package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"os"
)

func readInt8(fp *os.File)(int8, error){
	buff := make([]byte, 1)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	reader := bytes.NewReader(buff)
	n := int8(0)
	return n, binary.Read(reader, binary.LittleEndian, &n)
}

func readInt16(fp *os.File)(int16, error){
	buff := make([]byte, 2)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	reader := bytes.NewReader(buff)
	n := int16(0)
	return n, binary.Read(reader, binary.LittleEndian, &n)
}

func readInt32(fp *os.File)(int32, error){
	buff := make([]byte, 4)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	reader := bytes.NewReader(buff)
	n := int32(0)
	return n, binary.Read(reader, binary.LittleEndian, &n)
}

func readInt64(fp *os.File)(int64, error){
	buff := make([]byte, 8)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	reader := bytes.NewReader(buff)
	n := int64(0)
	return n, binary.Read(reader, binary.LittleEndian, &n)
}

func readUint8(fp *os.File)(uint8, error){
	buff := make([]byte, 1)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	return uint8(buff[0]), nil
}

func readUint16(fp *os.File)(uint16, error){
	buff := make([]byte, 2)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint16(buff), nil
}

func readUint32(fp *os.File)(uint32, error){
	buff := make([]byte, 4)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint32(buff), nil
}

func readUint64(fp *os.File)(uint64, error){
	buff := make([]byte, 8)
	_, err := fp.Read(buff)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint64(buff), nil
}

func readString(fp *os.File, len int)(string, error){
	buff := make([]byte, len)
	_, err := fp.Read(buff)
	if err != nil {
		return "", err
	}
	return string(buff), nil
}

func readBytes(fp *os.File, len int)([]byte, error){
	buff := make([]byte, len)
	_, err := fp.Read(buff)
	if err != nil {
		return []byte{}, err
	}
	return buff, nil
}

func readFontSearchArgs(fp *os.File)([]uint16, error){
	ret := []uint16{}
	arg1, err := readUint16(fp)
	if err != nil {
		return ret, err
	}
	ret = append(ret, arg1)
	arg1, err = readUint16(fp)
	if err != nil {
		return ret, err
	}
	ret = append(ret, arg1)
	arg1, err = readUint16(fp)
	if err != nil {
		return ret, err
	}
	ret = append(ret, arg1)
	return ret, nil
}

type ftable struct{
	Name string
	Sum uint32
	Offset uint32
	Length uint32
}

func readFontTables(fp *os.File, tables uint16)([]ftable, error){
	ret := []ftable{}
	for i := 0; i < int(tables); i++ {
		t :=ftable{}
		name, err := readString(fp, 4)
		if err != nil {
			return ret, err
		}
		t.Name = name
		sum, err := readUint32(fp)
		if err != nil {
			return ret, err
		}
		t.Sum = sum
		addr, err := readUint32(fp)
		if err != nil {
			return ret, err
		}
		t.Offset = addr
		length, err := readUint32(fp)
		if err != nil {
			return ret, err
		}
		t.Length = length
		ret = append(ret, t)
	}
	return ret, nil
}

func readNameTable(fp *os.File, offset uint32, length uint32)([]byte, error){
	fp.Seek(int64(offset), 0)
	return readBytes(fp, int(length))
}

func readFont(file string)error{
	fp, err := os.OpenFile(file, os.O_RDONLY, os.ModePerm)
	if err != nil {
		return err
	}
	defer fp.Close()
	verB, err := readUint16(fp)
	if err != nil {
		return err
	}
	verL, err := readInt16(fp)
	if err != nil {
		return err
	}
	fmt.Println("font head version is", fmt.Sprintf("%d.%d", verB, verL))
	tableCount, err := readUint16(fp)
	if err != nil {
		return err
	}
	fmt.Println("font table count is", tableCount)
	args, err := readFontSearchArgs(fp)
	if err != nil {
		return err
	}
	fmt.Println("font search args is", args)
	tables, err := readFontTables(fp, tableCount)
	if err != nil {
		return err
	}
	nameBuff := []byte{}
	for n, table := range tables {
		fmt.Println("font table", n+1, ": name: ", table.Name, "  sum: ", table.Sum, "  offset: ", table.Offset, "  length: ", table.Length)
		if table.Name == "name" {
			nameBuff, err = readNameTable(fp, table.Offset, table.Length)
		}
	}
	if err != nil {
		return err
	}
	fmt.Println("name table: ", string(nameBuff))
	return nil
}

func main(){
	err := readFont("文鼎中黑.ttf")
	if err != nil {
		panic(err)
	}
}