This is simplest possible backtracking SAT solver written in Python (not a DPLL one). It uses the same backtracking algorithm you can find in many simple Sudoku and 8 queens solvers. It works significantly slower, but due to its extreme simplicity, it can also count solutions. For example, it count all solutions of 8 queens problem. Also, there are 70 solutions for POPCNT4 function (the function is true if any 4 of its input 8 variables are true):

SAT -1 -2 -3 -4 5 6 7 8 0 SAT -1 -2 -3 4 -5 6 7 8 0 SAT -1 -2 -3 4 5 -6 7 8 0 SAT -1 -2 -3 4 5 6 -7 8 0 ... SAT 1 2 3 -4 -5 6 -7 -8 0 SAT 1 2 3 -4 5 -6 -7 -8 0 SAT 1 2 3 4 -5 -6 -7 -8 0 UNSAT solutions= 70

It was also tested on my Minesweeper cracker, and finishes in reasonable time (though, slower than MiniSat by a factor of ~10).

On bigger CNF instances, it gets stuck.

Source code:

#!/usr/bin/env python count_solutions=True #count_solutions=False import sys def read_text_file (fname): with open(fname) as f: content = f.readlines() return [x.strip() for x in content] def read_DIMACS (fname): content=read_text_file(fname) header=content[0].split(" ") assert header[0]=="p" and header[1]=="cnf" variables_total, clauses_total = int(header[2]), int(header[3]) # array idx=number (of line) of clause # val=list of terms # term can be negative signed integer clauses=[] for c in content[1:]: clause=[] for var_s in c.split(" "): var=int(var_s) if var!=0: clause.append(var) clauses.append(clause) # this is variables index. # for each variable, it has list of clauses, where this variable is used. # key=variable # val=list of numbers of clause variables_idx={} for i in range(len(clauses)): for term in clauses[i]: variables_idx.setdefault(abs(term), []).append(i) return clauses, variables_idx # clause=list of terms. signed integer. -x means negated. # values=list of values: from 0th: [F,F,F,F,T,F,T,T...] def eval_clause (terms, values): try: # we search for at least one True for t in terms: # variable is non-negated: if t>0 and values[t-1]==True: return True # variable is negated: if t<0 and values[(-t)-1]==False: return True # all terms enumerated at this point return False except IndexError: # values[] has not enough values # None means "maybe" return None def chk_vals(clauses, variables_idx, vals): # check only clauses which affected by the last (new/changed) value, ignore the rest # because since we already got here, all other values are correct, so no need to recheck them idx_of_last_var=len(vals) # variable can be absent in index, because no clause uses it: if idx_of_last_var not in variables_idx: return True # enumerate clauses which has this variable: for clause_n in variables_idx[idx_of_last_var]: clause=clauses[clause_n] # if any clause evaluated to False, stop checking, new value is incorrect: if eval_clause (clause, vals)==False: return False # all clauses evaluated to True or None ("maybe") return True def print_vals(vals): # enumerate all vals[] # prepend "-" if vals[i] is False (i.e., negated). print "".join([["-",""][vals[i]] + str(i+1) + " " for i in range(len(vals))])+"0" clauses, variables_idx = read_DIMACS(sys.argv[1]) solutions=0 def backtrack(vals): global solutions if len(vals)==len(variables_idx): # we reached end - all values are correct print "SAT" print_vals(vals) if count_solutions: solutions=solutions+1 # go back, if we need more solutions: return else: exit(10) # as in MiniSat return for next in [False, True]: # add new value: new_vals=vals+[next] if chk_vals(clauses, variables_idx, new_vals): # new value is correct, try add another one: backtrack(new_vals) else: # new value (False) is not correct, now try True: continue # try to find all values: backtrack([]) print "UNSAT" if count_solutions: print "solutions=", solutions exit(20) # as in MiniSat

As you can see, all it does is enumerate all possible solutions, but prunes search tree as early as possible. This is backtracking.

The files: https://github.com/dennis714/yurichev.com/tree/master/blog/SAT_backtrack.

My other notes about SAT/SMT: https://yurichev.com/writings/SAT_SMT_draft-EN.pdf.

Some comments: https://www.reddit.com/r/compsci/comments/6jn3th/simplest_sat_solver_in_120_lines/.

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