Introduce internal APT::StringView class

The class APT::StringView implements a drop-in replacement
for a subset of C++17 std::string_view() features. It will
be dropped at a later point and may not be used in public
interfaces.

apt-pkg/contrib/string_view.h

@@ -0,0 +1,112 @@
+/*
+ * Basic implementation of string_view
+ *
+ * (C) 2015 Julian Andres Klode <jak@debian.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#if !defined(APT_STRINGVIEW_H) && defined(APT_PKG_EXPOSE_STRING_VIEW)
+#define APT_STRINGVIEW_H
+#include <string.h>
+#include <string>
+
+namespace APT {
+
+/**
+ * \brief Simple subset of std::string_view from C++17
+ *
+ * This is an internal implementation of the subset of std::string_view
+ * used by APT. It is not meant to be used in programs, only inside the
+ * library for performance critical paths.
+ */
+class StringView {
+    const char *data_;
+    size_t size_;
+
+public:
+    static constexpr size_t npos = static_cast<size_t>(-1);
+
+    /* Constructors */
+    constexpr StringView() : data_(""), size_(0) {}
+    constexpr StringView(const char *data, size_t size) : data_(data), size_(size) {}
+
+    StringView(const char *data) : data_(data), size_(strlen(data)) {}
+    StringView(std::string const & str): data_(str.data()), size_(str.size()) {}
+
+
+    /* Viewers */
+    constexpr StringView substr(size_t pos, size_t n = npos) const {
+        return StringView(data_ + pos, n > (size_ - pos) ? (size_ - pos) : n);
+    }
+
+    size_t find(int c, size_t pos=0) const {
+        if (pos != 0)
+            return substr(pos).find(c);
+
+        const char *found = static_cast<const char*>(memchr(data_, c, size_));
+
+        if (found == NULL)
+            return npos;
+
+        return found - data_;
+    }
+
+    size_t rfind(int c, size_t pos=0) const {
+        if (pos != 0)
+            return substr(pos).find(c);
+
+        const char *found = static_cast<const char*>(memrchr(data_, c, size_));
+
+        if (found == NULL)
+            return npos;
+
+        return found - data_;
+    }
+
+    /* Conversions */
+    std::string to_string() const {
+        return std::string(data_, size_);
+    }
+
+    /* Comparisons */
+    int compare(size_t pos, size_t n, StringView other) const {
+        return substr(pos, n).compare(other);
+    }
+
+    int compare(StringView other) const {
+        int res;
+
+        res = memcmp(data_, other.data_, std::min(size_, other.size_));
+        if (res != 0)
+            return res;
+        if (size_ == other.size_)
+            return res;
+
+        return (size_ > other.size_) ? 1 : -1;
+    }
+
+    /* Optimization: If size not equal, string cannot be equal */
+    bool operator ==(StringView other) const { return size_ == other.size_ && compare(other) == 0; }
+    bool operator !=(StringView other) const { return !(*this == other); }
+
+    /* Accessors */
+    constexpr bool empty() const { return size_ == 0; }
+    constexpr const char* data() const { return data_; }
+    constexpr const char* begin() const { return data_; }
+    constexpr const char* end() const { return data_ + size_; }
+    constexpr char operator [](size_t i) const { return data_[i]; }
+    constexpr size_t size() const { return size_; }
+    constexpr size_t length() const { return size_; }
+};
+
+
+}
+
+inline bool operator ==(const char *other, APT::StringView that);
+inline bool operator ==(const char *other, APT::StringView that) { return that.compare(other) == 0; }
+
+#endif