Random: ensure seed is non-zero

Random has accepted a seed value of zero and then thrown an exception.
This is not necessary, since we can simply use a non-zero seed value.

include/lsst/afw/math/Random.h

@@ -100,32 +100,28 @@ class Random {
     // -- Constructor --------
     /**
      * Creates a random number generator that uses the given algorithm to produce random numbers,
-     * and seeds it with the specified value. Passing a seed-value of zero will cause the
-     * generator to be seeded with an algorithm specific default value. The default value for
+     * and seeds it with the specified value. The default value for
      * `algorithm` is MT19937, corresponding to the "Mersenne Twister" algorithm by
      * Makoto Matsumoto and Takuji Nishimura.
      *
      * @param[in] algorithm     the algorithm to use for random number generation
      * @param[in] seed          the seed value to initialize the generator with
      *
      * @throws lsst::pex::exceptions::InvalidParameterError
-     *      Thrown if the requested algorithm is not supported or a seed value of zero
-     *      (corresponding to an algorithm specific seed) is chosen.
+     *      Thrown if the requested algorithm is not supported.
      * @throws lsst::pex::exceptions::MemoryError
      *      Thrown if memory allocation for internal generator state fails.
      */
     explicit Random(Algorithm algorithm = MT19937, unsigned long seed = 1);
     /**
      * Creates a random number generator that uses the algorithm with the given name to produce
-     * random numbers, and seeds it with the specified value. Passing a seed-value of zero will
-     * cause the generator to be seeded with an algorithm specific default value.
+     * random numbers, and seeds it with the specified value.
      *
      * @param[in] algorithm     the name of the algorithm to use for random number generation
      * @param[in] seed          the seed value to initialize the generator with
      *
      * @throws lsst::pex::exceptions::InvalidParameterError
-     *      Thrown if the requested algorithm is not supported or a seed value of zero
-     *      (corresponding to an algorithm specific seed) is chosen.
+     *      Thrown if the requested algorithm is not supported.
      * @throws lsst::pex::exceptions::MemoryError
      *      Thrown if memory allocation for internal generator state fails.
      */
@@ -145,8 +141,6 @@ class Random {
      *      Thrown if the requested algorithm is not supported.
      * @throws lsst::pex::exceptions::MemoryError
      *      Thrown if memory allocation for internal generator state fails.
-     * @throws lsst::pex::exceptions::RuntimeError
-     *      Thrown if the "rngSeed" policy value cannot be converted to an unsigned long int.
      */
     explicit Random(std::shared_ptr<pex::policy::Policy> const policy);
     // Use compiler generated destructor and shallow copy constructor/assignment operator
@@ -194,8 +188,7 @@ class Random {
     static std::vector<std::string> const &getAlgorithmNames();
     /**
      * @returns  The integer this random number generator was seeded with.
-     * @note    A seed value of 0 indicates that the random number generator
-     *          was seeded with an algorithm specific default value.
+     * @note    The seed is guaranteed not to be zero.
      */
     unsigned long getSeed() const;
 
@@ -290,9 +283,6 @@ class Random {
 
     /**
      * Initializes the underlying GSL random number generator.
-     *
-     * @throws lsst::pex::exceptions::InvalidParameterError
-     *      Thrown if a seed value of zero (corresponding to an algorithm specific seed) is chosen.
      */
     void initialize();
     /**
@@ -301,8 +291,7 @@ class Random {
      * @param[in] algorithm     the algorithm to use for random number generation
      *
      * @throws lsst::pex::exceptions::InvalidParameterError
-     *      Thrown if the requested algorithm is not supported or a seed value of zero
-     *      (corresponding to an algorithm specific seed) is chosen.
+     *      Thrown if the requested algorithm is not supported.
      */
     void initialize(std::string const &);
 };

src/math/Random.cc

@@ -27,6 +27,7 @@
  */
 
 #include <cstdlib>
+#include <limits>
 
 #include "boost/format.hpp"
 
@@ -62,14 +63,14 @@ char const *const Random::_seedEnvVarName = "LSST_RNG_SEED";
 // -- Private helper functions --------
 
 void Random::initialize() {
-    if (_seed == 0) {
-        throw LSST_EXCEPT(ex::InvalidParameterError, (boost::format("Invalid RNG seed: %lu") % _seed).str());
-    }
     ::gsl_rng *rng = ::gsl_rng_alloc(_gslRngTypes[_algorithm]);
     if (rng == 0) {
         throw LSST_EXCEPT(ex::MemoryError, "gsl_rng_alloc() failed");
     }
-    ::gsl_rng_set(rng, _seed);
+    // This seed is guaranteed to be non-zero.
+    // We want to give a non-zero seed to GSL to avoid it choosing its own.
+    unsigned long int useSeed = _seed == 0 ? std::numeric_limits<unsigned long int>::max() : _seed;
+    ::gsl_rng_set(rng, useSeed);
     _rng.reset(rng, ::gsl_rng_free);
 }