Implement 'command minus n'.

src/utils/history.ts

@@ -26,14 +26,29 @@ export function push(command: string): History {
 
 export function getCommandIfHistoryExpansion(input): string {
   let command = undefined;
-  const regExpExecArray = /!(?:(\d+)|!)/.exec(input);
+  const regExpExecArray = /!(-?)(?:(\d+)|!)/.exec(input);
   if (regExpExecArray) {
-    const index = regExpExecArray[1];
-    if (index === '!') {
+    const isMinus =
+      Boolean(regExpExecArray[1]) && regExpExecArray[1].length > 0;
+
+    /**
+     * either digits or '!'
+     */
+    const n = regExpExecArray[2];
+    const index = parseInt(n);
+    if (Number.isNaN(index)) {
       command = getLast();
     } else {
-      // parseInt here is guaranteed to succeed I suppose.
-      command = get(parseInt(index));
+      /**
+       * !0, !-0 are invalid
+       */
+      if (index > 0) {
+        if (isMinus) {
+          command = getMinus(index);
+        } else {
+          command = get(index);
+        }
+      }
     }
   }
 
@@ -44,6 +59,11 @@ function get(index: number): string {
   return history[index];
 }
 
+function getMinus(index: number): string {
+  // '!!' is a synonym for '!-1' so + 1
+  return history[historyIndex - index + 1];
+}
+
 function getLast(): string {
   return history[historyIndex];
 }