package main

import (
	"bytes"
	"fmt"
	"go/format"
	"os"
	"path"
	"path/filepath"
	"sort"
	"strings"
)

// Generate renders the discovery aggregator for the module directory dir. It
// blank-imports every sub-directory that holds Go source, in sorted order, so
// the aggregator is deterministic (SPEC-008 AC-2) and the DIRECTORY is the
// source of truth (D-028). importBase is the Go import path of dir.
func Generate(dir, importBase string) ([]byte, error) {
	names, err := moduleDirs(dir)
	if err != nil {
		return nil, err
	}

	var buf bytes.Buffer
	buf.WriteString("// Code generated by go generate; DO NOT EDIT.\n")
	buf.WriteString("//\n")
	buf.WriteString("// Produced by `go generate ./internal/modules` (internal/modules/generate.go).\n")
	buf.WriteString("// Every sub-directory that holds Go source is blank-imported here so its\n")
	buf.WriteString("// init() self-registration runs at startup (D-028): THE DIRECTORY IS THE\n")
	buf.WriteString("// SOURCE OF TRUTH for the module set. Add a directory and re-run go generate.\n")
	buf.WriteString("package modules\n\n")
	if len(names) == 0 {
		buf.WriteString("// No module directories were found.\n")
	} else {
		buf.WriteString("import (\n")
		for _, n := range names {
			fmt.Fprintf(&buf, "\t_ %q\n", path.Join(importBase, n))
		}
		buf.WriteString(")\n")
	}

	src, err := format.Source(buf.Bytes())
	if err != nil {
		return nil, fmt.Errorf("gendiscovery: format output: %w", err)
	}
	return src, nil
}

// moduleDirs lists the module directories under dir, sorted.
func moduleDirs(dir string) ([]string, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, err
	}
	var names []string
	for _, e := range entries {
		if !e.IsDir() || skipDir(e.Name()) {
			continue
		}
		ok, err := hasGoSource(filepath.Join(dir, e.Name()))
		if err != nil {
			return nil, err
		}
		if ok {
			names = append(names, e.Name())
		}
	}
	sort.Strings(names)
	return names, nil
}

// skipDir reports whether a directory is tooling, not a module.
func skipDir(name string) bool {
	switch name {
	case "gen", "testdata":
		return true
	}
	return strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_")
}

// hasGoSource reports whether dir holds at least one non-test Go file.
func hasGoSource(dir string) (bool, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return false, err
	}
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		n := e.Name()
		if strings.HasSuffix(n, ".go") && !strings.HasSuffix(n, "_test.go") {
			return true, nil
		}
	}
	return false, nil
}