remove unused LockFreeRingBuffer functionality

Summary:
Nothing uses the skipFraction in LockFreeRingBuffer::currentTail, and
floating point math for discrete fractions can be hard to reason
about, so remove this dead code.

Differential Revision: D27717906

fbshipit-source-id: b25e2bea25ab66c59e8c76a80d75140db474c46d

folly/experimental/LockFreeRingBuffer.h

@@ -17,7 +17,6 @@
 #pragma once
 
 #include <atomic>
-#include <cmath>
 #include <cstring>
 #include <memory>
 #include <type_traits>
@@ -152,21 +151,12 @@ class LockFreeRingBuffer {
   /// Returns a Cursor pointing to the first write that has not occurred yet.
   Cursor currentHead() noexcept { return Cursor(ticket_.load()); }
 
-  /// Returns a Cursor pointing to a currently readable write.
-  /// skipFraction is a value in the [0, 1] range indicating how far into the
-  /// currently readable window to place the cursor. 0 means the
-  /// earliest readable write, 1 means the latest readable write (if any).
-  Cursor currentTail(double skipFraction = 0.0) noexcept {
-    assert(skipFraction >= 0.0 && skipFraction <= 1.0);
+  /// Returns a Cursor pointing to the earliest readable write.
+  Cursor currentTail() noexcept {
     uint64_t ticket = ticket_.load();
 
-    uint64_t backStep = llround((1.0 - skipFraction) * capacity_);
-
-    // always try to move at least one step backward to something readable
-    backStep = std::max<uint64_t>(1, backStep);
-
     // can't go back more steps than we've taken
-    backStep = std::min(ticket, backStep);
+    uint64_t backStep = std::min<uint64_t>(ticket, capacity_);
 
     return Cursor(ticket - backStep);
   }

folly/experimental/test/LockFreeRingBufferTest.cpp

@@ -177,42 +177,6 @@ TEST(LockFreeRingBuffer, readerCanDetectSkips) {
   cursor = rb.currentTail();
   EXPECT_TRUE(rb.tryRead(result, cursor));
   EXPECT_EQ(capacity * (rounds - 1), result);
-
-  cursor = rb.currentTail(1.0);
-  EXPECT_TRUE(rb.tryRead(result, cursor));
-  EXPECT_EQ((capacity * rounds) - 1, result);
-}
-
-TEST(LockFreeRingBuffer, currentTailRange) {
-  const int capacity = 4;
-  LockFreeRingBuffer<int> rb(capacity);
-
-  // Workaround for template deduction failure
-  auto (&cursorValue)(value<int, std::atomic>);
-
-  // Empty buffer - everything points to 0
-  EXPECT_EQ(0, cursorValue(rb.currentTail(0)));
-  EXPECT_EQ(0, cursorValue(rb.currentTail(0.5)));
-  EXPECT_EQ(0, cursorValue(rb.currentTail(1)));
-
-  // Half-full
-  int val = 5;
-  rb.write(val);
-  rb.write(val);
-
-  EXPECT_EQ(0, cursorValue(rb.currentTail(0)));
-  EXPECT_EQ(1, cursorValue(rb.currentTail(1)));
-
-  // Full
-  rb.write(val);
-  rb.write(val);
-
-  EXPECT_EQ(0, cursorValue(rb.currentTail(0)));
-  EXPECT_EQ(3, cursorValue(rb.currentTail(1)));
-
-  auto midvalue = cursorValue(rb.currentTail(0.5));
-  // both rounding behaviours are acceptable
-  EXPECT_TRUE(midvalue == 1 || midvalue == 2);
 }
 
 TEST(LockFreeRingBuffer, cursorFromWrites) {