feat(power): Add power module

Author: sargalias

src/recursion-practice/power/power.js

@@ -0,0 +1,50 @@
+// Power
+const power = (base, exponent) => {
+  if (exponent === 0) {
+    return 1;
+  }
+  return base * power(base, exponent - 1);
+};
+
+// Power proper tail call
+const powerPTC = (base, exponent, resultSoFar = 1) => {
+  if (exponent === 0) {
+    return resultSoFar;
+  }
+  const newResultSoFar = resultSoFar * base;
+  return powerPTC(base, exponent - 1, newResultSoFar);
+};
+
+// Power continuous passing style
+const identity = x => x;
+const powerCPS = (base, exponent, cont = identity) => {
+  if (exponent === 0) {
+    return cont(1);
+  }
+  return powerCPS(base, exponent - 1, result => cont(result * base));
+};
+
+// Power with trampolining
+const trampoline = fn => (...args) => {
+  let result = fn(...args);
+
+  while (typeof result === 'function') {
+    result = result();
+  }
+
+  return result;
+};
+
+const powerTrampoline = trampoline(function _power(
+  base,
+  exponent,
+  resultSoFar = 1,
+) {
+  if (exponent === 0) {
+    return resultSoFar;
+  }
+  const newResultSoFar = resultSoFar * base;
+  return () => powerPTC(base, exponent - 1, newResultSoFar);
+});
+
+export { power, powerPTC, powerCPS, powerTrampoline };

src/recursion-practice/power/power.test.js

@@ -0,0 +1,22 @@
+import each from 'jest-each';
+import { power, powerPTC, powerCPS, powerTrampoline } from './power';
+
+[power, powerPTC, powerCPS, powerTrampoline].forEach(fn => {
+  describe(`${fn.name}`, () => {
+    each`
+      base    | exponent  | expected
+      ${1}    | ${0}      | ${1}
+      ${1}    | ${1}      | ${1}
+      ${1}    | ${2}      | ${1}
+      ${2}    | ${0}      | ${1}
+      ${2}    | ${1}      | ${2}
+      ${2}    | ${2}      | ${4}
+      ${2}    | ${4}      | ${16}
+    `.test(
+      'should return $expected when called with $base and $exponent',
+      ({ base, exponent, expected }) => {
+        expect(fn(base, exponent)).toBe(expected);
+      },
+    );
+  });
+});