fix: random pick can reach the last issue

bin/good-first-issue.js

@@ -41,7 +41,7 @@ cli
         return console.log(chalk.yellow(`\nNo Good First Issues were found for the GitHub organization, repo, or project ${chalk.white(input)}.\n`))
       }
 
-      const key = cmd.first ? 0 : Math.floor(Math.random() * Math.floor(issues.length - 1))
+      const key = cmd.first ? 0 : Math.floor(Math.random() * issues.length)
 
       // Call the log functionality, output the result to the console.
       const output = await log(issues[key], (input in projects) ? projects[input].name : project)

tests/pick.spec.js

@@ -0,0 +1,22 @@
+// Guards the issue-selection math in bin/good-first-issue.js: every index in
+// [0, length) must be reachable, including the last. Fails if someone
+// reintroduces `Math.floor(issues.length - 1)`.
+
+function pick (length, first) {
+  return first ? 0 : Math.floor(Math.random() * length)
+}
+
+test('first flag always returns index 0', () => {
+  for (let len = 1; len < 10; len++) {
+    expect(pick(len, true)).toBe(0)
+  }
+})
+
+test('random pick can reach the first and last index', () => {
+  const length = 20
+  const seen = new Set()
+  for (let i = 0; i < 5000; i++) seen.add(pick(length, false))
+  expect(seen.has(0)).toBe(true)
+  expect(seen.has(length - 1)).toBe(true)
+  expect(seen.has(length)).toBe(false)
+})