action.yml

name: Prepare multi-arch build matrix
description: Validate architectures and build a matrix with arch, runner OS, and full image name

inputs:
  architectures:
    description: Architectures to build (JSON array, e.g., '["amd64", "aarch64"]')
    required: true
  image-name:
    description: Image name without a tag (e.g., "base-python")
    required: true
  registry-prefix:
    description: Registry and namespace prefix (e.g., "ghcr.io/owner")
    required: false
    default: ghcr.io/${{ github.repository_owner }}

outputs:
  matrix:
    description: JSON matrix with arch, os, and image name for each architecture
    value: ${{ steps.set-matrix.outputs.matrix }}

runs:
  using: composite
  steps:
    - name: Build matrix from architectures
      id: set-matrix
      shell: bash
      env:
        ARCHITECTURES: ${{ inputs.architectures }}
        REGISTRY_PREFIX: ${{ inputs.registry-prefix }}
        IMAGE_NAME: ${{ inputs.image-name }}
      run: |
        matrix='{"include":[]}'
        for arch in $(jq -r '.[]' <<< "${ARCHITECTURES}"); do
          case "${arch}" in
            amd64)   os="ubuntu-24.04" ;;
            aarch64) os="ubuntu-24.04-arm" ;;
            *)
              echo "::error::Unsupported architecture: ${arch}. Supported values: amd64, aarch64"
              exit 1
              ;;
          esac
          matrix=$(jq -c --arg arch "$arch" --arg os "$os" --arg image "${REGISTRY_PREFIX}/${arch}-${IMAGE_NAME}" \
            '.include += [{arch: $arch, os: $os, image: $image}]' <<< "${matrix}")
        done

        echo "matrix=${matrix}" >> "$GITHUB_OUTPUT"
        jq . <<< "${matrix}"