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

package asm

import (
	"bytes"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sync"
)

// GOObjectRISCV emits a GOOBJ object file for RISC-V.  The layout is the
// shared one in goobj.go — the toolchain preamble, the go120ld header with
// its block offsets, the string table, the symbol definitions and the
// reloc/aux/data index arrays — with the RISC-V preamble, the MinLC of 2 for
// the pc-value deltas, and the single R_RISCV_PCREL_ITYPE/STYPE relocation
// per AUIPC pair, matching `go tool asm`'s model (each pair is one 8-byte
// relocation, not the ELF HI20/LO12 pair).
func (img *Image) GOObjectRISCV(pkgPath, srcPath string) ([]byte, error) {
	pre, err := toolchainObjectPreambleRISCV()
	if err != nil {
		return nil, err
	}
	return img.emitGOObject(pkgPath, srcPath, pre, 2, func(r Reloc) (uint16, uint8) {
		switch r.Kind {
		case RelRISCVPCRELSType:
			return relocRISCVPcrelStype, 8
		case RelRISCVJal:
			return relocRISCVJal, 4
		default:
			return relocRISCVPcrelItype, 8
		}
	})
}

// RISC-V relocation types (cmd/internal/objabi).  The Go linker applies
// R_RISCV_PCREL_ITYPE/STYPE to an AUIPC + I/S-type instruction pair as a
// single 8-byte field; R_RISCV_JAL covers a single 4-byte J-type instruction.
const (
	relocRISCVJal        = 59 // R_RISCV_JAL
	relocRISCVPcrelItype = 62 // R_RISCV_PCREL_ITYPE
	relocRISCVPcrelStype = 63 // R_RISCV_PCREL_STYPE
)

// toolchainObjectPreambleRISCV returns the "go object ...\n!\n" header
// the installed go tool asm writes for riscv64, captured by assembling a
// one-instruction probe (see toolchainObjectPreamble).
var (
	preambleRISCVOnce sync.Once
	preambleRISCV     []byte
	preambleRISCVErr  error
)

func toolchainObjectPreambleRISCV() ([]byte, error) {
	preambleRISCVOnce.Do(func() {
		goBin, err := exec.LookPath("go")
		if err != nil {
			preambleRISCVErr = fmt.Errorf("GOOBJ emission needs the Go toolchain: %w", err)
			return
		}
		dir, err := os.MkdirTemp("", "gasm-preamble-riscv")
		if err != nil {
			preambleRISCVErr = err
			return
		}
		defer os.RemoveAll(dir)
		src := filepath.Join(dir, "probe_riscv64.s")
		if err := os.WriteFile(src, []byte("TEXT \u00b7x(SB), $0-0\n\tRET\n"), 0o644); err != nil {
			preambleRISCVErr = err
			return
		}
		obj := filepath.Join(dir, "probe.o")
		cmd := exec.Command(goBin, "tool", "asm", "-p", "probe", "-o", obj, src)
		cmd.Env = append(os.Environ(), "GOARCH=riscv64")
		if out, err := cmd.CombinedOutput(); err != nil {
			preambleRISCVErr = fmt.Errorf("probing the assembler for the object header: %v\n%s", err, out)
			return
		}
		data, err := os.ReadFile(obj)
		if err != nil {
			preambleRISCVErr = err
			return
		}
		i := bytes.Index(data, []byte("\n!\n"))
		if i < 0 || !bytes.HasPrefix(data[i+3:], []byte(goobjMagic)) {
			preambleRISCVErr = fmt.Errorf("unrecognised assembler object layout")
			return
		}
		preambleRISCV = data[:i+3]
	})
	return preambleRISCV, preambleRISCVErr
}