// Package tools provides performance regression detection utilities.
package tools

import (
	"bufio"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
	"text/tabwriter"
	"time"
)

// PerformanceRegressionDetector detects performance regressions in benchmark results
type PerformanceRegressionDetector struct {
	BaselineFile string
	Threshold    float64
}

// ParseGoBenchFile reads a file containing `go test -bench` output and returns parsed benchmark results.
func ParseGoBenchFile(path string) ([]BenchmarkResult, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("failed to open benchmark file: %w", err)
	}
	defer func() { _ = f.Close() }()

	return ParseGoBenchOutput(f)
}

// ParseGoBenchOutput parses `go test -bench` output.
//
// It extracts these metrics when present:
// - ns/op
// - B/op
// - allocs/op
//
// Each metric becomes a separate BenchmarkResult where Name is suffixed with the unit.
func ParseGoBenchOutput(r io.Reader) ([]BenchmarkResult, error) {
	scanner := bufio.NewScanner(r)
	results := make([]BenchmarkResult, 0)
	now := time.Now()

	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if !strings.HasPrefix(line, "Benchmark") {
			continue
		}

		fields := strings.Fields(line)
		if len(fields) < 4 {
			continue
		}

		benchName := fields[0]

		for i := 2; i+1 < len(fields); i++ {
			valStr := fields[i]
			unit := fields[i+1]
			if unit != "ns/op" && unit != "B/op" && unit != "allocs/op" {
				continue
			}

			v, err := strconv.ParseFloat(valStr, 64)
			if err != nil {
				continue
			}

			results = append(results, BenchmarkResult{
				Name:      benchName + "/" + unit,
				Value:     v,
				Unit:      unit,
				Timestamp: now,
			})
		}
	}

	if err := scanner.Err(); err != nil {
		return nil, fmt.Errorf("failed reading benchmark output: %w", err)
	}

	return results, nil
}

// BenchmarkResult represents a single benchmark result
type BenchmarkResult struct {
	Timestamp time.Time `json:"timestamp"`
	Name      string    `json:"name"`
	Unit      string    `json:"unit"`
	Value     float64   `json:"value"`
}

// RegressionReport contains regression analysis results
type RegressionReport struct {
	Summary      string        `json:"summary"`
	Regressions  []Regression  `json:"regressions"`
	Improvements []Improvement `json:"improvements"`
}

// Regression represents a performance regression
type Regression struct {
	Benchmark     string  `json:"benchmark"`
	Severity      string  `json:"severity"`
	CurrentValue  float64 `json:"current_value"`
	BaselineValue float64 `json:"baseline_value"`
	PercentChange float64 `json:"percent_change"`
}

// Improvement represents a performance improvement
type Improvement struct {
	Benchmark     string  `json:"benchmark"`
	CurrentValue  float64 `json:"current_value"`
	BaselineValue float64 `json:"baseline_value"`
	PercentChange float64 `json:"percent_change"`
}

// NewPerformanceRegressionDetector creates a new detector instance
func NewPerformanceRegressionDetector(
	baselineFile string,
	threshold float64,
) *PerformanceRegressionDetector {
	return &PerformanceRegressionDetector{
		BaselineFile: baselineFile,
		Threshold:    threshold,
	}
}

// LoadBaseline loads baseline benchmark results from file
func (prd *PerformanceRegressionDetector) LoadBaseline() ([]BenchmarkResult, error) {
	if _, err := os.Stat(prd.BaselineFile); os.IsNotExist(err) {
		return nil, fmt.Errorf("baseline file not found: %s", prd.BaselineFile)
	}

	data, err := os.ReadFile(prd.BaselineFile)
	if err != nil {
		return nil, fmt.Errorf("failed to read baseline file: %w", err)
	}

	var results []BenchmarkResult
	if err := json.Unmarshal(data, &results); err != nil {
		return nil, fmt.Errorf("failed to parse baseline file: %w", err)
	}

	return results, nil
}

// AnalyzeResults analyzes current results against baseline
func (prd *PerformanceRegressionDetector) AnalyzeResults(
	current []BenchmarkResult,
) (*RegressionReport, error) {
	baseline, err := prd.LoadBaseline()
	if err != nil {
		return nil, fmt.Errorf("failed to load baseline: %w", err)
	}

	return prd.AnalyzeParsedResults(baseline, current)
}

// AnalyzeParsedResults analyzes current results against a provided baseline.
func (prd *PerformanceRegressionDetector) AnalyzeParsedResults(
	baseline []BenchmarkResult,
	current []BenchmarkResult,
) (*RegressionReport, error) {

	report := &RegressionReport{
		Regressions:  []Regression{},
		Improvements: []Improvement{},
	}

	baselineMap := make(map[string]BenchmarkResult)
	for _, result := range baseline {
		baselineMap[result.Name] = result
	}

	for _, currentResult := range current {
		baselineResult, exists := baselineMap[currentResult.Name]
		if !exists {
			continue // Skip new benchmarks without baseline
		}

		if baselineResult.Value == 0 {
			continue
		}

		percentChange := ((currentResult.Value - baselineResult.Value) / baselineResult.Value) * 100

		if percentChange > prd.Threshold {
			// Performance regression detected
			severity := "minor"
			if percentChange > prd.Threshold*2 {
				severity = "major"
			}
			if percentChange > prd.Threshold*3 {
				severity = "critical"
			}

			report.Regressions = append(report.Regressions, Regression{
				Benchmark:     currentResult.Name,
				CurrentValue:  currentResult.Value,
				BaselineValue: baselineResult.Value,
				PercentChange: percentChange,
				Severity:      severity,
			})
		} else if percentChange < -prd.Threshold {
			// Performance improvement detected
			report.Improvements = append(report.Improvements, Improvement{
				Benchmark:     currentResult.Name,
				CurrentValue:  currentResult.Value,
				BaselineValue: baselineResult.Value,
				PercentChange: percentChange,
			})
		}
	}

	// Generate summary
	regressionCount := len(report.Regressions)
	improvementCount := len(report.Improvements)

	if regressionCount == 0 && improvementCount == 0 {
		report.Summary = "No significant performance changes detected"
	} else {
		report.Summary = fmt.Sprintf("Detected %d regression(s) and %d improvement(s)",
			regressionCount, improvementCount)
	}

	return report, nil
}

// SaveBaseline saves current results as new baseline
func (prd *PerformanceRegressionDetector) SaveBaseline(results []BenchmarkResult) error {
	data, err := json.MarshalIndent(results, "", "  ")
	if err != nil {
		return fmt.Errorf("failed to marshal results: %w", err)
	}

	err = os.WriteFile(prd.BaselineFile, data, 0600)
	if err != nil {
		return fmt.Errorf("failed to write baseline file: %w", err)
	}

	return nil
}

// PrintReport prints a formatted regression report
func (prd *PerformanceRegressionDetector) PrintReport(report *RegressionReport) {
	fmt.Printf("Performance Regression Analysis Report\n")
	fmt.Printf("=====================================\n\n")
	fmt.Printf("Summary:\t%s\n", report.Summary)
	fmt.Printf("Threshold:\t%.1f%%\n\n", prd.Threshold)

	if len(report.Regressions) > 0 {
		fmt.Printf("Regressions (%d)\n", len(report.Regressions))
		w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
		_, _ = fmt.Fprintln(w, "Severity\tBenchmark\tBaseline\tCurrent\tChange")
		_, _ = fmt.Fprintln(w, "--------\t---------\t--------\t-------\t------")
		for _, regression := range report.Regressions {
			_, _ = fmt.Fprintf(
				w,
				"%s\t%s\t%.2f\t%.2f\t%.1f%% worse\n",
				regression.Severity,
				regression.Benchmark,
				regression.BaselineValue,
				regression.CurrentValue,
				regression.PercentChange,
			)
		}
		_ = w.Flush()
		fmt.Println()
	}

	if len(report.Improvements) > 0 {
		fmt.Printf("Improvements (%d)\n", len(report.Improvements))
		w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
		_, _ = fmt.Fprintln(w, "Benchmark\tBaseline\tCurrent\tChange")
		_, _ = fmt.Fprintln(w, "---------\t--------\t-------\t------")
		for _, improvement := range report.Improvements {
			_, _ = fmt.Fprintf(
				w,
				"%s\t%.2f\t%.2f\t%.1f%% better\n",
				improvement.Benchmark,
				improvement.BaselineValue,
				improvement.CurrentValue,
				-improvement.PercentChange,
			)
		}
		_ = w.Flush()
		fmt.Println()
	}
}