[ADT] llvm::bsearch, binary search for mere mortals

Summary:
Add to STLExtras a binary search function with a simple mental model:
You provide a range and a predicate which is true above a certain point.
bsearch() tells you that point.
Overloads are provided for integers, iterators, and containers.

This is more suitable than std:: alternatives in many cases:
 - std::binary_search only indicates presence/absence
 - upper_bound/lower_bound give you the opportunity to pick the wrong one
 - all of the options have confusing names and definitions when your predicate
   doesn't have simple "less than" semantics
 - all of the options require iterators
 - we plumb around a useless `value` parameter that should be a lambda capture

The API is inspired by Go's standard library, but we add an extra parameter as
well as some overloads and templates to show how clever C++ is.

Reviewers: ilya-biryukov, gribozavr

Subscribers: dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60779

llvm-svn: 358540

llvm/include/llvm/ADT/STLExtras.h

@@ -1304,6 +1304,47 @@ auto upper_bound(R &&Range, T &&Value, Compare C)
   return std::upper_bound(adl_begin(Range), adl_end(Range),
                           std::forward<T>(Value), C);
 }
+
+/// Binary search for the first index where a predicate is true.
+/// Returns the first I in [Lo, Hi) where C(I) is true, or Hi if it never is.
+/// Requires that C is always false below some limit, and always true above it.
+///
+/// Example:
+///   size_t DawnModernEra = bsearch(1776, 2050, [](size_t Year){
+///     return Presidents.for(Year).twitterHandle() != None;
+///   });
+///
+/// Note the return value differs from std::binary_search!
+template <typename Predicate>
+size_t bsearch(size_t Lo, size_t Hi, Predicate P) {
+  while (Lo != Hi) {
+    assert(Hi > Lo);
+    size_t Mid = Lo + (Hi - Lo) / 2;
+    if (P(Mid))
+      Hi = Mid;
+    else
+      Lo = Mid + 1;
+  }
+  return Hi;
+}
+
+/// Binary search for the first iterator where a predicate is true.
+/// Returns the first I in [Lo, Hi) where C(*I) is true, or Hi if it never is.
+/// Requires that C is always false below some limit, and always true above it.
+template <typename It, typename Predicate,
+          typename Val = decltype(*std::declval<It>())>
+It bsearch(It Lo, It Hi, Predicate P) {
+  return std::lower_bound(Lo, Hi, 0u,
+                          [&](const Val &V, unsigned) { return !P(V); });
+}
+
+/// Binary search for the first iterator in a range where a predicate is true.
+/// Requires that C is always false below some limit, and always true above it.
+template <typename R, typename Predicate>
+auto bsearch(R &&Range, Predicate P) -> decltype(adl_begin(Range)) {
+  return bsearch(adl_begin(Range), adl_end(Range), P);
+}
+
 /// Wrapper function around std::equal to detect if all elements
 /// in a container are same.
 template <typename R>

llvm/unittests/ADT/STLExtrasTest.cpp

@@ -469,4 +469,25 @@ TEST(STLExtrasTest, to_address) {
   EXPECT_EQ(V1, to_address(V3));
 }
 
+TEST(STLExtrasTest, bsearch) {
+  // Integer version.
+  EXPECT_EQ(7u, bsearch(5, 10, [](unsigned X) { return X >= 7; }));
+  EXPECT_EQ(5u, bsearch(5, 10, [](unsigned X) { return X >= 0; }));
+  EXPECT_EQ(10u, bsearch(5, 10, [](unsigned X) { return X >= 50; }));
+
+  // Iterator version.
+  std::vector<int> V = {1, 3, 5, 7, 9};
+  EXPECT_EQ(V.begin() + 3,
+            bsearch(V.begin(), V.end(), [](unsigned X) { return X >= 7; }));
+  EXPECT_EQ(V.begin(),
+            bsearch(V.begin(), V.end(), [](unsigned X) { return X >= 0; }));
+  EXPECT_EQ(V.end(),
+            bsearch(V.begin(), V.end(), [](unsigned X) { return X >= 50; }));
+
+  // Range version.
+  EXPECT_EQ(V.begin() + 3, bsearch(V, [](unsigned X) { return X >= 7; }));
+  EXPECT_EQ(V.begin(), bsearch(V, [](unsigned X) { return X >= 0; }));
+  EXPECT_EQ(V.end(), bsearch(V, [](unsigned X) { return X >= 50; }));
+}
+
 } // namespace