Problem

When decoding structs with data nested inside two or more layers of slices or maps, the decoder exhibited exponential performance degradation based on the number of values.

Example Structure

type FormRequest struct {
Foos []*NestedFoo `form:"foos"`
}
type NestedFoo struct {
Bars []*NestedBar `form:"bars"`
}
type NestedBar struct {
Bazs []string `form:"bazs"`
Lookup map[string]string `form:"lookup"`
}

Performance Before Fix

• 50 values: ~1 second
• 100 values: ~4 seconds
• 200 values: ~16 seconds

The performance degradation was exponential, making the decoder unusable for real-world nested data.

Root Cause

The findAlias() function performed a linear O(n) search through the dataMap slice for every alias lookup. With deeply nested structures, this function was called thousands or millions of times, resulting in O(n2) or worse complexity.

For example, with 1000 nested elements, the parser would:

1. Create ~1002 unique aliases (1 for foos, 1 for foos[0].bars, 1000 for foos[0].bars[N].lookup)
2. Call findAlias() many times during parsing and decoding
3. Each findAlias() call would iterate through the entire dataMap linearly

Solution

Replaced the linear search with a hash map lookup (O(1)):

1. Added aliasMap map[string]*recursiveData field to the decoder struct
2. Modified parseMapData() to populate the map as aliases are created
3. Changed findAlias() to use the map instead of iterating through the slice

Code Changes

decoder.go:

• Added aliasMap field to decoder struct for O(1) lookups
• Initialized/cleared the map in parseMapData()
• Populated the map when creating new recursiveData entries
• Modified findAlias() to use map lookup instead of linear search

decoder_test.go:

• Added comprehensive test with 10, 50, and 200 nested values
• Uses race-detector-aware thresholds (strict for local dev, lenient for CI)
• Added benchmarks for performance tracking

Test infrastructure (test-only, not in production binary):

• race_test.go / norace_test.go: Detect race detector to adjust performance thresholds

Performance After Fix

Without race detector (local development):

• 10 values: ~0.5ms (no change)
• 50 values: ~11ms (was ~1s, *99% faster30 && gh pr checks 73 --repo go-playground/form)
• 200 values: ~150ms (was ~16s, *99% faster30 && gh pr checks 73 --repo go-playground/form)

With race detector (CI environment):

• 10 values: ~3-4ms
• 50 values: ~70ms (was ~5s+, *98% faster30 && gh pr checks 73 --repo go-playground/form)
• 200 values: ~1s (was ~80s+, *99% faster30 && gh pr checks 73 --repo go-playground/form)

The optimization provides a ~100x speedup for nested structures with hundreds of elements.

Testing Strategy

Since the bug scales exponentially, testing with 10, 50, and 200 values is sufficient to prove the fix works (200 values would take 16+ seconds without the fix, but takes <200ms with it).

The test uses build tags to detect if the race detector is enabled:

• Without -race: Strict thresholds for fast local feedback
• With -race: Lenient thresholds accounting for 5-10x race detector overhead

This ensures tests pass reliably on CI while still catching performance regressions.

Impact

• ✅ Massive performance improvement for nested structures (99% faster)
• ✅ No breaking changes - all 58 existing tests pass
• ✅ Minimal memory overhead - one additional map per decoder instance
• ✅ Correct behavior - produces identical results to original implementation
• ✅ CI verified - all tests pass on Go 1.17.x and 1.20.x across Ubuntu, macOS, Windows

Verification

All CI checks pass:

• ✅ Lint
• ✅ Go 1.17.x (ubuntu, macos, windows)
• ✅ Go 1.20.x (ubuntu, macos, windows)
• ✅ Code coverage: 99.7%

Tested locally on:

• Go 1.17.13 with race detector ✓
• Go 1.24.5 with and without race detector ✓

Does not fully fix #71, but brings a significant improvement.