// Package scanner discovers Git repositories on disk and produces a status
// snapshot for each of them.
package scanner

import (
	"context"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
)

// Discoverer walks a directory tree and reports the repository roots it finds.
type Discoverer struct {
	excludePatterns []string
	maxDepth        int // 0 means unlimited
}

// DiscoverOption configures a Discoverer (functional options pattern).
type DiscoverOption func(*Discoverer)

// WithExclude adds glob patterns that are skipped during traversal. Patterns
// are matched against both the full path and the base name of each entry.
func WithExclude(patterns ...string) DiscoverOption {
	return func(d *Discoverer) { d.excludePatterns = append(d.excludePatterns, patterns...) }
}

// WithMaxDepth limits how many directory levels below the root are scanned.
// A value of 0 means unlimited. Repositories nested deeper than the limit
// are ignored.
func WithMaxDepth(depth int) DiscoverOption {
	return func(d *Discoverer) { d.maxDepth = depth }
}

// NewDiscoverer builds a Discoverer with the given options.
func NewDiscoverer(opts ...DiscoverOption) *Discoverer {
	d := &Discoverer{}
	for _, opt := range opts {
		opt(d)
	}
	return d
}

// Discover returns the absolute paths of all repository roots under root.
//
// Working trees are detected via their .git directory or .git pointer file
// (linked worktrees and submodule checkouts), bare repositories via a
// directory named *.git that carries its own HEAD/objects/refs. .git
// internals are never descended into.
//
// Walk errors (e.g. permission denied) are aggregated and returned alongside
// whatever repositories were found, so a partially-scanned result is still
// useful to the caller.
func (d *Discoverer) Discover(ctx context.Context, root string) ([]string, error) {
	info, err := os.Stat(root)
	if err != nil {
		return nil, fmt.Errorf("discover %s: %w", root, err)
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("discover %s: not a directory", root)
	}
	root, err = filepath.Abs(root)
	if err != nil {
		return nil, fmt.Errorf("discover %s: %w", root, err)
	}

	var (
		repos    []string
		walkErrs []error
	)

	walkFn := func(path string, entry fs.DirEntry, err error) error {
		if err != nil {
			walkErrs = append(walkErrs, fmt.Errorf("walk %s: %w", path, err))
			return nil // keep walking siblings
		}
		if err := ctx.Err(); err != nil {
			return err
		}

		if !entry.IsDir() {
			// A .git file marks a linked worktree or submodule checkout;
			// its parent directory is the working tree root.
			if entry.Name() == ".git" {
				repos = append(repos, filepath.Dir(path))
			}
			return nil
		}

		name := entry.Name()
		path = filepath.Clean(path)

		// The .git directory inside a working tree: report the parent and
		// never descend into git internals.
		if name == ".git" {
			repos = append(repos, filepath.Dir(path))
			return filepath.SkipDir
		}

		// A bare repository: a directory named *.git (other than the .git
		// handled above) that carries its own git metadata.
		if strings.HasSuffix(name, ".git") && isBareRepoDir(path) {
			repos = append(repos, path)
			return filepath.SkipDir
		}

		if d.excluded(path, name) {
			return filepath.SkipDir
		}

		if d.maxDepth > 0 && depthBelow(root, path) > d.maxDepth {
			return filepath.SkipDir
		}
		return nil
	}

	if err := filepath.WalkDir(root, walkFn); err != nil {
		// Distinguish cancellation from a walk-level failure.
		if errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
			return repos, err
		}
		walkErrs = append(walkErrs, err)
	}
	return repos, errors.Join(walkErrs...)
}

// isBareRepoDir reports whether path looks like a bare repository: it holds
// its own HEAD, objects, and refs entries.
func isBareRepoDir(path string) bool {
	if _, err := os.Stat(filepath.Join(path, "HEAD")); err != nil {
		return false
	}
	if info, err := os.Stat(filepath.Join(path, "objects")); err != nil || !info.IsDir() {
		return false
	}
	if info, err := os.Stat(filepath.Join(path, "refs")); err != nil || !info.IsDir() {
		return false
	}
	return true
}

// excluded reports whether path or its base name matches any pattern.
func (d *Discoverer) excluded(path, name string) bool {
	for _, pat := range d.excludePatterns {
		if ok, _ := filepath.Match(pat, name); ok {
			return true
		}
		if ok, _ := filepath.Match(pat, path); ok {
			return true
		}
	}
	return false
}

// depthBelow returns how many directory levels path sits below root.
// The root itself is depth 0.
func depthBelow(root, path string) int {
	rel, err := filepath.Rel(root, path)
	if err != nil || rel == "." {
		return 0
	}
	return strings.Count(rel, string(filepath.Separator)) + 1
}