package distance

import (
	"math"
	"strings"
	"sync"

	"github.com/milvus-io/milvus/pkg/v3/util/merr"
)

/**
 * Delete in #25663 Remove calc_distance
 * Add back partially as clustering feature needs to calculate distance between search vector and clustering center
 */
const (
	// L2 represents the Euclidean distance
	L2 = "L2"
	// IP represents the inner product distance
	IP = "IP"
	// COSINE represents the cosine distance
	COSINE = "COSINE"
)

func L2ImplPure(a []float32, b []float32) float32 {
	var sum float32

	for i := range a {
		sum += (a[i] - b[i]) * (a[i] - b[i])
	}

	return sum
}

func IPImplPure(a []float32, b []float32) float32 {
	var sum float32

	for i := range a {
		sum += a[i] * b[i]
	}

	return sum
}

func CosineImplPure(a []float32, b []float32) float32 {
	var sum, normA, normB float32

	for i := range a {
		sum += a[i] * b[i]
		normA += a[i] * a[i]
		normB += b[i] * b[i]
	}

	return sum / float32(math.Sqrt(float64(normA)*float64(normB)))
}

var (
	L2Impl     func(a []float32, b []float32) float32 = L2ImplPure
	IPImpl     func(a []float32, b []float32) float32 = IPImplPure
	CosineImpl func(a []float32, b []float32) float32 = CosineImplPure
)

// ValidateMetricType returns metric text or error
func ValidateMetricType(metric string) (string, error) {
	if metric == "" {
		err := merr.WrapErrParameterInvalidMsg("metric type is empty")
		return "", err
	}

	m := strings.ToUpper(metric)
	if m == L2 || m == IP || m == COSINE {
		return m, nil
	}

	err := merr.WrapErrParameterInvalidMsg("invalid metric type")
	return metric, err
}

// ValidateFloatArrayLength is used validate float vector length
func ValidateFloatArrayLength(dim int64, length int) error {
	if length == 0 || int64(length)%dim != 0 {
		err := merr.WrapErrParameterInvalidMsg("invalid float vector length")
		return err
	}

	return nil
}

// CalcFFBatch calculate the distance of @left & @right vectors in batch by given @metic, store result in @result
func CalcFFBatch(dim int64, left []float32, lIndex int64, right []float32, metric string, result *[]float32) {
	rightNum := int64(len(right)) / dim
	for i := int64(0); i < rightNum; i++ {
		var distance float32 = -1.0
		switch metric {
		case L2:
			distance = L2Impl(left[lIndex*dim:lIndex*dim+dim], right[i*dim:i*dim+dim])
		case IP:
			distance = IPImpl(left[lIndex*dim:lIndex*dim+dim], right[i*dim:i*dim+dim])
		case COSINE:
			distance = CosineImpl(left[lIndex*dim:lIndex*dim+dim], right[i*dim:i*dim+dim])
		}
		(*result)[lIndex*rightNum+i] = distance
	}
}

// CalcFloatDistance calculate float distance by given metric
// it will checks input, and calculate the distance concurrently
func CalcFloatDistance(dim int64, left, right []float32, metric string) ([]float32, error) {
	if dim <= 0 {
		err := merr.WrapErrParameterInvalidMsg("invalid dimension")
		return nil, err
	}

	metricUpper := strings.ToUpper(metric)
	if metricUpper != L2 && metricUpper != IP && metricUpper != COSINE {
		err := merr.WrapErrParameterInvalidMsg("invalid metric type")
		return nil, err
	}

	err := ValidateFloatArrayLength(dim, len(left))
	if err != nil {
		return nil, err
	}

	err = ValidateFloatArrayLength(dim, len(right))
	if err != nil {
		return nil, err
	}

	leftNum := int64(len(left)) / dim
	rightNum := int64(len(right)) / dim

	distArray := make([]float32, leftNum*rightNum)

	// Multi-threads to calculate distance. TODO: avoid too many go routines
	var waitGroup sync.WaitGroup
	CalcWorker := func(index int64) {
		CalcFFBatch(dim, left, index, right, metricUpper, &distArray)
		waitGroup.Done()
	}
	for i := int64(0); i < leftNum; i++ {
		waitGroup.Add(1)
		go CalcWorker(i)
	}
	waitGroup.Wait()

	return distArray, nil
}
