From 0248a96e72699a42a0e56c9b0a2bef1b5d050faf Mon Sep 17 00:00:00 2001
From: Seb35 <seb35@seb35.fr>
Date: Sat, 27 Oct 2018 17:51:50 +0200
Subject: [PATCH] Memory optimisation

Only the current and the previous rows are needed. This trick is a
great improvement for long texts: it reduces space from O(mn) to
O(n). (It could be still slightly improved to O(min(m,n)) by
swapping the two texts.)
---
 src/MatchesSolver.php |  6 +++---
 src/Solver.php        | 16 ++++++++--------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/src/MatchesSolver.php b/src/MatchesSolver.php
index 5079be1..7bf12f7 100644
--- a/src/MatchesSolver.php
+++ b/src/MatchesSolver.php
@@ -22,10 +22,10 @@ protected function newIndex(array $matrix, int $i, int $j)
     {
         return new Match(
             [
-                $i - $matrix[$i][$j] + 1,
-                $j - $matrix[$i][$j] + 1,
+                $i - $matrix[$i%2][$j] + 1,
+                $j - $matrix[$i%2][$j] + 1,
             ],
-            $matrix[$i][$j]
+            $matrix[$i%2][$j]
         );
     }
 
diff --git a/src/Solver.php b/src/Solver.php
index 74ee735..ae6f36d 100644
--- a/src/Solver.php
+++ b/src/Solver.php
@@ -18,7 +18,7 @@ class Solver implements SolverInterface
      */
     protected function newIndex(array $matrix, int $i, int $j)
     {
-        return $i - $matrix[$i][$j] + 1;
+        return $i - $matrix[$i%2][$j] + 1;
     }
 
     /**
@@ -58,7 +58,7 @@ public function solve(string $stringA, string $stringB)
         $charsA = str_split($stringA);
         $charsB = str_split($stringB);
 
-        $matrix = array_fill_keys(array_keys($charsA), array_fill_keys(array_keys($charsB), 0));
+        $matrix = array_fill_keys([0, 1], array_fill_keys(array_keys($charsB), 0));
         $longestLength = 0;
         $longestIndexes = [];
 
@@ -66,20 +66,20 @@ public function solve(string $stringA, string $stringB)
             foreach ($charsB as $j => $charB) {
                 if ($charA === $charB) {
                     if (0 === $i || 0 === $j) {
-                        $matrix[$i][$j] = 1;
+                        $matrix[$i%2][$j] = 1;
                     } else {
-                        $matrix[$i][$j] = $matrix[$i - 1][$j - 1] + 1;
+                        $matrix[$i%2][$j] = $matrix[($i-1)%2][$j - 1] + 1;
                     }
 
                     $newIndex = $this->newIndex($matrix, $i, $j);
-                    if ($matrix[$i][$j] > $longestLength) {
-                        $longestLength = $matrix[$i][$j];
+                    if ($matrix[$i%2][$j] > $longestLength) {
+                        $longestLength = $matrix[$i%2][$j];
                         $longestIndexes = [$newIndex];
-                    } elseif ($matrix[$i][$j] === $longestLength) {
+                    } elseif ($matrix[$i%2][$j] === $longestLength) {
                         $longestIndexes[] = $newIndex;
                     }
                 } else {
-                    $matrix[$i][$j] = 0;
+                    $matrix[$i%2][$j] = 0;
                 }
             }
         }