// mklipo merges two Mach-O binaries into one macOS universal (fat) binary.
package main

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

const (
	fatMagic      = 0xCAFEBABE
	cpuTypeX86_64 = 0x01000007
	cpuTypeARM64  = 0x0100000c
	fatArchSize   = 20
)

type slice struct {
	cpu, sub uint32
	data     []byte
}

func main() {
	if len(os.Args) != 4 {
		fmt.Fprintln(os.Stderr, "usage: mklipo <amd64-bin> <arm64-bin> <out>")
		os.Exit(2)
	}
	amd, err := os.ReadFile(os.Args[1])
	if err != nil {
		fatal(err)
	}
	arm, err := os.ReadFile(os.Args[2])
	if err != nil {
		fatal(err)
	}
	if err := writeFat(os.Args[3], []slice{
		{cpuTypeX86_64, 3, amd},
		{cpuTypeARM64, 0, arm},
	}); err != nil {
		fatal(err)
	}
}

func writeFat(path string, parts []slice) error {
	const align uint32 = 0x4000
	hdr := 8 + fatArchSize*len(parts)
	offsets := make([]uint32, len(parts))
	cur := uint32(hdr)
	for i, p := range parts {
		pad := (align - (cur % align)) % align
		cur += pad
		offsets[i] = cur
		cur += uint32(len(p.data))
	}

	out, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o755)
	if err != nil {
		return err
	}
	defer out.Close()

	if err := binary.Write(out, binary.BigEndian, uint32(fatMagic)); err != nil {
		return err
	}
	if err := binary.Write(out, binary.BigEndian, uint32(len(parts))); err != nil {
		return err
	}
	for i, p := range parts {
		if err := binary.Write(out, binary.BigEndian, p.cpu); err != nil {
			return err
		}
		if err := binary.Write(out, binary.BigEndian, p.sub); err != nil {
			return err
		}
		if err := binary.Write(out, binary.BigEndian, offsets[i]); err != nil {
			return err
		}
		if err := binary.Write(out, binary.BigEndian, uint32(len(p.data))); err != nil {
			return err
		}
		if err := binary.Write(out, binary.BigEndian, align); err != nil {
			return err
		}
	}

	cur = uint32(hdr)
	for _, p := range parts {
		pad := (align - (cur % align)) % align
		if pad > 0 {
			if _, err := out.Write(make([]byte, pad)); err != nil {
				return err
			}
			cur += pad
		}
		if _, err := out.Write(p.data); err != nil {
			return err
		}
		cur += uint32(len(p.data))
	}
	return nil
}

func fatal(err error) {
	fmt.Fprintln(os.Stderr, "mklipo:", err)
	os.Exit(1)
}