Adding a method to get varint encoding size

Summary: #title

Reviewed By: yfeldblum

Differential Revision: D6864158

fbshipit-source-id: 13c98c3e44db6febae844f099baa9fbecc9db673

folly/Varint.h

@@ -21,6 +21,7 @@
 #include <folly/Conv.h>
 #include <folly/Expected.h>
 #include <folly/Likely.h>
+#include <folly/Portability.h>
 #include <folly/Range.h>
 
 namespace folly {
@@ -56,6 +57,13 @@ constexpr size_t kMaxVarintLength64 = 10;
  */
 size_t encodeVarint(uint64_t val, uint8_t* buf);
 
+/**
+ * Determine the number of bytes needed to represent "val".
+ * 32-bit values need at most 5 bytes.
+ * 64-bit values need at most 10 bytes.
+ */
+int encodeVarintSize(uint64_t val);
+
 /**
  * Decode a value from a given buffer, advances data past the returned value.
  * Throws on error.
@@ -108,6 +116,21 @@ inline size_t encodeVarint(uint64_t val, uint8_t* buf) {
   return size_t(p - buf);
 }
 
+inline int encodeVarintSize(uint64_t val) {
+  if (folly::kIsArchAmd64) {
+    // __builtin_clzll is undefined for 0
+    int highBit = 64 - __builtin_clzll(val | 1);
+    return (highBit + 6) / 7;
+  } else {
+    int s = 1;
+    while (val >= 128) {
+      ++s;
+      val >>= 7;
+    }
+    return s;
+  }
+}
+
 template <class T>
 inline uint64_t decodeVarint(Range<T*>& data) {
   auto expected = tryDecodeVarint(data);

folly/test/VarintTest.cpp

@@ -39,6 +39,7 @@ void testVarint(uint64_t val, std::initializer_list<uint8_t> bytes) {
     uint8_t buf[kMaxVarintLength64];
     EXPECT_EQ(expected.size(), encodeVarint(val, buf));
     EXPECT_TRUE(ByteRange(buf, expected.size()) == expected);
+    EXPECT_EQ(expected.size(), encodeVarintSize(val));
   }
 
   {