I assume that you have some sorter function with signature
List<String> sortFooBar(List<String> list)
I see at least five properties that sortFooBar(list)
should fulfill:
- Keep all items - and only those - in the list
- No item before first "foo"
- No other items between first and last "foo"
- No item after last "bar"
- No other item between first and last "bar"
In a real functional language those properties are all rather easy to formulate in Java it requires a bit of code. So here's my take on the problem using jqwik as PBT framework and AssertJ for assertions:
import java.util.*;
import java.util.function.*;
import org.assertj.core.api.*;
import net.jqwik.api.*;
class MySorterProperties {
@Property
void allItemsAreKept(@ForAll List<@From("withFooBars") String> list) {
List<String> sorted = MySorter.sortFooBar(list);
Assertions.assertThat(sorted).containsExactlyInAnyOrderElementsOf(list);
}
@Property
void noItemBeforeFoo(@ForAll List<@From("withFooBars") String> list) {
List<String> sorted = MySorter.sortFooBar(list);
int firstFoo = findFirst(sorted, item -> item.startsWith("foo"));
if (firstFoo < 0) return;
Assertions.assertThat(sorted.stream().limit(firstFoo)).isEmpty();
}
@Property
void noItemBetweenFoos(@ForAll List<@From("withFooBars") String> list) {
List<String> sorted = MySorter.sortFooBar(list);
int firstFoo = findFirst(sorted, item -> item.startsWith("foo"));
int lastFoo = findLast(sorted, item -> item.startsWith("foo"));
if (firstFoo < 0 && lastFoo < 0) return;
List<String> allFoos = sorted.subList(
Math.max(firstFoo, 0),
lastFoo >= 0 ? lastFoo + 1 : sorted.size()
);
Assertions.assertThat(allFoos).allMatch(item -> item.startsWith("foo"));
}
@Property
void noItemAfterBar(@ForAll List<@From("withFooBars") String> list) {
List<String> sorted = MySorter.sortFooBar(list);
int lastBar = findLast(sorted, item -> item.startsWith("bar"));
if (lastBar < 0) return;
Assertions.assertThat(sorted.stream().skip(lastBar + 1)).isEmpty();
}
@Property
void noItemBetweenBars(@ForAll List<@From("withFooBars") String> list) {
List<String> sorted = MySorter.sortFooBar(list);
int firstBar = findFirst(sorted, item -> item.startsWith("bar"));
int lastBar = findLast(sorted, item -> item.startsWith("bar"));
if (firstBar < 0 && lastBar < 0) return;
List<String> allFoos = sorted.subList(
Math.max(firstBar, 0),
lastBar >= 0 ? lastBar + 1 : sorted.size()
);
Assertions.assertThat(allFoos).allMatch(item -> item.startsWith("bar"));
}
@Provide
Arbitrary<String> withFooBars() {
Arbitrary<String> postFix = Arbitraries.strings().alpha().ofMaxLength(10);
return Arbitraries.oneOf(
postFix, postFix.map(post -> "foo" + post), postFix.map(post -> "bar" + post)
);
}
int findFirst(List<String> list, Predicate<String> condition) {
for (int i = 0; i < list.size(); i++) {
String item = list.get(i);
if (condition.test(item)) {
return i;
}
}
return -1;
}
int findLast(List<String> list, Predicate<String> condition) {
for (int i = list.size() - 1; i >= 0; i--) {
String item = list.get(i);
if (condition.test(item)) {
return i;
}
}
return -1;
}
}
And this is a naive implementation that is consistent with the spec:
class MySorter {
static List<String> sortFooBar(List<String> in) {
ArrayList<String> result = new ArrayList<>();
int countFoos = 0;
for (String item : in) {
if (item.startsWith("foo")) {
result.add(0, item);
countFoos++;
} else if (item.startsWith("bar")) {
result.add(result.size(), item);
} else {
result.add(countFoos, item);
}
}
return result;
}
}
In this example the code for the properties exceeds the amount of code for the implementation. This might be good or bad depending on how tricky the desired behaviour is.