Generate runtime graph when building runtime

tools/RuntimeGraphGenerator/RuntimeGraphManager.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.DotNet.ProjectModel.Compilation;
+using Microsoft.DotNet.ProjectModel.Graph;
+using NuGet.RuntimeModel;
+using System.IO;
+using Microsoft.Extensions.DependencyModel;
+
+namespace Microsoft.DotNet.ProjectModel
+{
+    public class RuntimeGraphManager
+    {
+        private const string RuntimeJsonFileName = "runtime.json";
+
+        public NuGet.RuntimeModel.RuntimeGraph Collect(IEnumerable<LibraryExport> exports)
+        {
+            var graph = RuntimeGraph.Empty;
+            foreach (var export in exports)
+            {
+                if (export.Library.Identity.Type == LibraryType.Package)
+                {
+                    var runtimeJson =  ((PackageDescription) export.Library).Library.Files.FirstOrDefault(f => f == RuntimeJsonFileName);
+                    if (runtimeJson != null)
+                    {
+                        var runtimeJsonFullName = Path.Combine(export.Library.Path, runtimeJson);
+                        graph = RuntimeGraph.Merge(graph, JsonRuntimeFormat.ReadRuntimeGraph(runtimeJsonFullName));
+                    }
+                }
+            }
+            return graph;
+        }
+
+        public IEnumerable<RuntimeFallbacks> Expand(RuntimeGraph runtimeGraph, IEnumerable<string> runtimes)
+        {
+            foreach (var runtime in runtimes)
+            {
+                var importers = FindImporters(runtimeGraph, runtime);
+                foreach (var importer in importers)
+                {
+                    // ExpandRuntime return runtime itself as first item so we are skiping it
+                    yield return new RuntimeFallbacks(importer, runtimeGraph.ExpandRuntime(importer).Skip(1));
+                }
+            }
+        }
+
+        private IEnumerable<string> FindImporters(RuntimeGraph runtimeGraph, string runtime)
+        {
+            foreach (var runtimePair in runtimeGraph.Runtimes)
+            {
+                var expanded = runtimeGraph.ExpandRuntime(runtimePair.Key);
+                if (expanded.Contains(runtime))
+                {
+                    yield return runtimePair.Key;
+                }
+            }
+        }
+    }
+}