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

//go:build linux && amd64

package debug

import (
	"fmt"
	"syscall"
	"unsafe"
)

// StopReason describes why the debuggee stopped.
type StopReason int

const (
	StopNone       StopReason = iota
	StopBreakpoint            // INT3 breakpoint hit
	StopWatchpoint            // hardware watchpoint triggered
	StopSingleStep            // single-step completed
	StopSignal                // stopped by a signal
	StopExited                // process exited
)

// siginfo_t layout (Linux amd64): si_signo, si_errno, si_code, then union.
type siginfoT struct {
	SiSigno int32
	SiErrno int32
	SiCode  int32
	_pad    [125]byte
}

const (
	trapBRKPT   = 1 // INT3 breakpoint
	trapHWBRKPT = 4 // hardware watchpoint
)

// StopInfo returns the reason the debuggee stopped and the faulting address
// (for watchpoints, the watched address that was accessed).
func (s *Session) StopInfo() (StopReason, uint64) {
	if s.exited {
		return StopExited, 0
	}
	var info siginfoT
	_, _, errno := syscall.Syscall6(
		syscall.SYS_PTRACE,
		uintptr(syscall.PTRACE_GETSIGINFO),
		uintptr(s.pid),
		0,
		uintptr(unsafe.Pointer(&info)),
		0, 0,
	)
	if errno != 0 {
		return StopNone, 0
	}
	if info.SiSigno != int32(syscall.SIGTRAP) {
		return StopSignal, uint64(info.SiCode)
	}
	switch info.SiCode {
	case trapBRKPT:
		return StopBreakpoint, 0
	case trapHWBRKPT:
		// The faulting address is in si_addr (offset 16 in siginfo_t on amd64).
		addr := *(*uint64)(unsafe.Pointer(uintptr(unsafe.Pointer(&info)) + 16))
		return StopWatchpoint, addr
	default:
		return StopSingleStep, 0
	}
}

// SetReg modifies a register value in the debuggee.
func (s *Session) SetReg(name string, value uint64) error {
	regs, err := s.GetRegs()
	if err != nil {
		return err
	}
	switch name {
	case "rax", "eax", "ax", "al":
		regs.RAX = value
	case "rbx", "ebx", "bx", "bl":
		regs.RBX = value
	case "rcx", "ecx", "cx", "cl":
		regs.RCX = value
	case "rdx", "edx", "dx", "dl":
		regs.RDX = value
	case "rsi", "esi", "si":
		regs.RSI = value
	case "rdi", "edi", "di":
		regs.RDI = value
	case "rbp", "ebp", "bp":
		regs.RBP = value
	case "rsp", "esp", "sp":
		regs.RSP = value
	case "r8":
		regs.R8 = value
	case "r9":
		regs.R9 = value
	case "r10":
		regs.R10 = value
	case "r11":
		regs.R11 = value
	case "r12":
		regs.R12 = value
	case "r13":
		regs.R13 = value
	case "r14":
		regs.R14 = value
	case "r15":
		regs.R15 = value
	case "rip", "eip":
		regs.RIP = value
	default:
		return fmt.Errorf("debug: unknown register %q", name)
	}
	return s.SetRegs(&regs)
}