dotnet-installer/tools/RuntimeGraphGenerator/RuntimeGraphManager.cs

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).PackageLibrary.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;
                }
            }
        }
    }
}