// Copyright 2014 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ppc64asm

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

const debugDecode = false

// instFormat is a decoding rule for one specific instruction form.
// a uint32 instruction ins matches the rule if ins&Mask == Value
// DontCare bits should be zero, but the machine might not reject
// ones in those bits, they are mainly reserved for future expansion
// of the instruction set.
// The Args are stored in the same order as the instruction manual.
type instFormat struct {
	Op       Op
	Mask     uint32
	Value    uint32
	DontCare uint32
	Args     [5]*argField
}

// argField indicate how to decode an argument to an instruction.
// First parse the value from the BitFields, shift it left by Shift
// bits to get the actual numerical value.
type argField struct {
	Type  ArgType
	Shift uint8
	BitFields
}

// Parse parses the Arg out from the given binary instruction i.
func (a argField) Parse(i uint32) Arg {
	switch a.Type {
	default:
		return nil
	case TypeUnknown:
		return nil
	case TypeReg:
		return R0 + Reg(a.BitFields.Parse(i))
	case TypeCondRegBit:
		return Cond0LT + CondReg(a.BitFields.Parse(i))
	case TypeCondRegField:
		return CR0 + CondReg(a.BitFields.Parse(i))
	case TypeFPReg:
		return F0 + Reg(a.BitFields.Parse(i))
	case TypeVecReg:
		return V0 + Reg(a.BitFields.Parse(i))
	case TypeVecSReg:
		return VS0 + Reg(a.BitFields.Parse(i))
	case TypeSpReg:
		return SpReg(a.BitFields.Parse(i))
	case TypeImmSigned:
		return Imm(a.BitFields.ParseSigned(i) << a.Shift)
	case TypeImmUnsigned:
		return Imm(a.BitFields.Parse(i) << a.Shift)
	case TypePCRel:
		return PCRel(a.BitFields.ParseSigned(i) << a.Shift)
	case TypeLabel:
		return Label(a.BitFields.ParseSigned(i) << a.Shift)
	case TypeOffset:
		return Offset(a.BitFields.ParseSigned(i) << a.Shift)
	}
}

type ArgType int8

const (
	TypeUnknown      ArgType = iota
	TypePCRel                // PC-relative address
	TypeLabel                // absolute address
	TypeReg                  // integer register
	TypeCondRegBit           // conditional register bit (0-31)
	TypeCondRegField         // conditional register field (0-7)
	TypeFPReg                // floating point register
	TypeVecReg               // vector register
	TypeVecSReg              // VSX register
	TypeSpReg                // special register (depends on Op)
	TypeImmSigned            // signed immediate
	TypeImmUnsigned          // unsigned immediate/flag/mask, this is the catch-all type
	TypeOffset               // signed offset in load/store
	TypeLast                 // must be the last one
)

func (t ArgType) String() string {
	switch t {
	default:
		return fmt.Sprintf("ArgType(%d)", int(t))
	case TypeUnknown:
		return "Unknown"
	case TypeReg:
		return "Reg"
	case TypeCondRegBit:
		return "CondRegBit"
	case TypeCondRegField:
		return "CondRegField"
	case TypeFPReg:
		return "FPReg"
	case TypeVecReg:
		return "VecReg"
	case TypeVecSReg:
		return "VecSReg"
	case TypeSpReg:
		return "SpReg"
	case TypeImmSigned:
		return "ImmSigned"
	case TypeImmUnsigned:
		return "ImmUnsigned"
	case TypePCRel:
		return "PCRel"
	case TypeLabel:
		return "Label"
	case TypeOffset:
		return "Offset"
	}
}

func (t ArgType) GoString() string {
	s := t.String()
	if t > 0 && t < TypeLast {
		return "Type" + s
	}
	return s
}

var (
	// Errors
	errShort   = fmt.Errorf("truncated instruction")
	errUnknown = fmt.Errorf("unknown instruction")
)

var decoderCover []bool

// Decode decodes the leading bytes in src as a single instruction using
// byte order ord.
func Decode(src []byte, ord binary.ByteOrder) (inst Inst, err error) {
	if len(src) < 4 {
		return inst, errShort
	}
	if decoderCover == nil {
		decoderCover = make([]bool, len(instFormats))
	}
	inst.Len = 4 // only 4-byte instructions are supported
	ui := ord.Uint32(src[:inst.Len])
	inst.Enc = ui
	for i, iform := range instFormats {
		if ui&iform.Mask != iform.Value {
			continue
		}
		if ui&iform.DontCare != 0 {
			if debugDecode {
				log.Printf("Decode(%#x): unused bit is 1 for Op %s", ui, iform.Op)
			}
			// to match GNU objdump (libopcodes), we ignore don't care bits
		}
		for i, argfield := range iform.Args {
			if argfield == nil {
				break
			}
			inst.Args[i] = argfield.Parse(ui)
		}
		inst.Op = iform.Op
		if debugDecode {
			log.Printf("%#x: search entry %d", ui, i)
			continue
		}
		break
	}
	if inst.Op == 0 && inst.Enc != 0 {
		return inst, errUnknown
	}
	return inst, nil
}
