package bytesize

import (
	"errors"
	"fmt"
)

type Size int64

// Byte size prefix.
const (
	Byte     Size = 1                // byte
	Kibibyte Size = 1 << (10 * iota) // kibibyte
	Mebibyte                         // mebibyte
	Gibibyte                         // gibibyte
	Tebibyte                         // tebibyte
	Pebibyte                         // pebibyte

	Kilobyte Size = 1000 * Byte     // kilobyte
	Megabyte Size = 1000 * Kilobyte // megabyte
	Gigabyte Size = 1000 * Megabyte // gigabyte
	Terabyte Size = 1000 * Gigabyte // terabyte
	Petabyte Size = 1000 * Terabyte // petabyte
)

var unitMap = map[string]uint64{
	"B":   uint64(Byte),
	"KiB": uint64(Kibibyte),
	"MiB": uint64(Mebibyte),
	"GiB": uint64(Gibibyte),
	"TiB": uint64(Tebibyte),
	"PiB": uint64(Pebibyte),

	"kB": uint64(Kilobyte),
	"KB": uint64(Kilobyte),
	"MB": uint64(Megabyte),
	"GB": uint64(Gigabyte),
	"TB": uint64(Terabyte),
	"PB": uint64(Petabyte),
}

var (
	ErrInvalidSize = errors.New("bytesize: invalid size")
	ErrMissingUnit = errors.New("bytesize: missing unit")
	ErrUnknownUnit = errors.New("bytesize: unknown unit")
)

// Parse parses a size string.
// A byte size string is a possibly signed sequence of
// decimal numbers, each with optional fraction and a unit suffix,
// such as "300kB", "-1.5GiB" or "2GB45MB".
// Valid units are "B", "KiB", "MiB", "GiB", "TiB", "PiB" for binary size.
// Valid units are "B", "kB", "MB", "GB", "TB", "PB" for human size.
//
// nolint: funlen,gocognit,gocyclo,cyclop
func Parse(in string) (Size, error) {
	// [-+]?([0-9]*(\.[0-9]*)?[a-z]+)+
	orig := in
	neg := false

	var res uint64

	// Consume [-+]?
	if in != "" {
		c := in[0]
		if c == '-' || c == '+' {
			neg = c == '-'
			in = in[1:]
		}
	}
	// Special case: if all that is left is "0", this is zero.
	if in == "0" {
		return 0, nil
	}

	if in == "" {
		return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
	}

	for in != "" {
		var (
			val, fVal uint64      // integers before, after decimal point
			scale     float64 = 1 // value = v + f/scale
		)

		var err error

		// The next character must be [0-9.]
		if !(in[0] == '.' || '0' <= in[0] && in[0] <= '9') {
			return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
		}

		// Consume [0-9]*
		pl := len(in)

		val, in, err = leadingInt(in)
		if err != nil {
			return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
		}

		pre := pl != len(in) // whether we consumed anything before a period

		// Consume (\.[0-9]*)?
		post := false

		if in != "" && in[0] == '.' {
			in = in[1:]
			pl := len(in)
			fVal, scale, in = leadingFraction(in)
			post = pl != len(in)
		}

		if !pre && !post {
			// no digits (e.g. ".s" or "-.s")
			return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
		}

		// Consume unit.
		i := 0
		for ; i < len(in); i++ {
			c := in[i]
			if c == '.' || '0' <= c && c <= '9' {
				break
			}
		}

		if i == 0 {
			return 0, fmt.Errorf("%w %q", ErrMissingUnit, orig)
		}

		u := in[:i]
		in = in[i:]
		unit, ok := unitMap[u]

		if !ok {
			return 0, fmt.Errorf("%w %s in size %q", ErrMissingUnit, u, orig)
		}

		if val > 1<<63/unit {
			// overflow
			return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
		}

		val *= unit
		if fVal > 0 {
			// float64 is needed to be nanosecond accurate for fractions of hours.
			// v >= 0 && (f*unit/scale) <= 3.6e+12 (ns/h, h is the largest unit)
			val += uint64(float64(fVal) * (float64(unit) / scale))
			if val > 1<<63 {
				// overflow
				return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
			}
		}

		res += val
		if res > 1<<63 {
			return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
		}
	}

	if neg {
		return -Size(res), nil
	}

	if res > 1<<63-1 {
		return 0, fmt.Errorf("%w %q", ErrInvalidSize, orig)
	}

	return Size(res), nil
}

// leadingFraction consumes the leading [0-9]* from s.
// It is used only for fractions, so does not return an error on overflow,
// it just stops accumulating precision.
func leadingFraction(in string) (uint64, float64, string) {
	i := 0
	scale := float64(1)
	overflow := false

	var res uint64

	for ; i < len(in); i++ {
		cur := in[i]
		if cur < '0' || cur > '9' {
			break
		}

		if overflow {
			continue
		}

		if res > (1<<63-1)/10 {
			// It's possible for overflow to give a positive number, so take care.
			overflow = true

			continue
		}

		pres := res*10 + uint64(cur) - '0'
		if pres > 1<<63 {
			overflow = true

			continue
		}

		res = pres
		scale *= 10
	}

	return res, scale, in[i:]
}

var errLeadingInt = errors.New("bytesize: bad [0-9]*")

// leadingInt consumes the leading [0-9]* from in.
func leadingInt[bytes []byte | string](in bytes) (uint64, bytes, error) {
	i := 0

	var (
		res uint64
		rem bytes
	)

	for ; i < len(in); i++ {
		curr := in[i]

		if curr < '0' || curr > '9' {
			break
		}

		if res > 1<<63/10 {
			// overflow
			return 0, rem, errLeadingInt
		}

		res = res*10 + uint64(curr) - '0'
		if res > 1<<63 {
			// overflow
			return 0, rem, errLeadingInt
		}
	}

	return res, in[i:], nil
}

func (s Size) String() string {
	var arr [32]byte
	n := s.format(&arr)

	return string(arr[n:])
}

// format formats the representation of d into the end of buf and
// returns the offset of the first character.
// nolint: gosec
func (s Size) format(buf *[32]byte) int {
	wLen := len(buf)

	data := uint64(s)
	neg := s < 0

	if neg {
		data = -data
	}

	prec := 3
	wLen--
	buf[wLen] = 'B'
	wLen--

	switch {
	case data == 0:
		buf[wLen] = '0'

		return wLen
	case data < uint64(Kilobyte):
		// print bytes
		wLen++
		prec = 0
	case data < uint64(Megabyte):
		// print kipobytes
		buf[wLen] = 'k'
	case data < uint64(Gigabyte):
		// print megabytes
		prec = 6
		buf[wLen] = 'M'
	case data < uint64(Terabyte):
		// print gigabytes
		prec = 9
		buf[wLen] = 'G'
	case data < uint64(Petabyte):
		// print terabytes
		prec = 12
		buf[wLen] = 'T'
	default:
		// print petobytes
		prec = 15
		buf[wLen] = 'P'
	}

	wLen, data = fmtFrac(buf[:wLen], data, prec)
	wLen = fmtInt(buf[:wLen], data)

	if neg {
		wLen--
		buf[wLen] = '-'
	}

	return wLen
}

// fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
// tail of buf, omitting trailing zeros. It omits the decimal
// point too when the fraction is 0. It returns the index where the
// output bytes begin and the value v/10**prec.
func fmtFrac(buf []byte, val uint64, prec int) (int, uint64) {
	// Omit trailing zeros up to and including decimal point.
	wLen := len(buf)
	show := false

	for range prec {
		digit := val % 10
		show = show || digit != 0

		if show {
			wLen--
			buf[wLen] = byte(digit) + '0'
		}

		val /= 10
	}

	if show {
		wLen--
		buf[wLen] = '.'
	}

	return wLen, val
}

// fmtInt formats v into the tail of buf.
// It returns the index where the output begins.
func fmtInt(buf []byte, val uint64) int {
	wLen := len(buf)
	if val == 0 {
		wLen--
		buf[wLen] = '0'
	} else {
		for val > 0 {
			wLen--
			buf[wLen] = byte(val%10) + '0'
			val /= 10
		}
	}

	return wLen
}