Add function for computing the edit distance of two arrays.

Accomplished by moving the body of StringRef::edit_distance into
a separate function that accepts two ArrayRefs, and making
StringRef::edit_distance a wrapper around the new function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150621 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/edit_distance.h

@@ -0,0 +1,102 @@
+//===-- llvm/ADT/edit_distance.h - Array edit distance function --- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines a Levenshtein distance function that works for any two
+// sequences, with each element of each sequence being analogous to a character
+// in a string.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_EDIT_DISTANCE_H
+#define LLVM_ADT_EDIT_DISTANCE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/OwningPtr.h"
+#include <algorithm>
+
+namespace llvm {
+
+/// \brief Determine the edit distance between two sequences.
+///
+/// \param FromArray the first sequence to compare.
+///
+/// \param ToArray the second sequence to compare.
+///
+/// \param AllowReplacements whether to allow element replacements (change one
+/// element into another) as a single operation, rather than as two operations
+/// (an insertion and a removal).
+///
+/// \param MaxEditDistance If non-zero, the maximum edit distance that this
+/// routine is allowed to compute. If the edit distance will exceed that
+/// maximum, returns \c MaxEditDistance+1.
+///
+/// \returns the minimum number of element insertions, removals, or (if
+/// \p AllowReplacements is \c true) replacements needed to transform one of
+/// the given sequences into the other. If zero, the sequences are identical.
+template<typename T>
+unsigned ComputeEditDistance(ArrayRef<T> FromArray, ArrayRef<T> ToArray,
+                             bool AllowReplacements = true,
+                             unsigned MaxEditDistance = 0) {
+  // The algorithm implemented below is the "classic"
+  // dynamic-programming algorithm for computing the Levenshtein
+  // distance, which is described here:
+  //
+  //   http://en.wikipedia.org/wiki/Levenshtein_distance
+  //
+  // Although the algorithm is typically described using an m x n
+  // array, only two rows are used at a time, so this implemenation
+  // just keeps two separate vectors for those two rows.
+  typename ArrayRef<T>::size_type m = FromArray.size();
+  typename ArrayRef<T>::size_type n = ToArray.size();
+
+  const unsigned SmallBufferSize = 64;
+  unsigned SmallBuffer[SmallBufferSize];
+  llvm::OwningArrayPtr<unsigned> Allocated;
+  unsigned *Previous = SmallBuffer;
+  if (2*(n + 1) > SmallBufferSize) {
+    Previous = new unsigned [2*(n+1)];
+    Allocated.reset(Previous);
+  }
+  unsigned *Current = Previous + (n + 1);
+
+  for (unsigned i = 0; i <= n; ++i)
+    Previous[i] = i;
+
+  for (typename ArrayRef<T>::size_type y = 1; y <= m; ++y) {
+    Current[0] = y;
+    unsigned BestThisRow = Current[0];
+
+    for (typename ArrayRef<T>::size_type x = 1; x <= n; ++x) {
+      if (AllowReplacements) {
+        Current[x] = std::min(
+            Previous[x-1] + (FromArray[y-1] == ToArray[x-1] ? 0u : 1u),
+            std::min(Current[x-1], Previous[x])+1);
+      }
+      else {
+        if (FromArray[y-1] == ToArray[x-1]) Current[x] = Previous[x-1];
+        else Current[x] = std::min(Current[x-1], Previous[x]) + 1;
+      }
+      BestThisRow = std::min(BestThisRow, Current[x]);
+    }
+
+    if (MaxEditDistance && BestThisRow > MaxEditDistance)
+      return MaxEditDistance + 1;
+
+    unsigned *tmp = Current;
+    Current = Previous;
+    Previous = tmp;
+  }
+
+  unsigned Result = Previous[n];
+  return Result;
+}
+
+} // End llvm namespace
+
+#endif