feat: Add pretty assert (denoland/deno_std#187)

Original: ddafcc6572

testing/pretty.ts

@@ -0,0 +1,75 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+
+import { equal } from "./mod.ts";
+import { red, green, white, gray, bold } from "../colors/mod.ts";
+import diff, { DiffType, DiffResult } from "./diff.ts";
+import { format } from "./format.ts";
+
+const CAN_NOT_DISPLAY = "[Cannot display]";
+
+function createStr(v: unknown): string {
+  try {
+    return format(v);
+  } catch (e) {
+    return red(CAN_NOT_DISPLAY);
+  }
+}
+
+function createColor(diffType: DiffType) {
+  switch (diffType) {
+    case "added":
+      return (s: string) => green(bold(s));
+    case "removed":
+      return (s: string) => red(bold(s));
+    default:
+      return white;
+  }
+}
+
+function createSign(diffType: DiffType) {
+  switch (diffType) {
+    case "added":
+      return "+   ";
+    case "removed":
+      return "-   ";
+    default:
+      return "    ";
+  }
+}
+
+function buildMessage(diffResult: ReadonlyArray<DiffResult<string>>) {
+  const messages = [];
+  messages.push("");
+  messages.push("");
+  messages.push(
+    `    ${gray(bold("[Diff]"))} ${red(bold("Left"))} / ${green(bold("Right"))}`
+  );
+  messages.push("");
+  messages.push("");
+  diffResult.forEach((result: DiffResult<string>) => {
+    const c = createColor(result.type);
+    messages.push(c(`${createSign(result.type)}${result.value}`));
+  });
+  messages.push("");
+
+  return messages;
+}
+
+export function assertEqual(actual: unknown, expected: unknown) {
+  if (equal(actual, expected)) {
+    return;
+  }
+  let message = "";
+  const actualString = createStr(actual);
+  const expectedString = createStr(expected);
+  try {
+    const diffResult = diff(
+      actualString.split("\n"),
+      expectedString.split("\n")
+    );
+    message = buildMessage(diffResult).join("\n");
+  } catch (e) {
+    message = `\n${red(CAN_NOT_DISPLAY)} + \n\n`;
+  }
+  throw new Error(message);
+}