Add help messages when passing unknown flags

src/command.ts

@@ -1,6 +1,7 @@
 import type { ArgumentType } from "./types.ts";
 import { ArgumentError, HelpError } from "./error.ts";
 import { leftPad } from "./fmt.ts";
+import { closest } from "./distance.ts";
 
 export interface ArgumentOptions {
   /**
@@ -139,7 +140,13 @@ export class Command<T = Record<never, never>> {
   }
 
   private _fmtUnknownFlag(arg: string): string {
-    return `Unknown flag '${arg.replaceAll("'", "\\'")}'`;
+    const unknownFlag = `Unknown flag '${arg.replaceAll("'", "\\'")}'`;
+    const closestFlag = closest(arg, this._allFlags);
+    if (closestFlag) {
+      return `${unknownFlag}\n\nHELP:\n\tDid you mean ${closestFlag}?`;
+    } else {
+      return unknownFlag;
+    }
   }
 
   private _normalizeFlags(flags: string[]): string[] {

src/distance.ts

@@ -0,0 +1,26 @@
+function intersection<T>(a: Set<T>, b: Set<T>): number {
+  let intersection = 0;
+  for (const item of a) if (b.has(item)) intersection += 1;
+  return intersection;
+}
+
+function union<T>(a: Set<T>, b: Set<T>): number {
+  let union = a.size;
+  for (const item of b) if (!a.has(item)) union += 1;
+  return union;
+}
+
+export function closest(string: string, options: Set<string>): string | null {
+  const stringSet = new Set(string);
+  let bestItem = null;
+  let bestScore = 0;
+  for (const item of options) {
+    const itemSet = new Set(item);
+    const score = intersection(stringSet, itemSet) / union(stringSet, itemSet);
+    if (bestScore <= score) {
+      bestScore = score;
+      bestItem = item;
+    }
+  }
+  return bestItem;
+}