5 pipeline with unit tests (#10)

---------

Co-authored-by: Julian Piribauer <julian.piribauer@gmail.com>
Co-authored-by: julian <julian.piribauer@gmail.com>
Reviewed-on: #10

.gitea/ci-image/Dockerfile

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+# Custom CI image for the arm64 (Raspberry Pi) Gitea Actions runner,
+# since the official sagemath/sagemath image is amd64-only.
+#
+# Build & push commands:
+#   docker build -t gitea.piribauer.ch/julian/sage-ci:latest -f .gitea/ci-image/Dockerfile .
+#   docker login gitea.piribauer.ch -u julian
+#   docker push gitea.piribauer.ch/julian/sage-ci:latest
+
+FROM condaforge/miniforge3:latest
+
+RUN mamba install -y -c conda-forge sage ruff pytest \
+    && mamba clean -afy
+
+RUN mamba install -y -c conda-forge nodejs \
+    && mamba clean -afy
+
+WORKDIR /workspace

.gitea/preparse.sh

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+#!/usr/bin/env bash
+
+# This script is run in the CI container to preparse all .sage files into .sage.py files
+# and can also be run locally for local linting/testing.
+
+set -e
+
+if command -v python >/dev/null 2>&1 && python -c "import sage.repl" >/dev/null 2>&1; then
+  run_python() { python "$@"; }
+else
+  run_python() { sage --python "$@"; }
+fi
+
+for f in $(find . -name "*.sage" -not -path "./playground/*"); do
+  run_python -c "
+import sys
+from sage.repl.preparse import preparse_file
+
+path = sys.argv[1]
+with open(path) as fh:
+    src = fh.read()
+with open(path + '.py', 'w') as fh:
+    fh.write('from sage.all import *  # noqa: F401,F403\n')
+    fh.write(preparse_file(src))
+" "$f"
+done

.gitea/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    # Custom image (see .gitea/ci-image/Dockerfile)
+    container:
+      image: gitea.piribauer.ch/julian/sage-ci:latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Preparse .sage files
+        run: .gitea/preparse.sh
+
+      - name: ruff check
+        run: python -m ruff check --no-respect-gitignore .
+
+  test:
+    runs-on: ubuntu-latest
+    needs: lint
+    container:
+      image: gitea.piribauer.ch/julian/sage-ci:latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run tests
+        run: python -m pytest tests/