From 3e3dce5f4a775c64154fb987bfdd10c628dfbd6d Mon Sep 17 00:00:00 2001
From: Manish <manish@radii.dev>
Date: Tue, 22 Aug 2023 10:51:35 +1000
Subject: [PATCH] Algorithms: Pairwise

---
 Algorithms/pairwise.js | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 Algorithms/pairwise.js

diff --git a/Algorithms/pairwise.js b/Algorithms/pairwise.js
new file mode 100644
index 0000000..e318bfb
--- /dev/null
+++ b/Algorithms/pairwise.js
@@ -0,0 +1,30 @@
+// https://www.freecodecamp.org/learn/coding-interview-prep/algorithms/pairwise
+
+function pairwise(arr, sum) {
+  const arr2 = arr.map((num, index) => ({ num: num, index: index }));
+  arr2.sort(({ num: numA, index: indexA }, { num: numB, index: indexB }) => {
+    if (numA === numB) {
+      return indexA - indexB;
+    } else {
+      return numA - numB;
+    }
+  });
+
+  let [i, reverseI, pairsIndexSum] = [0, arr2.length - 1, 0];
+  while (i < reverseI) {
+    if (arr2[i].num + arr2[reverseI].num === sum) {
+      if (arr2[i].num === arr2[reverseI].num) {
+        pairsIndexSum += arr2[i++].index + arr2[i++].index;
+      } else {
+        pairsIndexSum += arr2[i++].index + arr2[reverseI--].index;
+      }
+    } else if (arr2[i].num + arr2[reverseI].num < sum) {
+      i++;
+    } else {
+      reverseI--;
+    }
+  }
+  return pairsIndexSum;
+}
+
+console.log(pairwise([1, 1, 1], 2));