package rrdformat

import (
	"encoding/binary"
	"fmt"
	"math"

	"git.lukeshu.com/go/librrd/rrdformat/rrdbinary"
)

func SniffArchitecture(data []byte) (rrdbinary.Architecture, error) {
	var arch rrdbinary.Architecture

	var header struct {
		Cookie  rrdbinary.String `rrdbinary:"size=4" xml:"-"`
		Version rrdbinary.String `rrdbinary:"size=5" xml:"version"`
	}
	if err := rrdbinary.NewDecoder(arch, data).Decode(&header, ""); err != nil {
		return rrdbinary.Architecture{}, err
	}

	// 1. File header magic number
	if header.Cookie != "RRD" {
		return rrdbinary.Architecture{}, rrdbinary.NewBinError("not an RRD file: wrong magic number", data, 0, 4)
	}

	// 1. File format version string
	switch header.Version {
	case "0001", "0002", "0003", "0004", "0005":
		// do nothing
	default:
		return rrdbinary.Architecture{}, rrdbinary.NewBinError(fmt.Sprintf("can't handle RRD file version %q", header.Version), data, 4, 5)
	}

	// 3. float cookie
	//
	// Assume IEEE 754 doubles.  C doesn't assume 754 doubles, but anything that doesn't use 754 doubles is exotic
	// enough that I'm OK saying "you're going to need to use `rrdtool dump`".  This lets us assume that:
	//  - a 'double' is 8 bytes wide
	//  - the value will be exactly equal, and we don't need to worry about weird rounding.
	arch.DoubleWidth = 8
	magicFloat := float64(8.642135e130)
	floatAddrPacked := 9
	floatAddr32 := ((floatAddrPacked + 3) / 4) * 4
	floatAddr64 := ((floatAddrPacked + 7) / 8) * 8
	var restOffset int
	switch {
	case len(data) < floatAddr32+arch.DoubleWidth:
		return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end-of-file", data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
	case math.Float64frombits(binary.LittleEndian.Uint64(data[floatAddr32:])) == magicFloat:
		arch.ByteOrder = binary.LittleEndian
		arch.DoubleAlign = 4
		restOffset = floatAddr32 + arch.DoubleWidth
	case math.Float64frombits(binary.BigEndian.Uint64(data[floatAddr32:])) == magicFloat:
		arch.ByteOrder = binary.BigEndian
		arch.DoubleAlign = 4
		restOffset = floatAddr32 + arch.DoubleWidth
	case len(data) < floatAddr64+arch.DoubleWidth:
		return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end-of-file", data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
	case math.Float64frombits(binary.LittleEndian.Uint64(data[floatAddr64:])) == magicFloat:
		arch.ByteOrder = binary.LittleEndian
		arch.DoubleAlign = 8
		restOffset = floatAddr64 + arch.DoubleWidth
	case math.Float64frombits(binary.BigEndian.Uint64(data[floatAddr64:])) == magicFloat:
		arch.ByteOrder = binary.BigEndian
		arch.DoubleAlign = 8
		restOffset = floatAddr64 + arch.DoubleWidth
	default:
		return rrdbinary.Architecture{}, rrdbinary.NewBinError("failed to sniff byte-order and float-alignment",
			data, floatAddrPacked, floatAddr64+arch.DoubleWidth-floatAddrPacked)
	}

	// 5, 6. ds_cnt, rra_cnt
	switch arch.DoubleAlign {
	case 4:
		// Assume that if floats are only 32-bit aligned, then everything is 32-bit
		arch.LongWidth = 4
		arch.LongAlign = 4
	case 8:
		// If floats are 64-bit aligned, then this might be all-in on 64-bit, or it might 32-bit ints.

		// (The following heuristic is borrowed from javascriptRRD, and adjusted to also work with big-endian.)
		//
		// The next 2 things after the float_cookie are ds_cnt and rra_cnt (both 'unsigned long'--which may be
		// either 32 or 64 bit).  We'll inspect the bytes a bit to guess how long a long is.
		//
		// By assuming
		//  1. ds_cnt <= math.MaxUint32
		//  2l. rra_cnt > 0 (relevant if little-endian)
		//  2b. ds_cnt > 0 (relevant if big-endian)
		// we can inspect the 4 bytes (marked "big" or "little" below) that are either
		//  - the most significant bits of 64-bit ds_cnt, or
		//  - the entirety of 32-bit rra_cnt (if littlen-endian) or 32-bit ds_cnt (if big-endian)
		// If we see that those 4 bytes are all 0, then we assume that it's part of a 64-bit ds_cnt.
		//
		//       |           |           |           |           |           |           |    big    |  little   |
		//       |00|01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
		//  32   | R| R| D|\0| 0| 0| 0| 3|\0|                    |<----doublecookie----->|<--ds_cnt->|<-rra_cnt->|
		//  64le | R| R| D|\0| 0| 0| 0| 3|\0|                    |<----doublecookie----->|<1111----ds_cnt---0000>|
		//  64be | R| R| D|\0| 0| 0| 0| 3|\0|                    |<----doublecookie----->|<0000----ds_cnt---1111>|
		if len(data) < restOffset+8 {
			return rrdbinary.Architecture{}, rrdbinary.NewBinError("unexpected end of file", data, restOffset, 8)
		}
		offset := map[binary.ByteOrder]int{
			binary.BigEndian:    restOffset,     // 24 in the above diagram
			binary.LittleEndian: restOffset + 4, // 28 in the above diagram
		}[arch.ByteOrder]
		if arch.ByteOrder.Uint32(data[offset:]) == 0 {
			arch.LongWidth = 8
			arch.LongAlign = 8
		} else {
			arch.LongWidth = 4
			arch.LongAlign = 4
		}
	}

	// The above just os happens that DoubleXXX >= LongXXX, so we
	// can just set the Unival stuff to the Double Stuff.
	arch.UnivalWidth = arch.DoubleWidth // max(DoubleWidth, LongWidth)
	arch.UnivalAlign = arch.DoubleAlign // max(DoubleAlign, LongAlign)

	// FIXME: Figure out how to sniff the sizeof(time_t).
	//
	// javascriptRRD assumes that it's the same as sizeof(long),
	// which his historically been true, but
	//  - isn't true of proprietary 32-bit Unixen that are 2038-safe
	//  - isn't true of the Linux kernel with the x32 ABI
	arch.TimeWidth = arch.LongWidth
	arch.TimeAlign = arch.LongAlign

	return arch, nil
}