package build

import (
	"context"
	"errors"
	"fmt"
	"go/token"
	"io"
	"os"
	"path/filepath"
	"regexp"
)

// appNameRe is the public `summer new <name>` rule: lower-case letters and
// digits, starting with a letter. The name is also the second part of the
// starter plugin ID golem15.<name>.
var appNameRe = regexp.MustCompile(`^[a-z][a-z0-9]*$`)

// NewOptions configures NewApp.
type NewOptions struct {
	// Name is the application name; the app is written to sm-<Name>-app.
	Name string
	// Parent is the directory that receives sm-<Name>-app. Empty means the
	// current directory.
	Parent string
	// Framework is the path to the framework checkout, resolved against
	// Parent when relative.
	Framework string
}

// genFile is one file NewApp writes itself, relative to the app root.
type genFile struct {
	Rel  string
	Body []byte
	Mode os.FileMode
}

// newData feeds the new/* templates.
type newData struct {
	Name      string
	Module    string
	Framework string
	GoVersion string
	Toolchain string
	Replace   string
}

// ValidateAppName reports whether name is usable as `summer new <name>`.
func ValidateAppName(name string) error {
	if !appNameRe.MatchString(name) {
		return fmt.Errorf("build: app name %q must be lower-case letters and digits, starting with a letter", name)
	}
	if token.IsKeyword(name) {
		return fmt.Errorf("build: app name %q is a Go keyword; the starter plugin package would not compile", name)
	}
	switch name {
	case "main":
		return fmt.Errorf("build: app name %q is reserved: plugins/main would be a program, not an importable package", name)
	case "internal":
		return fmt.Errorf("build: app name %q is reserved: Go forbids importing plugins/internal from the app", name)
	}
	return nil
}

// appModulePath is the module path of a generated application.
func appModulePath(name string) string {
	return "git.golem15.com/golem15/sm-" + name + "-app"
}

// appDirName is the directory a generated application lives in.
func appDirName(name string) string {
	return "sm-" + name + "-app"
}

// NewApp writes a ready-to-run application to <Parent>/sm-<Name>-app and
// returns its path. It refuses a target that exists in any form.
func NewApp(ctx context.Context, opts NewOptions, out io.Writer) (string, error) {
	if out == nil {
		out = io.Discard
	}
	if err := ValidateAppName(opts.Name); err != nil {
		return "", err
	}
	parent := opts.Parent
	if parent == "" {
		parent = "."
	}
	parent, err := filepath.Abs(parent)
	if err != nil {
		return "", fmt.Errorf("build: resolve parent directory: %w", err)
	}
	target := filepath.Join(parent, appDirName(opts.Name))
	if _, err := os.Lstat(target); err == nil {
		return "", fmt.Errorf("build: %s already exists; choose another name or remove it", target)
	} else if !errors.Is(err, os.ErrNotExist) {
		return "", fmt.Errorf("build: stat %s: %w", target, err)
	}
	if opts.Framework == "" {
		return "", fmt.Errorf("build: framework checkout not found; pass --framework <path>")
	}
	framework := opts.Framework
	if !filepath.IsAbs(framework) {
		framework = filepath.Join(parent, framework)
	}
	framework = filepath.Clean(framework)
	if err := checkFramework(framework); err != nil {
		return "", err
	}

	files, err := planNewApp(opts, target, framework)
	if err != nil {
		return "", err
	}
	if err := os.Mkdir(target, 0o755); err != nil {
		return "", fmt.Errorf("build: create %s: %w", target, err)
	}
	if err := populateApp(ctx, opts, target, files); err != nil {
		return "", err
	}
	fmt.Fprintf(out, "created %s\n", target)
	return target, nil
}

// checkFramework requires dir/go.mod to declare the framework module.
func checkFramework(dir string) error {
	module, _, err := parseGoMod(filepath.Join(dir, "go.mod"))
	if err != nil {
		return fmt.Errorf("build: framework checkout %s: %w", dir, err)
	}
	if module != frameworkModule {
		return fmt.Errorf("build: %s declares module %q, not the framework %s", dir, module, frameworkModule)
	}
	return nil
}

// planNewApp renders every file NewApp writes from its own templates. It
// reads nothing but the framework go.mod, so the output can be compared
// byte for byte.
func planNewApp(opts NewOptions, target, framework string) ([]genFile, error) {
	if err := ValidateAppName(opts.Name); err != nil {
		return nil, err
	}
	if err := checkFramework(framework); err != nil {
		return nil, err
	}
	replace, err := relModulePath(target, framework)
	if err != nil {
		return nil, fmt.Errorf("build: framework replace: %w", err)
	}
	data := newData{
		Name:      opts.Name,
		Module:    appModulePath(opts.Name),
		Framework: frameworkModule,
		GoVersion: goVersionLine,
		Toolchain: toolchainLine,
		Replace:   replace,
	}
	plan := []struct {
		rel  string
		tmpl string
		mode os.FileMode
	}{
		{"go.mod", "new/go.mod", 0o644},
		{"config/app.yaml", "new/config/app.yaml", 0o644},
		{"config/database.yaml", "new/config/database.yaml", 0o644},
		{"config/http.yaml", "new/config/http.yaml", 0o644},
		{"config/storage.yaml", "new/config/storage.yaml", 0o644},
	}
	files := make([]genFile, 0, len(plan))
	for _, p := range plan {
		body, err := renderStub(p.tmpl, data)
		if err != nil {
			return nil, err
		}
		files = append(files, genFile{Rel: p.rel, Body: body, Mode: p.mode})
	}
	return files, nil
}

// populateApp writes the planned files, the workspace, the manifest and the
// starter plugin into target, then generates main.go and plugins.gen.go.
func populateApp(ctx context.Context, opts NewOptions, target string, files []genFile) error {
	for _, f := range files {
		path := filepath.Join(target, filepath.FromSlash(f.Rel))
		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
			return fmt.Errorf("build: mkdir %s: %w", filepath.Dir(path), err)
		}
		if err := os.WriteFile(path, f.Body, f.Mode); err != nil {
			return fmt.Errorf("build: write %s: %w", path, err)
		}
	}
	// The app's own go.work comes before the plugin step so a go.work in an
	// ancestor directory is never picked up.
	if err := writeGoWork(filepath.Join(target, "go.work")); err != nil {
		return err
	}
	manifestPath := filepath.Join(target, manifestName)
	if err := SaveManifest(manifestPath, Manifest{Module: appModulePath(opts.Name), Binary: opts.Name}); err != nil {
		return err
	}
	if _, err := makePlugin(ctx, target, "golem15."+opts.Name, pluginOptions{StarterRoute: true}); err != nil {
		return err
	}
	if err := AddPlugin(ctx, target, filepath.Join("plugins", opts.Name)); err != nil {
		return err
	}
	m, err := LoadManifest(manifestPath)
	if err != nil {
		return err
	}
	return generate(target, m)
}