Add ADT/IntEqClasses.h as a light-weight implementation of EquivalenceClasses.h.

This implementation already exists as ConnectedVNInfoEqClasses in
LiveInterval.cpp, and it seems to be generally useful to have a light-weight way
of forming equivalence classes of small integers.

IntEqClasses doesn't allow enumeration of the elements in a class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122293 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/IntEqClasses.h

@@ -0,0 +1,81 @@
+//===-- llvm/ADT/IntEqClasses.h - Equiv. Classes of Integers ----*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Equivalence classes for small integers. This is a mapping of the integers
+// 0 .. N-1 into M equivalence classes numbered 0 .. M-1.
+//
+// Initially each integer has its own equivalence class. Classes are joined by
+// passing a representative member of each class to join().
+//
+// Once the classes are built, compress() will number them 0 .. M-1 and prevent
+// further changes.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_INTEQCLASSES_H
+#define LLVM_ADT_INTEQCLASSES_H
+
+#include "llvm/ADT/SmallVector.h"
+
+namespace llvm {
+
+class IntEqClasses {
+  /// EC - When uncompressed, map each integer to a smaller member of its
+  /// equivalence class. The class leader is the smallest member and maps to
+  /// itself.
+  ///
+  /// When compressed, EC[i] is the equivalence class of i.
+  SmallVector<unsigned, 8> EC;
+
+  /// NumClasses - The number of equivalence classes when compressed, or 0 when
+  /// uncompressed.
+  unsigned NumClasses;
+
+public:
+  /// IntEqClasses - Create an equivalence class mapping for 0 .. N-1.
+  IntEqClasses(unsigned N) : NumClasses(0) { grow(N); }
+
+  /// grow - Increase capacity to hold 0 .. N-1, putting new integers in unique
+  /// equivalence classes.
+  /// This requires an uncompressed map.
+  void grow(unsigned N);
+
+  /// join - Join the equivalence classes of a and b. After joining classes,
+  /// findLeader(a) == findLeader(b).
+  /// This requires an uncompressed map.
+  void join(unsigned a, unsigned b);
+
+  /// findLeader - Compute the leader of a's equivalence class. This is the
+  /// smallest member of the class.
+  /// This requires an uncompressed map.
+  unsigned findLeader(unsigned a) const;
+
+  /// compress - Compress equivalence classes by numbering them 0 .. M.
+  /// This makes the equivalence class map immutable.
+  void compress();
+
+  /// getNumClasses - Return the number of equivalence classes after compress()
+  /// was called.
+  unsigned getNumClasses() const { return NumClasses; }
+
+  /// operator[] - Return a's equivalence class number, 0 .. getNumClasses()-1.
+  /// This requires a compressed map.
+  unsigned operator[](unsigned a) const {
+    assert(NumClasses && "operator[] called before compress()");
+    return EC[a];
+  }
+
+  /// uncompress - Change back to the uncompressed representation that allows
+  /// editing.
+  void uncompress();
+};
+
+} // End llvm namespace
+
+#endif