// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

//go:build freebsd && (amd64 || arm64 || riscv64)

package debug

import (
	"encoding/hex"
	"fmt"
	"os"
	"runtime"
	"strconv"
	"strings"
	"syscall"
	"unsafe"

	"golang.org/x/sys/unix"

	"sourcedock.dev/petrbalvin/gasm-devkit/asm"
	"sourcedock.dev/petrbalvin/gasm-devkit/parser"
	"sourcedock.dev/petrbalvin/gasm-devkit/verify"
)

// mapRWX maps code into a read-write-execute region.
func mapRWX(code []byte) ([]byte, error) {
	const pageSize = 4096
	size := (len(code) + pageSize - 1) &^ (pageSize - 1)
	mem, err := syscall.Mmap(-1, 0, size,
		syscall.PROT_READ|syscall.PROT_WRITE|syscall.PROT_EXEC,
		syscall.MAP_PRIVATE|syscall.MAP_ANON)
	if err != nil {
		return nil, err
	}
	copy(mem, code)
	return mem, nil
}

// setupBuffers allocates buffers in the debuggee's memory.
func setupBuffers(spec string, args []byte, tmpDir string) ([]byte, error) {
	type bufSpec struct {
		name    string
		size    int
		pattern string
	}
	var specs []bufSpec
	for part := range strings.SplitSeq(spec, ",") {
		fields := strings.SplitN(part, ":", 3)
		if len(fields) != 3 {
			continue
		}
		size, err := strconv.Atoi(fields[1])
		if err != nil || size <= 0 {
			continue
		}
		specs = append(specs, bufSpec{name: fields[0], size: size, pattern: fields[2]})
	}

	if len(specs) == 0 {
		return args, nil
	}

	var bufAddrs []uint64
	for _, s := range specs {
		buf, err := syscall.Mmap(-1, 0, s.size,
			syscall.PROT_READ|syscall.PROT_WRITE,
			syscall.MAP_PRIVATE|syscall.MAP_ANON)
		if err != nil {
			return nil, fmt.Errorf("mmap buffer %s: %w", s.name, err)
		}
		fillBuffer(buf, s.pattern)
		bufAddrs = append(bufAddrs, uint64(uintptr(unsafe.Pointer(&buf[0]))))
	}

	addrFile, err := os.Create(tmpDir + "/bufaddrs")
	if err != nil {
		return nil, err
	}
	for _, addr := range bufAddrs {
		fmt.Fprintf(addrFile, "%d\n", addr)
	}
	addrFile.Close()

	return args, nil
}

// fillBuffer fills a buffer with the specified pattern.
func fillBuffer(buf []byte, pattern string) {
	switch pattern {
	case "zero":
	case "ones":
		for i := range buf {
			buf[i] = 0xFF
		}
	case "seq":
		for i := range buf {
			buf[i] = byte(i)
		}
	default:
		if data, err := hex.DecodeString(pattern); err == nil && len(data) > 0 {
			for i := range buf {
				buf[i] = data[i%len(data)]
			}
		}
	}
}

// RunTarget is the debuggee entry point (gasm debug --target).
func RunTarget(asmPath, funcName, argsFile, tmpDir string) error {
	src, err := os.ReadFile(asmPath)
	if err != nil {
		return fmt.Errorf("debug target: %w", err)
	}
	file, errs := parser.Parse(asmPath, string(src))
	if len(errs) > 0 {
		return fmt.Errorf("debug target: parse: %v", errs[0])
	}
	img, err := asm.AssembleFile(file)
	if err != nil {
		return fmt.Errorf("debug target: assemble: %w", err)
	}

	var fl *asm.FuncLayout
	for i := range img.Funcs {
		if img.Funcs[i].Name == funcName {
			fl = &img.Funcs[i]
			break
		}
	}
	if fl == nil {
		return fmt.Errorf("debug target: function %q not found", funcName)
	}

	code := img.Bytes()
	exec, err := mapRWX(code)
	if err != nil {
		return fmt.Errorf("debug target: mmap: %w", err)
	}

	codeBase := uintptr(unsafe.Pointer(&exec[0]))
	if err := os.WriteFile(tmpDir+"/codebase", []byte(fmt.Sprintf("%d", codeBase)), 0o644); err != nil {
		return fmt.Errorf("debug target: write codebase: %w", err)
	}

	meta := fmt.Sprintf("%d %d %d", fl.Offset, fl.Size, fl.Args)
	os.WriteFile(tmpDir+"/funcmeta", []byte(meta), 0o644)

	labelsFile, _ := os.Create(tmpDir + "/labels")
	if labelsFile != nil {
		for label, off := range fl.Labels {
			fmt.Fprintf(labelsFile, "%s %d\n", label, off)
		}
		labelsFile.Close()
	}

	args, err := os.ReadFile(argsFile)
	if err != nil {
		return fmt.Errorf("debug target: read args: %w", err)
	}
	if len(args) < fl.Args {
		padded := make([]byte, fl.Args)
		copy(padded, args)
		args = padded
	}

	bufSpecFile := tmpDir + "/bufspec"
	if bufSpec, err := os.ReadFile(bufSpecFile); err == nil && len(bufSpec) > 0 {
		args, err = setupBuffers(string(bufSpec), args, tmpDir)
		if err != nil {
			return fmt.Errorf("debug target: setup buffers: %w", err)
		}
	}

	runtime.LockOSThread()

	if _, _, errno := unix.RawSyscall(unix.SYS_PTRACE, uintptr(unix.PT_TRACE_ME), 0, 0); errno != 0 {
		return fmt.Errorf("debug target: PT_TRACE_ME: %v", errno)
	}
	os.WriteFile(tmpDir+"/ready", []byte("ok"), 0o644)
	syscall.Kill(syscall.Getpid(), syscall.SIGSTOP)

	os.WriteFile(tmpDir+"/entry", []byte("ok"), 0o644)
	syscall.Kill(syscall.Getpid(), syscall.SIGSTOP)

	fnAddr := codeBase + uintptr(fl.Offset)
	stackArgs := make([]byte, fl.Args)
	copy(stackArgs, args)

	if _, callErr := verify.Call(fnAddr, stackArgs); callErr != nil {
		os.Exit(1)
	}
	// Success returns to the caller, which exits with status 0; the JIT
	// code has already run to its own trampoline by the time Call returns.
	return nil
}