// Command gendiscovery writes the module discovery aggregator
// (internal/modules/imports_gen.go) from the directories under
// internal/modules (D-028). It is invoked by `go generate ./internal/modules`.
//
// Usage: gendiscovery [dir]   (dir defaults to ".")
package main

import (
	"fmt"
	"os"
	"path"
	"path/filepath"
	"strings"
)

func main() {
	dir := "."
	if len(os.Args) > 1 {
		dir = os.Args[1]
	}
	if err := run(dir); err != nil {
		fmt.Fprintln(os.Stderr, "gendiscovery:", err)
		os.Exit(1)
	}
}

func run(dir string) error {
	abs, err := filepath.Abs(dir)
	if err != nil {
		return err
	}
	root, modulePath, err := findModule(abs)
	if err != nil {
		return err
	}
	rel, err := filepath.Rel(root, abs)
	if err != nil {
		return err
	}
	importBase := path.Join(modulePath, filepath.ToSlash(rel))

	src, err := Generate(dir, importBase)
	if err != nil {
		return err
	}
	out := filepath.Join(dir, "imports_gen.go")
	if err := os.WriteFile(out, src, 0o644); err != nil {
		return err
	}
	fmt.Fprintln(os.Stderr, "gendiscovery: wrote", out)
	return nil
}

// findModule walks up from dir to the nearest go.mod and returns its directory
// and module path.
func findModule(dir string) (root, modulePath string, err error) {
	cur := dir
	for {
		candidate := filepath.Join(cur, "go.mod")
		if data, readErr := os.ReadFile(candidate); readErr == nil {
			if mp := parseModulePath(string(data)); mp != "" {
				return cur, mp, nil
			}
			return "", "", fmt.Errorf("go.mod at %s declares no module path", candidate)
		}
		parent := filepath.Dir(cur)
		if parent == cur {
			return "", "", fmt.Errorf("no go.mod found above %s", dir)
		}
		cur = parent
	}
}

// parseModulePath extracts the `module <path>` line from a go.mod.
func parseModulePath(goMod string) string {
	for _, line := range strings.Split(goMod, "\n") {
		line = strings.TrimSpace(line)
		if strings.HasPrefix(line, "module ") {
			return strings.TrimSpace(strings.TrimPrefix(line, "module "))
		}
	}
	return ""
}