// Copyright 2019 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.

/*
The digraph command performs queries over unlabelled directed graphs
represented in text form.  It is intended to integrate nicely with
typical UNIX command pipelines.

Usage:

	your-application | digraph [command]

The support commands are:

	nodes
		the set of all nodes
	degree
		the in-degree and out-degree of each node
	preds <node> ...
		the set of immediate predecessors of the specified nodes
	succs <node> ...
		the set of immediate successors of the specified nodes
	forward <node> ...
		the set of nodes transitively reachable from the specified nodes
	reverse <node> ...
		the set of nodes that transitively reach the specified nodes
	somepath <node> <node>
		the list of nodes on some arbitrary path from the first node to the second
	allpaths <node> <node>
		the set of nodes on all paths from the first node to the second
	sccs
		all strongly connected components (one per line)
	scc <node>
		the set of nodes nodes strongly connected to the specified one

Input format:

Each line contains zero or more words. Words are separated by unquoted
whitespace; words may contain Go-style double-quoted portions, allowing spaces
and other characters to be expressed.

Each word declares a node, and if there are more than one, an edge from the
first to each subsequent one. The graph is provided on the standard input.

For instance, the following (acyclic) graph specifies a partial order among the
subtasks of getting dressed:

	$ cat clothes.txt
	socks shoes
	"boxer shorts" pants
	pants belt shoes
	shirt tie sweater
	sweater jacket
	hat

The line "shirt tie sweater" indicates the two edges shirt -> tie and
shirt -> sweater, not shirt -> tie -> sweater.

Example usage:

Using digraph with existing Go tools:

	$ go mod graph | digraph nodes # Operate on the Go module graph.
	$ go list -m all | digraph nodes # Operate on the Go package graph.

Show the transitive closure of imports of the digraph tool itself:
	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' ... | digraph forward golang.org/x/tools/cmd/digraph

Show which clothes (see above) must be donned before a jacket:
	$ digraph reverse jacket

*/
package main // import "golang.org/x/tools/cmd/digraph"

// TODO(adonovan):
// - support input files other than stdin
// - support alternative formats (AT&T GraphViz, CSV, etc),
//   a comment syntax, etc.
// - allow queries to nest, like Blaze query language.

import (
	"bufio"
	"bytes"
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"sort"
	"strconv"
	"unicode"
	"unicode/utf8"
)

func usage() {
	fmt.Fprintf(os.Stderr, `Usage: your-application | digraph [command]

The support commands are:
	nodes
		the set of all nodes
	degree
		the in-degree and out-degree of each node
	preds <node> ...
		the set of immediate predecessors of the specified nodes
	succs <node> ...
		the set of immediate successors of the specified nodes
	forward <node> ...
		the set of nodes transitively reachable from the specified nodes
	reverse <node> ...
		the set of nodes that transitively reach the specified nodes
	somepath <node> <node>
		the list of nodes on some arbitrary path from the first node to the second
	allpaths <node> <node>
		the set of nodes on all paths from the first node to the second
	sccs
		all strongly connected components (one per line)
	scc <node>
		the set of nodes nodes strongly connected to the specified one
`)
	os.Exit(2)
}

func main() {
	flag.Usage = usage
	flag.Parse()

	args := flag.Args()
	if len(args) == 0 {
		usage()
	}

	if err := digraph(args[0], args[1:]); err != nil {
		fmt.Fprintf(os.Stderr, "digraph: %s\n", err)
		os.Exit(1)
	}
}

type nodelist []string

func (l nodelist) println(sep string) {
	for i, label := range l {
		if i > 0 {
			fmt.Fprint(stdout, sep)
		}
		fmt.Fprint(stdout, label)
	}
	fmt.Fprintln(stdout)
}

type nodeset map[string]bool

func (s nodeset) sort() nodelist {
	labels := make(nodelist, len(s))
	var i int
	for label := range s {
		labels[i] = label
		i++
	}
	sort.Strings(labels)
	return labels
}

func (s nodeset) addAll(x nodeset) {
	for label := range x {
		s[label] = true
	}
}

// A graph maps nodes to the non-nil set of their immediate successors.
type graph map[string]nodeset

func (g graph) addNode(label string) nodeset {
	edges := g[label]
	if edges == nil {
		edges = make(nodeset)
		g[label] = edges
	}
	return edges
}

func (g graph) addEdges(from string, to ...string) {
	edges := g.addNode(from)
	for _, to := range to {
		g.addNode(to)
		edges[to] = true
	}
}

func (g graph) reachableFrom(roots nodeset) nodeset {
	seen := make(nodeset)
	var visit func(label string)
	visit = func(label string) {
		if !seen[label] {
			seen[label] = true
			for e := range g[label] {
				visit(e)
			}
		}
	}
	for root := range roots {
		visit(root)
	}
	return seen
}

func (g graph) transpose() graph {
	rev := make(graph)
	for label, edges := range g {
		rev.addNode(label)
		for succ := range edges {
			rev.addEdges(succ, label)
		}
	}
	return rev
}

func (g graph) sccs() []nodeset {
	// Kosaraju's algorithm---Tarjan is overkill here.

	// Forward pass.
	S := make(nodelist, 0, len(g)) // postorder stack
	seen := make(nodeset)
	var visit func(label string)
	visit = func(label string) {
		if !seen[label] {
			seen[label] = true
			for e := range g[label] {
				visit(e)
			}
			S = append(S, label)
		}
	}
	for label := range g {
		visit(label)
	}

	// Reverse pass.
	rev := g.transpose()
	var scc nodeset
	seen = make(nodeset)
	var rvisit func(label string)
	rvisit = func(label string) {
		if !seen[label] {
			seen[label] = true
			scc[label] = true
			for e := range rev[label] {
				rvisit(e)
			}
		}
	}
	var sccs []nodeset
	for len(S) > 0 {
		top := S[len(S)-1]
		S = S[:len(S)-1] // pop
		if !seen[top] {
			scc = make(nodeset)
			rvisit(top)
			sccs = append(sccs, scc)
		}
	}
	return sccs
}

func (g graph) allpaths(from, to string) error {
	// Mark all nodes to "to".
	seen := make(nodeset) // value of seen[x] indicates whether x is on some path to "to"
	var visit func(node string) bool
	visit = func(node string) bool {
		reachesTo, ok := seen[node]
		if !ok {
			reachesTo = node == to
			seen[node] = reachesTo
			for e := range g[node] {
				if visit(e) {
					reachesTo = true
				}
			}
			if reachesTo && node != to {
				seen[node] = true
			}
		}
		return reachesTo
	}
	visit(from)

	// For each marked node, collect its marked successors.
	var edges []string
	for n := range seen {
		for succ := range g[n] {
			if seen[succ] {
				edges = append(edges, n+" "+succ)
			}
		}
	}

	// Sort (so that this method is deterministic) and print edges.
	sort.Strings(edges)
	for _, e := range edges {
		fmt.Fprintln(stdout, e)
	}

	return nil
}

func (g graph) somepath(from, to string) error {
	type edge struct{ from, to string }
	seen := make(nodeset)
	var dfs func(path []edge, from string) bool
	dfs = func(path []edge, from string) bool {
		if !seen[from] {
			seen[from] = true
			if from == to {
				// fmt.Println(path, len(path), cap(path))
				// Print and unwind.
				for _, e := range path {
					fmt.Fprintln(stdout, e.from+" "+e.to)
				}
				return true
			}
			for e := range g[from] {
				if dfs(append(path, edge{from: from, to: e}), e) {
					return true
				}
			}
		}
		return false
	}
	maxEdgesInGraph := len(g) * (len(g) - 1)
	if !dfs(make([]edge, 0, maxEdgesInGraph), from) {
		return fmt.Errorf("no path from %q to %q", from, to)
	}
	return nil
}

func parse(rd io.Reader) (graph, error) {
	g := make(graph)

	var linenum int
	in := bufio.NewScanner(rd)
	for in.Scan() {
		linenum++
		// Split into words, honoring double-quotes per Go spec.
		words, err := split(in.Text())
		if err != nil {
			return nil, fmt.Errorf("at line %d: %v", linenum, err)
		}
		if len(words) > 0 {
			g.addEdges(words[0], words[1:]...)
		}
	}
	if err := in.Err(); err != nil {
		return nil, err
	}
	return g, nil
}

// Overridable for testing purposes.
var stdin io.Reader = os.Stdin
var stdout io.Writer = os.Stdout

func digraph(cmd string, args []string) error {
	// Parse the input graph.
	g, err := parse(stdin)
	if err != nil {
		return err
	}

	// Parse the command line.
	switch cmd {
	case "nodes":
		if len(args) != 0 {
			return fmt.Errorf("usage: digraph nodes")
		}
		nodes := make(nodeset)
		for label := range g {
			nodes[label] = true
		}
		nodes.sort().println("\n")

	case "degree":
		if len(args) != 0 {
			return fmt.Errorf("usage: digraph degree")
		}
		nodes := make(nodeset)
		for label := range g {
			nodes[label] = true
		}
		rev := g.transpose()
		for _, label := range nodes.sort() {
			fmt.Fprintf(stdout, "%d\t%d\t%s\n", len(rev[label]), len(g[label]), label)
		}

	case "succs", "preds":
		if len(args) == 0 {
			return fmt.Errorf("usage: digraph %s <label> ...", cmd)
		}
		g := g
		if cmd == "preds" {
			g = g.transpose()
		}
		result := make(nodeset)
		for _, root := range args {
			edges := g[root]
			if edges == nil {
				return fmt.Errorf("no such node %q", root)
			}
			result.addAll(edges)
		}
		result.sort().println("\n")

	case "forward", "reverse":
		if len(args) == 0 {
			return fmt.Errorf("usage: digraph %s <label> ...", cmd)
		}
		roots := make(nodeset)
		for _, root := range args {
			if g[root] == nil {
				return fmt.Errorf("no such node %q", root)
			}
			roots[root] = true
		}
		g := g
		if cmd == "reverse" {
			g = g.transpose()
		}
		g.reachableFrom(roots).sort().println("\n")

	case "somepath":
		if len(args) != 2 {
			return fmt.Errorf("usage: digraph somepath <from> <to>")
		}
		from, to := args[0], args[1]
		if g[from] == nil {
			return fmt.Errorf("no such 'from' node %q", from)
		}
		if g[to] == nil {
			return fmt.Errorf("no such 'to' node %q", to)
		}
		if err := g.somepath(from, to); err != nil {
			return err
		}

	case "allpaths":
		if len(args) != 2 {
			return fmt.Errorf("usage: digraph allpaths <from> <to>")
		}
		from, to := args[0], args[1]
		if g[from] == nil {
			return fmt.Errorf("no such 'from' node %q", from)
		}
		if g[to] == nil {
			return fmt.Errorf("no such 'to' node %q", to)
		}
		if err := g.allpaths(from, to); err != nil {
			return err
		}

	case "sccs":
		if len(args) != 0 {
			return fmt.Errorf("usage: digraph sccs")
		}
		for _, scc := range g.sccs() {
			scc.sort().println(" ")
		}

	case "scc":
		if len(args) != 1 {
			return fmt.Errorf("usage: digraph scc <label>")
		}
		label := args[0]
		if g[label] == nil {
			return fmt.Errorf("no such node %q", label)
		}
		for _, scc := range g.sccs() {
			if scc[label] {
				scc.sort().println("\n")
				break
			}
		}

	default:
		return fmt.Errorf("no such command %q", cmd)
	}

	return nil
}

// -- Utilities --------------------------------------------------------

// split splits a line into words, which are generally separated by
// spaces, but Go-style double-quoted string literals are also supported.
// (This approximates the behaviour of the Bourne shell.)
//
//   `one "two three"` -> ["one" "two three"]
//   `a"\n"b` -> ["a\nb"]
//
func split(line string) ([]string, error) {
	var (
		words   []string
		inWord  bool
		current bytes.Buffer
	)

	for len(line) > 0 {
		r, size := utf8.DecodeRuneInString(line)
		if unicode.IsSpace(r) {
			if inWord {
				words = append(words, current.String())
				current.Reset()
				inWord = false
			}
		} else if r == '"' {
			var ok bool
			size, ok = quotedLength(line)
			if !ok {
				return nil, errors.New("invalid quotation")
			}
			s, err := strconv.Unquote(line[:size])
			if err != nil {
				return nil, err
			}
			current.WriteString(s)
			inWord = true
		} else {
			current.WriteRune(r)
			inWord = true
		}
		line = line[size:]
	}
	if inWord {
		words = append(words, current.String())
	}
	return words, nil
}

// quotedLength returns the length in bytes of the prefix of input that
// contain a possibly-valid double-quoted Go string literal.
//
// On success, n is at least two (""); input[:n] may be passed to
// strconv.Unquote to interpret its value, and input[n:] contains the
// rest of the input.
//
// On failure, quotedLength returns false, and the entire input can be
// passed to strconv.Unquote if an informative error message is desired.
//
// quotedLength does not and need not detect all errors, such as
// invalid hex or octal escape sequences, since it assumes
// strconv.Unquote will be applied to the prefix.  It guarantees only
// that if there is a prefix of input containing a valid string literal,
// its length is returned.
//
// TODO(adonovan): move this into a strconv-like utility package.
//
func quotedLength(input string) (n int, ok bool) {
	var offset int

	// next returns the rune at offset, or -1 on EOF.
	// offset advances to just after that rune.
	next := func() rune {
		if offset < len(input) {
			r, size := utf8.DecodeRuneInString(input[offset:])
			offset += size
			return r
		}
		return -1
	}

	if next() != '"' {
		return // error: not a quotation
	}

	for {
		r := next()
		if r == '\n' || r < 0 {
			return // error: string literal not terminated
		}
		if r == '"' {
			return offset, true // success
		}
		if r == '\\' {
			var skip int
			switch next() {
			case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', '"':
				skip = 0
			case '0', '1', '2', '3', '4', '5', '6', '7':
				skip = 2
			case 'x':
				skip = 2
			case 'u':
				skip = 4
			case 'U':
				skip = 8
			default:
				return // error: invalid escape
			}

			for i := 0; i < skip; i++ {
				next()
			}
		}
	}
}