{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "3fa0020a",
   "metadata": {},
   "source": [
    "# mt_fillHistos\n",
    "Fill histograms in parallel and write them on file\n",
    "with a multithreaded approach using std::thread.\n",
    "This is the simplest meaningful example which shows \n",
    "ROOT thread awareness.\n",
    "\n",
    "\n",
    "\n",
    "\n",
    "**Author:** Danilo Piparo  \n",
    "<i><small>This notebook tutorial was automatically generated with <a href= \"https://github.com/root-project/root/blob/master/documentation/doxygen/converttonotebook.py\">ROOTBOOK-izer</a> from the macro found in the ROOT repository  on Tuesday, May 19, 2026 at 08:11 PM.</small></i>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "56c76c41",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:10.987269Z",
     "iopub.status.busy": "2026-05-19T20:11:10.987100Z",
     "iopub.status.idle": "2026-05-19T20:11:11.508672Z",
     "shell.execute_reply": "2026-05-19T20:11:11.487013Z"
    }
   },
   "outputs": [],
   "source": [
    "const UInt_t nNumbers = 20000000U;\n",
    "\n",
    "const UInt_t nWorkers = 4U;"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "3f2371c4",
   "metadata": {},
   "source": [
    "The first, fundamental operation to be performed in order to make ROOT\n",
    "thread-aware."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "eeb198ab",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:11.530263Z",
     "iopub.status.busy": "2026-05-19T20:11:11.530104Z",
     "iopub.status.idle": "2026-05-19T20:11:11.744528Z",
     "shell.execute_reply": "2026-05-19T20:11:11.743881Z"
    }
   },
   "outputs": [],
   "source": [
    "ROOT::EnableThreadSafety();"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "a31309a4",
   "metadata": {},
   "source": [
    "We define our work item"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "bd0670c3",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:11.746157Z",
     "iopub.status.busy": "2026-05-19T20:11:11.746034Z",
     "iopub.status.idle": "2026-05-19T20:11:11.948759Z",
     "shell.execute_reply": "2026-05-19T20:11:11.947572Z"
    }
   },
   "outputs": [],
   "source": [
    "auto workItem = [](UInt_t workerID) {\n",
    "   // One generator, file and ntuple per worker\n",
    "   TRandom3 workerRndm(workerID); // Change the seed\n",
    "   TFile f(Form(\"myFile_mt001_%u.root\", workerID), \"RECREATE\");\n",
    "   TH1F h(Form(\"myHisto_%u\", workerID), \"The Histogram\", 64, -4, 4);\n",
    "   for (UInt_t i = 0; i < nNumbers; ++i) {\n",
    "      h.Fill(workerRndm.Gaus());\n",
    "   }\n",
    "   h.Write();\n",
    "};"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "785f07ea",
   "metadata": {},
   "source": [
    "Create the collection which will hold the threads, our \"pool\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "068ce2e0",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:11.950193Z",
     "iopub.status.busy": "2026-05-19T20:11:11.950065Z",
     "iopub.status.idle": "2026-05-19T20:11:12.153903Z",
     "shell.execute_reply": "2026-05-19T20:11:12.153207Z"
    }
   },
   "outputs": [],
   "source": [
    "std::vector<std::thread> workers;"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "472c7c66",
   "metadata": {},
   "source": [
    "Fill the \"pool\" with workers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "a485c20f",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:12.155258Z",
     "iopub.status.busy": "2026-05-19T20:11:12.155141Z",
     "iopub.status.idle": "2026-05-19T20:11:12.365083Z",
     "shell.execute_reply": "2026-05-19T20:11:12.364707Z"
    }
   },
   "outputs": [],
   "source": [
    "for (auto workerID : ROOT::TSeqI(nWorkers)) {\n",
    "   workers.emplace_back(workItem, workerID);\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "097fe654",
   "metadata": {},
   "source": [
    "Now join them"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "id": "ef935a1a",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:12.401447Z",
     "iopub.status.busy": "2026-05-19T20:11:12.401299Z",
     "iopub.status.idle": "2026-05-19T20:11:13.325028Z",
     "shell.execute_reply": "2026-05-19T20:11:13.318362Z"
    }
   },
   "outputs": [],
   "source": [
    "for (auto &&worker : workers)\n",
    "   worker.join();\n",
    "\n",
    "return 0;"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "35f0ac0e",
   "metadata": {},
   "source": [
    "Draw all canvases "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "id": "44721299",
   "metadata": {
    "collapsed": false,
    "execution": {
     "iopub.execute_input": "2026-05-19T20:11:13.330409Z",
     "iopub.status.busy": "2026-05-19T20:11:13.330262Z",
     "iopub.status.idle": "2026-05-19T20:11:13.543307Z",
     "shell.execute_reply": "2026-05-19T20:11:13.542827Z"
    }
   },
   "outputs": [],
   "source": [
    "gROOT->GetListOfCanvases()->Draw()"
   ]
  }
 ],
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