fix: hadnle zeros at the endpoints

Author: vil02

Diff for: Maths/BisectionMethod.js

@@ -23,7 +23,7 @@ const findRoot = (a, b, func, numberOfIterations) => {
 
   // Bolzano theorem
   const hasRoot = (a, b, func) => {
-    return func(a) * func(b) < 0
+    return func(a) * func(b) <= 0
   }
   if (hasRoot(a, b, func) === false) {
     throw Error(
@@ -45,10 +45,9 @@ const findRoot = (a, b, func, numberOfIterations) => {
   const prod2 = fm * func(b)
 
   // Depending on the sign of the products above, decide which position will m fill (a's or b's)
-  if (prod1 > 0 && prod2 < 0) return findRoot(m, b, func, --numberOfIterations)
-  else if (prod1 < 0 && prod2 > 0)
-    return findRoot(a, m, func, --numberOfIterations)
-  else throw Error('Unexpected behavior')
+  if (prod2 <= 0) return findRoot(m, b, func, --numberOfIterations)
+
+  return findRoot(a, m, func, --numberOfIterations)
 }
 
 export { findRoot }

Diff for: Maths/test/BisectionMethod.test.js

@@ -35,3 +35,28 @@ test('Equation f(x) = sqrt(x) + e^(2*x) - 8*x = 0, has root x = 0.93945851 in [a
   )
   expect(Number(Number(root).toPrecision(8))).toBe(0.93945851)
 })
+
+test('Equation f(x) = x^3 = 0, has root x = 0.0 in [a, b] = [-1.0, 1.0]', () => {
+  const root = findRoot(
+    -1.0,
+    1.0,
+    (x) => {
+      return Math.pow(x, 3)
+    },
+    32
+  )
+  expect(root).toBeCloseTo(0.0, 5)
+})
+
+test('Throws an error when function does not change sign', () => {
+  expect(() =>
+    findRoot(
+      -1.0,
+      1.0,
+      (x) => {
+        return Math.pow(x, 2)
+      },
+      10
+    )
+  ).toThrowError()
+})