[wpiutil] StringExtras: Add substr() (#3742)

Unlike std::string and std::string_view, this substr() allows a start
greater than the length of the string, in which case an empty string
is returned.  This matches llvm::StringRef behavior.

wpiutil/src/main/native/cpp/HttpUtil.cpp

@@ -232,7 +232,7 @@ bool FindMultipartBoundary(raw_istream& is, std::string_view boundary,
 
     // Did we find the boundary?
     if (searchBuf[0] == '-' && searchBuf[1] == '-' &&
-        searchBuf.substr(2) == boundary) {
+        wpi::substr(searchBuf, 2) == boundary) {
       return true;
     }
 

wpiutil/src/main/native/cpp/MimeTypes.cpp

@@ -4,6 +4,7 @@
 
 #include "wpi/MimeTypes.h"
 
+#include "wpi/StringExtras.h"
 #include "wpi/StringMap.h"
 
 namespace wpi {
@@ -53,11 +54,11 @@ std::string_view MimeTypeFromPath(std::string_view path) {
 
   auto pos = path.find_last_of("/");
   if (pos != std::string_view::npos) {
-    path = path.substr(pos + 1);
+    path = wpi::substr(path, pos + 1);
   }
   auto dot_pos = path.find_last_of(".");
   if (dot_pos > 0 && dot_pos != std::string_view::npos) {
-    auto type = mimeTypes.find(path.substr(dot_pos + 1));
+    auto type = mimeTypes.find(wpi::substr(path, dot_pos + 1));
     if (type != mimeTypes.end()) {
       return type->getValue();
     }

wpiutil/src/main/native/cpp/StringExtras.cpp

@@ -65,7 +65,7 @@ std::string_view::size_type wpi::find_lower(
 std::string_view::size_type wpi::find_lower(
     std::string_view str, std::string_view other,
     std::string_view::size_type from) noexcept {
-  auto s = str.substr(from);
+  auto s = substr(str, from);
   while (s.size() >= other.size()) {
     if (starts_with_lower(s, other)) {
       return from;
@@ -98,7 +98,7 @@ std::string_view::size_type wpi::rfind_lower(std::string_view str,
   }
   for (size_t i = str.size() - n + 1, e = 0; i != e;) {
     --i;
-    if (equals_lower(str.substr(i, n), other)) {
+    if (equals_lower(substr(str, i, n), other)) {
       return i;
     }
   }

wpiutil/src/main/native/include/wpi/SmallString.h

@@ -181,21 +181,6 @@ public:
   }
 
   /// @}
-  /// @name Substring Operations
-  /// @{
-
-  /// Return a reference to the substring from [Start, Start + N).
-  ///
-  /// \param Start The index of the starting character in the substring; if
-  /// the index is npos or greater than the length of the string then the
-  /// empty substring will be returned.
-  ///
-  /// \param N The number of characters to included in the substring. If \p N
-  /// exceeds the number of characters remaining in the string, the string
-  /// suffix (starting with \p Start) will be returned.
-  std::string_view substr(size_t Start, size_t N = std::string_view::npos) const {
-    return str().substr(Start, N);
-  }
 
   // Extra methods.
 

wpiutil/src/main/native/include/wpi/StringExtras.h

@@ -207,12 +207,31 @@ inline std::string_view::size_type rfind_lower(std::string_view str,
   return rfind_lower(str, std::string_view{other});
 }
 
+/**
+ * Returns the substring of @p str from [start, start + n).
+ *
+ * @param start The index of the starting character in the substring; if
+ * the index is npos or greater than the length of the string then the
+ * empty substring will be returned.
+ *
+ * @param n The number of characters to included in the substring. If n
+ * exceeds the number of characters remaining in the string, the string
+ * suffix (starting with @p start) will be returned.
+ */
+constexpr std::string_view substr(
+    std::string_view str, std::string_view::size_type start,
+    std::string_view::size_type n = std::string_view::npos) noexcept {
+  auto length = str.size();
+  start = (std::min)(start, length);
+  return {str.data() + start, (std::min)(n, length - start)};
+}
+
 /**
  * Checks if @p str starts with the given @p prefix.
  */
 constexpr bool starts_with(std::string_view str,
                            std::string_view prefix) noexcept {
-  return str.substr(0, prefix.size()) == prefix;
+  return substr(str, 0, prefix.size()) == prefix;
 }
 
 /**
@@ -354,6 +373,34 @@ constexpr std::string_view drop_back(
   return str;
 }
 
+/**
+ * Returns a view equal to @p str but with only the first @p n
+ * elements remaining.  If @p n is greater than the length of the
+ * string, the entire string is returned.
+ */
+constexpr std::string_view take_front(
+    std::string_view str, std::string_view::size_type n = 1) noexcept {
+  auto length = str.size();
+  if (n >= length) {
+    return str;
+  }
+  return drop_back(str, length - n);
+}
+
+/**
+ * Returns a view equal to @p str but with only the last @p n
+ * elements remaining.  If @p n is greater than the length of the
+ * string, the entire string is returned.
+ */
+constexpr std::string_view take_back(
+    std::string_view str, std::string_view::size_type n = 1) noexcept {
+  auto length = str.size();
+  if (n >= length) {
+    return str;
+  }
+  return drop_front(str, length - n);
+}
+
 /**
  * Returns a reference to the substring of @p str from [start, end).
  *

wpiutil/src/main/native/include/wpi/UrlParser.h

@@ -7,6 +7,7 @@
 
 #include <string_view>
 
+#include "wpi/StringExtras.h"
 #include "wpi/http_parser.h"
 
 namespace wpi {
@@ -52,35 +53,35 @@ class UrlParser {
   }
 
   std::string_view GetSchema() const {
-    return m_data.substr(m_url.field_data[UF_SCHEMA].off,
-                         m_url.field_data[UF_SCHEMA].len);
+    return wpi::substr(m_data, m_url.field_data[UF_SCHEMA].off,
+                       m_url.field_data[UF_SCHEMA].len);
   }
 
   std::string_view GetHost() const {
-    return m_data.substr(m_url.field_data[UF_HOST].off,
-                         m_url.field_data[UF_HOST].len);
+    return wpi::substr(m_data, m_url.field_data[UF_HOST].off,
+                       m_url.field_data[UF_HOST].len);
   }
 
   unsigned int GetPort() const { return m_url.port; }
 
   std::string_view GetPath() const {
-    return m_data.substr(m_url.field_data[UF_PATH].off,
-                         m_url.field_data[UF_PATH].len);
+    return wpi::substr(m_data, m_url.field_data[UF_PATH].off,
+                       m_url.field_data[UF_PATH].len);
   }
 
   std::string_view GetQuery() const {
-    return m_data.substr(m_url.field_data[UF_QUERY].off,
-                         m_url.field_data[UF_QUERY].len);
+    return wpi::substr(m_data, m_url.field_data[UF_QUERY].off,
+                       m_url.field_data[UF_QUERY].len);
   }
 
   std::string_view GetFragment() const {
-    return m_data.substr(m_url.field_data[UF_FRAGMENT].off,
-                         m_url.field_data[UF_FRAGMENT].len);
+    return wpi::substr(m_data, m_url.field_data[UF_FRAGMENT].off,
+                       m_url.field_data[UF_FRAGMENT].len);
   }
 
   std::string_view GetUserInfo() const {
-    return m_data.substr(m_url.field_data[UF_USERINFO].off,
-                         m_url.field_data[UF_USERINFO].len);
+    return wpi::substr(m_data, m_url.field_data[UF_USERINFO].off,
+                       m_url.field_data[UF_USERINFO].len);
   }
 
  private:

wpiutil/src/main/native/linux/MulticastServiceResolver.cpp

@@ -6,6 +6,7 @@
 
 #include "AvahiClient.h"
 #include "wpi/SmallString.h"
+#include "wpi/StringExtras.h"
 #include "wpi/mutex.h"
 
 using namespace wpi;
@@ -61,8 +62,9 @@ static void ResolveCallback(AvahiServiceResolver* r, AvahiIfIndex interface,
           // Todo make this just do key
           continue;
         }
-        std::string_view key = value.substr(0, splitIndex);
-        value = value.substr(splitIndex + 1, value.size() - splitIndex - 1);
+        std::string_view key = wpi::substr(value, 0, splitIndex);
+        value =
+            wpi::substr(value, splitIndex + 1, value.size() - splitIndex - 1);
         data.txt.emplace_back(std::pair<std::string, std::string>{key, value});
       }
       wpi::SmallString<256> outputHostName;