fix: allow fixed length strings to be created

Refs: JF-73

Author: akutschera

src/main/java/com/github/nylle/javafixture/PseudoRandom.java

@@ -46,10 +46,17 @@ public Double nextDouble(boolean positiveOnly) {
     public String nextString(StringConstraints constraints) {
         validate(constraints.getMin(), constraints.getMax());
         return Stream.generate(() -> "" + letters[random.nextInt(letters.length)])
-                .limit(constraints.getMin() + Math.min(128, random.nextInt(constraints.getMax() - constraints.getMin())))
+                .limit(constraints.getMin() + computeMaxBound(constraints))
                 .collect(Collectors.joining());
     }
 
+    private int computeMaxBound(StringConstraints constraints) {
+        if (constraints.getMin() == constraints.getMax()) {
+            return 0;
+        }
+        return Math.min(128, random.nextInt(constraints.getMax() - constraints.getMin()));
+    }
+
     private void validate(int min, int max) {
         if (min < 0) {
             throw new SpecimenException("minimum must be non-negative");

src/test/java/com/github/nylle/javafixture/PseudoRandomTest.java

@@ -50,6 +50,7 @@ void nextDouble(boolean onlyPositive, double min, double max) {
     @TestCase(int1 = 0, int2 = Integer.MAX_VALUE, int3 = 128)
     @TestCase(int1 = 37, int2 = 52, int3 = 52)
     @TestCase(int1 = 0, int2 = 4, int3 = 4)
+    @TestCase(int1 = 4, int2 = 4, int3 = 4)
     @TestCase(int1 = 1024, int2 = Integer.MAX_VALUE, int3 = 1024+128)
     @TestCase(int1 = 1025, int2 = Integer.MAX_VALUE, int3 = 1025+128)
     @DisplayName("nextString will return a random string with at least min and at most min+128 (or max, whichever is less) characters")