diff --git a/lib/mnswpr.js b/lib/mnswpr.js
index 8eef78e..624e806 100644
--- a/lib/mnswpr.js
+++ b/lib/mnswpr.js
@@ -82,7 +82,8 @@ const Minesweeper = function(appId, version, hooks = undefined) {
   }
   storageService.saveToLocal('setting', setting)
   let flagsCount = setting.mines
-  let minesArray = []
+  // Mine positions stored as a Set of numeric keys (row * cols + col) for O(1) lookup
+  let mines = new Set()
 
   this.initialize = function() {
     const headingElement = document.createElement('h1')
@@ -188,7 +189,7 @@ const Minesweeper = function(appId, version, hooks = undefined) {
     grid.innerHTML = ''
     grid.oncontextmenu = () => false
     flagsCount = setting.mines
-    minesArray = []
+    mines.clear()
 
     for (let i = 0; i < setting.rows; i++) {
       let row = grid.insertRow(i)
@@ -447,7 +448,7 @@ const Minesweeper = function(appId, version, hooks = undefined) {
       if (isMine(cell)) {
         transferMine()
       } else {
-        minesArray.push([row, col])
+        mines.add(mineKey(row, col))
       }
       if (TEST_MODE){
         cell.innerHTML = 'X'
@@ -536,30 +537,12 @@ const Minesweeper = function(appId, version, hooks = undefined) {
     return getStatus(cell) == 'flagged'
   }
 
+  function mineKey(row, col) {
+    return row * setting.cols + col
+  }
+
   function isMine(cell) {
-    return getIndex(minesArray, cell) > -1
-  }
-
-  function removeItem(arr, cell) {
-    const index = getIndex(arr, cell)
-    if (index > -1) {
-      arr.splice(index, 1)
-    }
-  }
-
-  function getIndex(arr, cell) {
-    const row = cell.parentNode.rowIndex
-    const col = cell.cellIndex
-    let index = -1
-    for (let i = 0; i < arr.length; i++) {
-      let rowCol = arr[i]
-      if (rowCol[0] === row && rowCol[1] === col) {
-        index = i
-        break
-      }
-    }
-
-    return index
+    return mines.has(mineKey(getRow(cell), getCol(cell)))
   }
 
   function checkLevelCompletion() {
@@ -721,7 +704,7 @@ const Minesweeper = function(appId, version, hooks = undefined) {
       return
     } else if (isMine(cell) && firstClick) {
       // cell.setAttribute('data-mine', 'false');
-      removeItem(minesArray, cell)
+      mines.delete(mineKey(getRow(cell), getCol(cell)))
       transferMine(cell)
       if (TEST_MODE) printMines()
     }
@@ -749,7 +732,7 @@ const Minesweeper = function(appId, version, hooks = undefined) {
       if (isMine(transferMineToCell) || isNeighbor(cell, transferMineToCell)) {
         continue
       } else {
-        minesArray.push([row, col])
+        mines.add(mineKey(row, col))
         if (TEST_MODE){
           transferMineToCell.innerHTML = 'X'
           if (TEST_MODE) loggerService.debug('transferred mine to: ' + row + ', ' + col)