[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.

cscore/src/main/native/cpp/HttpCameraImpl.cpp

@@ -208,7 +208,7 @@ wpi::HttpConnection* HttpCameraImpl::DeviceStreamConnect(
     if (wpi::trim(key) == "boundary") {
       value = wpi::trim(wpi::trim(value), '"');  // value may be quoted
       if (wpi::starts_with(value, "--")) {
-        value = value.substr(2);
+        value = wpi::substr(value, 2);
       }
       boundary.append(value.begin(), value.end());
     }

cscore/src/main/native/cpp/JpegUtil.cpp

@@ -4,6 +4,7 @@
 
 #include "JpegUtil.h"
 
+#include <wpi/StringExtras.h>
 #include <wpi/raw_istream.h>
 
 namespace cs {
@@ -64,7 +65,7 @@ bool GetJpegSize(std::string_view data, int* width, int* height) {
     return false;
   }
 
-  data = data.substr(2);  // Get to the first block
+  data = wpi::substr(data, 2);  // Get to the first block
   for (;;) {
     if (data.size() < 4) {
       return false;  // EOF
@@ -89,7 +90,7 @@ bool GetJpegSize(std::string_view data, int* width, int* height) {
       return true;
     }
     // Go to the next block
-    data = data.substr(bytes[2] * 256 + bytes[3] + 2);
+    data = wpi::substr(data, bytes[2] * 256 + bytes[3] + 2);
   }
 }
 
@@ -102,7 +103,7 @@ bool JpegNeedsDHT(const char* data, size_t* size, size_t* locSOF) {
   *locSOF = *size;
 
   // Search until SOS for DHT tag
-  sdata = sdata.substr(2);  // Get to the first block
+  sdata = wpi::substr(sdata, 2);  // Get to the first block
   for (;;) {
     if (sdata.size() < 4) {
       return false;  // EOF
@@ -121,7 +122,7 @@ bool JpegNeedsDHT(const char* data, size_t* size, size_t* locSOF) {
       *locSOF = sdata.data() - data;  // SOF
     }
     // Go to the next block
-    sdata = sdata.substr(bytes[2] * 256 + bytes[3] + 2);
+    sdata = wpi::substr(sdata, bytes[2] * 256 + bytes[3] + 2);
   }
 
   // Only add DHT if we also found SOF (insertion point)

cscore/src/main/native/cpp/MjpegServerImpl.cpp

@@ -797,14 +797,14 @@ void MjpegServerImpl::ConnThread::ProcessRequest() {
   // compatibility, others are for Axis camera compatibility.
   if ((pos = req.find("POST /stream")) != std::string_view::npos) {
     kind = kStream;
-    parameters = req.substr(req.find('?', pos + 12)).substr(1);
+    parameters = wpi::substr(wpi::substr(req, req.find('?', pos + 12)), 1);
   } else if ((pos = req.find("GET /?action=stream")) !=
              std::string_view::npos) {
     kind = kStream;
-    parameters = req.substr(req.find('&', pos + 19)).substr(1);
+    parameters = wpi::substr(wpi::substr(req, req.find('&', pos + 19)), 1);
   } else if ((pos = req.find("GET /stream.mjpg")) != std::string_view::npos) {
     kind = kStream;
-    parameters = req.substr(req.find('?', pos + 16)).substr(1);
+    parameters = wpi::substr(wpi::substr(req, req.find('?', pos + 16)), 1);
   } else if (req.find("GET /settings") != std::string_view::npos &&
              req.find(".json") != std::string_view::npos) {
     kind = kGetSettings;
@@ -820,7 +820,7 @@ void MjpegServerImpl::ConnThread::ProcessRequest() {
   } else if ((pos = req.find("GET /?action=command")) !=
              std::string_view::npos) {
     kind = kCommand;
-    parameters = req.substr(req.find('&', pos + 20)).substr(1);
+    parameters = wpi::substr(wpi::substr(req, req.find('&', pos + 20)), 1);
   } else if (req.find("GET / ") != std::string_view::npos || req == "GET /\n") {
     kind = kRootPage;
   } else {
@@ -833,7 +833,7 @@ void MjpegServerImpl::ConnThread::ProcessRequest() {
   pos = parameters.find_first_not_of(
       "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_"
       "-=&1234567890%./");
-  parameters = parameters.substr(0, pos);
+  parameters = wpi::substr(parameters, 0, pos);
   SDEBUG("command parameters: \"{}\"", parameters);
 
   // Read the rest of the HTTP request.

cscore/src/main/native/linux/UsbCameraImpl.cpp

@@ -107,7 +107,7 @@ static __u32 FromPixelFormat(VideoMode::PixelFormat pixelFormat) {
 
 static bool IsPercentageProperty(std::string_view name) {
   if (wpi::starts_with(name, "raw_")) {
-    name = name.substr(4);
+    name = wpi::substr(name, 4);
   }
   return name == "brightness" || name == "contrast" || name == "saturation" ||
          name == "hue" || name == "sharpness" || name == "gain" ||
@@ -181,13 +181,13 @@ static bool GetVendorProduct(int dev, int* vendor, int* product) {
   }
   std::string_view readStr{readBuf};
   if (auto v = wpi::parse_integer<int>(
-          readStr.substr(readStr.find('v')).substr(1, 4), 16)) {
+          wpi::substr(wpi::substr(readStr, readStr.find('v')), 1, 4), 16)) {
     *vendor = v.value();
   } else {
     return false;
   }
   if (auto v = wpi::parse_integer<int>(
-          readStr.substr(readStr.find('p')).substr(1, 4), 16)) {
+          wpi::substr(wpi::substr(readStr, readStr.find('p')), 1, 4), 16)) {
     *product = v.value();
   } else {
     return false;
@@ -236,8 +236,8 @@ static bool GetDescriptionIoctl(const char* cpath, std::string* desc) {
   std::optional<int> vendor;
   std::optional<int> product;
   if (wpi::starts_with(card, "UVC Camera (") &&
-      (vendor = wpi::parse_integer<int>(card.substr(12, 4), 16)) &&
-      (product = wpi::parse_integer<int>(card.substr(17, 4), 16))) {
+      (vendor = wpi::parse_integer<int>(wpi::substr(card, 12, 4), 16)) &&
+      (product = wpi::parse_integer<int>(wpi::substr(card, 17, 4), 16))) {
     std::string card2 = GetUsbNameFromId(vendor.value(), product.value());
     if (!card2.empty()) {
       *desc = std::move(card2);
@@ -283,7 +283,7 @@ static int GetDeviceNum(const char* cpath) {
   if (!wpi::starts_with(fn, "video")) {
     return -1;
   }
-  if (auto dev = wpi::parse_integer<int>(fn.substr(5), 10)) {
+  if (auto dev = wpi::parse_integer<int>(wpi::substr(fn, 5), 10)) {
     return dev.value();
   }
   return -1;
@@ -1635,7 +1635,8 @@ std::vector<UsbCameraInfo> EnumerateUsbCameras(CS_Status* status) {
       }
 
       unsigned int dev = 0;
-      if (auto v = wpi::parse_integer<unsigned int>(fname.substr(5), 10)) {
+      if (auto v =
+              wpi::parse_integer<unsigned int>(wpi::substr(fname, 5), 10)) {
         dev = v.value();
       } else {
         continue;
@@ -1686,7 +1687,8 @@ std::vector<UsbCameraInfo> EnumerateUsbCameras(CS_Status* status) {
             std::string fname = fs::path{target}.filename();
             std::optional<unsigned int> dev;
             if (wpi::starts_with(fname, "video") &&
-                (dev = wpi::parse_integer<unsigned int>(fname.substr(5), 10)) &&
+                (dev = wpi::parse_integer<unsigned int>(wpi::substr(fname, 5),
+                                                        10)) &&
                 dev.value() < retval.size()) {
               retval[dev.value()].otherPaths.emplace_back(path.str());
             }

cscore/src/main/native/linux/UsbCameraProperty.cpp

@@ -6,6 +6,7 @@
 
 #include <fmt/format.h>
 #include <wpi/SmallString.h>
+#include <wpi/StringExtras.h>
 
 #include "UsbUtil.h"
 
@@ -93,7 +94,7 @@ static int GetStringCtrlIoctl(int fd, int id, int maximum, std::string* value) {
 
 static int SetStringCtrlIoctl(int fd, int id, int maximum,
                               std::string_view value) {
-  wpi::SmallString<64> str{value.substr(0, maximum)};
+  wpi::SmallString<64> str{wpi::substr(value, 0, maximum)};
 
   struct v4l2_ext_control ctrl;
   struct v4l2_ext_controls ctrls;

cscore/src/main/native/linux/UsbUtil.cpp

@@ -49,7 +49,7 @@ static std::string GetUsbNameFromFile(int vendor, int product) {
     // look for vendor at start of line
     if (wpi::starts_with(line, vendorStr)) {
       foundVendor = true;
-      buf += wpi::trim(line.substr(5));
+      buf += wpi::trim(wpi::substr(line, 5));
       buf += ' ';
       continue;
     }
@@ -62,8 +62,8 @@ static std::string GetUsbNameFromFile(int vendor, int product) {
       }
 
       // look for product
-      if (wpi::starts_with(line.substr(1), productStr)) {
-        buf += wpi::trim(line.substr(6));
+      if (wpi::starts_with(wpi::substr(line, 1), productStr)) {
+        buf += wpi::trim(wpi::substr(line, 6));
         return buf;
       }
     }

cscore/src/main/native/windows/UsbCameraImpl.cpp

@@ -269,7 +269,7 @@ void UsbCameraImpl::DeviceDisconnect() {
 
 static bool IsPercentageProperty(std::string_view name) {
   if (wpi::starts_with(name, "raw_"))
-    name = name.substr(4);
+    name = wpi::substr(name, 4);
   return name == "Brightness" || name == "Contrast" || name == "Saturation" ||
          name == "Hue" || name == "Sharpness" || name == "Gain" ||
          name == "Exposure";