Add AGENTS.md and copilot-setup-steps.yml workflow

.github/workflows/copilot-setup-steps.yml

@@ -0,0 +1,46 @@
+name: "Copilot Setup Steps"
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  pull_request:
+    branches: [main, dev]
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    # Set the permissions to the lowest permissions possible needed for your steps.
+    # Copilot will be given its own token for its operations.
+    permissions:
+      # If you want to clone the repository as part of your setup steps, for example to install dependencies, you'll need the `contents: read` permission. If you don't clone the repository in your setup steps, Copilot will do this for you automatically after the steps complete.
+      contents: read
+
+    # You can define any steps you want, and they will run before the agent starts.
+    # If you do not check out your code, Copilot will do this for you.
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+
+      - name: Setup Deno
+        uses: denoland/setup-deno@v2
+        with:
+          deno-version: v2.x
+          cache: true
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "lts/*"
+
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - name: Cache dependencies
+        run: deno install

AGENTS.md

@@ -0,0 +1,111 @@
+# Base64 - Agent Guide
+
+## Project Overview
+
+Base64 is a lightweight, zero-dependency base64 encoding/decoding library for JavaScript and TypeScript that works across Node.js, Deno, Bun, and browsers. It supports both regular base64 and base64url encoding with string and ArrayBuffer conversions.
+
+## Project Structure
+
+```
+base64/
+├── src/                  # Source code
+│   └── base64.ts        # Main entry point with all base64 functions
+├── tests/               # Test files
+│   └── base64.test.ts   # Test suite
+├── build/               # Build scripts
+├── docs/                # Documentation (Jekyll site)
+│   ├── index.md
+│   ├── installation.md
+│   ├── examples.md
+│   └── contributing.md
+└── deno.json            # Deno configuration and tasks
+```
+
+## Development Environment
+
+The project uses **Deno** as the primary development runtime, with cross-runtime support for Node.js and Bun.
+
+### Setup
+```bash
+# Install Deno dependencies
+deno install
+```
+
+## Contribution Guidelines
+
+### Pre-commit Checks
+
+Before committing changes, always run:
+```bash
+deno fmt
+deno lint
+deno check src/base64.ts
+```
+
+This ensures:
+- **Formatting**: Code follows style guidelines
+- **Linting**: Code quality checks pass
+- **Type checking**: TypeScript types are valid
+
+### Testing
+
+Run tests during development:
+```bash
+deno test
+```
+
+**Note**: The project uses CI workflows from `@cross-org/workflows` for cross-runtime testing (Node.js, Deno, and Bun). These workflows are imported in `.github/workflows/test.yml`.
+
+### Bun testing equivalent to CI
+
+```bash
+# Prerequisites
+bun x jsr add @cross/test @std/assert
+
+# Run tests
+bun test
+```
+
+### Node testing equivalent to CI
+
+package.json must be created with `"type": "module"`
+
+```bash
+# Prerequisites
+npx jsr add @cross/test @std/assert
+
+# Run tests
+npx --yes tsx --test tests/*.test.ts
+```
+
+### Full Build
+
+Before submitting a PR, run the full build to ensure all checks pass:
+```bash
+deno task build
+```
+
+This runs all tests, builds distribution files for npm, and validates the entire codebase.
+
+### Key Points
+
+- Base work on the `main` or `dev` branch
+- Add test cases for all changes
+- Zero dependencies - do not add external dependencies
+- Follow existing code style and patterns
+- Update documentation if changing public APIs
+- The library should work across Node.js, Deno, Bun, and browsers
+
+For detailed contribution guidelines, see [docs/contributing.md](docs/contributing.md).
+
+## API Overview
+
+The library exports a single `base64` object with the following methods:
+
+- `fromArrayBuffer(buffer, urlMode)` - Encodes ArrayBuffer to base64/base64url
+- `toArrayBuffer(str, urlMode)` - Decodes base64/base64url to ArrayBuffer
+- `fromString(str, urlMode)` - Encodes string to base64/base64url
+- `toString(str, urlMode)` - Decodes base64/base64url to string
+- `validate(str, urlMode)` - Validates base64/base64url strings
+
+All methods support an optional `urlMode` parameter to switch between standard base64 and base64url encoding.

src/base64.ts

@@ -122,7 +122,7 @@ function toString(str: string, urlMode?: boolean): string {
 string
  */
 function fromString(str: string, urlMode?: boolean): string {
-  return fromArrayBuffer(new TextEncoder().encode(str), urlMode);
+  return fromArrayBuffer(new TextEncoder().encode(str).buffer, urlMode);
 }
 
 /**