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

// Command gen regenerates the architecture instruction tables from the Go
// toolchain's own assembler source.  Go's Plan 9 assembler defines the exact,
// complete set of mnemonics it accepts for each architecture in
// $GOROOT/src/cmd/internal/obj/<arch>/anames.go; this tool extracts those
// names so gasm-devkit supports every instruction the real assembler does,
// with no hand-maintained (and therefore inevitably incomplete) lists.
//
// Usage (via the justfile):
//
//	just gen
//
// The generated files are committed; regenerating requires a Go installation
// but the toolkit itself has no dependency on the toolchain source at runtime.
package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
)

// archDirs maps a gasm-devkit architecture name to its obj sub-directory.
var archDirs = []struct {
	arch string
	sub  string
}{
	{"amd64", "x86"},
	{"arm64", "arm64"},
	{"riscv", "riscv"},
	{"loong64", "loong64"},
}

func main() {
	goroot := strings.TrimSpace(runGoEnvGOROOT())
	if goroot == "" {
		fatal("could not determine GOROOT")
	}
	// The common opcodes shared by every architecture (RET, JMP, NOP, CALL,
	// TEXT, FUNCDATA, …) live in cmd/internal/obj/util.go.
	commonPath := filepath.Join(goroot, "src", "cmd", "internal", "obj", "util.go")
	common, err := extractInstrs(commonPath)
	if err != nil {
		fatal("extract common: %v", err)
	}
	common = filterCommon(common)
	if err := writeCommon(common); err != nil {
		fatal("write common: %v", err)
	}
	fmt.Printf("%-8s %4d instructions -> arch/common_gen.go\n", "common", len(common))

	for _, a := range archDirs {
		path := filepath.Join(goroot, "src", "cmd", "internal", "obj", a.sub, "anames.go")
		names, err := extractInstrs(path)
		if err != nil {
			fatal("extract %s: %v", a.arch, err)
		}
		if err := writeGen(a.arch, a.sub, names); err != nil {
			fatal("write %s: %v", a.arch, err)
		}
		fmt.Printf("%-8s %4d instructions -> arch/%s_gen.go\n", a.arch, len(names), a.arch)
	}
}

// filterCommon drops opcode names that are not user-writable instructions.
func filterCommon(names []string) []string {
	drop := map[string]bool{"XXX": true, "LAST": true}
	var out []string
	for _, n := range names {
		if !drop[n] {
			out = append(out, n)
		}
	}
	return out
}

// writeCommon emits arch/common_gen.go.
func writeCommon(names []string) error {
	var b strings.Builder
	b.WriteString("// Code generated by gasm-devkit _gen; DO NOT EDIT.\n")
	b.WriteString("// Source: cmd/internal/obj/util.go from the Go toolchain.\n")
	b.WriteString("//\n")
	b.WriteString("// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)\n")
	b.WriteString("// SPDX-License-Identifier: BSD-3-Clause\n\n")
	b.WriteString("package arch\n\n")
	b.WriteString("// commonGeneratedInstrs is the set of opcodes shared by every architecture\n")
	b.WriteString("// (RET, JMP, NOP, CALL, TEXT, FUNCDATA, PCDATA, …).\n")
	b.WriteString("var commonGeneratedInstrs = []string{\n")
	for _, n := range names {
		fmt.Fprintf(&b, "\t%q,\n", n)
	}
	b.WriteString("}\n")
	return os.WriteFile(filepath.Join("arch", "common_gen.go"), []byte(b.String()), 0o644)
}

// extractInstrs parses an anames.go file and returns the sorted, de-duplicated
// instruction names from its `var Anames = []string{...}` literal.
func extractInstrs(path string) ([]string, error) {
	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, path, nil, 0)
	if err != nil {
		return nil, err
	}
	seen := map[string]bool{}
	var names []string
	for _, decl := range f.Decls {
		gd, ok := decl.(*ast.GenDecl)
		if !ok || gd.Tok != token.VAR {
			continue
		}
		for _, spec := range gd.Specs {
			vs, ok := spec.(*ast.ValueSpec)
			if !ok || len(vs.Names) == 0 || vs.Names[0].Name != "Anames" {
				continue
			}
			for _, val := range vs.Values {
				cl, ok := val.(*ast.CompositeLit)
				if !ok {
					continue
				}
				for _, elt := range cl.Elts {
					if lit := stringLit(elt); lit != "" && lit != "LAST" && !seen[lit] {
						seen[lit] = true
						names = append(names, lit)
					}
				}
			}
		}
	}
	sort.Strings(names)
	return names, nil
}

// stringLit returns the string value of a composite-literal element, whether it
// is a plain literal or a keyed entry such as `obj.A_ARCHSPECIFIC: "AAA"`.
func stringLit(elt ast.Expr) string {
	switch e := elt.(type) {
	case *ast.BasicLit:
		if e.Kind == token.STRING {
			return strings.Trim(e.Value, `"`)
		}
	case *ast.KeyValueExpr:
		return stringLit(e.Value)
	}
	return ""
}

// writeGen emits arch/<arch>_gen.go.
func writeGen(arch, sub string, names []string) error {
	var b strings.Builder
	b.WriteString("// Code generated by gasm-devkit _gen; DO NOT EDIT.\n")
	b.WriteString("// Source: cmd/internal/obj/" + sub + "/anames.go from the Go toolchain.\n")
	b.WriteString("//\n")
	b.WriteString("// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)\n")
	b.WriteString("// SPDX-License-Identifier: BSD-3-Clause\n\n")
	b.WriteString("package arch\n\n")
	b.WriteString("// " + arch + "GeneratedInstrs is the complete set of " + arch +
		" mnemonics accepted by\n// Go's Plan 9 assembler.\n")
	b.WriteString("var " + arch + "GeneratedInstrs = []string{\n")
	for _, n := range names {
		fmt.Fprintf(&b, "\t%q,\n", n)
	}
	b.WriteString("}\n")
	return os.WriteFile(filepath.Join("arch", arch+"_gen.go"), []byte(b.String()), 0o644)
}

func runGoEnvGOROOT() string {
	out, err := exec.Command("go", "env", "GOROOT").Output()
	if err != nil {
		return ""
	}
	return string(out)
}

func fatal(format string, args ...any) {
	fmt.Fprintf(os.Stderr, "gen: "+format+"\n", args...)
	os.Exit(1)
}