From 42ff4307bafdd29c186623407041dc502fae7d31 Mon Sep 17 00:00:00 2001
From: gikf <60067306+gikf@users.noreply.github.com>
Date: Mon, 31 May 2021 19:34:05 +0200
Subject: [PATCH] fix(curriculum): rework Project Euler 94 (#42243)

Co-authored-by: Sem Bauke <46919888+Sembauke@users.noreply.github.com>
---
 ...problem-94-almost-equilateral-triangles.md | 54 +++++++++++++++----
 1 file changed, 45 insertions(+), 9 deletions(-)

diff --git a/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-94-almost-equilateral-triangles.md b/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-94-almost-equilateral-triangles.md
index fbd3f04a8b..ef0c095e03 100644
--- a/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-94-almost-equilateral-triangles.md
+++ b/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-94-almost-equilateral-triangles.md
@@ -8,24 +8,42 @@ dashedName: problem-94-almost-equilateral-triangles
 
 # --description--
 
-It is easily proved that no equilateral triangle exists with integral length sides and integral area. However, the <dfn>almost equilateral triangle</dfn> 5-5-6 has an area of 12 square units.
+It is easily proved that no equilateral triangle exists with integral length sides and integral area. However, the almost equilateral triangle 5-5-6 has an area of 12 square units.
 
 We shall define an <dfn>almost equilateral triangle</dfn> to be a triangle for which two sides are equal and the third differs by no more than one unit.
 
-Find the sum of the perimeters of all <dfn>almost equilateral triangle</dfn> with integral side lengths and area and whose perimeters do not exceed one billion (1,000,000,000).
+Find the sum of the perimeters of all almost equilateral triangles with integral side lengths and area and whose perimeters do not exceed `limit`.
 
 # --hints--
 
-`almostEquilateralTriangles()` should return a number.
+`almostEquilateralTriangles(50)` should return a number.
 
 ```js
-assert(typeof almostEquilateralTriangles() === 'number');
+assert(typeof almostEquilateralTriangles(50) === 'number');
 ```
 
-`almostEquilateralTriangles()` should return 518408346.
+`almostEquilateralTriangles(50)` should return `66`.
 
 ```js
-assert.strictEqual(almostEquilateralTriangles(), 518408346);
+assert.strictEqual(almostEquilateralTriangles(50), 66);
+```
+
+`almostEquilateralTriangles(10000)` should return `3688`.
+
+```js
+assert.strictEqual(almostEquilateralTriangles(10000), 3688);
+```
+
+`almostEquilateralTriangles(10000000)` should return `9973078`.
+
+```js
+assert.strictEqual(almostEquilateralTriangles(10000000), 9973078);
+```
+
+`almostEquilateralTriangles(1000000000)` should return `518408346`.
+
+```js
+assert.strictEqual(almostEquilateralTriangles(1000000000), 518408346);
 ```
 
 # --seed--
@@ -33,16 +51,34 @@ assert.strictEqual(almostEquilateralTriangles(), 518408346);
 ## --seed-contents--
 
 ```js
-function almostEquilateralTriangles() {
+function almostEquilateralTriangles(limit) {
 
   return true;
 }
 
-almostEquilateralTriangles();
+almostEquilateralTriangles(50);
 ```
 
 # --solutions--
 
 ```js
-// solution required
+function almostEquilateralTriangles(limit) {
+  // Based on https://blog.dreamshire.com/project-euler-94-solution/
+  let perimetersSum = 0;
+
+  let sidesAB = 1;
+  let sideC = 1;
+  let perimeter = 0;
+  let perimeterOffset = 1;
+
+  while (perimeter <= limit) {
+    [sidesAB, sideC] = [4 * sidesAB - sideC + 2 * perimeterOffset, sidesAB];
+    perimeterOffset = -perimeterOffset;
+
+    perimetersSum += perimeter;
+    perimeter = 3 * sidesAB - perimeterOffset;
+  }
+
+  return perimetersSum;
+}
 ```