package pricing

import (
	"bytes"
	"compress/gzip"
	"embed"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"log"
	"slices"
	"strings"
	"sync"
)

const fallbackVersionUnknown = "0"
const litellmSnapshotPath = "snapshot/litellm_snapshot.json.gz"
const maxFallbackSnapshotCompressedBytes = 1 << 20
const maxFallbackSnapshotJSONBytes = 8 << 20
const maxFallbackSnapshotModels = 100_000

//go:generate go run ./cmd/litellm-snapshot -out snapshot/litellm_snapshot.json.gz

//go:embed snapshot/litellm_snapshot.json.gz
var litellmSnapshotFS embed.FS

type litellmFallbackSnapshot struct {
	Version string         `json:"version"`
	Models  []ModelPricing `json:"models"`
}

var (
	fallbackPricingErr  error
	fallbackPricing     []ModelPricing
	fallbackPricingOnce sync.Once
)

// FallbackVersion is derived from the embedded snapshot; regenerate litellm_snapshot.json.gz to change it.
var FallbackVersion = fallbackVersionUnknown

func init() {
	fallbackPricingOnce.Do(initFallbackPricing)
}

// FallbackPricing returns offline pricing from the embedded LiteLLM snapshot.
// Data is copied for caller safety and deterministic DB seeding.
func FallbackPricing() []ModelPricing {
	fallbackPricingOnce.Do(initFallbackPricing)
	if fallbackPricingErr != nil {
		panic(fallbackPricingErr)
	}

	pricing := make([]ModelPricing, len(fallbackPricing))
	copy(pricing, fallbackPricing)
	return pricing
}

func initFallbackPricing() {
	snapshot, err := decodeFallbackSnapshot()
	if err != nil {
		fallbackPricingErr = fmt.Errorf("loading liteLLM snapshot: %w", err)
		FallbackVersion = fallbackVersionUnknown
		log.Panicf("pricing: %v", fallbackPricingErr)
	}

	fallbackPricing = slices.Clone(snapshot.Models)
	FallbackVersion = snapshot.Version
}

func decodeFallbackSnapshot() (litellmFallbackSnapshot, error) {
	return decodeFallbackSnapshotFromFS(litellmSnapshotFS)
}

func decodeFallbackSnapshotFromFS(fsys fs.FS) (litellmFallbackSnapshot, error) {
	blob, err := fs.ReadFile(fsys, litellmSnapshotPath)
	if errors.Is(err, fs.ErrNotExist) {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"embedded LiteLLM snapshot is missing; run make pricing-snapshot",
		)
	}
	if err != nil {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"reading snapshot: %w", err,
		)
	}
	if len(blob) == 0 {
		return litellmFallbackSnapshot{}, fmt.Errorf("empty snapshot")
	}
	if len(blob) > maxFallbackSnapshotCompressedBytes {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"compressed snapshot exceeds %d bytes",
			maxFallbackSnapshotCompressedBytes,
		)
	}

	reader, err := gzip.NewReader(bytes.NewReader(blob))
	if err != nil {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"creating reader: %w", err,
		)
	}
	defer reader.Close()

	raw, err := readLimitedSnapshot(reader, maxFallbackSnapshotJSONBytes)
	if err != nil {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"decompressing snapshot: %w", err,
		)
	}

	var snapshot litellmFallbackSnapshot
	if err := json.Unmarshal(raw, &snapshot); err != nil {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"parsing snapshot json: %w", err,
		)
	}
	if snapshot.Version == "" {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"missing snapshot version",
		)
	}
	if len(snapshot.Models) == 0 {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"missing snapshot models",
		)
	}
	if len(snapshot.Models) > maxFallbackSnapshotModels {
		return litellmFallbackSnapshot{}, fmt.Errorf(
			"snapshot models exceed %d entries",
			maxFallbackSnapshotModels,
		)
	}
	for _, model := range snapshot.Models {
		if strings.TrimSpace(model.ModelPattern) == "" {
			return litellmFallbackSnapshot{}, fmt.Errorf(
				"snapshot contains model with empty pattern",
			)
		}
	}

	slices.SortFunc(snapshot.Models, func(a, b ModelPricing) int {
		return strings.Compare(a.ModelPattern, b.ModelPattern)
	})

	return snapshot, nil
}

func readLimitedSnapshot(reader io.Reader, limit int64) ([]byte, error) {
	limited := io.LimitReader(reader, limit+1)
	raw, err := io.ReadAll(limited)
	if err != nil {
		return nil, err
	}
	if int64(len(raw)) > limit {
		return nil, fmt.Errorf("decompressed snapshot exceeds %d bytes", limit)
	}
	return raw, nil
}