package frame

import (
	"encoding/binary"
	"fmt"
	"sort"
)

// CommonInformationEntry represents a Common Information Entry in
// the Dwarf .debug_frame section.
type CommonInformationEntry struct {
	Length                uint32
	CIE_id                uint32
	Version               uint8
	Augmentation          string
	CodeAlignmentFactor   uint64
	DataAlignmentFactor   int64
	ReturnAddressRegister uint64
	InitialInstructions   []byte
	staticBase            uint64

	// eh_frame pointer encoding
	ptrEncAddr ptrEnc
}

// FrameDescriptionEntry represents a Frame Descriptor Entry in the
// Dwarf .debug_frame section.
type FrameDescriptionEntry struct {
	Length       uint32
	CIE          *CommonInformationEntry
	Instructions []byte
	begin, size  uint64
	order        binary.ByteOrder
}

// Cover returns whether or not the given address is within the
// bounds of this frame.
func (fde *FrameDescriptionEntry) Cover(addr uint64) bool {
	return (addr - fde.begin) < fde.size
}

// Begin returns address of first location for this frame.
func (fde *FrameDescriptionEntry) Begin() uint64 {
	return fde.begin
}

// End returns address of last location for this frame.
func (fde *FrameDescriptionEntry) End() uint64 {
	return fde.begin + fde.size
}

// Translate moves the beginning of fde forward by delta.
func (fde *FrameDescriptionEntry) Translate(delta uint64) {
	fde.begin += delta
}

// EstablishFrame set up frame for the given PC.
func (fde *FrameDescriptionEntry) EstablishFrame(pc uint64) (*FrameContext, error) {
	fctxt := executeDwarfProgramUntilPC(fde, pc)
	if fctxt.err != nil {
		return nil, fctxt.err
	}
	return fctxt, nil
}

type FrameDescriptionEntries []*FrameDescriptionEntry

func newFrameIndex() FrameDescriptionEntries {
	return make(FrameDescriptionEntries, 0, 1000)
}

// ErrNoFDEForPC FDE for PC not found error
type ErrNoFDEForPC struct {
	PC uint64
}

func (err *ErrNoFDEForPC) Error() string {
	return fmt.Sprintf("could not find FDE for PC %#v", err.PC)
}

// FDEForPC returns the Frame Description Entry for the given PC.
func (fdes FrameDescriptionEntries) FDEForPC(pc uint64) (*FrameDescriptionEntry, error) {
	idx := sort.Search(len(fdes), func(i int) bool {
		return fdes[i].Cover(pc) || fdes[i].Begin() >= pc
	})
	if idx == len(fdes) || !fdes[idx].Cover(pc) {
		return nil, &ErrNoFDEForPC{pc}
	}
	return fdes[idx], nil
}

// Append appends otherFDEs to fdes and returns the result.
func (fdes FrameDescriptionEntries) Append(otherFDEs FrameDescriptionEntries) FrameDescriptionEntries {
	r := append(fdes, otherFDEs...)
	sort.SliceStable(r, func(i, j int) bool {
		return r[i].Begin() < r[j].Begin()
	})
	if len(r) < 2 { // fast path, no duplicates
		return r
	}

	// remove duplicates
	slow := 1
	for fast := 1; fast < len(r); fast++ {
		if r[fast].Begin() != r[fast-1].Begin() || r[fast].End() != r[fast-1].End() {
			if slow != fast {
				r[slow] = r[fast]
			}
			slow++
		}
	}
	return r[:slow]
}

// ptrEnc represents a pointer encoding value, used during eh_frame decoding
// to determine how pointers were encoded.
// Least significant 4 (0xf) bytes encode the size  as well as its
// signed-ness,  most significant 4 bytes (0xf0 == ptrEncFlagsMask) are flags
// describing how the value should be interpreted (absolute, relative...)
// See https://www.airs.com/blog/archives/460.
type ptrEnc uint8

const (
	ptrEncAbs    ptrEnc = 0x00 // pointer-sized unsigned integer
	ptrEncOmit   ptrEnc = 0xff // omitted
	ptrEncUleb   ptrEnc = 0x01 // ULEB128
	ptrEncUdata2 ptrEnc = 0x02 // 2 bytes
	ptrEncUdata4 ptrEnc = 0x03 // 4 bytes
	ptrEncUdata8 ptrEnc = 0x04 // 8 bytes
	ptrEncSigned ptrEnc = 0x08 // pointer-sized signed integer
	ptrEncSleb   ptrEnc = 0x09 // SLEB128
	ptrEncSdata2 ptrEnc = 0x0a // 2 bytes, signed
	ptrEncSdata4 ptrEnc = 0x0b // 4 bytes, signed
	ptrEncSdata8 ptrEnc = 0x0c // 8 bytes, signed

	ptrEncFlagsMask ptrEnc = 0xf0

	ptrEncPCRel    ptrEnc = 0x10 // value is relative to the memory address where it appears
	ptrEncTextRel  ptrEnc = 0x20 // value is relative to the address of the text section
	ptrEncDataRel  ptrEnc = 0x30 // value is relative to the address of the data section
	ptrEncFuncRel  ptrEnc = 0x40 // value is relative to the start of the function
	ptrEncAligned  ptrEnc = 0x50 // value should be aligned
	ptrEncIndirect ptrEnc = 0x80 // value is an address where the real value of the pointer is stored

	ptrEncSupportedFlags = ptrEncPCRel
)

// Supported returns true if this pointer encoding is supported.
func (ptrEnc ptrEnc) Supported() bool {
	if ptrEnc != ptrEncOmit {
		szenc := ptrEnc & 0x0f
		if ((szenc > ptrEncUdata8) && (szenc < ptrEncSigned)) || (szenc > ptrEncSdata8) {
			// These values aren't defined at the moment
			return false
		}
		if (ptrEnc&ptrEncFlagsMask)&^ptrEncSupportedFlags != 0 {
			// Currently only the PC relative flag is supported
			return false
		}
	}
	return true
}
