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Z3 excels at solving constraint satisfaction problems (CSPs) where you need to find values that satisfy a set of constraints. This use case is fundamental to scheduling, resource allocation, planning, and configuration management.

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When to Use Constraint Solving

Use Z3 for constraint solving when you have:
  • Scheduling problems: Assign tasks to time slots or resources
  • Planning problems: Find sequences of actions to achieve a goal
  • Configuration problems: Determine valid system configurations
  • Resource allocation: Distribute limited resources optimally
  • Puzzle solving: Sudoku, n-queens, graph coloring, etc.

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Core Concepts

Constraint solving in Z3 involves:
  1. Variables: Declare decision variables with appropriate types (Int, Bool, BitVec, etc.)
  2. Domains: Specify the valid range of values for each variable
  3. Constraints: Express relationships between variables
  4. Solver: Check satisfiability and extract solutions

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Example: Sudoku Solver

Sudoku is a classic constraint satisfaction problem where you fill a 9×9 grid with digits 1-9 such that each row, column, and 3×3 box contains all digits exactly once. 
from z3 import *

# Create 9x9 matrix of integer variables
X = [[Int(f"x_{i}_{j}") for j in range(9)] for i in range(9)]

s = Solver()

# Each cell contains a value in {1, ..., 9}
for i in range(9):
    for j in range(9):
        s.add(And(1 <= X[i][j], X[i][j] <= 9))

# Each row contains distinct values
for i in range(9):
    s.add(Distinct(X[i]))

# Each column contains distinct values
for j in range(9):
    s.add(Distinct([X[i][j] for i in range(9)]))

# Each 3x3 square contains distinct values
for i0 in range(3):
    for j0 in range(3):
        square = [X[3*i0 + i][3*j0 + j] for i in range(3) for j in range(3)]
        s.add(Distinct(square))

# Add initial values (puzzle instance)
instance = [
    [0, 0, 0, 0, 9, 4, 0, 3, 0],
    [0, 0, 0, 5, 1, 0, 0, 0, 7],
    # ... more rows
]

for i in range(9):
    for j in range(9):
        if instance[i][j] != 0:
            s.add(X[i][j] == instance[i][j])

# Solve and extract solution
if s.check() == sat:
    m = s.model()
    for i in range(9):
        for j in range(9):
            print(m.evaluate(X[i][j]), end=" ")
        print()

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Example: Scheduling Problem

Schedule tasks with dependencies and resource constraints: 
from z3 import *

# Task durations
tasks = {
    'A': 3,  # Task A takes 3 time units
    'B': 2,
    'C': 4,
    'D': 1
}

# Task start times (decision variables)
start = {name: Int(f"start_{name}") for name in tasks}

s = Solver()

# Tasks must start at non-negative times
for name in tasks:
    s.add(start[name] >= 0)

# Precedence constraints: B must follow A
s.add(start['B'] >= start['A'] + tasks['A'])

# Resource constraints: B and C cannot overlap (same resource)
s.add(Or(
    start['B'] + tasks['B'] <= start['C'],
    start['C'] + tasks['C'] <= start['B']
))

# Minimize total completion time
completion_time = Int('completion_time')
for name in tasks:
    s.add(completion_time >= start[name] + tasks[name])

if s.check() == sat:
    m = s.model()
    print("Schedule:")
    for name in tasks:
        print(f"Task {name}: start at {m.evaluate(start[name])}")
    print(f"Total time: {m.evaluate(completion_time)}")

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Example: All-Interval Series

The All-Interval Series Problem finds sequences where adjacent differences form a permutation: 
from z3 import *

def all_interval_series(n):
    # Create boolean variables x[i][j]: integer i is at position j
    xij = [[Bool(f"x_{i}_{j}") for j in range(n)] for i in range(n)]
    
    s = SolverFor("QF_FD")  # Finite domain solver
    
    # Each position has exactly one integer
    for i in range(n):
        s.add(AtMost(xij[i] + [1]))  # At most one true
        s.add(Or(xij[i]))             # At least one true
    
    # Each integer appears exactly once
    for j in range(n):
        xi = [xij[i][j] for i in range(n)]
        s.add(AtMost(xi + [1]))
        s.add(Or(xi))
    
    # Constraint on differences between adjacent positions
    def diff_at_j_is_i(xs, j, i):
        return Or(
            [And(xs[j][k], xs[j+1][k-i]) for k in range(i, n)] +
            [And(xs[j][k], xs[j+1][k+i]) for k in range(0, n-i)]
        )
    
    dji = [[diff_at_j_is_i(xij, j, i+1) for i in range(n-1)] 
           for j in range(n-1)]
    
    # Each difference appears exactly once
    for j in range(n-1):
        s.add(AtMost(dji[j] + [1]))
        s.add(Or(dji[j]))
    
    for i in range(n-1):
        dj = [dji[j][i] for j in range(n-1)]
        s.add(AtMost(dj + [1]))
        s.add(Or(dj))
    
    return s, xij

s, xij = all_interval_series(8)
if s.check() == sat:
    m = s.model()
    sequence = []
    for i in range(8):
        for j in range(8):
            if is_true(m.eval(xij[i][j])):
                sequence.append(j)
    print(f"Solution: {sequence}")

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Choosing the Right Solver

Z3 provides specialized solvers for different constraint types:
  • Solver(): General-purpose SMT solver
  • SolverFor("QF_FD"): Finite domain solver for Boolean, bit-vector, and bounded integer constraints
  • Optimize(): For optimization problems with objective functions
For pure constraint satisfaction with finite domains, use SolverFor("QF_FD") for better performance.

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Optimization Techniques

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Breaking Symmetries

Add constraints to eliminate symmetric solutions: 
# For n-queens, fix first queen position
s.add(queens[0] == 0)

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Using Cardinality Constraints

Use AtMost, AtLeast, and PbEq for efficient encoding: 
# Exactly k of the booleans must be true
s.add(PbEq([(b, 1) for b in bools], k))

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Incremental Solving

Use push/pop for exploring variations: 
s.push()
s.add(additional_constraint)
if s.check() == sat:
    # Process solution
s.pop()
  • Solvers: Understanding solver APIs
  • Models: Extracting and interpreting solutions
  • Tactics: Solver strategies and preprocessing

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Further Examples

See the Z3 repository for more constraint solving examples:
  • examples/python/trafficjam.py: Rush Hour puzzle solver using Datalog
  • examples/python/all_interval_series.py: Complete implementation
  • examples/java/JavaExample.java: Sudoku solver in Java

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