TST: Thoroughly test prng for entropy and uniform values

tests/prng.bats

@@ -6,24 +6,37 @@ true
 # shellcheck source=../parallel_opts.sh
 . "${BATS_TEST_DIRNAME}/../examples/script_that_sometimes_fails.sh"
 
-@test 'prng fails at least 4 times' {
+@test 'prng produces a uniform distribution with a single zero, and has entropy' {
     # shellcheck disable=2034
     SLURM_NTASKS=5
 
+    not_sorted=0
     for seed in {0..9}
     do
 	result=()
-	sum=0
+	num_zeros=0
 	for ID in {0..4}
 	do
 	    x=$(prng)
-	    if (( x % SLURM_NTASKS != 0 ))
+	    if (( x % SLURM_NTASKS == 0 ))
 	    then
-		sum=$(( sum + 1 ))
+		num_zeros=$(( num_zeros + 1 ))
 	    fi
 	    result[ID]=$x
 	done
-	echo "seed: $seed, prng: ${result[*]}, sum: $sum"
-	[[ $sum -ge 4 ]]
+	echo "seed: $seed, prng: ${result[*]}, num_zeros: $num_zeros"
+	[[ $num_zeros -eq 1 ]]
+	# Verify it contains all the values / is a uniform distribution.
+	expected=( $(seq 0 4) )
+	IFS=$'\n' sorted=($(sort <<<"${result[*]}"))
+	unset IFS
+	[[ "${sorted[*]}" = "${expected[*]}" ]]
+	if [[ "${sorted[*]}" != "${result[*]}" ]]
+	then
+	    not_sorted=$(( not_sorted + 1))
+	fi
     done
+    # Check for entropy.
+    echo "not_sorted: $not_sorted"
+    [ $not_sorted -ge 6 ]
 }