Add support for defaulting to latest version and improve code structure

pkg/registry/fetcher.go

@@ -0,0 +1,75 @@
+// Package registry provides interfaces and implementations for fetching dependencies
+// from various package registries (npm, pypi, go, etc.)
+package registry
+
+import (
+	"context"
+	"fmt"
+	"sync"
+
+	"github.com/safedep/pmg/pkg/models"
+)
+
+// Fetcher defines the interface for registry dependency fetchers
+type Fetcher interface {
+	// GetDependencyTree returns the complete dependency tree for a package
+	GetDependencyTree(ctx context.Context, pkg models.Package) (*models.DependencyNode, error)
+
+	// GetFlattenedDependencies returns a list of all dependencies as package@version strings
+	GetFlattenedDependencies(ctx context.Context, packageName, version string) ([]string, error)
+}
+
+// BaseFetcher implements common functionality for all registry fetchers
+type BaseFetcher struct {
+	visitedMu sync.RWMutex
+	visited   map[string]bool
+	client    RegistryClient
+}
+
+// NewBaseFetcher creates a new BaseFetcher with the specified registry client
+func NewBaseFetcher(client RegistryClient) *BaseFetcher {
+	return &BaseFetcher{
+		visited: make(map[string]bool),
+		client:  client,
+	}
+}
+
+// isVisited checks if a package has already been visited
+func (bf *BaseFetcher) isVisited(key string) bool {
+	bf.visitedMu.RLock()
+	defer bf.visitedMu.RUnlock()
+	return bf.visited[key]
+}
+
+// markVisited marks a package as visited
+func (bf *BaseFetcher) markVisited(key string) {
+	bf.visitedMu.Lock()
+	defer bf.visitedMu.Unlock()
+	bf.visited[key] = true
+}
+
+// cacheKey generates a unique key for a package
+func cacheKey(pkg models.Package) string {
+	return fmt.Sprintf("%s@%s", pkg.Name, pkg.Version)
+}
+
+// flattenDependencyTree recursively converts a dependency tree to a flat list of strings
+func flattenDependencyTree(node *models.DependencyNode, result *[]string) {
+	if node == nil {
+		return
+	}
+
+	depString := fmt.Sprintf("%s@%s", node.Name, node.Version)
+	*result = append(*result, depString)
+
+	for _, dep := range node.Dependencies {
+		flattenDependencyTree(dep, result)
+	}
+}
+
+// resetVisited resets the visited packages map
+func (bf *BaseFetcher) resetVisited() {
+	bf.visitedMu.Lock()
+	defer bf.visitedMu.Unlock()
+	bf.visited = make(map[string]bool)
+}