires { size = 32 } requires { n = 8 } = s := 0; queens (BV32.of_int 8) end

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# Why3 Proof Results for Project "queens_bv"

## Theory "queens_bv.Solution": fully verified in 0.08 s

Obligations | Alt-Ergo (0.99.1) | CVC3 (2.4.1) | CVC4 (1.4) |

partial_solution_eq_prefix | 0.03 | --- | --- |

no_duplicate | --- | 0.02 | 0.03 |

## Theory "queens_bv.Bits": fully verified in 2.08 s

Obligations | Alt-Ergo (0.99.1) | Alt-Ergo (1.10.prv) | CVC4 (1.4) | CVC4 (1.4 noBV) | ||

1. VC for empty | 0.03 | --- | --- | --- | ||

2. VC for is_empty | 0.11 | --- | --- | --- | ||

3. VC for remove_singleton | 0.98 | --- | --- | --- | ||

4. VC for add_singleton | --- | --- | --- | 0.08 | ||

5. VC for mul2 | --- | --- | --- | --- | ||

split_goal_wp | ||||||

1. type invariant | --- | --- | --- | --- | ||

split_goal_wp | ||||||

1. type invariant | 0.20 | --- | --- | --- | ||

2. type invariant | --- | --- | 0.07 | --- | ||

3. type invariant | --- | --- | 0.11 | --- | ||

4. type invariant | --- | 0.48 | --- | 0.14 | ||

6. VC for div2 | 0.36 | --- | --- | --- | ||

7. VC for diff | 0.46 | --- | --- | --- | ||

8. VC for rightmost_bit_trick | --- | --- | --- | --- | ||

split_goal_wp | ||||||

1. assertion | --- | --- | --- | 0.08 | ||

2. assertion | 0.16 | --- | --- | --- | ||

3. assertion | --- | --- | 0.08 | --- | ||

4. assertion | 0.16 | --- | --- | --- | ||

5. assertion | --- | --- | 0.13 | --- | ||

6. assertion | 0.56 | --- | --- | --- | ||

7. type invariant | --- | --- | --- | --- | ||

split_goal_wp | ||||||

1. type invariant | 0.11 | --- | --- | --- | ||

2. type invariant | --- | --- | 0.12 | --- | ||

3. type invariant | --- | --- | 0.12 | --- | ||

4. type invariant | 0.52 | --- | --- | --- | ||

8. postcondition | --- | --- | 0.03 | --- | ||

9. VC for below | --- | --- | --- | 0.06 |

## Theory "queens_bv.NQueensBits": fully verified in 0.00 s

Obligations | Alt-Ergo (0.99.1) | Alt-Ergo (1.01) | Alt-Ergo (1.30) | CVC4 (1.4) | CVC4 (1.4 noBV) | ||

1. VC for t | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. assertion | --- | 0.93 | 0.32 | --- | --- | ||

2. loop invariant init | --- | --- | --- | 0.06 | --- | ||

3. loop invariant init | --- | --- | --- | 0.02 | --- | ||

4. loop invariant init | --- | --- | --- | 0.03 | --- | ||

5. loop invariant init | --- | --- | --- | 0.03 | --- | ||

6. loop invariant init | --- | --- | --- | 0.04 | --- | ||

7. loop invariant init | --- | --- | --- | 0.03 | --- | ||

8. loop invariant init | --- | --- | --- | 0.03 | --- | ||

9. loop invariant init | --- | --- | --- | 0.06 | --- | ||

10. loop invariant init | --- | --- | --- | 0.08 | --- | ||

11. loop invariant init | --- | --- | --- | 0.07 | --- | ||

12. type invariant | --- | --- | --- | 0.05 | --- | ||

13. type invariant | --- | --- | --- | 0.10 | --- | ||

14. precondition | --- | --- | --- | 0.08 | --- | ||

15. assertion | --- | --- | --- | 0.12 | --- | ||

16. assertion | --- | --- | --- | 0.45 | --- | ||

17. assertion | --- | --- | --- | 0.09 | --- | ||

18. assertion | --- | --- | --- | 0.13 | --- | ||

19. assertion | --- | --- | --- | 0.11 | --- | ||

20. assertion | --- | --- | --- | 0.70 | --- | ||

21. precondition | --- | --- | --- | 0.14 | --- | ||

22. assertion | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. assertion | --- | --- | --- | 0.29 | --- | ||

2. assertion | --- | --- | --- | 5.80 | --- | ||

23. precondition | --- | --- | --- | 0.09 | --- | ||

24. assertion | --- | --- | --- | 1.46 | --- | ||

25. precondition | --- | --- | --- | 0.10 | --- | ||

26. precondition | --- | --- | --- | 0.10 | --- | ||

27. precondition | --- | --- | --- | 0.10 | --- | ||

28. precondition | --- | --- | --- | 0.12 | --- | ||

29. variant decrease | --- | --- | --- | 0.14 | --- | ||

30. precondition | --- | --- | --- | 0.11 | --- | ||

31. precondition | --- | --- | --- | 0.11 | --- | ||

32. precondition | --- | --- | --- | 0.18 | --- | ||

33. precondition | --- | --- | --- | 0.11 | --- | ||

34. precondition | --- | --- | --- | 0.22 | --- | ||

35. precondition | --- | --- | --- | 0.36 | --- | ||

36. precondition | --- | --- | --- | 0.34 | --- | ||

37. precondition | --- | --- | --- | 0.36 | --- | ||

38. assertion | --- | --- | --- | 0.84 | --- | ||

39. type invariant | --- | --- | --- | 0.18 | --- | ||

40. precondition | --- | --- | --- | 0.15 | --- | ||

41. precondition | --- | --- | --- | 0.21 | --- | ||

42. loop invariant preservation | --- | --- | --- | 0.17 | --- | ||

43. loop invariant preservation | --- | --- | --- | 0.15 | --- | ||

44. loop invariant preservation | --- | --- | --- | 0.10 | --- | ||

45. loop invariant preservation | --- | --- | --- | 0.16 | --- | ||

46. loop invariant preservation | --- | --- | --- | 0.15 | --- | ||

47. loop invariant preservation | --- | --- | --- | 0.20 | --- | ||

48. loop invariant preservation | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. loop invariant preservation | --- | --- | --- | 0.15 | --- | ||

2. loop invariant preservation | --- | --- | --- | 2.64 | --- | ||

3. loop invariant preservation | --- | --- | --- | 0.29 | --- | ||

4. loop invariant preservation | --- | --- | --- | 0.79 | --- | ||

49. loop invariant preservation | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. loop invariant preservation | --- | --- | --- | 0.44 | --- | ||

2. loop invariant preservation | --- | --- | --- | 0.11 | --- | ||

3. loop invariant preservation | --- | --- | --- | 32.21 | 6.44 | ||

50. loop invariant preservation | --- | --- | --- | 0.11 | --- | ||

51. loop invariant preservation | --- | --- | --- | 0.10 | --- | ||

52. loop variant decrease | --- | --- | --- | 0.18 | --- | ||

53. assertion | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. assertion | --- | --- | --- | 0.09 | --- | ||

2. assertion | --- | --- | --- | 0.04 | --- | ||

3. assertion | --- | --- | --- | 0.06 | --- | ||

54. postcondition | --- | --- | --- | 0.08 | --- | ||

55. postcondition | --- | --- | --- | 0.08 | --- | ||

56. postcondition | --- | --- | --- | 0.09 | --- | ||

57. postcondition | --- | --- | --- | 0.05 | --- | ||

58. postcondition | --- | --- | --- | 0.07 | --- | ||

59. postcondition | --- | --- | --- | 0.07 | --- | ||

60. postcondition | --- | --- | --- | 0.04 | --- | ||

61. postcondition | --- | --- | --- | 0.07 | --- | ||

62. postcondition | 0.18 | --- | --- | --- | --- | ||

63. postcondition | 0.19 | --- | --- | --- | --- | ||

64. postcondition | --- | --- | --- | 0.07 | --- | ||

65. postcondition | --- | --- | --- | 0.08 | --- | ||

2. VC for queens | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. precondition | --- | --- | --- | 0.06 | --- | ||

2. precondition | --- | --- | --- | 0.08 | --- | ||

3. precondition | --- | --- | --- | 0.04 | --- | ||

4. precondition | --- | --- | --- | 0.12 | --- | ||

5. precondition | --- | --- | --- | 0.04 | --- | ||

6. precondition | --- | --- | --- | 0.10 | --- | ||

7. precondition | --- | --- | --- | 0.10 | --- | ||

8. precondition | --- | --- | --- | 0.10 | --- | ||

9. precondition | --- | --- | --- | 0.09 | --- | ||

10. postcondition | --- | --- | --- | 0.08 | --- | ||

11. postcondition | --- | --- | --- | 0.08 | --- | ||

12. postcondition | --- | --- | --- | 0.87 | --- | ||

3. VC for test8 | --- | --- | --- | --- | --- | ||

split_goal_wp | |||||||

1. precondition | --- | --- | --- | 0.04 | --- | ||

2. precondition | --- | --- | --- | 0.04 | --- | ||

3. precondition | --- | --- | --- | 0.05 | --- |