From 0d547721d7e0d5c6bdcd319711157b4f5303bb12 Mon Sep 17 00:00:00 2001
From: Arye Lukashevski <aryel@wix.com>
Date: Thu, 28 Jun 2018 15:27:41 +0300
Subject: [PATCH] change backtrack to be iterative

---
 src/filters/lcs.js | 68 ++++++++++++++++++++++------------------------
 test/index.js      | 17 ++++++++++++
 2 files changed, 50 insertions(+), 35 deletions(-)

diff --git a/src/filters/lcs.js b/src/filters/lcs.js
index 857275a1..fc4b6889 100644
--- a/src/filters/lcs.js
+++ b/src/filters/lcs.js
@@ -7,7 +7,7 @@ reference: http://en.wikipedia.org/wiki/Longest_common_subsequence_problem
 */
 
 const defaultMatch = function(array1, array2, index1, index2) {
-  return array1[index1] === array2[index2];
+  return array1[ index1 ] === array2[ index2 ];
 };
 
 const lengthMatrix = function(array1, array2, match, context) {
@@ -16,11 +16,11 @@ const lengthMatrix = function(array1, array2, match, context) {
   let x, y;
 
   // initialize empty matrix of len1+1 x len2+1
-  let matrix = [len1 + 1];
+  let matrix = [ len1 + 1 ];
   for (x = 0; x < len1 + 1; x++) {
-    matrix[x] = [len2 + 1];
+    matrix[ x ] = [ len2 + 1 ];
     for (y = 0; y < len2 + 1; y++) {
-      matrix[x][y] = 0;
+      matrix[ x ][ y ] = 0;
     }
   }
   matrix.match = match;
@@ -28,44 +28,44 @@ const lengthMatrix = function(array1, array2, match, context) {
   for (x = 1; x < len1 + 1; x++) {
     for (y = 1; y < len2 + 1; y++) {
       if (match(array1, array2, x - 1, y - 1, context)) {
-        matrix[x][y] = matrix[x - 1][y - 1] + 1;
+        matrix[ x ][ y ] = matrix[ x - 1 ][ y - 1 ] + 1;
       } else {
-        matrix[x][y] = Math.max(matrix[x - 1][y], matrix[x][y - 1]);
+        matrix[ x ][ y ] = Math.max(matrix[ x - 1 ][ y ], matrix[ x ][ y - 1 ]);
       }
     }
   }
   return matrix;
 };
 
-const backtrack = function(matrix, array1, array2, index1, index2, context) {
-  if (index1 === 0 || index2 === 0) {
-    return {
-      sequence: [],
-      indices1: [],
-      indices2: [],
-    };
-  }
-
-  if (matrix.match(array1, array2, index1 - 1, index2 - 1, context)) {
-    const subsequence = backtrack(
-      matrix,
-      array1,
-      array2,
-      index1 - 1,
-      index2 - 1,
-      context
-    );
-    subsequence.sequence.push(array1[index1 - 1]);
-    subsequence.indices1.push(index1 - 1);
-    subsequence.indices2.push(index2 - 1);
-    return subsequence;
-  }
+const backtrack = function(matrix, array1, array2, context) {
+  let index1 = array1.length;
+  let index2 = array2.length;
+  const subsequence = {
+    sequence: [],
+    indices1: [],
+    indices2: [],
+  };
 
-  if (matrix[index1][index2 - 1] > matrix[index1 - 1][index2]) {
-    return backtrack(matrix, array1, array2, index1, index2 - 1, context);
-  } else {
-    return backtrack(matrix, array1, array2, index1 - 1, index2, context);
+  while (index1 !== 0 && index2 !== 0) {
+    const sameLetter =
+      matrix.match(array1, array2, index1 - 1, index2 - 1, context);
+    if (sameLetter) {
+      subsequence.sequence.unshift(array1[ index1 - 1 ]);
+      subsequence.indices1.unshift(index1 - 1);
+      subsequence.indices2.unshift(index2 - 1);
+      --index1;
+      --index2;
+    } else {
+      const valueAtMatrixAbove = matrix[ index1 ][ index2 - 1 ];
+      const valueAtMatrixLeft = matrix[ index1 - 1 ][ index2 ];
+      if (valueAtMatrixAbove > valueAtMatrixLeft) {
+        --index2;
+      } else {
+        --index1;
+      }
+    }
   }
+  return subsequence;
 };
 
 const get = function(array1, array2, match, context) {
@@ -80,8 +80,6 @@ const get = function(array1, array2, match, context) {
     matrix,
     array1,
     array2,
-    array1.length,
-    array2.length,
     innerContext
   );
   if (typeof array1 === 'string' && typeof array2 === 'string') {
diff --git a/test/index.js b/test/index.js
index 85fcd381..136583f7 100644
--- a/test/index.js
+++ b/test/index.js
@@ -751,4 +751,21 @@ describe('lcs', () => {
       indices2: [1],
     });
   });
+
+  it('should compute diff for large array', () => {
+    const ARRAY_LENGTH = 5000; // js stack is about 50k
+    function randomArray() {
+      let result = [];
+      for (let i = 0; i < ARRAY_LENGTH; i++) {
+        if (Math.random() > 0.5) {
+          result.push('A');
+        } else {
+          result.push('B');
+        }
+      }
+      return result;
+    }
+
+    lcs.get(randomArray(), randomArray());
+  });
 });