Pull request for the Extensions interface only (#655)

* Modify halsim to be able to load extension libraries if they are available.

It will read the list of libraries to try from the HALSIM_EXTENSIONS
environment variable.  Multiple libraries can be given if separated
by ';' (Windows) or ':' (Unix).

The library must have an 'HALSIM_InitExtension' method that returns >= 0 on success.

The library is expected to use the interface expressed by
  hal/src/src/main/native/include/MockData

* Add a simple halsim library that just prints robot values.

This makes a good test bed for cross platform purposes,
and provides the ultimate in light weight simulators.

This initial version only prints PWM values.

simulation/halsim_print/build.gradle

@@ -0,0 +1,26 @@
+description = "A simulation shared object that simply prints robot behaviors"
+
+apply plugin: 'edu.wpi.first.NativeUtils'
+apply plugin: 'cpp'
+
+ext.skipAthena = true
+
+apply from: "../../config.gradle"
+
+
+model {
+    components {
+        halsim_print(NativeLibrarySpec)
+    }
+
+    binaries {
+        all {
+            project(':hal').addHalCompilerArguments(it)
+            project(':hal').addHalToLinker(it)
+        }
+        withType(StaticLibraryBinarySpec) {
+            it.buildable = false
+        }
+    }
+}
+apply from: 'publish.gradle'

simulation/halsim_print/publish.gradle

@@ -0,0 +1,43 @@
+apply plugin: 'maven-publish'
+apply plugin: 'edu.wpi.first.wpilib.versioning.WPILibVersioningPlugin'
+
+if (!hasProperty('releaseType')) {
+    WPILibVersion {
+        releaseType = 'dev'
+    }
+}
+
+def pubVersion = ''
+if (project.hasProperty("publishVersion")) {
+    pubVersion = project.publishVersion
+} else {
+    pubVersion = WPILibVersion.version
+}
+
+def baseArtifactId = 'halsim-print'
+def artifactGroupId = 'edu.wpi.first.halsim'
+
+
+model {
+    publishing {
+        def libSpec
+        $.components.each {
+            if (it in NativeLibrarySpec) {
+                $.binaries.each {
+                    if (it in SharedLibraryBinarySpec) {
+                        libSpec = it.sharedLibraryFile
+                    }
+                }
+            }
+        }
+
+        publications {
+            cpp(MavenPublication) {
+                artifact libSpec
+                artifactId = baseArtifactId
+                groupId artifactGroupId
+                version pubVersion
+            }
+        }
+    }
+}

simulation/halsim_print/src/halsim_print/cpp/PrintPWM.cpp

@@ -0,0 +1,29 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "PrintPWM.h"
+
+#include <iostream>
+
+#include "MockData/HAL_Value.h"
+#include "MockData/NotifyListener.h"
+#include "MockData/PWMData.h"
+
+static void PWMCallback(const char* name, void* param,
+                        const struct HAL_Value* value) {
+  auto pwm = static_cast<PrintPWM*>(param);
+  pwm->Publish(value->data.v_double);
+}
+
+PrintPWM::PrintPWM(int port) {
+  m_port = port;
+  HALSIM_RegisterPWMSpeedCallback(port, PWMCallback, this, false);
+}
+
+void PrintPWM::Publish(double value) {
+  std::cout << "PWM " << m_port << ": " << value << std::endl;
+}

simulation/halsim_print/src/halsim_print/cpp/main.cpp

@@ -0,0 +1,34 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include <iostream>
+
+#include <HAL/Ports.h>
+
+#include "HALSimPrint.h"
+#include "PrintPWM.h"
+
+/**
+ * Currently, robots never terminate, so we keep a single static object
+ * and it is never properly released or cleaned up.
+ */
+static HALSimPrint halsim;
+
+extern "C" {
+#if defined(WIN32) || defined(_WIN32)
+__declspec(dllexport)
+#endif
+    int HALSIM_InitExtension(void) {
+  std::cout << "Print Simulator Initializing." << std::endl;
+
+  int pwmCount = HAL_GetNumPWMChannels();
+  halsim.m_pwms.reserve(pwmCount);
+  for (int i = 0; i < pwmCount; i++) halsim.m_pwms.emplace_back(i);
+
+  return 0;
+}
+}  // extern "C"

simulation/halsim_print/src/halsim_print/headers/HALSimPrint.h

@@ -0,0 +1,17 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <vector>
+
+class PrintPWM;
+
+class HALSimPrint {
+ public:
+  std::vector<PrintPWM> m_pwms;
+};

simulation/halsim_print/src/halsim_print/headers/PrintPWM.h

@@ -0,0 +1,19 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "HALSimPrint.h"
+
+class PrintPWM {
+ public:
+  explicit PrintPWM(int port);
+  void Publish(double value);
+
+ private:
+  int m_port;
+};