Add unified diff support for luatest.assert_equals

[mandesero]

This patch adds unified diff output for t.assert_equals() failures when the test suite is run with the --diff option, using a vendored Lua implementation of google/diff-match-patch (luatest/vendor/diff_match_patch.lua taken from ¹).

Closes #412

---

Here is the output for the example from the issue:

code

local t = require('luatest')

local g = t.group()

g.test_table_assertion = function()
    local a = {
      a = {
        ["a"] = 1,
        ["b"] = 2,
        ["c"] = 3,
      },
    }

    local b = {
      a = {
        ["a"] = 1,
        ["b"] = 5,
        ["c"] = 8,
      },
    }
    t.assert_equals(a, b, 'compare tables')
end

g.test_string_assertion = function()
    local a = [[
aaaaa
bbbbb
ccccc
]]
    local b = [[
aaaaa
ddddd
ccccc
]]

    t.assert_equals(a, b, 'compare multiline strings')
end

g.test_msgpack_assertion = function()
    local msgpack = require('msgpack')
    local a = {1, 2, 3}
    local b = {1, 4, 3}

    t.assert_equals(msgpack.encode(a), msgpack.encode(b), 'compare encoded msgpack')
end

Failed tests:
-------------

1) tmp.test_table_assertion
/home/ubuntu/luatest/test/tmp_test.lua:21: compare tables
expected: {a = {a = 1, b = 5, c = 8}}
actual: {a = {a = 1, b = 2, c = 3}}
diff:
@@ -1,32 +1,32 @@
 ---
 a:
-  b: 5
  a: 1
  c: 8
+  b: 2
  a: 1
  c: 3
 ...

2) tmp.test_string_assertion
/home/ubuntu/luatest/test/tmp_test.lua:36: compare multiline strings
expected:
"aaaaa\
ddddd\
ccccc\
"
actual:
"aaaaa\
bbbbb\
ccccc\
"
diff:
@@ -1,34 +1,34 @@
 --- |
  aaaaa
-  ddddd
+  bbbbb
   ccccc
 ...

3) tmp.test_msgpack_assertion
/home/ubuntu/luatest/test/tmp_test.lua:44: compare encoded msgpack
expected: "�\1\4\3"
actual: "�\1\2\3"
diff:
@@ -1,20 +1,20 @@
 ---
- 1
-- 4
+- 2
 - 3
 ...

Footnotes

1. https://github.com/google/diff-match-patch/blob/master/lua/diff_match_patch.lua ↩

[ligurio]

Max, thanks for the patch!
Please take a look on my comments.

[ligurio]

LGTM, thanks!

[mandesero]

@locker
I implemented a custom line-based diff and pushed it as a separate commit - please take a look and let me know if it’s OK or not.

[locker]

@mandesero Thanks for implementing the diff algo. I like it much more than using a third-party module.

[locker]

AFAICS pp.tostring never returns nil.

[locker]

I presume you used this wiki page as a reference? It'd be nice to put a link to it somewhere in a comment.

[locker]

It doesn't look like there's much point in splitting this function in two. I'd simply merge prettify_patch into diff_by_lines.

[locker]

How can it be? Why do we need to care, anyways?

[locker]

I don't think we need this check. actual and expected aren't supposed to be the same, but even if they are, we can print the diff without +-.

[locker]

I don't think we need this special case, either.

[locker]

All these tests checking various Lua types look pointless now, when we use pp.tostring. They should already be covered by pp.tostring tests.