Mutation Testing for Test Quality

/* Automotive-relevant mutation operators for safety-critical code */

/* Original code */
if (speed_kmh > SPEED_LIMIT_KMH) {
    FaultManager_SetFault(FAULT_OVERSPEED);
}

/* Mutation 1: relational operator change (> to >=) */
if (speed_kmh >= SPEED_LIMIT_KMH) {  // MUTANT -- boundary error
    FaultManager_SetFault(FAULT_OVERSPEED);
}

/* Mutation 2: logical operator change (&& to ||) */
/* Original: fault only when speed AND sensor valid */
if (speed_ok && sensor_valid) {  // original
if (speed_ok || sensor_valid) {  // MUTANT -- fault condition weakened

/* Mutation 3: constant replacement */
#define TIMEOUT_MS 500   // original
#define TIMEOUT_MS 499   // MUTANT -- off-by-one in timeout

/* Mutation 4: negation removal */
if (!fault_acknowledged) {  // original: only act if NOT acknowledged
if (fault_acknowledged)  {  // MUTANT -- inverted condition

/* These 4 mutation types cover the most safety-critical errors.
 * A good test suite must kill ALL of these mutants. */

#!/bin/bash
# Run mutation testing on SpeedController SiL binary
# Tool: mutmut (Python-based) or custom C mutator

# For C code: use custom mutation script
python3 mutate_c.py \
    --source src/SpeedController.c \
    --operators "relational,logical,arithmetic,constant" \
    --output mutations/

# Run tests against each mutant
TOTAL=0; KILLED=0
for mutant in mutations/*.c; do
    TOTAL=$((TOTAL+1))
    # Build mutant binary
    cp "$mutant" src/SpeedController.c
    cmake --build build/ --target SpeedController_sil -q 2>/dev/null

    # Run test suite against mutant
    if ! python3 -m pytest tests/ -q 2>/dev/null; then
        KILLED=$((KILLED+1))
    else
        echo "SURVIVED: $mutant"
    fi
done

# Restore original
git checkout src/SpeedController.c

SCORE=$(echo "scale=1; $KILLED * 100 / $TOTAL" | bc)
echo "Mutation score: $SCORE% ($KILLED/$TOTAL killed)"
[ "${SCORE%.*}" -ge 85 ] || { echo "FAIL: score < 85%"; exit 1; }