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GrayCode.java
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GrayCode.java
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package epi;
import epi.test_framework.EpiTest;
import epi.test_framework.GenericTest;
import epi.test_framework.TestFailure;
import epi.test_framework.TimedExecutor;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class GrayCode {
public static List<Integer> grayCode(int numBits) {
if (numBits == 0) {
return new ArrayList<>(List.of(0));
}
// These implicitly begin with 0 at bit-index (numBits - 1).
List<Integer> grayCodeNumBitsMinus1 = grayCode(numBits - 1);
// Now, add a 1 at bit-index (numBits - 1) to all entries in
// grayCodeNumBitsMinus1.
int leadingBitOne = 1 << (numBits - 1);
// Process in reverse order to achieve reflection of grayCodeNumBitsMinus1.
for (int i = grayCodeNumBitsMinus1.size() - 1; i >= 0; --i) {
grayCodeNumBitsMinus1.add(leadingBitOne | grayCodeNumBitsMinus1.get(i));
}
return grayCodeNumBitsMinus1;
}
private static boolean differsByOneBit(int x, int y) {
int bitDifference = x ^ y;
return bitDifference != 0 && (bitDifference & (bitDifference - 1)) == 0;
}
@EpiTest(testDataFile = "gray_code.tsv")
public static void grayCodeWrapper(TimedExecutor executor, int numBits)
throws Exception {
List<Integer> result = executor.run(() -> grayCode(numBits));
int expectedSize = (1 << numBits);
if (result.size() != expectedSize) {
throw new TestFailure("Length mismatch: expected " +
String.valueOf(expectedSize) + ", got " +
String.valueOf(result.size()));
}
for (int i = 1; i < result.size(); i++)
if (!differsByOneBit(result.get(i - 1), result.get(i))) {
if (result.get(i - 1).equals(result.get(i))) {
throw new TestFailure("Two adjacent entries are equal");
} else {
throw new TestFailure(
"Two adjacent entries differ by more than 1 bit");
}
}
Set<Integer> uniq = new HashSet<>(result);
if (uniq.size() != result.size()) {
throw new TestFailure("Not all entries are distinct: found " +
String.valueOf(result.size() - uniq.size()) +
" duplicates");
}
}
public static void main(String[] args) {
System.exit(
GenericTest
.runFromAnnotations(args, "GrayCode.java",
new Object() {}.getClass().getEnclosingClass())
.ordinal());
}
}