From aede226d6e01c2f105ec9bdc4d464d022ace4d29 Mon Sep 17 00:00:00 2001
From: shane <psh40963@naver.com>
Date: Sat, 6 May 2023 16:09:33 +0900
Subject: [PATCH] Create BinaryExponentiation.java

---
 .../number/BinaryExponentiation.java          | 92 +++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 projects/java/src/main/java/com/tistory/shanepark/number/BinaryExponentiation.java

diff --git a/projects/java/src/main/java/com/tistory/shanepark/number/BinaryExponentiation.java b/projects/java/src/main/java/com/tistory/shanepark/number/BinaryExponentiation.java
new file mode 100644
index 0000000..fd473ee
--- /dev/null
+++ b/projects/java/src/main/java/com/tistory/shanepark/number/BinaryExponentiation.java
@@ -0,0 +1,92 @@
+package com.tistory.shanepark.number;
+
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.springframework.util.StopWatch;
+
+import java.math.BigInteger;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public class BinaryExponentiation {
+
+    @Test
+    @DisplayName("Speed Test")
+    void test() {
+        StopWatch stopWatch = new StopWatch();
+        Set<Integer> answers = new HashSet<>();
+
+        answers.add(measure(new IterateCalculator(), stopWatch));
+        answers.add(measure(new BigIntegerCalculator(), stopWatch));
+        answers.add(measure(new BinaryExponentiationCalculator(), stopWatch));
+
+        Assertions.assertThat(answers).hasSize(1);
+        System.out.println("stopwach = " + stopWatch.prettyPrint());
+    }
+
+    @Test
+    @DisplayName("Test 2^13")
+    void test2() {
+        BinaryExponentiationCalculator calc = new BinaryExponentiationCalculator();
+        int result = calc.calc(2, 13, 1000000007);
+        Assertions.assertThat(result).isEqualTo((int) Math.pow(2, 8) * (int) Math.pow(2, 4) * (int) Math.pow(2, 1));
+    }
+
+    private int measure(Calculator calculator, StopWatch stopWatch) {
+        final int BASE = 2;
+        final int POW = 10000;
+        final int MOD = (int) 1e9 + 7;
+
+        stopWatch.start(calculator.getClass().getSimpleName());
+        List<Integer> answers = new ArrayList<>();
+        for (int i = 0; i < 10000; i++) {
+            answers.add(calculator.calc(BASE, POW, MOD));
+        }
+        stopWatch.stop();
+        return answers.get(0);
+    }
+
+    interface Calculator {
+        int calc(int base, int pow, int mod);
+    }
+
+    class IterateCalculator implements Calculator {
+        @Override
+        public int calc(int base, int pow, int mod) {
+            long cur = 1L;
+            for (int i = 0; i < pow; i++) {
+                cur = (cur * base) % mod;
+            }
+            return (int) cur;
+        }
+    }
+
+    class BigIntegerCalculator implements Calculator {
+        @Override
+        public int calc(int base, int pow, int mod) {
+            return BigInteger.valueOf(base)
+                    .pow(pow)
+                    .mod(BigInteger.valueOf(mod))
+                    .intValue();
+        }
+    }
+
+    class BinaryExponentiationCalculator implements Calculator {
+        @Override
+        public int calc(int base, int pow, int mod) {
+            long result = 1;
+            for (; pow > 0; pow /= 2) {
+                if (pow % 2 == 1) {
+                    result = (result * base) % mod;
+                }
+                base = (int) ((long) base * base % mod);
+            }
+            return (int) result;
+        }
+    }
+
+
+}