package binary

import (
	"bytes"
	"fmt"

	"github.com/tetratelabs/wabin/wasm"
)

// decodeGlobal returns the wasm.Global decoded with the WebAssembly Binary Format.
//
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#binary-global
func decodeGlobal(r *bytes.Reader, features wasm.CoreFeatures) (*wasm.Global, error) {
	gt, err := decodeGlobalType(r)
	if err != nil {
		return nil, err
	}

	init, err := decodeConstantExpression(r, features)
	if err != nil {
		return nil, err
	}

	return &wasm.Global{Type: gt, Init: init}, nil
}

// decodeGlobalType returns the wasm.GlobalType decoded with the WebAssembly Binary Format.
//
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#binary-globaltype
func decodeGlobalType(r *bytes.Reader) (*wasm.GlobalType, error) {
	vt, err := decodeValueTypes(r, 1)
	if err != nil {
		return nil, fmt.Errorf("read value type: %w", err)
	}

	ret := &wasm.GlobalType{
		ValType: vt[0],
	}

	b, err := r.ReadByte()
	if err != nil {
		return nil, fmt.Errorf("read mutablity: %w", err)
	}

	switch mut := b; mut {
	case 0x00: // not mutable
	case 0x01: // mutable
		ret.Mutable = true
	default:
		return nil, fmt.Errorf("%w for mutability: %#x != 0x00 or 0x01", ErrInvalidByte, mut)
	}
	return ret, nil
}

// encodeGlobal returns the wasm.Global encoded in WebAssembly Binary Format.
//
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#global-section%E2%91%A0
func encodeGlobal(g *wasm.Global) (data []byte) {
	var mutable byte
	if g.Type.Mutable {
		mutable = 1
	}
	data = []byte{g.Type.ValType, mutable}
	data = append(data, encodeConstantExpression(g.Init)...)
	return
}