Add line diff support

This patch adds a simple line-by-line diff output for `t.assert_equals()`
and `t.assert_covers()` failures.

Closes #412

Author: mandesero

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Added a simple line-by-line diff to `t.assert_equals()` and `t.assert_covers()`
+  failure messages (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_line_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",
@@ -470,7 +477,19 @@ end
 function M.assert_covers(actual, expected, message)
     if not table_covers(actual, expected) then
         local str_actual, str_expected = prettystr_pairs(actual, expected)
-        failure(string.format('expected %s to cover %s', str_actual, str_expected), message, 2)
+        local sliced_actual = table_slice(actual, expected)
+
+        local parts = {
+            string.format('expected %s to cover %s', str_actual, str_expected),
+        }
+
+        local diff_result = diff.build_line_diff(expected, sliced_actual)
+        if diff_result then
+            table.insert(parts, 'diff:')
+            table.insert(parts, diff_result)
+        end
+
+        failure(table.concat(parts, '\n'), message, 2)
     end
 end
 

luatest/diff.lua

@@ -0,0 +1,104 @@
+local pp = require("luatest.pp")
+
+local M = {}
+
+local function diff_by_lines(text1, text2)
+    local lines1 = string.split(text1, '\n')
+    local lines2 = string.split(text2, '\n')
+
+    local m = #lines1
+    local n = #lines2
+    local lcs = {}
+
+    for i = 0, m do
+        lcs[i] = {}
+        lcs[i][0] = 0
+    end
+
+    for j = 0, n do
+        lcs[0][j] = 0
+    end
+
+    for i = 1, m do
+        for j = 1, n do
+            if lines1[i] == lines2[j] then
+                lcs[i][j] = lcs[i - 1][j - 1] + 1
+            else
+                local left = lcs[i - 1][j]
+                local top = lcs[i][j - 1]
+                lcs[i][j] = left >= top and left or top
+            end
+        end
+    end
+
+    local diffs = {}
+    local i = m
+    local j = n
+
+    while i > 0 or j > 0 do
+        if i > 0 and j > 0 and lines1[i] == lines2[j] then
+            table.insert(diffs, 1, {'equal', lines1[i]})
+            i = i - 1
+            j = j - 1
+        elseif j > 0 and (i == 0 or lcs[i][j - 1] >= lcs[i - 1][j]) then
+            table.insert(diffs, 1, {'insert', lines2[j]})
+            j = j - 1
+        else
+            table.insert(diffs, 1, {'delete', lines1[i]})
+            i = i - 1
+        end
+    end
+
+    return diffs
+end
+
+local function prettify_patch(diffs)
+    local out = {}
+
+    for _, diff in ipairs(diffs) do
+        local tag, line = diff[1], diff[2]
+        if tag == 'equal' then
+            table.insert(out, ' ' .. line)
+        elseif tag == 'delete' then
+            table.insert(out, '-' .. line)
+        elseif tag == 'insert' then
+            table.insert(out, '+' .. line)
+        end
+    end
+
+    if #out == 0 then
+        return nil
+    end
+
+    return table.concat(out, '\n')
+end
+
+--- Build a simple line-by-line diff for expected and actual values
+-- serialized to text. Returns nil when values can't be serialized
+-- or there is no diff.
+function M.build_line_diff(expected, actual)
+    local old = pp.LINE_LENGTH
+    pp.LINE_LENGTH = 0
+    local expected_text = pp.tostring(expected)
+    local actual_text = pp.tostring(actual)
+    pp.LINE_LENGTH = old
+
+    if expected_text == nil or actual_text == nil then
+        return nil
+    end
+
+    if expected_text == actual_text then
+        return nil
+    end
+
+    local diffs = diff_by_lines(expected_text, actual_text)
+    local patch_text = prettify_patch(diffs)
+
+    if patch_text == '' or patch_text == nil then
+        return nil
+    end
+
+    return patch_text
+end
+
+return M