feat: implement pretty report generator with golden tests

[pfrederiksen]

Summary

Implement human-friendly pretty report generator with comprehensive golden tests for output validation.

Features

Report Generation

• Summary Statistics: Count of changes by type (added, removed, modified, moved)
• Detailed Listings: Each change with symbol, path, and values
• Configurable Output: Compact vs detailed, show/hide values, value length limits

Smart Value Formatting

Numbers:    42 (whole), 3.14 (decimal)
Strings:    "hello"
Objects:    {...} (2 keys)
Arrays:     [...] (5 items)
Null:       null
Truncated:  "this i... (when exceeds max length)

Change Symbols

+  Added
-  Removed
~  Modified
↔  Moved

Output Modes

Detailed (default):

Summary: +1 added, -1 removed, ~1 modified (3 total)

Changes:
  + /spec/replicas = 5

  ~ /spec/image: "nginx:1.19" → "nginx:1.20"

  - /metadata/annotations/deprecated (was: "true")

Compact:

Summary: +2 added, -1 removed (3 total)
Changes:
  + /newKey
  + /another
  - /oldKey

API

// Use default detailed report (auto-generated in Result)
result, _ := configdiff.DiffYAML(oldYAML, newYAML, opts)
fmt.Println(result.Report)

// Generate custom reports
report.Generate(changes, report.Options{
    Compact:        false,
    ShowValues:     true,
    MaxValueLength: 80,
})

// Convenience functions
report.GenerateCompact(changes)   // Minimal output
report.GenerateDetailed(changes)  // Full output

Golden Tests

10 golden test files validate output:

• empty.txt - No changes
• single_add.txt - Single addition
• single_remove.txt - Single removal
• single_modify.txt - Single modification
• multiple_changes.txt - Mixed changes
• complex_values.txt - Objects and arrays
• compact_format.txt - Compact mode
• without_values.txt - Values hidden
• value_truncation.txt - Long value handling
• number_formatting.txt - Number display

Run go test ./report/... -update to regenerate golden files.

Integration

• Automatically generates detailed report in DiffTrees()
• Result.Report field now populated
• Users get pretty output by default

Test Coverage

Report Package: 93.8%
Overall Project: 84.8%

Test Categories

• ✅ Golden file tests (all output formats)
• ✅ Summary generation and formatting
• ✅ Change formatting with all types
• ✅ Value formatting (all node types)
• ✅ Truncation handling
• ✅ Number formatting (whole vs decimal)
• ✅ Compact vs detailed modes

Example Output

See golden files in testdata/report/ for exact formatting.

Test Plan

• All tests pass locally
• Golden tests validate output
• Race detector passes
• Coverage exceeds 80% threshold (84.8%)
• Linting passes
• Integration with main API verified

🤖 Generated with Claude Code