From d2c121b00f2e1eb2ea8cc3a23a5039b3a4425bea Mon Sep 17 00:00:00 2001
From: thrien <39382079+thrien@users.noreply.github.com>
Date: Mon, 12 Jul 2021 21:25:08 +0200
Subject: [PATCH] feat: update Levenshtein to Damerau-Levenshtein (#1973)

---
 lib/utils/levenshtein.ts | 24 +++++++++++++++++-------
 test/yargs.cjs           | 12 ++++++++++++
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/lib/utils/levenshtein.ts b/lib/utils/levenshtein.ts
index aa4e9c94a..760c68c77 100644
--- a/lib/utils/levenshtein.ts
+++ b/lib/utils/levenshtein.ts
@@ -21,6 +21,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 // levenshtein distance algorithm, pulled from Andrei Mackenzie's MIT licensed.
 // gist, which can be found here: https://gist.github.com/andrei-m/982927
+// extended to compute damerau-levenshtein distance
 
 // Compute the edit distance between the two given strings
 export function levenshtein(a: string, b: string) {
@@ -47,13 +48,22 @@ export function levenshtein(a: string, b: string) {
       if (b.charAt(i - 1) === a.charAt(j - 1)) {
         matrix[i][j] = matrix[i - 1][j - 1];
       } else {
-        matrix[i][j] = Math.min(
-          matrix[i - 1][j - 1] + 1, // substitution
-          Math.min(
-            matrix[i][j - 1] + 1, // insertion
-            matrix[i - 1][j] + 1
-          )
-        ); // deletion
+        if (
+          i > 1 &&
+          j > 1 &&
+          b.charAt(i - 2) === a.charAt(j - 1) &&
+          b.charAt(i - 1) === a.charAt(j - 2)
+        ) {
+          matrix[i][j] = matrix[i - 2][j - 2] + 1; // transposition
+        } else {
+          matrix[i][j] = Math.min(
+            matrix[i - 1][j - 1] + 1, // substitution
+            Math.min(
+              matrix[i][j - 1] + 1, // insertion
+              matrix[i - 1][j] + 1
+            )
+          ); // deletion
+        }
       }
     }
   }
diff --git a/test/yargs.cjs b/test/yargs.cjs
index ee6091e3e..e743a70da 100644
--- a/test/yargs.cjs
+++ b/test/yargs.cjs
@@ -423,6 +423,18 @@ describe('yargs dsl tests', () => {
       r.errors[2].should.match(/Did you mean goat/);
     });
 
+    it('counts tranposition as one mistake', () => {
+      const r = checkOutput(() => {
+        yargs(['baot'])
+          .command('boat')
+          .command('bot')
+          .recommendCommands()
+          .parse();
+      });
+
+      r.errors[2].should.match(/Did you mean boat/);
+    });
+
     // see: https://github.com/yargs/yargs/issues/822
     it('does not print command recommendation if help message will be shown', done => {
       const parser = yargs().command('goat').help().recommendCommands();