action.yml

@@ -18,6 +18,8 @@ description: |
       comment-on-pr: 'true'
       generate-artifact: 'false'
       artifact-name: 'size-report.md'
+      base-branch: ${{ github.base_ref }} # set to e.g. main if not triggered by a pull_request
+      job-name: '' # set to the name of this job if you want to enable comparisons
   ```
 inputs:
   comment-on-pr:
@@ -29,6 +31,21 @@ inputs:
   artifact-name:
     description: The name of the generated artifact.
     default: report.json
+  base-branch:
+    description: |
+      The name of the base branch, defaults to github.base_ref which is only
+      available in pull_request triggered workflows, for other workflows
+      specify it manually.
+
+      It will try to download the generated artifact from this branch so make
+      sure `generate-artifacte: true` when the workflow is running on that
+      branch.
+    default: ${{ github.base_ref }}
+  job-name:
+    description: |
+      The name of the job running this action. If not set, no comparisons can
+      be made.
+    default: ''
 outputs:
 runs:
   using: 'composite'
@@ -44,5 +61,33 @@ runs:
         name: ${{ inputs.artifact-name }}
     - name: Comment Report
       if: inputs.comment-on-pr == 'true'
+      env:
+        ARTIFACT_NAME: ${{ inputs.artifact-name }}
+        HEAD_BRANCH: ${{ inputs.base-branch }}
+        JOB_NAME: ${{ inputs.job-name }}
       run: |
+        # USAGE: base_report_url
+        base_report_url() {
+          curl -X 'GET' \
+            "$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/actions/tasks" \
+            -H "Authorization: token $GITHUB_TOKEN" \
+            -H 'accept: application/json' |
+              jq --raw-output '[.workflow_runs[] | select(.name == '"$JOB_NAME"' and .head_branch == '"$HEAD_BRANCH"')] | first | .url'
+        }
+        # USAGE: has_report <RUN_URL>
+        has_report() {
+          http_code=$(curl -X 'GET' -o /dev/null --silent -Iw '%{http_code}' "$1/artifacts/$ARTIFACT_NAME")
+          test "$http_code" = '200'
+        }
+
+        # If we have a previous report compare against it
+        if [ "$JOB_NAME" ] && [ "$HEAD_BRANCH" ]; then
+          url=$(base_report_url)
+          if has_report "$url"; then
+            curl -X 'GET' -o old-report.json "$url/artifacts/$ARTIFACT_NAME"
+            "$GITHUB_ACTION_PATH/comment_on_pr.sh" report.json old-report.json
+            exit 0
+          fi
+        fi
+
         "$GITHUB_ACTION_PATH/comment_on_pr.sh" report.json