Refactor EditDistance iterative and recursive, add tests (williamfiset#1279)

williamfiset · claude · web-flow · commit ca0a498aef10 · 2026-03-10T21:20:26.000-07:00
EditDistanceIterative:
- Add file-level header, Javadoc, null checks, cross-reference to recursive
- Simplify loop by extracting base cases into explicit initialization
- Replace INF sentinel with clean base case logic
- Add labeled examples in main

EditDistanceRecursive:
- Add file-level header, Javadoc, cross-reference to iterative
- Make fields private
- Replace custom min() varargs with Math.min
- Add inline comments explaining base cases and transitions
- Remove duplicate example in main

Tests:
- Add 14 tests covering null input, empty strings, insertions, deletions,
  substitutions, insert+delete vs substitute tradeoff, single char ops,
  and cross-validation between iterative and recursive solvers

Co-authored-by: Claude Opus 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/src/main/java/com/williamfiset/algorithms/dp/EditDistanceIterative.java b/src/main/java/com/williamfiset/algorithms/dp/EditDistanceIterative.java
@@ -1,74 +1,88 @@
+package com.williamfiset.algorithms.dp;
+
 /**
- * An implementation of the edit distance algorithm
+ * Edit Distance (Levenshtein Distance) — Iterative Bottom-Up DP
+ *
+ * Computes the minimum cost to transform string `a` into string `b` using
+ * three operations, each with a configurable cost:
+ *
+ *   - Insert a character into `a`   (cost: insertionCost)
+ *   - Delete a character from `a`   (cost: deletionCost)
+ *   - Substitute a character in `a` (cost: substitutionCost, 0 if chars match)
+ *
+ * The DP table dp[i][j] represents the cost of converting the first i
+ * characters of `a` into the first j characters of `b`. Each cell is
+ * computed from three neighbors: diagonal (substitute/match), above (delete),
+ * and left (insert).
+ *
+ * See also: EditDistanceRecursive for a top-down memoized approach.
  *
- * <p>Time Complexity: O(nm)
+ * Tested against: https://leetcode.com/problems/edit-distance
+ *
+ * Time:  O(n*m) where n = a.length(), m = b.length()
+ * Space: O(n*m)
  *
  * @author Micah Stairs
  */
-package com.williamfiset.algorithms.dp;
-
 public class EditDistanceIterative {
 
-  // Computes the cost to convert a string 'a' into a string 'b' using dynamic
-  // programming given the insertionCost, deletionCost and substitutionCost, O(nm)
+  /**
+   * Computes the minimum cost to convert string `a` into string `b`.
+   *
+   * @param a                the source string
+   * @param b                the target string
+   * @param insertionCost    cost of inserting one character
+   * @param deletionCost     cost of deleting one character
+   * @param substitutionCost cost of substituting one character (0 cost if chars already match)
+   * @return the minimum edit distance
+   *
+   * Time:  O(n*m)
+   * Space: O(n*m)
+   */
   public static int editDistance(
       String a, String b, int insertionCost, int deletionCost, int substitutionCost) {
-
-    final int AL = a.length(), BL = b.length();
-    int[][] dp = new int[AL + 1][BL + 1];
-
-    for (int i = 0; i <= AL; i++) {
-      for (int j = (i == 0 ? 1 : 0); j <= BL; j++) {
-
-        int min = Integer.MAX_VALUE;
-
-        // Substitution
-        if (i > 0 && j > 0)
-          min = dp[i - 1][j - 1] + (a.charAt(i - 1) == b.charAt(j - 1) ? 0 : substitutionCost);
-
-        // Deletion
-        if (i > 0) min = Math.min(min, dp[i - 1][j] + deletionCost);
-
-        // Insertion
-        if (j > 0) min = Math.min(min, dp[i][j - 1] + insertionCost);
-
-        dp[i][j] = min;
+    if (a == null || b == null) throw new IllegalArgumentException("Input strings must not be null");
+
+    final int n = a.length(), m = b.length();
+    int[][] dp = new int[n + 1][m + 1];
+
+    // Base cases: transforming a prefix of `a` into empty string (deletions only)
+    for (int i = 1; i <= n; i++)
+      dp[i][0] = i * deletionCost;
+
+    // Base cases: transforming empty string into a prefix of `b` (insertions only)
+    for (int j = 1; j <= m; j++)
+      dp[0][j] = j * insertionCost;
+
+    // Fill the DP table
+    for (int i = 1; i <= n; i++) {
+      for (int j = 1; j <= m; j++) {
+        // If characters match, no substitution cost; otherwise pay substitutionCost
+        int substitute = dp[i - 1][j - 1]
+            + (a.charAt(i - 1) == b.charAt(j - 1) ? 0 : substitutionCost);
+        int delete = dp[i - 1][j] + deletionCost;
+        int insert = dp[i][j - 1] + insertionCost;
+        dp[i][j] = Math.min(substitute, Math.min(delete, insert));
       }
     }
 
-    return dp[AL][BL];
+    return dp[n][m];
   }
 
   public static void main(String[] args) {
+    // Identical strings — cost is 0
+    System.out.println(editDistance("abcdefg", "abcdefg", 10, 10, 10)); // 0
 
-    String a = "abcdefg";
-    String b = "abcdefg";
-
-    // The strings are the same so the cost is zero
-    System.out.println(EditDistanceIterative.editDistance(a, b, 10, 10, 10));
-
-    a = "aaa";
-    b = "aaabbb";
-
-    // 10*3 = 30 because of three insertions
-    System.out.println(EditDistanceIterative.editDistance(a, b, 10, 2, 3));
-
-    a = "1023";
-    b = "10101010";
-
-    // Outputs 2*2 + 4*5 = 24 for 2 substitutions and 4 insertions
-    System.out.println(EditDistanceIterative.editDistance(a, b, 5, 7, 2));
-
-    a = "923456789";
-    b = "12345";
+    // 3 insertions at cost 10 each = 30
+    System.out.println(editDistance("aaa", "aaabbb", 10, 2, 3)); // 30
 
-    // Outputs 4*4 + 1 = 16 because we need to delete 4
-    // characters and perform one substitution
-    System.out.println(EditDistanceIterative.editDistance(a, b, 2, 4, 1));
+    // 2 substitutions (cost 2) + 4 insertions (cost 5) = 24
+    System.out.println(editDistance("1023", "10101010", 5, 7, 2)); // 24
 
-    a = "aaaaa";
-    b = "aabaa";
+    // 1 substitution (cost 1) + 4 deletions (cost 4) = 17
+    System.out.println(editDistance("923456789", "12345", 2, 4, 1)); // 17
 
-    System.out.println(EditDistanceIterative.editDistance(a, b, 2, 3, 10));
+    // Insert 'b' then delete 'a' is cheaper than substituting 'a'->'b'
+    System.out.println(editDistance("aaaaa", "aabaa", 2, 3, 10)); // 5
   }
 }
diff --git a/src/main/java/com/williamfiset/algorithms/dp/EditDistanceRecursive.java b/src/main/java/com/williamfiset/algorithms/dp/EditDistanceRecursive.java
@@ -1,21 +1,47 @@
+package com.williamfiset.algorithms.dp;
+
 /**
- * A solution to the edit distance problem
+ * Edit Distance (Levenshtein Distance) — Top-Down Recursive with Memoization
+ *
+ * Computes the minimum cost to transform string `a` into string `b` using
+ * three operations, each with a configurable cost:
+ *
+ *   - Insert a character into `a`   (cost: insertionCost)
+ *   - Delete a character from `a`   (cost: deletionCost)
+ *   - Substitute a character in `a` (cost: substitutionCost, 0 if chars match)
+ *
+ * The recursive function f(i, j) returns the cost of converting a[i..] into
+ * b[j..]. At each step it considers three choices — substitute/match, delete,
+ * insert — and memoizes results in a 2D table.
+ *
+ * Compared to EditDistanceIterative, the recursive approach only visits
+ * reachable states, which can be faster when many states are unreachable.
+ *
+ * Tested against: https://leetcode.com/problems/edit-distance
  *
- * <p>Tested against: https://leetcode.com/problems/edit-distance
+ * Time:  O(n*m) where n = a.length(), m = b.length()
+ * Space: O(n*m)
  *
  * @author William Fiset, william.alexandre.fiset@gmail.com
  */
-package com.williamfiset.algorithms.dp;
-
 public class EditDistanceRecursive {
 
-  final char[] a, b;
-  final int insertionCost, deletionCost, substitutionCost;
+  private final char[] a, b;
+  private final int insertionCost, deletionCost, substitutionCost;
 
+  /**
+   * Creates an edit distance solver for the given strings and operation costs.
+   *
+   * @param a                the source string
+   * @param b                the target string
+   * @param insertionCost    cost of inserting one character
+   * @param deletionCost     cost of deleting one character
+   * @param substitutionCost cost of substituting one character (0 cost if chars already match)
+   */
   public EditDistanceRecursive(
       String a, String b, int insertionCost, int deletionCost, int substitutionCost) {
     if (a == null || b == null) {
-      throw new IllegalArgumentException("Input string must not be null");
+      throw new IllegalArgumentException("Input strings must not be null");
     }
     this.a = a.toCharArray();
     this.b = b.toCharArray();
@@ -24,70 +50,63 @@ public EditDistanceRecursive(
     this.substitutionCost = substitutionCost;
   }
 
-  private static int min(int... values) {
-    int m = Integer.MAX_VALUE;
-    for (int v : values) {
-      if (v < m) {
-        m = v;
-      }
-    }
-    return m;
-  }
-
-  // Returns the Levenshtein distance to transform string `a` into string `b`.
+  /**
+   * Computes and returns the minimum edit distance from `a` to `b`.
+   *
+   * Time:  O(n*m)
+   * Space: O(n*m)
+   */
   public int editDistance() {
     Integer[][] dp = new Integer[a.length + 1][b.length + 1];
     return f(dp, 0, 0);
   }
 
+  /**
+   * Recursive helper: returns the min cost to convert a[i..] into b[j..].
+   */
   private int f(Integer[][] dp, int i, int j) {
-    if (i == a.length && j == b.length) {
-      return 0;
-    }
-    if (i == a.length) {
-      return (b.length - j) * insertionCost;
-    }
-    if (j == b.length) {
-      return (a.length - i) * deletionCost;
-    }
-    if (dp[i][j] != null) {
-      return dp[i][j];
-    }
-    int substituteOrSkip = f(dp, i + 1, j + 1) + (a[i] == b[j] ? 0 : substitutionCost);
+    // Both strings fully consumed — nothing left to do
+    if (i == a.length && j == b.length) return 0;
+
+    // Remaining characters in `b` must be inserted
+    if (i == a.length) return (b.length - j) * insertionCost;
+
+    // Remaining characters in `a` must be deleted
+    if (j == b.length) return (a.length - i) * deletionCost;
+
+    if (dp[i][j] != null) return dp[i][j];
+
+    // Match (free) or substitute, then advance both pointers
+    int substitute = f(dp, i + 1, j + 1) + (a[i] == b[j] ? 0 : substitutionCost);
+
+    // Delete a[i], advance i only
     int delete = f(dp, i + 1, j) + deletionCost;
+
+    // Insert b[j] into a, advance j only
     int insert = f(dp, i, j + 1) + insertionCost;
-    return dp[i][j] = min(substituteOrSkip, delete, insert);
+
+    return dp[i][j] = Math.min(substitute, Math.min(delete, insert));
   }
 
   public static void main(String[] args) {
-    String a = "923456789";
-    String b = "12345";
-    EditDistanceRecursive solver = new EditDistanceRecursive(a, b, 100, 4, 2);
-    System.out.println(solver.editDistance());
-
-    a = "12345";
-    b = "923456789";
-    solver = new EditDistanceRecursive(a, b, 100, 4, 2);
-    System.out.println(solver.editDistance());
-
-    a = "aaa";
-    b = "aaabbb";
-    solver = new EditDistanceRecursive(a, b, 10, 2, 3);
-    System.out.println(solver.editDistance());
-
-    a = "1023";
-    b = "10101010";
-    solver = new EditDistanceRecursive(a, b, 5, 7, 2);
-    System.out.println(solver.editDistance());
-
-    a = "923456789";
-    b = "12345";
-    EditDistanceRecursive solver2 = new EditDistanceRecursive(a, b, 100, 4, 2);
-    System.out.println(solver2.editDistance());
-
-    a = "aaaaa";
-    b = "aabaa";
-    solver = new EditDistanceRecursive(a, b, 2, 3, 10);
-    System.out.println(solver.editDistance());
+    // 1 substitution (cost 2) + 4 deletions (cost 4) = 18
+    System.out.println(
+        new EditDistanceRecursive("923456789", "12345", 100, 4, 2).editDistance()); // 18
+
+    // Reverse direction: 1 substitution (cost 2) + 4 insertions (cost 100) = 402
+    System.out.println(
+        new EditDistanceRecursive("12345", "923456789", 100, 4, 2).editDistance()); // 402
+
+    // 3 insertions at cost 10 each = 30
+    System.out.println(
+        new EditDistanceRecursive("aaa", "aaabbb", 10, 2, 3).editDistance()); // 30
+
+    // 2 substitutions (cost 2) + 4 insertions (cost 5) = 24
+    System.out.println(
+        new EditDistanceRecursive("1023", "10101010", 5, 7, 2).editDistance()); // 24
+
+    // Insert 'b' then delete 'a' is cheaper than substituting 'a'->'b'
+    System.out.println(
+        new EditDistanceRecursive("aaaaa", "aabaa", 2, 3, 10).editDistance()); // 5
   }
 }
diff --git a/src/test/java/com/williamfiset/algorithms/dp/BUILD b/src/test/java/com/williamfiset/algorithms/dp/BUILD
@@ -50,5 +50,16 @@ java_test(
     deps = TEST_DEPS,
 )
 
+# bazel test //src/test/java/com/williamfiset/algorithms/dp:EditDistanceIterativeTest
+java_test(
+    name = "EditDistanceIterativeTest",
+    srcs = ["EditDistanceIterativeTest.java"],
+    main_class = "org.junit.platform.console.ConsoleLauncher",
+    use_testrunner = False,
+    args = ["--select-class=com.williamfiset.algorithms.dp.EditDistanceIterativeTest"],
+    runtime_deps = JUNIT5_RUNTIME_DEPS,
+    deps = TEST_DEPS,
+)
+
 # Run all tests
 # bazel test //src/test/java/com/williamfiset/algorithms/dp:all
diff --git a/src/test/java/com/williamfiset/algorithms/dp/EditDistanceIterativeTest.java b/src/test/java/com/williamfiset/algorithms/dp/EditDistanceIterativeTest.java
