.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/vm/contracts/plot_01_vending_machine.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_api_gallery_vm_contracts_plot_01_vending_machine.py>`
        to download the full example code or to run this example in your browser via JupyterLite.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_api_gallery_vm_contracts_plot_01_vending_machine.py:


Smart contracts: the vending machine (Szabo 1994)
=================================================

Nick Szabo coined *smart contract* for terms of an agreement enforced by a
mechanism rather than by courts, and gave the vending machine as the
primordial example: anyone who inserts enough money gets the item and the
change, and nobody can take the goods without paying. A blockchain contract
is such a machine whose code and state every node can check.

What to look for
----------------

Every outcome follows from the code: a sale moves exactly one item and the
price, underpaying or buying from an empty machine changes nothing, and the
machine's books always balance: revenue equals price times items sold.

The history behind this experiment: :doc:`/history/vm_breakthroughs`.

.. GENERATED FROM PYTHON SOURCE LINES 22-24

Run a day of customers
----------------------

.. GENERATED FROM PYTHON SOURCE LINES 24-59

.. code-block:: Python

    from random import Random

    import matplotlib.pyplot as plt

    import blockchainkit as bk

    machine = bk.vm.vending_machine()
    STOCK, PRICE, REVENUE = 0, 1, 2
    storage = {STOCK: 10, PRICE: 3, REVENUE: 0}
    rng = Random(4)
    history = []
    for _ in range(25):
        payment = rng.randint(0, 6)
        try:
            result = bk.vm.execute(machine, arguments=(payment,), storage=storage)
            storage = dict(result.storage)
            history.append(("sale", payment, result.stack[0]))
        except bk.vm.VMError:
            history.append(("refused", payment, payment))  # Nothing happened; money returned.
        sold = 10 - storage[STOCK]
        assert storage.get(REVENUE, 0) == sold * storage[PRICE]
    sales = [h for h in history if h[0] == "sale"]
    print(len(sales), "sales;", storage)
    assert all(change == paid - 3 for _, paid, change in sales)
    assert storage[STOCK] == 0 or all(h[1] < 3 for h in history if h[0] == "refused")

    fig, ax = plt.subplots(figsize=(8, 3.5))
    colors = {"sale": "#16a34a", "refused": "#dc2626"}
    for i, (kind, paid, _) in enumerate(history):
        ax.bar(i, paid, color=colors[kind])
    ax.axhline(3, color="black", linestyle="--", label="price")
    ax.set(xlabel="customer", ylabel="payment", title="Green: sale, red: refused (nothing changes)")
    ax.legend()
    fig.tight_layout()




.. image-sg:: /api/gallery/vm/contracts/images/sphx_glr_plot_01_vending_machine_001.png
   :alt: Green: sale, red: refused (nothing changes)
   :srcset: /api/gallery/vm/contracts/images/sphx_glr_plot_01_vending_machine_001.png
   :class: sphx-glr-single-img


.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    10 sales; {0: 0, 1: 3, 2: 30}




.. GENERATED FROM PYTHON SOURCE LINES 60-65

Exercise
--------
Add an owner withdrawal: a second program that moves the revenue out and
resets it to zero. What stops a customer from running it? (This machine
has no notion of a caller; what would the contract need to know?)


.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 0.134 seconds)


.. _sphx_glr_download_api_gallery_vm_contracts_plot_01_vending_machine.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: lite-badge

      .. image:: images/jupyterlite_badge_logo.svg
        :target: ../../../../lite/lab/index.html?path=api/gallery/vm/contracts/plot_01_vending_machine.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: plot_01_vending_machine.ipynb <plot_01_vending_machine.ipynb>`

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: plot_01_vending_machine.py <plot_01_vending_machine.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: plot_01_vending_machine.zip <plot_01_vending_machine.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_