feat: add a schema for serialization

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
 
     - name: Install package
-      run: python -m pip install .[test]
+      run: python -m pip install .[test,schema]
 
     - name: Test
       run: python -m pytest -ra

.pre-commit-config.yaml

@@ -39,7 +39,6 @@ repos:
         - importlib_metadata
         - importlib_resources
 
-
   - repo: https://github.com/pre-commit/pygrep-hooks
     rev: v1.10.0
     hooks:
@@ -72,3 +71,9 @@ repos:
     hooks:
       - id: check-readthedocs
       - id: check-github-workflows
+      - id: check-metaschema
+        files: ^src/uhi/resources/histogram.json$
+      - id: check-jsonschema
+        name: Validate Histogram examples
+        args: [--schemafile, src/uhi/resources/histogram.json]
+        files: ^tests/resources/.*\.json

docs/conf.py

@@ -32,6 +32,7 @@
 # ones.
 extensions = [
     "myst_parser",
+    "sphinx-jsonschema",
     "sphinx.ext.napoleon",
     "sphinx_copybutton",
     "sphinx_github_changelog",

docs/index.rst

@@ -30,6 +30,7 @@ to plot a histogram, including error bars.
    indexing.rst
    indexing+.rst
    plotting.rst
+   serialization.md
    changelog.md
 
 

docs/serialization.md

@@ -0,0 +1,118 @@
+# Serialization
+
+
+## Introduction
+
+Histogram serialization has to cover a wide range of formats. As such, we
+describe a form for serialization that covers the metadata structure as
+JSON-like, with a provided JSON-schema. The data (bins and/or variable edges)
+is stored out-of-band in a binary format based on what type of data file you
+are in.  For very small (primarily 1D) histograms, data is allowed inline as
+well.
+
+The following formats are being targeted:
+
+```
+┌────────┐ ┌────────┐ ┌───────┐
+│  ROOT  │ │  HDF5  │ │  ZIP  │
+└────────┘ └────────┘ └───────┘
+```
+
+Other formats can be used as well, assuming they support out-of-band data and
+text attributes or files for the metadata.
+
+## Caveats
+
+This structure was based heavily on boost-histogram, but it is intended to be
+general, and can be expanded in the future as needed. As such, the following
+limitations are required:
+
+* Serialization followed by deserialisation may cause axis changes. Axis types
+  may change to an equivalent but less performant axis, growth status will be
+  lost, etc.
+* Metadata must be expressible as JSON. It should also be reasonably sized; some
+  formats like HDF5 may limit the size of attributes to 64K.
+* Floating point errors could be incurred on conversion, as the storage format
+  uses a stable but different representation.
+* Axis `name` is only part of the metadata, and is not standardized. This is
+  due to lack of support from boost-histogram.
+
+## Design
+
+The following axes types are supported:
+
+* `"regular"`: A regularly spaced set of even bins. Boost-histogram's "integer"
+  axes maps to this axis as well. Has `upper`, `lower`, `bins`, `underflow`,
+  `overflow`, and `circular` properties. `circular` defaults to False if not
+  present.
+* `"variable"`: A continuous axis defined by bins+1 edges. Has `edges`, which
+  is either an in-line list of numbers or a string pointing to an out-of-band data source.
+  Also has `underflow`, `overflow`, and `circular` properties. `circular`
+  defaults to False if not present.
+* `"category_int"`: A list of integer bins, non-continuous. Has `categories`,
+  which is an in-line list of integers. Also has `flow`.
+* `"category_str"`: A list of string bins. Has `categories`,
+  which is an in-line list of strings. Also has `flow`.
+* `"boolean"`: A true/false axis.
+
+Axes with gaps are currently not supported.
+
+All axes support `metadata`, a string-valued dictionary of arbitrary, JSON-like data.
+
+The following storages are supported:
+
+* `"int"`: A collection of integers. Boost-histogram's Int64 and AtomicInt64
+  map to this, and sometimes Unlimited.
+* `"double"`: A collection of 64-bit floating point values. Boost-histogram's
+  Double storage maps to this, and sometimes Unlimited.
+* `"weighted"`: A collection of two arrays of 64-bit floating point values,
+  `"value"` and `"variance"`. Boost-histogram's Weight storage maps to this.
+* `"mean"`: A collection of three arrays of 64-bit floating point values,
+  "count", "value", and "variance". Boost-histogram's Mean storage maps to
+  this.
+* `"weighted_mean"`: A collection of four arrays of 64-bit floating point
+  values, `"sum_of_weights"`, `"sum_of_weights_squared"`, `"values"`, and
+  `"variances"`. Boost-histogram's WeighedMean storage maps to this.
+
+## CLI/API
+
+You can currently test a JSON file against the schema by running:
+
+```console
+$ python -m uhi.schema some/file.json
+```
+
+Or with code:
+
+```python
+import uhi.schema
+
+uhi.schema.validate("some/file.json")
+```
+
+Eventually this should also be usable for JSON's inside zip, HDF5 attributes,
+and maybe more.
+
+```{warning}
+
+Currently, this spec describes **how to prepare the metadata** for one of the
+targeted backends. It does not yet cover backend specific details, like how to
+define and use the binary resource locator strings or how to store the data.
+JSON is not a target spec, but just part of the ZIP spec, meaning the files
+that currently "pass" the tool above would be valid inside a `.zip` file
+eventually, but are not valid by themselves.
+```
+
+## Rendered schema
+
+```{jsonschema} ../src/uhi/resources/histogram.json
+```
+
+
+## Full schema
+
+The full schema is below:
+
+```{literalinclude} ../src/uhi/resources/histogram.json
+:language: json
+```

noxfile.py

@@ -28,7 +28,7 @@ def tests(session):
     """
     Run the unit and regular tests.
     """
-    session.install("-e.[test]")
+    session.install("-e.[test,schema]")
     session.run("pytest", *session.posargs)
 
 

pyproject.toml

@@ -44,10 +44,15 @@ Documentation = "https://uhi.readthedocs.io/en/latest/"
 Changelog = "https://github.com/scikit-hep/uhi/releases"
 
 [project.optional-dependencies]
+schema = [
+  "fastjsonschema",
+  "importlib-resources; python_version<'3.9'",
+]
 docs = [
   "sphinx>=4.0",
   "furo",
   "sphinx-copybutton>=0.3.1",
+  "sphinx-jsonschema",
   "myst-parser",
   "sphinx_github_changelog",
 ]
@@ -88,6 +93,10 @@ show_error_codes = true
 warn_unreachable = true
 enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 
+[[tool.mypy.overrides]]
+module = ["fastjsonschema"]
+ignore_missing_imports = true
+
 [tool.ruff]
 select = [
   "E", "F", "W", # flake8