diff --git a/src/main/java/org/apache/commons/text/similarity/DamerauLevenshteinDistance.java b/src/main/java/org/apache/commons/text/similarity/DamerauLevenshteinDistance.java
new file mode 100644
index 0000000000..d7685ebc29
--- /dev/null
+++ b/src/main/java/org/apache/commons/text/similarity/DamerauLevenshteinDistance.java
@@ -0,0 +1,326 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.text.similarity;
+
+/**
+ * An algorithm for measuring the difference between two character sequences using the
+ * Damerau-Levenshtein Distance.
+ *
+ *
+ * This is the number of changes needed to change one sequence into another, where each change is a single character
+ * modification (deletion, insertion, substitution, or transposition of two adjacent characters).
+ *
+ *
+ * @see Damerau-Levenshtein Distance on Wikipedia
+ * @since 1.15.0
+ */
+public class DamerauLevenshteinDistance implements EditDistance {
+
+ /**
+ * Utility function to ensure distance is valid according to threshold.
+ *
+ * @param distance The distance value
+ * @param threshold The threshold value
+ * @return The distance value, or {@code -1} if distance is greater than threshold
+ */
+ private static int clampDistance(final int distance, final int threshold) {
+ return distance > threshold ? -1 : distance;
+ }
+
+ /**
+ * Finds the Damerau-Levenshtein distance between two CharSequences if it's less than or equal to a given threshold.
+ *
+ * @param left the first SimilarityInput, must not be null.
+ * @param right the second SimilarityInput, must not be null.
+ * @param threshold the target threshold, must not be negative.
+ * @return result distance, or -1 if distance exceeds threshold
+ */
+ private static int limitedCompare(SimilarityInput left, SimilarityInput right, final int threshold) {
+ if (left == null || right == null) {
+ throw new IllegalArgumentException("Left/right inputs must not be null");
+ }
+
+ if (threshold < 0) {
+ throw new IllegalArgumentException("Threshold can not be negative");
+ }
+
+ // Implementation based on https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance#Optimal_string_alignment_distance
+
+ int leftLength = left.length();
+ int rightLength = right.length();
+
+ if (leftLength == 0) {
+ return clampDistance(rightLength, threshold);
+ }
+
+ if (rightLength == 0) {
+ return clampDistance(leftLength, threshold);
+ }
+
+ // Inspired by LevenshteinDistance impl; swap the input strings to consume less memory
+ if (rightLength > leftLength) {
+ final SimilarityInput tmp = left;
+ left = right;
+ right = tmp;
+ leftLength = rightLength;
+ rightLength = right.length();
+ }
+
+ // If the difference between the lengths of the strings is greater than the threshold, we must at least do
+ // threshold operations so we can return early
+ if (leftLength - rightLength > threshold) {
+ return -1;
+ }
+
+ // Use three arrays of minimum possible size to reduce memory usage. This avoids having to create a 2D
+ // array of size leftLength * rightLength
+ int[] curr = new int[rightLength + 1];
+ int[] prev = new int[rightLength + 1];
+ int[] prevPrev = new int[rightLength + 1];
+ int[] temp; // Temp variable use to shuffle arrays at the end of each iteration
+
+ int rightIndex, leftIndex, cost, minCost;
+
+ // Changing empty sequence to [0..i] requires i insertions
+ for (rightIndex = 0; rightIndex <= rightLength; rightIndex++) {
+ prev[rightIndex] = rightIndex;
+ }
+
+ // Calculate how many operations it takes to change right[0..rightIndex] into left[0..leftIndex]
+ // For each iteration
+ // - curr[i] contains the cost of changing right[0..i] into left[0..leftIndex]
+ // (computed in current iteration)
+ // - prev[i] contains the cost of changing right[0..i] into left[0..leftIndex - 1]
+ // (computed in previous iteration)
+ // - prevPrev[i] contains the cost of changing right[0..i] into left[0..leftIndex - 2]
+ // (computed in iteration before previous)
+ for (leftIndex = 1; leftIndex <= leftLength; leftIndex++) {
+ // For right[0..0] we must insert leftIndex characters, which means the cost is always leftIndex
+ curr[0] = leftIndex;
+
+ minCost = Integer.MAX_VALUE;
+
+ for (rightIndex = 1; rightIndex <= rightLength; rightIndex++) {
+ cost = (left.at(leftIndex - 1) == right.at(rightIndex - 1)) ? 0 : 1;
+
+ // Select cheapest operation
+ curr[rightIndex] = Math.min(
+ Math.min(
+ prev[rightIndex] + 1, // Delete current character
+ curr[rightIndex - 1] + 1 // Insert current character
+ ),
+ prev[rightIndex - 1] + cost // Replace (or no cost if same character)
+ );
+
+ // Check if adjacent characters are the same -> transpose if cheaper
+ if (leftIndex > 1
+ && rightIndex > 1
+ && left.at(leftIndex - 1) == right.at(rightIndex - 2)
+ && left.at(leftIndex - 2) == right.at(rightIndex - 1)) {
+ // Use cost here, to properly handle two subsequent equal letters
+ curr[rightIndex] = Math.min(curr[rightIndex], prevPrev[rightIndex - 2] + cost);
+ }
+
+ minCost = Math.min(curr[rightIndex], minCost);
+ }
+
+ // If there was no total cost for this entire iteration to transform right to left[0..leftIndex], there
+ // can not be a way to do it below threshold. This is because we have no way to reduce the overall cost
+ // in later operations.
+ if (minCost > threshold) {
+ return -1;
+ }
+
+ // Rotate arrays for next iteration
+ temp = prevPrev;
+ prevPrev = prev;
+ prev = curr;
+ curr = temp;
+ }
+
+ // Prev contains the value computed in the latest iteration
+ return clampDistance(prev[rightLength], threshold);
+ }
+
+ /**
+ * Finds the Damerau-Levenshtein distance between two inputs using optimal string alignment.
+ *
+ * @param left the first CharSequence, must not be null.
+ * @param right the second CharSequence, must not be null.
+ * @return result distance.
+ * @throws IllegalArgumentException if either CharSequence input is {@code null}.
+ */
+ private static int unlimitedCompare(SimilarityInput left, SimilarityInput right) {
+ if (left == null || right == null) {
+ throw new IllegalArgumentException("Left/right inputs must not be null");
+ }
+
+ /*
+ * Implementation based on https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance#Optimal_string_alignment_distance
+ */
+
+ int leftLength = left.length();
+ int rightLength = right.length();
+
+ if (leftLength == 0) {
+ return rightLength;
+ }
+
+ if (rightLength == 0) {
+ return leftLength;
+ }
+
+ // Inspired by LevenshteinDistance impl; swap the input strings to consume less memory
+ if (rightLength > leftLength) {
+ final SimilarityInput tmp = left;
+ left = right;
+ right = tmp;
+ leftLength = rightLength;
+ rightLength = right.length();
+ }
+
+ // Use three arrays of minimum possible size to reduce memory usage. This avoids having to create a 2D
+ // array of size leftLength * rightLength
+ int[] curr = new int[rightLength + 1];
+ int[] prev = new int[rightLength + 1];
+ int[] prevPrev = new int[rightLength + 1];
+ int[] temp; // Temp variable use to shuffle arrays at the end of each iteration
+
+ int rightIndex, leftIndex, cost;
+
+ // Changing empty sequence to [0..i] requires i insertions
+ for (rightIndex = 0; rightIndex <= rightLength; rightIndex++) {
+ prev[rightIndex] = rightIndex;
+ }
+
+ // Calculate how many operations it takes to change right[0..rightIndex] into left[0..leftIndex]
+ // For each iteration
+ // - curr[i] contains the cost of changing right[0..i] into left[0..leftIndex]
+ // (computed in current iteration)
+ // - prev[i] contains the cost of changing right[0..i] into left[0..leftIndex - 1]
+ // (computed in previous iteration)
+ // - prevPrev[i] contains the cost of changing right[0..i] into left[0..leftIndex - 2]
+ // (computed in iteration before previous)
+ for (leftIndex = 1; leftIndex <= leftLength; leftIndex++) {
+ // For right[0..0] we must insert leftIndex characters, which means the cost is always leftIndex
+ curr[0] = leftIndex;
+
+ for (rightIndex = 1; rightIndex <= rightLength; rightIndex++) {
+ cost = (left.at(leftIndex - 1) == right.at(rightIndex - 1)) ? 0 : 1;
+
+ // Select cheapest operation
+ curr[rightIndex] = Math.min(
+ Math.min(
+ prev[rightIndex] + 1, // Delete current character
+ curr[rightIndex - 1] + 1 // Insert current character
+ ),
+ prev[rightIndex - 1] + cost // Replace (or no cost if same character)
+ );
+
+ // Check if adjacent characters are the same -> transpose if cheaper
+ if (leftIndex > 1
+ && rightIndex > 1
+ && left.at(leftIndex - 1) == right.at(rightIndex - 2)
+ && left.at(leftIndex - 2) == right.at(rightIndex - 1)) {
+ // Use cost here, to properly handle two subsequent equal letters
+ curr[rightIndex] = Math.min(curr[rightIndex], prevPrev[rightIndex - 2] + cost);
+ }
+ }
+
+ // Rotate arrays for next iteration
+ temp = prevPrev;
+ prevPrev = prev;
+ prev = curr;
+ curr = temp;
+ }
+
+ // Prev contains the value computed in the latest iteration
+ return prev[rightLength];
+ }
+
+ /**
+ * Threshold.
+ */
+ private final Integer threshold;
+
+ /**
+ * Constructs a default instance that uses a version of the algorithm that does not use a threshold parameter.
+ */
+ public DamerauLevenshteinDistance() {
+ this(null);
+ }
+
+ /**
+ * Constructs a new instance. If the threshold is not null, distance calculations will be limited to a maximum length.
+ * If the threshold is null, the unlimited version of the algorithm will be used.
+ *
+ * @param threshold If this is null then distances calculations will not be limited. This may not be negative.
+ */
+ public DamerauLevenshteinDistance(final Integer threshold) {
+ if (threshold != null && threshold < 0) {
+ throw new IllegalArgumentException("Threshold must not be negative");
+ }
+ this.threshold = threshold;
+ }
+
+ /**
+ * Computes the Damerau-Levenshtein distance between two Strings.
+ *
+ *
+ * A higher score indicates a greater distance.
+ *
+ *
+ * @param left the first input, must not be null.
+ * @param right the second input, must not be null.
+ * @return result distance, or -1 if threshold is exceeded.
+ * @throws IllegalArgumentException if either String input {@code null}.
+ */
+ @Override
+ public Integer apply(final CharSequence left, final CharSequence right) {
+ return apply(SimilarityInput.input(left), SimilarityInput.input(right));
+ }
+
+ /**
+ * Computes the Damerau-Levenshtein distance between two inputs.
+ *
+ *
+ * A higher score indicates a greater distance.
+ *
+ *
+ * @param The type of similarity score unit.
+ * @param left the first input, must not be null.
+ * @param right the second input, must not be null.
+ * @return result distance, or -1 if threshold is exceeded.
+ * @throws IllegalArgumentException if either String input {@code null}.
+ * @since 1.13.0
+ */
+ public Integer apply(final SimilarityInput left, final SimilarityInput right) {
+ if (threshold != null) {
+ return limitedCompare(left, right, threshold);
+ }
+ return unlimitedCompare(left, right);
+ }
+
+ /**
+ * Gets the distance threshold.
+ *
+ * @return The distance threshold.
+ */
+ public Integer getThreshold() {
+ return threshold;
+ }
+}
diff --git a/src/test/java/org/apache/commons/text/similarity/DamerauLevenshteinDistanceTest.java b/src/test/java/org/apache/commons/text/similarity/DamerauLevenshteinDistanceTest.java
new file mode 100644
index 0000000000..d1c1dcadc6
--- /dev/null
+++ b/src/test/java/org/apache/commons/text/similarity/DamerauLevenshteinDistanceTest.java
@@ -0,0 +1,220 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.text.similarity;
+
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Arrays;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+public class DamerauLevenshteinDistanceTest {
+
+ private static DamerauLevenshteinDistance defaultInstance;
+
+ @BeforeAll
+ static void createInstance() {
+ defaultInstance = new DamerauLevenshteinDistance();
+ }
+
+ @Test
+ void testGetThresholdDirectlyAfterObjectInstantiation() {
+ assertNull(defaultInstance.getThreshold());
+ }
+
+ @Test
+ void testGetThresholdIsCorrect() {
+ DamerauLevenshteinDistance distance = new DamerauLevenshteinDistance(10);
+
+ assertEquals(10, distance.getThreshold());
+ }
+
+ @Test
+ void testNullInputsThrowUnlimited() {
+ assertThrows(IllegalArgumentException.class, () -> defaultInstance.apply(null, "test"));
+ assertThrows(IllegalArgumentException.class, () -> defaultInstance.apply("test", null));
+ assertThrows(IllegalArgumentException.class, () -> defaultInstance.apply(null, SimilarityInput.input("test")));
+ assertThrows(IllegalArgumentException.class, () -> defaultInstance.apply(SimilarityInput.input("test"), null));
+ }
+
+ @Test
+ void testNullInputsThrowLimited() {
+ DamerauLevenshteinDistance instance = new DamerauLevenshteinDistance(10);
+
+ assertThrows(IllegalArgumentException.class, () -> instance.apply(null, "test"));
+ assertThrows(IllegalArgumentException.class, () -> instance.apply("test", null));
+ assertThrows(IllegalArgumentException.class, () -> instance.apply(null, SimilarityInput.input("test")));
+ assertThrows(IllegalArgumentException.class, () -> instance.apply(SimilarityInput.input("test"), null));
+ }
+
+ @Test
+ void testInvalidThresholdThrows() {
+ assertThrows(IllegalArgumentException.class, () -> new DamerauLevenshteinDistance(-1));
+ }
+
+ @ParameterizedTest(name = "DamerauLevenshteinDistance.unlimitedCompare(\"{0}\", \"{1}\") should return {2}")
+ @MethodSource("unlimitedDamerauLevenshteinDistanceTestCases")
+ void testCalculateDamerauLevenshteinDistance(String left, String right, int expectedDistance) {
+ int leftRightDistance = defaultInstance.apply(left, right);
+ int rightLeftDistance = defaultInstance.apply(right, left);
+
+ assertEquals(expectedDistance, leftRightDistance);
+ assertEquals(expectedDistance, rightLeftDistance);
+ }
+
+ @ParameterizedTest(name = "DamerauLevenshteinDistance.unlimitedCompare(\"{0}\", \"{1}\") should return {2} ({3})")
+ @MethodSource("unlimitedDamerauLevenshteinDistanceTestCases_SimilarityInput")
+ void testCalculateDamerauLevenshteinDistance_SimilarityInput(String left, String right, int expectedDistance, final Class cls) {
+ SimilarityInput