Add unified diff support for luatest.assert_equals

This patch adds unified diff output for `t.assert_equals()` failures,
using a vendored Lua implementation of google/diff-match-patch
(`luatest/vendor/diff_match_patch.lua` taken from [^1]).

Closes #412

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

Author: mandesero

.luacheckrc

@@ -1,4 +1,4 @@
 include_files = {"**/*.lua", "*.rockspec", "*.luacheckrc"}
-exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/"}
+exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/", "luatest/vendor/"}
 
 max_line_length = 120

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Added support for unified diff output in `t.assert_equals()` failure messages
+  when expected and actual values are YAML-serializable (gh-412).
 - Fixed a bug when the JUnit reporter generated invalid XML for parameterized
   tests with string arguments (gh-407).
 - Group and suite hooks must now be registered using the call-style

luatest/assertions.lua

@@ -6,6 +6,7 @@
 local math = require('math')
 
 local comparator = require('luatest.comparator')
+local diff = require('luatest.diff')
 local mismatch_formatter = require('luatest.mismatch_formatter')
 local pp = require('luatest.pp')
 local log = require('luatest.log')
@@ -83,6 +84,12 @@ local function error_msg_equality(actual, expected, deep_analysis)
         if success then
             result = table.concat({result, mismatchResult}, '\n')
         end
+
+        local diff_result = diff.build_unified_diff(expected, actual)
+        if diff_result then
+            result = table.concat({result, 'diff:', diff_result}, '\n')
+        end
+
         return result
     end
     return string.format("expected: %s, actual: %s",

luatest/diff.lua

@@ -0,0 +1,270 @@
+local yaml = require('yaml')
+local uri = require('uri')
+
+-- diff_match_patch expects bit32
+if not rawget(_G, 'bit32') then
+    _G.bit32 = require('bit')
+end
+
+local diff_match_patch = require('luatest.vendor.diff_match_patch')
+
+diff_match_patch.settings({
+    Diff_Timeout = 0,
+    Patch_Margin = 1e9,
+})
+
+local M = {}
+
+-- Maximum number of distinct line IDs that can be encoded as single-byte chars.
+local MAX_LINE_ID = 0x100
+
+local function encode_line_id(id)
+    if id >= MAX_LINE_ID then
+        return nil
+    end
+
+    return string.char(id)
+end
+
+local function decode_line_id(encoded)
+    return encoded:byte(1)
+end
+
+-- Recursively normalize a value into something that:
+-- * is safe and stable for YAML encoding;
+-- * produces meaningful diffs for values that provide informative tostring();
+-- * does NOT produce noisy diffs for opaque userdata/cdata (newproxy, ffi types, etc).
+local function normalize_for_yaml(value)
+    local t = type(value)
+
+    if t == 'table' then
+        local res = {}
+        for k, v in pairs(value) do
+            local nk = normalize_for_yaml(k)
+            if nk == nil then
+                -- YAML keys must be representable; fallback to tostring.
+                nk = tostring(k)
+            end
+            res[nk] = normalize_for_yaml(v)
+        end
+        return res
+    end
+
+    if t == 'cdata' or t == 'userdata' then
+        local ok, s = pcall(tostring, value)
+        if ok and type(s) == 'string' then
+            return s
+        end
+
+        return '<unknown cdata/userdata>'
+    end
+
+    if t == 'function' or t == 'thread' then
+        return '<' .. t .. '>'
+    end
+
+    -- other primitive types.
+    return value
+end
+
+-- Encode a Lua value as YAML after normalizing it to a diff-friendly form.
+local function encode_yaml(value)
+    local ok, encoded = pcall(yaml.encode, normalize_for_yaml(value))
+    if ok then
+        return encoded
+    end
+end
+
+-- Convert a supported Lua value into a textual form suitable for diffing.
+--
+-- * Tables are serialized to YAML with recursive normalization.
+-- * Strings are used as-is.
+-- * Numbers / booleans are converted via tostring().
+-- * Top-level opaque userdata/cdata disable diffing when tostring() fails (return nil).
+local function as_yaml(value)
+    local t = type(value)
+
+    if t == 'cdata' or t == 'userdata' then
+        local ok, s = pcall(tostring, value)
+        if ok and type(s) == 'string' then
+            return s
+        end
+
+        return nil
+    end
+
+    if t == 'string' then
+        return value
+    end
+
+    local encoded = encode_yaml(value)
+    if encoded ~= nil then
+        return encoded
+    end
+
+    local ok, s = pcall(tostring, value)
+    if ok and type(s) == 'string' then
+        return s
+    end
+end
+
+-- Map two multiline texts to compact "char sequences" and shared line table.
+-- Returns nil if the number of unique lines exceeds MAX_LINE_ID.
+local function lines_to_chars(text1, text2)
+    local line_array = {}
+    local line_hash = {}
+
+    local function add_line(line)
+        local id = line_hash[line]
+        if id == nil then
+            id = #line_array + 1
+            local encoded = encode_line_id(id)
+            if encoded == nil then
+                return nil
+            end
+            line_array[id] = line
+            line_hash[line] = id
+        end
+
+        return encode_line_id(id)
+    end
+
+    local function munge(text)
+        local tokens = {}
+        local start = 1
+
+        while true do
+            local newline_pos = text:find('\n', start, true)
+            if newline_pos == nil then
+                local tail = text:sub(start)
+                if tail ~= '' then
+                    local token = add_line(tail)
+                    if token == nil then
+                        return nil
+                    end
+                    table.insert(tokens, token)
+                end
+                break
+            end
+
+            local token = add_line(text:sub(start, newline_pos))
+            if token == nil then
+                return nil
+            end
+            table.insert(tokens, token)
+            start = newline_pos + 1
+        end
+
+        return table.concat(tokens)
+    end
+
+    local chars1 = munge(text1)
+    if chars1 == nil then
+        return nil
+    end
+
+    local chars2 = munge(text2)
+    if chars2 == nil then
+        return nil
+    end
+
+    return chars1, chars2, line_array
+end
+
+-- Expand a "char sequence" produced by lines_to_chars back into full text.
+local function chars_to_lines(text, line_array)
+    local out = {}
+
+    for i = 1, #text do
+        local id = decode_line_id(text:sub(i, i))
+        local line = line_array[id]
+        if line == nil then
+            return nil
+        end
+        table.insert(out, line)
+    end
+
+    return table.concat(out)
+end
+
+-- Compute line-based diff using diff_match_patch, falling back to nil on failure.
+local function diff_by_lines(text1, text2)
+    local chars1, chars2, line_array = lines_to_chars(text1, text2)
+    if chars1 == nil then
+        return nil
+    end
+
+    local diffs = diff_match_patch.diff_main(chars1, chars2, false)
+    diff_match_patch.diff_cleanupSemantic(diffs)
+
+    for i, diff in ipairs(diffs) do
+        local text = chars_to_lines(diff[2], line_array)
+        if text == nil then
+            return nil
+        end
+        diffs[i][2] = text
+    end
+
+    return diffs
+end
+
+-- Normalize patch text from diff_match_patch: unescape it, drop junk lines,
+-- and ensure it is valid, readable unified diff.
+local function prettify_patch(patch_text)
+    -- patch_toText() escapes non-ascii symbols using URL escaping. Convert it
+    -- back to preserve the original values in unified diff output.
+    patch_text = uri.unescape(patch_text)
+
+    local out = {}
+
+    for line in (patch_text .. '\n'):gmatch('(.-)\n') do
+        if line ~= '' and line ~= ' ' then
+            local first = line:sub(1, 1)
+
+            if first ~= '@' and first ~= '+'
+               and first ~= '-' and first ~= ' ' then
+                line = ' ' .. line
+            end
+
+            table.insert(out, line)
+        end
+    end
+
+    return table.concat(out, '\n')
+end
+
+--- Build unified diff for expected and actual values serialized to YAML.
+-- Tries line-based diff first, falls back to char-based.
+-- Returns nil when values can't be serialized or there is no diff.
+function M.build_unified_diff(expected, actual)
+    local expected_text = as_yaml(expected)
+    local actual_text = as_yaml(actual)
+
+    if expected_text == nil or actual_text == nil then
+        return nil
+    end
+
+    local diffs = diff_by_lines(expected_text, actual_text)
+    local used_line_diff = true
+
+    if diffs == nil then
+        diffs = diff_match_patch.diff_main(expected_text, actual_text)
+        used_line_diff = false
+    end
+
+    if not used_line_diff then
+        diff_match_patch.diff_cleanupSemantic(diffs)
+    end
+
+    local patches = diff_match_patch.patch_make(expected_text,
+                                                actual_text, diffs)
+    local patch_text = diff_match_patch.patch_toText(patches)
+
+    if patch_text == '' then
+        return nil
+    end
+
+    return prettify_patch(patch_text)
+end
+
+return M