Create StringOrdinal to allow placement of strings in sorted lists

chrome/common/string_ordinal.cc

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/common/string_ordinal.h"
+
+#include <algorithm>
+#include <cstddef>
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+
+namespace {
+
+// Constants for StringOrdinal digits.
+const char kZeroDigit = 'a';
+const char kMinDigit = 'b';
+const char kMidDigit = 'n';
+const char kMaxDigit = 'z';
+const int kMidDigitValue = kMidDigit - kZeroDigit;
+const int kMaxDigitValue = kMaxDigit - kZeroDigit;
+const int kRadix = kMaxDigitValue + 1;
+COMPILE_ASSERT(kMidDigitValue == 13, kMidDigitValue_incorrect);
+COMPILE_ASSERT(kMaxDigitValue == 25, kMaxDigitValue_incorrect);
+COMPILE_ASSERT(kRadix == 26, kRadix_incorrect);
+
+// Helper Functions
+
+// Returns the length that string value.substr(0, length) would be with
+// trailing zeros removed.
+size_t GetLengthWithoutTrailingZeros(const std::string& value, size_t length) {
+  DCHECK(!value.empty());
+
+  size_t end_position = value.find_last_not_of(kZeroDigit, length - 1);
+
+  // If no non kZeroDigit is found then the string is a string of all zeros
+  // digits so we return 0 as the correct length.
+  if (end_position == std::string::npos)
+    return 0;
+
+  return end_position + 1;
+}
+
+// Return the digit value at position i, padding with kZeroDigit if required.
+int GetPositionValue(const std::string& str, size_t i) {
+  return (i < str.length()) ? (str[i] - kZeroDigit) : 0;
+}
+
+// Add kMidDigitValue to the value at position index. This returns false if
+// adding the half results in an overflow past the first digit, otherwise it
+// returns true. This is used by ComputeMidpoint.
+bool AddHalf(size_t position, std::string& value) {
+  DCHECK_GT(position, 0U);
+  DCHECK_LT(position, value.length());
+
+  // We can't perform this operation directly on the string because
+  // overflow can occur and mess up the values.
+  int new_position_value = value[position] + kMidDigitValue;
+
+  if (new_position_value <= kMaxDigit) {
+    value[position] = new_position_value;
+  } else {
+    value[position] = new_position_value - kRadix;
+    ++value[position - 1];
+
+    for (size_t i = position - 1; value[i] > kMaxDigit; --i) {
+      if (i == 0U) {
+        // If the first digit is too large we have no previous digit
+        // to increase, so we fail.
+        return false;
+      }
+      value[i] -= kRadix;
+      ++value[i - 1];
+    }
+  }
+
+  return true;
+}
+
+// Drops off the last digit of value and then all trailing zeros iff that
+// doesn't change its ordering as greater than |start|.
+void DropUnneededDigits(const std::string& start, std::string* value) {
+  CHECK_GT(*value, start);
+
+  size_t drop_length = GetLengthWithoutTrailingZeros(*value, value->length());
+  // See if the value can have its last digit removed without affecting
+  // the ordering.
+  if (drop_length > 1) {
+    // We must drop the trailing zeros before comparing |shorter_value| to
+    // |start| because if we don't we may have |shorter_value|=|start|+|a|*
+    // where |shorter_value| > |start| but not when it drops the trailing |a|s
+    // to become a valid StringOrdinal value.
+    int truncated_length = GetLengthWithoutTrailingZeros(*value,
+                                                         drop_length - 1);
+
+    if (truncated_length != 0 && value->compare(0, truncated_length, start) > 0)
+      drop_length = truncated_length;
+  }
+
+  value->resize(drop_length);
+}
+
+// Compute the midpoint string that is between |start| and |end|.
+std::string ComputeMidpoint(const std::string& start,
+                            const std::string& end) {
+  size_t max_size = std::max(start.length(), end.length()) + 1;
+  std::string midpoint(max_size, kZeroDigit);
+
+  bool add_half = false;
+  for (size_t i = 0; i < max_size; ++i) {
+    int char_value = GetPositionValue(start, i);
+    char_value += GetPositionValue(end, i);
+
+    midpoint[i] += (char_value / 2);
+    if (add_half) {
+      // AddHalf only returns false if (midpoint[0] > kMaxDigit), which
+      // implies the midpoint of two strings in (0, 1) is >= 1, which is a
+      // contradiction.
+      CHECK(AddHalf(i, midpoint));
+    }
+
+    add_half = (char_value % 2 == 1);
+  }
+  DCHECK(!add_half);
+
+  return midpoint;
+}
+
+// Create a StringOrdinal that is lexigraphically greater than |start| and
+// lexigraphically less than |end|. The returned StringOrdinal will be roughly
+// between |start| and |end|.
+StringOrdinal CreateStringOrdinalBetween(const StringOrdinal& start,
+                                         const StringOrdinal& end) {
+  CHECK(start.IsValid());
+  CHECK(end.IsValid());
+  CHECK(start.LessThan(end));
+  const std::string& start_string = start.ToString();
+  const std::string& end_string = end.ToString();
+  DCHECK_LT(start_string, end_string);
+
+  std::string midpoint = ComputeMidpoint(start_string, end_string);
+
+  DropUnneededDigits(start_string, &midpoint);
+
+  DCHECK_GT(midpoint, start_string);
+  DCHECK_LT(midpoint, end_string);
+
+  StringOrdinal midpoint_ordinal(midpoint);
+  DCHECK(midpoint_ordinal.IsValid());
+  return midpoint_ordinal;
+}
+
+// Returns true iff the input string matches the format [a-z]*[b-z].
+bool IsValidStringOrdinal(const std::string& value) {
+  if (value.empty())
+    return false;
+
+  for (size_t i = 0; i < value.length(); ++i) {
+    if (value[i] < kZeroDigit || value[i] > kMaxDigit)
+      return false;
+  }
+
+  return value[value.length() - 1] != kZeroDigit;
+}
+
+} // namespace
+
+StringOrdinal::StringOrdinal(const std::string& string_ordinal)
+    : string_ordinal_(string_ordinal),
+      is_valid_(IsValidStringOrdinal(string_ordinal_)) {
+}
+
+StringOrdinal::StringOrdinal() : string_ordinal_(""),
+                                 is_valid_(false) {
+}
+
+bool StringOrdinal::IsValid() const {
+  return is_valid_;
+}
+
+bool StringOrdinal::LessThan(const StringOrdinal& other) const {
+  CHECK(IsValid());
+  CHECK(other.IsValid());
+  return string_ordinal_ < other.string_ordinal_;
+}
+
+bool StringOrdinal::Equal(const StringOrdinal& other) const {
+  CHECK(IsValid());
+  CHECK(other.IsValid());
+  return string_ordinal_ == other.string_ordinal_;
+}
+
+StringOrdinal StringOrdinal::CreateBetween(const StringOrdinal& other) const {
+  CHECK(IsValid());
+  CHECK(other.IsValid());
+  CHECK(!Equal(other));
+
+  if (LessThan(other)) {
+    return CreateStringOrdinalBetween(*this, other);
+  } else {
+    return CreateStringOrdinalBetween(other, *this);
+  }
+}
+
+StringOrdinal StringOrdinal::CreateBefore() const {
+  CHECK(IsValid());
+  // Create the smallest valid StringOrdinal of the appropriate length
+  // to be the minimum boundary.
+  const size_t length = string_ordinal_.length();
+  std::string start(length, kZeroDigit);
+  start[length - 1] = kMinDigit;
+  if (start == string_ordinal_) {
+    start[length - 1] = kZeroDigit;
+    start += kMinDigit;
+  }
+
+  // Even though |start| is already a valid StringOrdinal that is less
+  // than |*this|, we don't return it because we wouldn't have much space in
+  // front of it to insert potential future values.
+  return CreateBetween(StringOrdinal(start));
+}
+
+StringOrdinal StringOrdinal::CreateAfter() const {
+  CHECK(IsValid());
+  // Create the largest valid StringOrdinal of the appropriate length to be
+  // the maximum boundary.
+  std::string end(string_ordinal_.length(), kMaxDigit);
+  if (end == string_ordinal_)
+    end += kMaxDigit;
+
+  // Even though |end| is already a valid StringOrdinal that is greater than
+  // |*this|, we don't return it because we wouldn't have much space after
+  // it to insert potential future values.
+  return CreateBetween(StringOrdinal(end));
+}
+
+std::string StringOrdinal::ToString() const {
+  CHECK(IsValid());
+  return string_ordinal_;
+}