allow running precommit only on changed files from a base ref

README.md

@@ -63,3 +63,30 @@ while you could _technically_ configure this for a public repository (using a
 personal access token), I can't think of a way to do this safely without
 exposing a privileged token to pull requests -- if you have any ideas, please
 leave an issue!
+
+### run precommit against changed files
+
+to use the `changed` parameter, you must make sure your checkout contains enough depth
+to compare against.
+
+here's an example which runs during pull requests and executes precommit against the files changed
+from the target branch:
+
+```yaml
+name: precommit
+
+on: [pull_request]
+jobs:
+  precommit:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout code
+        uses: actions/checkout@v2
+      - run: |
+          git fetch --no-tags --prune --unshallow
+      - uses: actions/setup-python@v2
+      - uses: pre-commit/action@v1.0.1
+        with:
+          changed: true
+          base: ${{ github.event.pull_request.head.sha }}
+```

action.yml

@@ -4,6 +4,14 @@ inputs:
   token:
     description: github token to clone / push with
     required: false
+  changed:
+    description: only run against changed files in this commit.  requires advanced checkout.
+    required: false
+    default: "false"
+  base:
+    description: base git ref for 'changed' parameter.  requires advanced checkout.
+    required: false
+    default: "master"
 runs:
-  using: 'node12'
+  using: "node12"
-  main: 'dist/index.js'
+  main: "dist/index.js"

index.js

@@ -2,24 +2,57 @@ const core = require('@actions/core');
 const exec = require('@actions/exec');
 const github = require('@actions/github');
 
-const ARGS = [
+const ALL_ARGS = [
     'run', '--all-files', '--show-diff-on-failure', '--color=always'
 ];
 
+const CHANGED_ARGS = [
+    'run', '--show-diff-on-failure', '--color=always', '--files'
+];
+
+
 function addToken(url, token) {
     return url.replace(/^https:\/\//, `https://x-access-token:${token}@`)
 }
 
+async function getChangedFiles() {
+
+    let changedFiles = '';
+    const baseRef = core.getInput('base');
+
+    const options = {};
+    options.listeners = {
+        stdout: (data) => {
+            changedFiles += data.toString();
+        },
+    };
+
+    await exec.exec('git', ['diff', baseRef, '--name-only'], options);
+
+    return changedFiles;
+}
+
 async function main() {
+    var ARGS
+
     await core.group('install pre-commit', async () => {
         await exec.exec('pip', ['install', 'pre-commit']);
         await exec.exec('pip', ['freeze', '--local']);
     });
 
+    if (core.getInput('changed')) {
+        ARGS = CHANGED_ARGS;
+        fileList = await getChangedFiles();
+        ARGS.push(fileList);
+    }
+    else {
+        ARGS = ALL_ARGS;
+    }
+
     const token = core.getInput('token');
     const pr = github.context.payload.pull_request;
     const push = !!token && !!pr;
-    const ret = await exec.exec('pre-commit', ARGS, {ignoreReturnCode: push});
+    const ret = await exec.exec('pre-commit', ARGS, { ignoreReturnCode: push });
     if (ret && push) {
         // actions do not run on pushes made by actions.
         // need to make absolute sure things are good before pushing
@@ -27,7 +60,7 @@ async function main() {
         await exec.exec('pre-commit', ARGS);
 
         const diff = await exec.exec(
-            'git', ['diff', '--quiet'], {ignoreReturnCode: true}
+            'git', ['diff', '--quiet'], { ignoreReturnCode: true }
         );
         if (diff) {
             await core.group('push fixes', async () => {