EPI: swap bits

Author: jsgoller1

elements-of-programming-interviews/cpp/swap_bits.cc

@@ -1,7 +1,41 @@
 #include "test_framework/generic_test.h"
+
+void printBin(unsigned long long val, size_t len, const std::string& suffix) {
+  std::string binval;
+  for (size_t i = 0; i < len; i++) {
+    binval.append(val & 1 ? "1" : "0");
+    val >>= 1;
+  }
+  reverse(binval.begin(), binval.end());
+  printf("%s (%s)\n", binval.c_str(), suffix.c_str());
+}
+
+long long SwapBitsBruteForce(long long x, int i, int j) {
+  // Not clever approach:
+  // 1) Isolate the bit values:
+  long long ival = x & (1LL << i);
+  long long jval = x & (1LL << j);
+
+  // 2) Clear them in the original string
+  x &= ~(1LL << i);
+  x &= ~(1LL << j);
+
+  // 3) Now use the isolated values to set them in the swapped places
+  if (ival) {
+    x |= (1LL << j);
+  }
+  if (jval) {
+    x |= (1LL << i);
+  }
+  return x;
+}
+
 long long SwapBits(long long x, int i, int j) {
-  // TODO - you fill in here.
-  return 0;
+  if (((x >> i) & 1) != ((x >> j) & 1)) {
+    long long mask = (1LL << i) | (1LL << j);
+    return x ^ mask;
+  }
+  return x;
 }
 
 int main(int argc, char* argv[]) {
@@ -10,3 +44,6 @@ int main(int argc, char* argv[]) {
   return GenericTestMain(args, "swap_bits.cc", "swap_bits.tsv", &SwapBits,
                          DefaultComparator{}, param_names);
 }
+
+// 0000000000000000000100010010101011010110100101010001100010101001
+// 00000000000000000001000x0010101011010110100101010001100010101001

elements-of-programming-interviews/problem_mapping.js

@@ -30,7 +30,7 @@ problem_mapping = {
         },
         "4.02 Swap bits": {
             "C++: swap_bits.cc": {
-                "passed": 0,
+                "passed": 10001,
                 "total": 10001
             },
             "Java: SwapBits.java": {