package datapoint

import (
	"archive/zip"
	"bytes"
	"encoding/json"
	"fmt"
	"io"
)

const (
	MaxBatchBytes     = 4 << 20  // Maximum ZIP payload size on the wire.
	MaxBatchJSONBytes = 16 << 20 // Maximum expanded JSON size.
)

// ZipBatch packages GPS fixes into an in-memory ZIP containing points.json.
func ZipBatch(points []DataPoint) ([]byte, error) {
	raw, err := json.Marshal(points)
	if err != nil {
		return nil, fmt.Errorf("encode points: %w", err)
	}
	if len(raw) > MaxBatchJSONBytes {
		return nil, fmt.Errorf("batch JSON exceeds %d bytes", MaxBatchJSONBytes)
	}
	var buf bytes.Buffer
	zw := zip.NewWriter(&buf)
	file, err := zw.Create("points.json")
	if err != nil {
		return nil, err
	}
	if _, err = file.Write(raw); err != nil {
		return nil, err
	}
	if err = zw.Close(); err != nil {
		return nil, err
	}
	if buf.Len() > MaxBatchBytes {
		return nil, fmt.Errorf("ZIP batch exceeds %d bytes", MaxBatchBytes)
	}
	return buf.Bytes(), nil
}

// UnzipBatch reads points in memory; archive entries are never written to disk.
// Legacy JSON batches are accepted so the server can be upgraded first.
func UnzipBatch(payload []byte) ([]DataPoint, error) {
	if len(payload) > MaxBatchBytes {
		return nil, fmt.Errorf("batch exceeds %d bytes", MaxBatchBytes)
	}
	raw := payload
	if !bytes.HasPrefix(bytes.TrimSpace(payload), []byte("[")) && !bytes.Equal(bytes.TrimSpace(payload), []byte("null")) {
		zr, err := zip.NewReader(bytes.NewReader(payload), int64(len(payload)))
		if err != nil {
			return nil, fmt.Errorf("open ZIP batch: %w", err)
		}
		if len(zr.File) != 1 || zr.File[0].Name != "points.json" {
			return nil, fmt.Errorf("ZIP batch must contain only points.json")
		}
		file := zr.File[0]
		if file.UncompressedSize64 > MaxBatchJSONBytes {
			return nil, fmt.Errorf("expanded batch exceeds %d bytes", MaxBatchJSONBytes)
		}
		r, err := file.Open()
		if err != nil {
			return nil, fmt.Errorf("open points.json: %w", err)
		}
		defer r.Close()
		raw, err = io.ReadAll(io.LimitReader(r, MaxBatchJSONBytes+1))
		if err != nil {
			return nil, fmt.Errorf("read points.json: %w", err)
		}
		if len(raw) > MaxBatchJSONBytes {
			return nil, fmt.Errorf("expanded batch exceeds %d bytes", MaxBatchJSONBytes)
		}
	}
	var points []DataPoint
	if err := json.Unmarshal(raw, &points); err != nil {
		return nil, fmt.Errorf("decode points: %w", err)
	}
	return points, nil
}