Move some definitions out of util.ts into a new github.ts module

src/github.ts

@@ -0,0 +1,62 @@
+import type { GitHubInstance } from "./api-client";
+import { ActionsEnvVars, type ReadOnlyEnv } from "./environment";
+import { ConfigurationError } from "./util";
+
+/** A string representing a GitHub instance address. */
+export type InstanceURL = string;
+
+/**
+ * The URL for github.com.
+ */
+export const GITHUB_DOTCOM_URL: InstanceURL = "https://github.com";
+
+/** A string representing a GitHub API instance address. */
+export type InstanceApiURL = string;
+
+/**
+ * The URL for api.github.com.
+ */
+export const GITHUB_DOTCOM_API_URL: InstanceApiURL = "https://api.github.com";
+
+/**
+ * Parses user input of a github.com or GHES URL to a canonical form.
+ * Removes any API prefix or suffix if one is present.
+ */
+export function parseGitHubUrl(inputUrl: string): string {
+  const originalUrl = inputUrl;
+  if (inputUrl.indexOf("://") === -1) {
+    inputUrl = `https://${inputUrl}`;
+  }
+  if (!inputUrl.startsWith("http://") && !inputUrl.startsWith("https://")) {
+    throw new ConfigurationError(`"${originalUrl}" is not a http or https URL`);
+  }
+
+  let url: URL;
+  try {
+    url = new URL(inputUrl);
+  } catch {
+    throw new ConfigurationError(`"${originalUrl}" is not a valid URL`);
+  }
+
+  // If we detect this is trying to be to github.com
+  // then return with a fixed canonical URL.
+  if (url.hostname === "github.com" || url.hostname === "api.github.com") {
+    return GITHUB_DOTCOM_URL;
+  }
+
+  // Remove the API prefix if it's present
+  if (url.pathname.indexOf("/api/v3") !== -1) {
+    url.pathname = url.pathname.substring(0, url.pathname.indexOf("/api/v3"));
+  }
+  // Also consider subdomain isolation on GHES
+  if (url.hostname.startsWith("api.")) {
+    url.hostname = url.hostname.substring(4);
+  }
+
+  // Normalise path to having a trailing slash for consistency
+  if (!url.pathname.endsWith("/")) {
+    url.pathname = `${url.pathname}/`;
+  }
+
+  return url.toString();
+}