package bitutil

import (
	"fmt"
	"math"

	"github.com/steakknife/hamming"
)

// CheckBit checks if a bit in a bit field is one.
func CheckBit(bitfield []byte, index int) (bool, error) {
	chunkLocation := (index + 1) / 8
	indexLocation := (index + 1) % 8
	if indexLocation == 0 {
		indexLocation = 8
	} else {
		chunkLocation++
	}

	if chunkLocation > len(bitfield) {
		return false, fmt.Errorf("index out of range for bitfield: length: %d, position: %d ",
			len(bitfield), chunkLocation-1)
	}

	field := bitfield[chunkLocation-1] >> (8 - uint(indexLocation))
	return field%2 != 0, nil
}

// BitSetCount counts the number of 1s in a byte using Hamming weight.
// See: https://en.wikipedia.org/wiki/Hamming_weight
func BitSetCount(b []byte) int {
	return hamming.CountBitsBytes(b)
}

// BitLength returns the length of the bitfield in bytes.
func BitLength(b int) int {
	return (b + 7) / 8
}

// SetBitfield takes an index and returns bitfield with the index flipped.
func SetBitfield(index int) []byte {
	chunkLocation := index / 8
	indexLocation := math.Pow(2, 7-float64(index%8))
	var bitfield []byte

	for i := 0; i < chunkLocation; i++ {
		bitfield = append(bitfield, byte(0))
	}
	bitfield = append(bitfield, byte(indexLocation))

	return bitfield
}

// FillBitfield returns a bitfield of length `count`, all set to true.
func FillBitfield(count int) []byte {
	numChunks := count/8 + 1
	bitfield := make([]byte, numChunks)
	for i := 0; i < numChunks; i++ {
		if i+1 == numChunks {
			bitfield[i] = fillNBits(uint64(count % 8))
		} else {
			bitfield[i] = byte(8)
		}
	}

	return bitfield
}

func fillNBits(numBits uint64) byte {
	result := byte(0)
	for i := uint64(0); i < numBits; i++ {
		result = fillBit(result, i)
	}

	return result
}

func fillBit(target byte, index uint64) byte {
	bitShift := 7 - index
	return target ^ (1 << bitShift)
}