pr-change-chart.yml

name: PR change chart

# Posts (and keeps updated) a single comment on each pull request showing the
# diff size per file: a Mermaid pie chart of each file's share of the churn
# plus a collapsible +/- breakdown table. Uses only first-party actions and
# the PR API, so there is no checkout, no build, and no third-party action.

on:
  pull_request:
    types: [opened, synchronize, reopened]

permissions:
  contents: read
  pull-requests: write

jobs:
  change-chart:
    runs-on: ubuntu-latest
    steps:
      - name: Build and upsert the change-summary comment
        uses: actions/github-script@v7
        with:
          script: |
            // Hidden marker so we can find and update our own comment instead
            // of posting a new one on every push.
            const MARKER = '<!-- pr-change-chart -->';
            const TOP = 15; // cap rows so the chart/table stay readable
            const { owner, repo } = context.repo;
            const pr = context.payload.pull_request;

            // Per-file additions/deletions come straight from the PR API.
            const files = await github.paginate(github.rest.pulls.listFiles, {
              owner, repo, pull_number: pr.number, per_page: 100,
            });

            let totalAdd = 0, totalDel = 0;
            const rows = files.map(f => {
              totalAdd += f.additions;
              totalDel += f.deletions;
              return { name: f.filename, add: f.additions, del: f.deletions, total: f.changes };
            }).sort((a, b) => b.total - a.total);

            const shown = rows.slice(0, TOP);
            const rest = rows.slice(TOP);
            const restTotal = rest.reduce((s, r) => s + r.total, 0);
            const churn = totalAdd + totalDel;

            // Mermaid pie of churn share per file (GitHub renders Mermaid
            // natively). Guarded: a pie with no slices would error, so we skip
            // it when the diff is all-binary / zero-line.
            let pie = '';
            if (churn > 0) {
              pie = '```mermaid\npie showData\n';
              pie += `    title Lines changed per file (${churn} total)\n`;
              for (const r of shown) {
                if (r.total <= 0) continue;
                const label = r.name.replace(/"/g, "'");
                pie += `    "${label}" : ${r.total}\n`;
              }
              if (restTotal > 0) pie += `    "… ${rest.length} more files" : ${restTotal}\n`;
              pie += '```';
            }

            // Collapsible per-file table with the +/- split.
            let table = '| File | + | − | total |\n|---|--:|--:|--:|\n';
            for (const r of shown) {
              table += `| \`${r.name}\` | ${r.add} | ${r.del} | ${r.total} |\n`;
            }
            if (rest.length) {
              table += `| _… ${rest.length} more files_ |  |  | ${restTotal} |\n`;
            }
            table += `| **Total (${files.length} files)** | **${totalAdd}** | **${totalDel}** | **${churn}** |\n`;

            const body = [
              MARKER,
              '### 📊 Change summary',
              `**${files.length}** files changed · **+${totalAdd}** / **−${totalDel}** lines`,
              '',
              pie,
              '',
              '<details><summary>Per-file breakdown</summary>',
              '',
              table,
              '</details>',
            ].join('\n');

            // Upsert: update our marked comment if it exists, else create it.
            const comments = await github.paginate(github.rest.issues.listComments, {
              owner, repo, issue_number: pr.number, per_page: 100,
            });
            const mine = comments.find(c => c.body && c.body.includes(MARKER));
            if (mine) {
              await github.rest.issues.updateComment({ owner, repo, comment_id: mine.id, body });
            } else {
              await github.rest.issues.createComment({ owner, repo, issue_number: pr.number, body });
            }