package plugin

import "fmt"

// TopologicalSort orders plugins so that dependencies come before dependents.
// Returns error on circular dependencies or missing required dependencies.
func TopologicalSort(plugins []PluginRegistration) ([]PluginRegistration, error) {
	byName := make(map[string]*PluginRegistration, len(plugins))
	for i := range plugins {
		byName[plugins[i].Name] = &plugins[i]
	}

	for _, p := range plugins {
		for _, dep := range p.Dependencies {
			if dep.Required {
				if _, ok := byName[dep.Plugin]; !ok {
					return nil, fmt.Errorf("plugin %q requires %q, but it is not available", p.Name, dep.Plugin)
				}
			}
		}
	}

	inDegree := make(map[string]int, len(plugins))
	dependents := make(map[string][]string)

	for _, p := range plugins {
		if _, ok := inDegree[p.Name]; !ok {
			inDegree[p.Name] = 0
		}
		for _, dep := range p.Dependencies {
			if _, ok := byName[dep.Plugin]; ok {
				inDegree[p.Name]++
				dependents[dep.Plugin] = append(dependents[dep.Plugin], p.Name)
			}
		}
	}

	var queue []string
	for _, p := range plugins {
		if inDegree[p.Name] == 0 {
			queue = append(queue, p.Name)
		}
	}

	var result []PluginRegistration
	for len(queue) > 0 {
		name := queue[0]
		queue = queue[1:]
		result = append(result, *byName[name])

		for _, dep := range dependents[name] {
			inDegree[dep]--
			if inDegree[dep] == 0 {
				queue = append(queue, dep)
			}
		}
	}

	if len(result) != len(plugins) {
		return nil, fmt.Errorf("circular dependency detected among plugins")
	}

	return result, nil
}