{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "2d51c41e",
   "metadata": {},
   "source": [
    "# 21 - Backend Comparison\n",
    "\n",
    "Compare the local simulator with the Aer backend.\n",
    "\n",
    "**Concepts:** Backend routing, qs.run, performance, consistency"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "1d873f7d",
   "metadata": {},
   "outputs": [],
   "source": [
    "import quantsdk as qs\n",
    "import time"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "ab3d7437",
   "metadata": {},
   "source": [
    "## Local Simulator"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "0f3c96ce",
   "metadata": {},
   "outputs": [],
   "source": [
    "circuit = qs.Circuit(4, name=\"test\")\n",
    "circuit.h(0).cx(0, 1).cx(1, 2).cx(2, 3).measure_all()\n",
    "\n",
    "# Run on local simulator (default)\n",
    "t0 = time.perf_counter()\n",
    "result_local = qs.run(circuit, shots=4000, seed=42, backend=\"local\")\n",
    "t_local = time.perf_counter() - t0\n",
    "\n",
    "print(\"Local Simulator Results:\")\n",
    "print(f\"  Time: {t_local*1000:.1f} ms\")\n",
    "print(f\"  Counts: {result_local.counts}\")\n",
    "print(f\"  Most likely: {result_local.most_likely}\")"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "5f806028",
   "metadata": {},
   "source": [
    "## Aer Backend (if available)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "201206ef",
   "metadata": {},
   "outputs": [],
   "source": [
    "try:\n",
    "    t0 = time.perf_counter()\n",
    "    result_aer = qs.run(circuit, shots=4000, seed=42, backend=\"aer\")\n",
    "    t_aer = time.perf_counter() - t0\n",
    "    \n",
    "    print(\"Aer Backend Results:\")\n",
    "    print(f\"  Time: {t_aer*1000:.1f} ms\")\n",
    "    print(f\"  Counts: {result_aer.counts}\")\n",
    "    print(f\"  Most likely: {result_aer.most_likely}\")\n",
    "except Exception as e:\n",
    "    print(f\"Aer backend not available: {e}\")\n",
    "    print(\"Install: pip install 'quantsdk[ibm]'\")"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "7baa3f41",
   "metadata": {},
   "source": [
    "## Consistency Check"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "21d43f83",
   "metadata": {},
   "outputs": [],
   "source": [
    "# Both backends should give consistent results for simple circuits\n",
    "test_circuits = [\n",
    "    (\"H|0>\", qs.Circuit(1).h(0).measure(0)),\n",
    "    (\"Bell\", qs.Circuit(2).h(0).cx(0, 1).measure_all()),\n",
    "    (\"GHZ-3\", qs.Circuit(3).h(0).cx(0, 1).cx(1, 2).measure_all()),\n",
    "]\n",
    "\n",
    "for name, circ in test_circuits:\n",
    "    r = qs.run(circ, shots=4000, seed=42, backend=\"local\")\n",
    "    print(f\"{name:8s}: {r.most_likely} | top_2={r.top_k(2)}\")"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "3ff29c15",
   "metadata": {},
   "source": [
    "## Scaling Benchmark"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "89698f8f",
   "metadata": {},
   "outputs": [],
   "source": [
    "# How does the local simulator scale with qubit count?\n",
    "for n in [4, 8, 12, 16, 20]:\n",
    "    circuit = qs.Circuit(n)\n",
    "    circuit.h(0)\n",
    "    for i in range(n - 1):\n",
    "        circuit.cx(i, i + 1)\n",
    "    circuit.measure_all()\n",
    "    \n",
    "    t0 = time.perf_counter()\n",
    "    result = qs.run(circuit, shots=1000, seed=42, backend=\"local\")\n",
    "    elapsed = time.perf_counter() - t0\n",
    "    \n",
    "    print(f\"n={n:2d}: {elapsed*1000:8.1f} ms | {result.most_likely[:8]}... | states=2^{n}={2**n}\")"
   ]
  }
 ],
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  "language_info": {
   "name": "python"
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