using System;
using Mono.Cecil;
using Mono.Cecil.Cil;

class P {
  static int Main(string[] a) {
    string src = a[0], dst = a[1];
    var m = ModuleDefinition.ReadModule(src);
    var tsAppDomain = new TypeReference("System", "AppDomain", m, m.TypeSystem.CoreLibrary, false);
    var getCurrentDomain = new MethodReference("get_CurrentDomain", tsAppDomain, tsAppDomain){ HasThis = false };
    var getBaseDir = new MethodReference("get_BaseDirectory", m.TypeSystem.String, tsAppDomain){ HasThis = true };
    var mGetCurrent = m.ImportReference(getCurrentDomain);
    var mGetBase = m.ImportReference(getBaseDir);
    int patched = 0;
    foreach (var t in m.GetTypes())
      foreach (var meth in t.Methods) {
        if (!meth.HasBody) continue;
        var il = meth.Body.Instructions;
        for (int i = 1; i < il.Count; i++) {
          var ins = il[i];
          if (ins.OpCode == OpCodes.Call && ins.Operand is MethodReference mr
              && mr.Name == "initialize" && mr.DeclaringType.Name == "Entry"
              && mr.Parameters.Count == 2 && il[i-1].OpCode == OpCodes.Ldnull) {
            // assumes the ldnull is not a branch or handler target; Replace
            // does not retarget operands pointing at the removed instruction
            var proc = meth.Body.GetILProcessor();
            var ldnull = il[i-1];
            var call1 = proc.Create(OpCodes.Call, mGetCurrent);
            var call2 = proc.Create(OpCodes.Callvirt, mGetBase);
            proc.Replace(ldnull, call1);
            proc.InsertAfter(call1, call2);
            patched++;
            Console.WriteLine($"Patched {t.FullName}.{meth.Name}: ldnull -> AppDomain.CurrentDomain.BaseDirectory");
          }
        }
      }
    if (patched == 0) { Console.WriteLine("No patch site found"); return 2; }
    m.Write(dst);
    Console.WriteLine($"Wrote {dst}, patched={patched}");
    return 0;
  }
}