// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package importgraph computes the forward and reverse import
// dependency graphs for all packages in a Go workspace.
package importgraph // import "golang.org/x/tools/refactor/importgraph"

import (
	"go/build"
	"sync"

	"golang.org/x/tools/go/buildutil"
)

// A Graph is an import dependency graph, either forward or reverse.
//
// The graph maps each node (a package import path) to the set of its
// successors in the graph.  For a forward graph, this is the set of
// imported packages (prerequisites); for a reverse graph, it is the set
// of importing packages (clients).
//
type Graph map[string]map[string]bool

func (g Graph) addEdge(from, to string) {
	edges := g[from]
	if edges == nil {
		edges = make(map[string]bool)
		g[from] = edges
	}
	edges[to] = true
}

// Search returns all the nodes of the graph reachable from
// any of the specified roots, by following edges forwards.
// Relationally, this is the reflexive transitive closure.
func (g Graph) Search(roots ...string) map[string]bool {
	seen := make(map[string]bool)
	var visit func(x string)
	visit = func(x string) {
		if !seen[x] {
			seen[x] = true
			for y := range g[x] {
				visit(y)
			}
		}
	}
	for _, root := range roots {
		visit(root)
	}
	return seen
}

// Build scans the specified Go workspace and builds the forward and
// reverse import dependency graphs for all its packages.
// It also returns a mapping from import paths to errors for packages
// whose loading was not entirely successful.
// A package may appear in the graph and in the errors mapping.
func Build(ctxt *build.Context) (forward, reverse Graph, errors map[string]error) {
	type importEdge struct {
		from, to string
	}
	type pathError struct {
		path string
		err  error
	}

	ch := make(chan interface{})

	var wg sync.WaitGroup
	buildutil.ForEachPackage(ctxt, func(path string, err error) {
		wg.Add(1)
		go func() {
			defer wg.Done()
			if err != nil {
				ch <- pathError{path, err}
				return
			}

			bp, err := ctxt.Import(path, "", 0)
			if err != nil {
				if _, ok := err.(*build.NoGoError); ok {
					// empty directory is not an error
				} else {
					ch <- pathError{path, err}
				}
				// Even in error cases, Import usually returns a package.
			}
			if bp != nil {
				for _, imp := range bp.Imports {
					ch <- importEdge{path, imp}
				}
				for _, imp := range bp.TestImports {
					ch <- importEdge{path, imp}
				}
				for _, imp := range bp.XTestImports {
					ch <- importEdge{path, imp}
				}
			}
		}()
	})
	go func() {
		wg.Wait()
		close(ch)
	}()

	forward = make(Graph)
	reverse = make(Graph)

	for e := range ch {
		switch e := e.(type) {
		case pathError:
			if errors == nil {
				errors = make(map[string]error)
			}
			errors[e.path] = e.err

		case importEdge:
			if e.to == "C" {
				continue // "C" is fake
			}
			forward.addEdge(e.from, e.to)
			reverse.addEdge(e.to, e.from)
		}
	}

	return forward, reverse, errors
}