// Copyright 2024 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package util

import "encoding/binary"

// GetValuesList is used to get `num` values between lower and upper value.
// To Simplify the explain, suppose lower and upper value type is int64, and lower=0, upper=100, num=10,
// then calculate the step=(upper-lower)/num=10, then the function should return 0+10, 10+10, 20+10... all together 9 (num-1) values.
// Then the function will return [10,20,30,40,50,60,70,80,90].
// The difference is the value type of upper, lower is []byte, So I use getUint64FromBytes to convert []byte to uint64.
func GetValuesList(lower, upper []byte, num int, valuesList [][]byte) [][]byte {
	commonPrefixIdx := longestCommonPrefixLen(lower, upper)
	step := getStepValue(lower[commonPrefixIdx:], upper[commonPrefixIdx:], num)
	startV := getUint64FromBytes(lower[commonPrefixIdx:], 0)
	// To get `num` regions, only need to split `num-1` idx keys.
	buf := make([]byte, 8)
	for range num - 1 {
		value := make([]byte, 0, commonPrefixIdx+8)
		value = append(value, lower[:commonPrefixIdx]...)
		startV += step
		binary.BigEndian.PutUint64(buf, startV)
		value = append(value, buf...)
		valuesList = append(valuesList, value)
	}
	return valuesList
}

// longestCommonPrefixLen gets the longest common prefix byte length.
func longestCommonPrefixLen(s1, s2 []byte) int {
	l := min(len(s1), len(s2))
	i := 0
	for ; i < l; i++ {
		if s1[i] != s2[i] {
			break
		}
	}
	return i
}

// getStepValue gets the step of between the lower and upper value. step = (upper-lower)/num.
// Convert byte slice to uint64 first.
func getStepValue(lower, upper []byte, num int) uint64 {
	lowerUint := getUint64FromBytes(lower, 0)
	upperUint := getUint64FromBytes(upper, 0xff)
	return (upperUint - lowerUint) / uint64(num)
}

// getUint64FromBytes gets a uint64 from the `bs` byte slice.
// If len(bs) < 8, then padding with `pad`.
func getUint64FromBytes(bs []byte, pad byte) uint64 {
	buf := bs
	if len(buf) < 8 {
		buf = make([]byte, 0, 8)
		buf = append(buf, bs...)
		for i := len(buf); i < 8; i++ {
			buf = append(buf, pad)
		}
	}
	return binary.BigEndian.Uint64(buf)
}