add section: File resolution (DavHau#434)

Readme.md

@@ -44,6 +44,7 @@ Table of Contents
       * [Why nix?](#why-nix)
       * [How does mach-nix work?](#how-does-mach-nix-work)
          * [Dependency resolution](#dependency-resolution)
+         * [File resolution](#file-resolution)
          * [Generating a nix expression](#generating-a-nix-expression)
       * [Contributing](#contributing)
       * [Limitations](#limitations)
@@ -203,6 +204,60 @@ As core for the resolving resolvelib is used: https://github.com/sarugaku/resolv
 
 Mach-nix supports multiple providers to retrieve python packages from. The user can specify which providers should be preferred. Packages from different providers can be mixed.
 
+### File resolution
+With `pypi-deps-db`, we have built a dependency graph, where each package is defined by name and version, for example `pillow-9.1.0`.
+
+To download a package, we need it's URL and hash. This is where [nix-pypi-fetcher](https://github.com/DavHau/nix-pypi-fetcher) comes in.
+`nix-pypi-fetcher` is another database, which allows us to resolve packages to their URL and hash.
+
+For example, the declaration "package pillow + version 9.1.0 + python 3.9 + linux" resolves to
+
+```json
+  "Pillow-9.1.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl": [
+    "gig6+ZwcOluh2kTGcpbVqtGfEcU1tVGlrlUyijF84zE=",
+    "cp39"
+  ],
+```
+
+The wheel URL is constructed in [pypi-crawlers/src/crawl_wheel_deps.py](pypi-crawlers/src/crawl_wheel_deps.py)
+
+```py
+def construct_url(name, pyver, filename: str):
+    base_url = "https://files.pythonhosted.org/packages/"
+    return f"{base_url}{pyver}/{name[0]}/{name}/{filename}"
+```
+
+So in this example, the full URL would be
+
+```
+https://files.pythonhosted.org/packages/cp39/p/pillow/Pillow-9.1.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl
+```
+
+Now we can call Nix's `fetchurl` like
+
+```nix
+builtins.fetchurl {
+  url = "https://files.pythonhosted.org/packages/cp39/p/pillow/Pillow-9.1.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl";
+  sha256 = "gig6+ZwcOluh2kTGcpbVqtGfEcU1tVGlrlUyijF84zE=";
+}
+```
+
+... and we get
+
+```
+$ unzip -l /nix/store/7gbbbrsqkw7f69axyh818abh9yw45fnb-Pillow-9.1.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl | head
+Archive:  /nix/store/7gbbbrsqkw7f69axyh818abh9yw45fnb-Pillow-9.1.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl
+  Length      Date    Time    Name
+---------  ---------- -----   ----
+        0  04-01-2022 10:08   PIL/
+        0  04-01-2022 10:08   Pillow.libs/
+        0  04-01-2022 10:08   Pillow-9.1.0.dist-info/
+     7311  04-01-2022 10:08   PIL/PdfImagePlugin.py
+   141328  04-01-2022 10:08   PIL/_imagingcms.cpython-39-x86_64-linux-gnu.so
+    46872  04-01-2022 10:08   PIL/_imagingtk.cpython-39-x86_64-linux-gnu.so
+     1513  04-01-2022 10:08   PIL/GribStubImagePlugin.py
+```
+
 ### Generating a nix expression
 After all python dependencies and their providers have been determined by the dependency resolver, mach-nix will generate a nix expression defining your python environment.