package kiosk

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"go/types"
	"os"
	"path/filepath"
	"regexp"
	"slices"
	"strconv"
	"strings"
)

// Identity is what a plugin's ID() and Requires() return.
type Identity struct {
	ID       string
	Requires []string
}

// ReadIdentity reads the plugin ID and the required plugin IDs from the
// Go sources in dir without building or running them. ID() and Requires()
// may return string literals or constants: a const of the same package, of
// another package of the same module, or of a package in a module go.mod
// requires (located with `go list -m -json`). Requires() may also return
// nil or an empty slice. Any other expression is an error naming ID() or
// Requires(). When only Requires() fails, the returned Identity still
// carries the ID.
func ReadIdentity(ctx context.Context, dir string) (Identity, error) {
	abs, err := filepath.Abs(dir)
	if err != nil {
		return Identity{}, fmt.Errorf("kiosk: resolve %s: %w", dir, err)
	}
	r := newResolver(ctx, abs)
	return r.identity(abs)
}

// resolver evaluates string constants across packages and modules. Module
// facts are loaded on first use, so a plugin whose ID is a literal costs
// no go command.
type resolver struct {
	ctx    context.Context
	start  string
	fset   *token.FileSet
	pkgs   map[string]*pkgSrc
	active map[string]bool

	modLoaded bool
	modErr    error
	root      string
	mod       goMod
	modDirs   map[string]listedModule
}

// pkgSrc is one parsed package directory.
type pkgSrc struct {
	name   string
	files  map[string]*ast.File
	order  []string
	consts map[string]constDef
}

type constDef struct {
	expr ast.Expr
	file *ast.File
}

// listedModule is the part of `go list -m -json` output the resolver reads.
type listedModule struct {
	Path    string
	Version string
	Dir     string
	GoMod   string
	Error   *struct{ Err string }
}

func newResolver(ctx context.Context, start string) *resolver {
	return &resolver{
		ctx:     ctx,
		start:   start,
		fset:    token.NewFileSet(),
		pkgs:    map[string]*pkgSrc{},
		active:  map[string]bool{},
		modDirs: map[string]listedModule{},
	}
}

func (r *resolver) identity(dir string) (Identity, error) {
	pkg, err := r.load(dir)
	if err != nil {
		return Identity{}, err
	}
	idFn, idFile, idPath := pkg.method("ID", "")
	if idFn == nil {
		return Identity{}, fmt.Errorf("kiosk: %s has no ID() method", dir)
	}
	var id Identity
	expr := lastReturn(idFn)
	if expr == nil {
		return Identity{}, fmt.Errorf("kiosk: %s: ID() must return a string literal or constant", idPath)
	}
	id.ID, err = r.str(dir, idFile, expr)
	if err != nil {
		return Identity{}, fmt.Errorf("kiosk: %s: ID() must return a string literal or constant: %w", idPath, err)
	}
	if id.ID == "" {
		return Identity{}, fmt.Errorf("kiosk: %s: ID() returns an empty string", idPath)
	}
	reqFn, reqFile, reqPath := pkg.method("Requires", recvType(idFn))
	if reqFn == nil {
		return id, nil
	}
	reqs, err := r.requires(dir, reqFile, lastReturn(reqFn))
	if err != nil {
		return id, fmt.Errorf("kiosk: %s: Requires() must return nil or a []string of string literals or constants: %w", reqPath, err)
	}
	id.Requires = reqs
	return id, nil
}

func (r *resolver) requires(dir string, file *ast.File, expr ast.Expr) ([]string, error) {
	switch e := expr.(type) {
	case nil:
		return nil, errors.New("no single-value return")
	case *ast.Ident:
		if e.Name == "nil" {
			return nil, nil
		}
	case *ast.CompositeLit:
		if at, ok := e.Type.(*ast.ArrayType); !ok || at.Len != nil || types.ExprString(at.Elt) != "string" {
			break
		}
		out := make([]string, 0, len(e.Elts))
		for _, el := range e.Elts {
			s, err := r.str(dir, file, el)
			if err != nil {
				return nil, err
			}
			out = append(out, s)
		}
		return out, nil
	}
	return nil, fmt.Errorf("%s is not a []string literal", types.ExprString(expr))
}

// str evaluates expr, found in file of package dir, to a string constant.
func (r *resolver) str(dir string, file *ast.File, expr ast.Expr) (string, error) {
	switch e := expr.(type) {
	case *ast.ParenExpr:
		return r.str(dir, file, e.X)
	case *ast.BasicLit:
		if e.Kind == token.STRING {
			return strconv.Unquote(e.Value)
		}
	case *ast.Ident:
		return r.constant(dir, e.Name)
	case *ast.SelectorExpr:
		if x, ok := e.X.(*ast.Ident); ok {
			pkgDir, err := r.importDir(dir, file, x.Name)
			if err != nil {
				return "", err
			}
			return r.constant(pkgDir, e.Sel.Name)
		}
	case *ast.BinaryExpr:
		if e.Op == token.ADD {
			a, err := r.str(dir, file, e.X)
			if err != nil {
				return "", err
			}
			b, err := r.str(dir, file, e.Y)
			if err != nil {
				return "", err
			}
			return a + b, nil
		}
	}
	return "", fmt.Errorf("%s is not a string constant", types.ExprString(expr))
}

// constant resolves the package-level const name of package dir.
func (r *resolver) constant(dir, name string) (string, error) {
	key := dir + "\x00" + name
	if r.active[key] {
		return "", fmt.Errorf("constant %s refers to itself", name)
	}
	r.active[key] = true
	defer delete(r.active, key)
	pkg, err := r.load(dir)
	if err != nil {
		return "", err
	}
	def, ok := pkg.consts[name]
	if !ok {
		return "", fmt.Errorf("%s is not a constant of package %s", name, pkg.name)
	}
	if def.expr == nil {
		return "", fmt.Errorf("constant %s has no explicit value", name)
	}
	return r.str(dir, def.file, def.expr)
}

// load parses the non-test Go files of dir once.
func (r *resolver) load(dir string) (*pkgSrc, error) {
	if p, ok := r.pkgs[dir]; ok {
		return p, nil
	}
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, fmt.Errorf("kiosk: read %s: %w", dir, err)
	}
	p := &pkgSrc{files: map[string]*ast.File{}, consts: map[string]constDef{}}
	for _, e := range entries {
		n := e.Name()
		if e.IsDir() || !strings.HasSuffix(n, ".go") || strings.HasSuffix(n, "_test.go") {
			continue
		}
		path := filepath.Join(dir, n)
		f, err := parser.ParseFile(r.fset, path, nil, parser.SkipObjectResolution)
		if err != nil {
			return nil, fmt.Errorf("kiosk: parse %s: %w", path, err)
		}
		if p.name == "" {
			p.name = f.Name.Name
		}
		p.files[path] = f
		p.order = append(p.order, path)
		for _, decl := range f.Decls {
			gd, ok := decl.(*ast.GenDecl)
			if !ok || gd.Tok != token.CONST {
				continue
			}
			for _, spec := range gd.Specs {
				vs := spec.(*ast.ValueSpec)
				for i, name := range vs.Names {
					def := constDef{file: f}
					if i < len(vs.Values) {
						def.expr = vs.Values[i]
					}
					p.consts[name.Name] = def
				}
			}
		}
	}
	if len(p.order) == 0 {
		return nil, fmt.Errorf("kiosk: %s has no Go files", dir)
	}
	// plugin.go first, then the other files in name order.
	slices.SortStableFunc(p.order, func(a, b string) int {
		ap, bp := filepath.Base(a) == "plugin.go", filepath.Base(b) == "plugin.go"
		switch {
		case ap && !bp:
			return -1
		case bp && !ap:
			return 1
		}
		return strings.Compare(a, b)
	})
	r.pkgs[dir] = p
	return p, nil
}

// method finds a method with a body named name, on the receiver type recv
// when recv is not empty, preferring plugin.go.
func (p *pkgSrc) method(name, recv string) (*ast.FuncDecl, *ast.File, string) {
	for _, path := range p.order {
		f := p.files[path]
		for _, decl := range f.Decls {
			fn, ok := decl.(*ast.FuncDecl)
			if !ok || fn.Recv == nil || fn.Body == nil || fn.Name.Name != name {
				continue
			}
			if recv != "" && recvType(fn) != recv {
				continue
			}
			return fn, f, path
		}
	}
	return nil, nil, ""
}

// recvType is the receiver's type name without pointer or type parameters.
func recvType(fn *ast.FuncDecl) string {
	if fn.Recv == nil || len(fn.Recv.List) == 0 {
		return ""
	}
	t := fn.Recv.List[0].Type
	if s, ok := t.(*ast.StarExpr); ok {
		t = s.X
	}
	switch x := t.(type) {
	case *ast.IndexExpr:
		t = x.X
	case *ast.IndexListExpr:
		t = x.X
	}
	if id, ok := t.(*ast.Ident); ok {
		return id.Name
	}
	return ""
}

var majorSuffix = regexp.MustCompile(`^v[0-9]+$`)

// importDir finds the directory of the package file imports as pkgName.
func (r *resolver) importDir(dir string, file *ast.File, pkgName string) (string, error) {
	var unnamed []string
	for _, imp := range file.Imports {
		path, err := strconv.Unquote(imp.Path.Value)
		if err != nil {
			continue
		}
		if imp.Name != nil {
			if imp.Name.Name == pkgName {
				return r.dirForImport(path)
			}
			continue
		}
		unnamed = append(unnamed, path)
	}
	// An unnamed import is usually named after its last path element.
	for _, path := range unnamed {
		if lastElem(path) == pkgName {
			if d, err := r.dirForImport(path); err == nil {
				if p, err := r.load(d); err == nil && p.name == pkgName {
					return d, nil
				}
			}
		}
	}
	// Otherwise read the package clause of every non-standard import.
	for _, path := range unnamed {
		first, _, _ := strings.Cut(path, "/")
		if !strings.Contains(first, ".") || lastElem(path) == pkgName {
			continue
		}
		d, err := r.dirForImport(path)
		if err != nil {
			continue
		}
		if p, err := r.load(d); err == nil && p.name == pkgName {
			return d, nil
		}
	}
	return "", fmt.Errorf("%s: no import provides package %s", r.fset.Position(file.Package).Filename, pkgName)
}

func lastElem(path string) string {
	parts := strings.Split(path, "/")
	last := parts[len(parts)-1]
	if majorSuffix.MatchString(last) && len(parts) > 1 {
		last = parts[len(parts)-2]
	}
	return strings.TrimPrefix(last, "go-")
}

// loadModule reads the module that contains the start directory.
func (r *resolver) loadModule() error {
	if r.modLoaded {
		return r.modErr
	}
	r.modLoaded = true
	r.root, r.modErr = moduleRoot(r.start)
	if r.modErr == nil {
		r.mod, r.modErr = readGoMod(r.ctx, r.root)
	}
	return r.modErr
}

// dirForImport maps an import path to a directory: inside this module, or
// inside a module go.mod requires.
func (r *resolver) dirForImport(path string) (string, error) {
	if err := r.loadModule(); err != nil {
		return "", err
	}
	if rest, ok := cutModule(path, r.mod.Module.Path); ok {
		return filepath.Join(r.root, filepath.FromSlash(rest)), nil
	}
	best := ""
	for _, req := range r.mod.Require {
		if _, ok := cutModule(path, req.Path); ok && len(req.Path) > len(best) {
			best = req.Path
		}
	}
	if best == "" {
		return "", fmt.Errorf("import %s is neither in module %s nor in a module its go.mod requires", path, r.mod.Module.Path)
	}
	mods, err := r.modules(best)
	if err != nil {
		return "", err
	}
	m := mods[0]
	if m.Dir == "" {
		return "", fmt.Errorf("module %s has no source directory", best)
	}
	rest, _ := cutModule(path, best)
	return filepath.Join(m.Dir, filepath.FromSlash(rest)), nil
}

// cutModule reports whether path is mod or inside it, and the rest.
func cutModule(path, mod string) (string, bool) {
	if path == mod {
		return "", true
	}
	if rest, ok := strings.CutPrefix(path, mod+"/"); ok {
		return rest, true
	}
	return "", false
}

// modules lists the given required modules with `go list -m -e -json`
// and downloads the ones whose source is not in the module cache yet.
func (r *resolver) modules(paths ...string) ([]listedModule, error) {
	res, err := r.list(paths)
	if err != nil {
		return nil, err
	}
	return res, r.download(res)
}

// list runs one `go list -m -e -json` for the paths not listed yet.
func (r *resolver) list(paths []string) ([]listedModule, error) {
	if err := r.loadModule(); err != nil {
		return nil, err
	}
	var missing []string
	for _, p := range paths {
		if _, ok := r.modDirs[p]; !ok {
			missing = append(missing, p)
		}
	}
	if len(missing) > 0 {
		args := append([]string{"list", "-m", "-e", "-json", "--"}, missing...)
		out, err := runGo(r.ctx, r.root, args...)
		if err != nil {
			return nil, err
		}
		dec := json.NewDecoder(bytes.NewReader(out))
		for dec.More() {
			var m listedModule
			if err := dec.Decode(&m); err != nil {
				return nil, fmt.Errorf("kiosk: go list -m: %w", err)
			}
			r.modDirs[m.Path] = m
		}
	}
	res := make([]listedModule, 0, len(paths))
	for _, p := range paths {
		m := r.modDirs[p]
		if m.Path == "" {
			m.Path = p
		}
		res = append(res, m)
	}
	return res, nil
}

// download fills Dir for listed modules whose source is not in the module
// cache, with one `go mod download -json` run.
func (r *resolver) download(mods []listedModule) error {
	var args []string
	for _, m := range mods {
		if m.Dir == "" && m.Error == nil && m.Version != "" {
			args = append(args, m.Path+"@"+m.Version)
		}
	}
	if len(args) == 0 {
		return nil
	}
	out, err := runGo(r.ctx, r.root, append([]string{"mod", "download", "-json", "--"}, args...)...)
	if err != nil {
		return err
	}
	dec := json.NewDecoder(bytes.NewReader(out))
	for dec.More() {
		var d listedModule
		if err := dec.Decode(&d); err != nil {
			return fmt.Errorf("kiosk: go mod download: %w", err)
		}
		if m, ok := r.modDirs[d.Path]; ok && d.Dir != "" {
			m.Dir = d.Dir
			r.modDirs[d.Path] = m
		}
	}
	for i := range mods {
		mods[i] = r.modDirs[mods[i].Path]
	}
	return nil
}

// minimums maps each required plugin ID to its minimum version. A plugin
// of the same module gets "", one found in a go.mod-required module gets
// that require's version, and one found nowhere gets "".
func (r *resolver) minimums(ids []string) (Requires, []string, error) {
	if len(ids) == 0 {
		return nil, nil, nil
	}
	if err := r.loadModule(); err != nil {
		return nil, nil, err
	}
	own := map[string]bool{}
	for _, d := range pluginDirs(r.root) {
		if id, _ := r.identity(d); id.ID != "" {
			own[id.ID] = true
		}
	}
	provider := map[string]string{}
	pending := 0
	for _, id := range ids {
		if !own[id] {
			pending++
		}
	}
	var warnings []string
	if pending > 0 {
		var paths []string
		for _, req := range r.mod.Require {
			if req.Path != FrameworkModule {
				paths = append(paths, req.Path)
			}
		}
		listed, err := r.list(paths)
		if err != nil {
			return nil, nil, err
		}
		// Only a module whose go.mod requires the framework can hold a plugin.
		var candidates []listedModule
		for _, m := range listed {
			if m.Error == nil && requiresFramework(m) {
				candidates = append(candidates, m)
			}
		}
		if err := r.download(candidates); err != nil {
			warnings = append(warnings, fmt.Sprintf("required plugins not matched to modules: %v", err))
		}
		for _, m := range candidates {
			m = r.modDirs[m.Path]
			if m.Dir == "" {
				continue
			}
			for _, d := range pluginDirs(m.Dir) {
				if id, _ := ReadIdentity(r.ctx, d); id.ID != "" {
					if _, seen := provider[id.ID]; !seen {
						provider[id.ID] = m.Path
					}
				}
			}
		}
	}
	out := Requires{}
	for _, id := range ids {
		if own[id] {
			out[id] = ""
			continue
		}
		modPath, ok := provider[id]
		if !ok {
			out[id] = ""
			continue
		}
		v, _ := r.mod.version(modPath)
		switch {
		case v == "v0.0.0":
			return nil, warnings, fmt.Errorf("kiosk: %s requires %s at v0.0.0 (module of %s), which is not a released version; require a tag", filepath.Join(r.root, "go.mod"), modPath, id)
		case isPseudoVersion(v):
			warnings = append(warnings, fmt.Sprintf("%s minimum %s is a pseudo-version; require a tag of %s before publishing", id, v, modPath))
		}
		out[id] = v
	}
	return out, warnings, nil
}

// requiresFramework reports whether the module's go.mod names the
// framework module.
func requiresFramework(m listedModule) bool {
	path := m.GoMod
	if path == "" && m.Dir != "" {
		path = filepath.Join(m.Dir, "go.mod")
	}
	if path == "" {
		return false
	}
	data, err := os.ReadFile(path)
	return err == nil && bytes.Contains(data, []byte(FrameworkModule))
}

// pluginDirs lists the directories under root that hold a plugin.go,
// skipping hidden, testdata, vendor and node_modules directories and
// nested modules.
func pluginDirs(root string) []string {
	var dirs []string
	_ = filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
		if err != nil {
			return nil
		}
		if d.IsDir() {
			if path == root {
				return nil
			}
			n := d.Name()
			if strings.HasPrefix(n, ".") || strings.HasPrefix(n, "_") || n == "testdata" || n == "vendor" || n == "node_modules" {
				return filepath.SkipDir
			}
			if _, err := os.Stat(filepath.Join(path, "go.mod")); err == nil {
				return filepath.SkipDir
			}
			return nil
		}
		if d.Name() == "plugin.go" {
			dirs = append(dirs, filepath.Dir(path))
		}
		return nil
	})
	return dirs
}