add fast-check package, use it, simplify code slightly

Author: josharian

packages/common/package.json

@@ -20,6 +20,7 @@
     "@types/lodash": "4.14.181",
     "@types/mocha": "^8.0.4",
     "@types/sinon": "^10.0.2",
+    "fast-check": "3.12.0",
     "js-yaml": "^4.1.0",
     "mocha": "^10.2.0",
     "sinon": "^11.1.1"

packages/common/src/testUtil/mockEditor.test.ts

@@ -1,5 +1,6 @@
 import * as assert from "assert";
 import { MockTextDocument, Range } from "..";
+import * as fc from "fast-check";
 
 suite("mockEditor", () => {
   test("basic", () => {
@@ -33,4 +34,29 @@ suite("mockEditor", () => {
     assert.equal(line1.rangeIncludingLineBreak.start.character, 0);
     assert.equal(line1.lastNonWhitespaceCharacterIndex, 0);
   });
+
+  test("fastcheck", () => {
+    fc.assert(
+      fc.property(fc.string(), (contents) => {
+        const doc: MockTextDocument = new MockTextDocument(
+          "test.txt",
+          "plaintext",
+          contents,
+        );
+        for (let i = 0; i < contents.length; i++) {
+          const pos = doc.positionAt(i);
+          // positions must be within the range of a line
+          if (pos.character > doc.lineAt(pos.line).range.end.character) {
+            return false;
+          }
+          const offset = doc.offsetAt(pos);
+          // positionAt and offsetAt are inverses
+          if (offset !== i) {
+            return false;
+          }
+          return true;
+        }
+      }),
+    );
+  });
 });

packages/common/src/testUtil/mockEditor.ts

@@ -78,11 +78,12 @@ export class MockTextDocument implements TextDocument {
     this.lines = rawLines.map((line, i) => {
       return new MockTextLine(i, line, "\n");
     });
+    const lastLineRange = this.lines[this.lines.length - 1].range;
     this.range = new Range(
       0,
       0,
-      this.lineCount - 1,
-      this.lineLength(this.lineCount - 1),
+      lastLineRange.end.line,
+      lastLineRange.end.character,
     );
     this.eol = "LF";
   }
@@ -91,10 +92,6 @@ export class MockTextDocument implements TextDocument {
     return this.lines.length;
   }
 
-  private lineLength(line: number): number {
-    return this.lines[line].text.length + 1; // EOF
-  }
-
   public lineAt(x: number | Position): TextLine {
     if (typeof x === "number") {
       return this.lines[x];