1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
//! Generate files suitable for use with [Graphviz](https://www.graphviz.org/)
//!
//! The `render` function generates output (e.g., an `output.dot` file) for
//! use with [Graphviz](https://www.graphviz.org/) by walking a labeled
//! graph. (Graphviz can then automatically lay out the nodes and edges
//! of the graph, and also optionally render the graph as an image or
//! other [output formats](https://www.graphviz.org/docs/outputs), such as SVG.)
//!
//! Rather than impose some particular graph data structure on clients,
//! this library exposes two traits that clients can implement on their
//! own structs before handing them over to the rendering function.
//!
//! Note: This library does not yet provide access to the full
//! expressiveness of the [DOT language](https://www.graphviz.org/doc/info/lang.html).
//! For example, there are many [attributes](https://www.graphviz.org/doc/info/attrs.html)
//! related to providing layout hints (e.g., left-to-right versus top-down, which
//! algorithm to use, etc). The current intention of this library is to
//! emit a human-readable .dot file with very regular structure suitable
//! for easy post-processing.
//!
//! # Examples
//!
//! The first example uses a very simple graph representation: a list of
//! pairs of ints, representing the edges (the node set is implicit).
//! Each node label is derived directly from the int representing the node,
//! while the edge labels are all empty strings.
//!
//! This example also illustrates how to use `Cow<[T]>` to return
//! an owned vector or a borrowed slice as appropriate: we construct the
//! node vector from scratch, but borrow the edge list (rather than
//! constructing a copy of all the edges from scratch).
//!
//! The output from this example renders five nodes, with the first four
//! forming a diamond-shaped acyclic graph and then pointing to the fifth
//! which is cyclic.
//!
//! ```rust
//! #![feature(rustc_private)]
//!
//! use std::io::Write;
//! use rustc_graphviz as dot;
//!
//! type Nd = isize;
//! type Ed = (isize,isize);
//! struct Edges(Vec<Ed>);
//!
//! pub fn render_to<W: Write>(output: &mut W) {
//!     let edges = Edges(vec![(0,1), (0,2), (1,3), (2,3), (3,4), (4,4)]);
//!     dot::render(&edges, output).unwrap()
//! }
//!
//! impl<'a> dot::Labeller<'a> for Edges {
//!     type Node = Nd;
//!     type Edge = Ed;
//!     fn graph_id(&'a self) -> dot::Id<'a> { dot::Id::new("example1").unwrap() }
//!
//!     fn node_id(&'a self, n: &Nd) -> dot::Id<'a> {
//!         dot::Id::new(format!("N{}", *n)).unwrap()
//!     }
//! }
//!
//! impl<'a> dot::GraphWalk<'a> for Edges {
//!     type Node = Nd;
//!     type Edge = Ed;
//!     fn nodes(&self) -> dot::Nodes<'a,Nd> {
//!         // (assumes that |N| \approxeq |E|)
//!         let &Edges(ref v) = self;
//!         let mut nodes = Vec::with_capacity(v.len());
//!         for &(s,t) in v {
//!             nodes.push(s); nodes.push(t);
//!         }
//!         nodes.sort();
//!         nodes.dedup();
//!         nodes.into()
//!     }
//!
//!     fn edges(&'a self) -> dot::Edges<'a,Ed> {
//!         let &Edges(ref edges) = self;
//!         (&edges[..]).into()
//!     }
//!
//!     fn source(&self, e: &Ed) -> Nd { let &(s,_) = e; s }
//!
//!     fn target(&self, e: &Ed) -> Nd { let &(_,t) = e; t }
//! }
//!
//! # pub fn main() { render_to(&mut Vec::new()) }
//! ```
//!
//! ```no_run
//! # pub fn render_to<W:std::io::Write>(output: &mut W) { unimplemented!() }
//! pub fn main() {
//!     use std::fs::File;
//!     let mut f = File::create("example1.dot").unwrap();
//!     render_to(&mut f)
//! }
//! ```
//!
//! Output from first example (in `example1.dot`):
//!
//! ```dot
//! digraph example1 {
//!     N0[label="N0"];
//!     N1[label="N1"];
//!     N2[label="N2"];
//!     N3[label="N3"];
//!     N4[label="N4"];
//!     N0 -> N1[label=""];
//!     N0 -> N2[label=""];
//!     N1 -> N3[label=""];
//!     N2 -> N3[label=""];
//!     N3 -> N4[label=""];
//!     N4 -> N4[label=""];
//! }
//! ```
//!
//! The second example illustrates using `node_label` and `edge_label` to
//! add labels to the nodes and edges in the rendered graph. The graph
//! here carries both `nodes` (the label text to use for rendering a
//! particular node), and `edges` (again a list of `(source,target)`
//! indices).
//!
//! This example also illustrates how to use a type (in this case the edge
//! type) that shares substructure with the graph: the edge type here is a
//! direct reference to the `(source,target)` pair stored in the graph's
//! internal vector (rather than passing around a copy of the pair
//! itself). Note that this implies that `fn edges(&'a self)` must
//! construct a fresh `Vec<&'a (usize,usize)>` from the `Vec<(usize,usize)>`
//! edges stored in `self`.
//!
//! Since both the set of nodes and the set of edges are always
//! constructed from scratch via iterators, we use the `collect()` method
//! from the `Iterator` trait to collect the nodes and edges into freshly
//! constructed growable `Vec` values (rather than using `Cow` as in the
//! first example above).
//!
//! The output from this example renders four nodes that make up the
//! Hasse-diagram for the subsets of the set `{x, y}`. Each edge is
//! labeled with the &sube; character (specified using the HTML character
//! entity `&sube`).
//!
//! ```rust
//! #![feature(rustc_private)]
//!
//! use std::io::Write;
//! use rustc_graphviz as dot;
//!
//! type Nd = usize;
//! type Ed<'a> = &'a (usize, usize);
//! struct Graph { nodes: Vec<&'static str>, edges: Vec<(usize,usize)> }
//!
//! pub fn render_to<W: Write>(output: &mut W) {
//!     let nodes = vec!["{x,y}","{x}","{y}","{}"];
//!     let edges = vec![(0,1), (0,2), (1,3), (2,3)];
//!     let graph = Graph { nodes: nodes, edges: edges };
//!
//!     dot::render(&graph, output).unwrap()
//! }
//!
//! impl<'a> dot::Labeller<'a> for Graph {
//!     type Node = Nd;
//!     type Edge = Ed<'a>;
//!     fn graph_id(&'a self) -> dot::Id<'a> { dot::Id::new("example2").unwrap() }
//!     fn node_id(&'a self, n: &Nd) -> dot::Id<'a> {
//!         dot::Id::new(format!("N{}", n)).unwrap()
//!     }
//!     fn node_label(&self, n: &Nd) -> dot::LabelText<'_> {
//!         dot::LabelText::LabelStr(self.nodes[*n].into())
//!     }
//!     fn edge_label(&self, _: &Ed<'_>) -> dot::LabelText<'_> {
//!         dot::LabelText::LabelStr("&sube;".into())
//!     }
//! }
//!
//! impl<'a> dot::GraphWalk<'a> for Graph {
//!     type Node = Nd;
//!     type Edge = Ed<'a>;
//!     fn nodes(&self) -> dot::Nodes<'a,Nd> { (0..self.nodes.len()).collect() }
//!     fn edges(&'a self) -> dot::Edges<'a,Ed<'a>> { self.edges.iter().collect() }
//!     fn source(&self, e: &Ed<'_>) -> Nd { let & &(s,_) = e; s }
//!     fn target(&self, e: &Ed<'_>) -> Nd { let & &(_,t) = e; t }
//! }
//!
//! # pub fn main() { render_to(&mut Vec::new()) }
//! ```
//!
//! ```no_run
//! # pub fn render_to<W:std::io::Write>(output: &mut W) { unimplemented!() }
//! pub fn main() {
//!     use std::fs::File;
//!     let mut f = File::create("example2.dot").unwrap();
//!     render_to(&mut f)
//! }
//! ```
//!
//! The third example is similar to the second, except now each node and
//! edge now carries a reference to the string label for each node as well
//! as that node's index. (This is another illustration of how to share
//! structure with the graph itself, and why one might want to do so.)
//!
//! The output from this example is the same as the second example: the
//! Hasse-diagram for the subsets of the set `{x, y}`.
//!
//! ```rust
//! #![feature(rustc_private)]
//!
//! use std::io::Write;
//! use rustc_graphviz as dot;
//!
//! type Nd<'a> = (usize, &'a str);
//! type Ed<'a> = (Nd<'a>, Nd<'a>);
//! struct Graph { nodes: Vec<&'static str>, edges: Vec<(usize,usize)> }
//!
//! pub fn render_to<W: Write>(output: &mut W) {
//!     let nodes = vec!["{x,y}","{x}","{y}","{}"];
//!     let edges = vec![(0,1), (0,2), (1,3), (2,3)];
//!     let graph = Graph { nodes: nodes, edges: edges };
//!
//!     dot::render(&graph, output).unwrap()
//! }
//!
//! impl<'a> dot::Labeller<'a> for Graph {
//!     type Node = Nd<'a>;
//!     type Edge = Ed<'a>;
//!     fn graph_id(&'a self) -> dot::Id<'a> { dot::Id::new("example3").unwrap() }
//!     fn node_id(&'a self, n: &Nd<'a>) -> dot::Id<'a> {
//!         dot::Id::new(format!("N{}", n.0)).unwrap()
//!     }
//!     fn node_label(&self, n: &Nd<'_>) -> dot::LabelText<'_> {
//!         let &(i, _) = n;
//!         dot::LabelText::LabelStr(self.nodes[i].into())
//!     }
//!     fn edge_label(&self, _: &Ed<'_>) -> dot::LabelText<'_> {
//!         dot::LabelText::LabelStr("&sube;".into())
//!     }
//! }
//!
//! impl<'a> dot::GraphWalk<'a> for Graph {
//!     type Node = Nd<'a>;
//!     type Edge = Ed<'a>;
//!     fn nodes(&'a self) -> dot::Nodes<'a,Nd<'a>> {
//!         self.nodes.iter().map(|s| &s[..]).enumerate().collect()
//!     }
//!     fn edges(&'a self) -> dot::Edges<'a,Ed<'a>> {
//!         self.edges.iter()
//!             .map(|&(i,j)|((i, &self.nodes[i][..]),
//!                           (j, &self.nodes[j][..])))
//!             .collect()
//!     }
//!     fn source(&self, e: &Ed<'a>) -> Nd<'a> { let &(s,_) = e; s }
//!     fn target(&self, e: &Ed<'a>) -> Nd<'a> { let &(_,t) = e; t }
//! }
//!
//! # pub fn main() { render_to(&mut Vec::new()) }
//! ```
//!
//! ```no_run
//! # pub fn render_to<W:std::io::Write>(output: &mut W) { unimplemented!() }
//! pub fn main() {
//!     use std::fs::File;
//!     let mut f = File::create("example3.dot").unwrap();
//!     render_to(&mut f)
//! }
//! ```
//!
//! # References
//!
//! * [Graphviz](https://www.graphviz.org/)
//!
//! * [DOT language](https://www.graphviz.org/doc/info/lang.html)

#![doc(
    html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
    test(attr(allow(unused_variables), deny(warnings)))
)]
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]

use LabelText::*;

use std::borrow::Cow;
use std::io;
use std::io::prelude::*;

/// The text for a graphviz label on a node or edge.
pub enum LabelText<'a> {
    /// This kind of label preserves the text directly as is.
    ///
    /// Occurrences of backslashes (`\`) are escaped, and thus appear
    /// as backslashes in the rendered label.
    LabelStr(Cow<'a, str>),

    /// This kind of label uses the graphviz label escString type:
    /// <https://www.graphviz.org/docs/attr-types/escString>
    ///
    /// Occurrences of backslashes (`\`) are not escaped; instead they
    /// are interpreted as initiating an escString escape sequence.
    ///
    /// Escape sequences of particular interest: in addition to `\n`
    /// to break a line (centering the line preceding the `\n`), there
    /// are also the escape sequences `\l` which left-justifies the
    /// preceding line and `\r` which right-justifies it.
    EscStr(Cow<'a, str>),

    /// This uses a graphviz [HTML string label][html]. The string is
    /// printed exactly as given, but between `<` and `>`. **No
    /// escaping is performed.**
    ///
    /// [html]: https://www.graphviz.org/doc/info/shapes.html#html
    HtmlStr(Cow<'a, str>),
}

/// The style for a node or edge.
/// See <https://www.graphviz.org/docs/attr-types/style/> for descriptions.
/// Note that some of these are not valid for edges.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub enum Style {
    None,
    Solid,
    Dashed,
    Dotted,
    Bold,
    Rounded,
    Diagonals,
    Filled,
    Striped,
    Wedged,
}

impl Style {
    pub fn as_slice(self) -> &'static str {
        match self {
            Style::None => "",
            Style::Solid => "solid",
            Style::Dashed => "dashed",
            Style::Dotted => "dotted",
            Style::Bold => "bold",
            Style::Rounded => "rounded",
            Style::Diagonals => "diagonals",
            Style::Filled => "filled",
            Style::Striped => "striped",
            Style::Wedged => "wedged",
        }
    }
}

// There is a tension in the design of the labelling API.
//
// For example, I considered making a `Labeller<T>` trait that
// provides labels for `T`, and then making the graph type `G`
// implement `Labeller<Node>` and `Labeller<Edge>`. However, this is
// not possible without functional dependencies. (One could work
// around that, but I did not explore that avenue heavily.)
//
// Another approach that I actually used for a while was to make a
// `Label<Context>` trait that is implemented by the client-specific
// Node and Edge types (as well as an implementation on Graph itself
// for the overall name for the graph). The main disadvantage of this
// second approach (compared to having the `G` type parameter
// implement a Labelling service) that I have encountered is that it
// makes it impossible to use types outside of the current crate
// directly as Nodes/Edges; you need to wrap them in newtype'd
// structs. See e.g., the `No` and `Ed` structs in the examples. (In
// practice clients using a graph in some other crate would need to
// provide some sort of adapter shim over the graph anyway to
// interface with this library).
//
// Another approach would be to make a single `Labeller<N,E>` trait
// that provides three methods (graph_label, node_label, edge_label),
// and then make `G` implement `Labeller<N,E>`. At first this did not
// appeal to me, since I had thought I would need separate methods on
// each data variant for dot-internal identifiers versus user-visible
// labels. However, the identifier/label distinction only arises for
// nodes; graphs themselves only have identifiers, and edges only have
// labels.
//
// So in the end I decided to use the third approach described above.

/// `Id` is a Graphviz `ID`.
pub struct Id<'a> {
    name: Cow<'a, str>,
}

impl<'a> Id<'a> {
    /// Creates an `Id` named `name`.
    ///
    /// The caller must ensure that the input conforms to an
    /// identifier format: it must be a non-empty string made up of
    /// alphanumeric or underscore characters, not beginning with a
    /// digit (i.e., the regular expression `[a-zA-Z_][a-zA-Z_0-9]*`).
    ///
    /// (Note: this format is a strict subset of the `ID` format
    /// defined by the DOT language. This function may change in the
    /// future to accept a broader subset, or the entirety, of DOT's
    /// `ID` format.)
    ///
    /// Passing an invalid string (containing spaces, brackets,
    /// quotes, ...) will return an empty `Err` value.
    pub fn new<Name: Into<Cow<'a, str>>>(name: Name) -> Result<Id<'a>, ()> {
        let name = name.into();
        match name.chars().next() {
            Some(c) if c.is_ascii_alphabetic() || c == '_' => {}
            _ => return Err(()),
        }
        if !name.chars().all(|c| c.is_ascii_alphanumeric() || c == '_') {
            return Err(());
        }

        Ok(Id { name })
    }

    pub fn as_slice(&'a self) -> &'a str {
        &self.name
    }
}

/// Each instance of a type that implements `Label<C>` maps to a
/// unique identifier with respect to `C`, which is used to identify
/// it in the generated .dot file. They can also provide more
/// elaborate (and non-unique) label text that is used in the graphviz
/// rendered output.

/// The graph instance is responsible for providing the DOT compatible
/// identifiers for the nodes and (optionally) rendered labels for the nodes and
/// edges, as well as an identifier for the graph itself.
pub trait Labeller<'a> {
    type Node;
    type Edge;

    /// Must return a DOT compatible identifier naming the graph.
    fn graph_id(&'a self) -> Id<'a>;

    /// Maps `n` to a unique identifier with respect to `self`. The
    /// implementor is responsible for ensuring that the returned name
    /// is a valid DOT identifier.
    fn node_id(&'a self, n: &Self::Node) -> Id<'a>;

    /// Maps `n` to one of the [graphviz `shape` names][1]. If `None`
    /// is returned, no `shape` attribute is specified.
    ///
    /// [1]: https://www.graphviz.org/doc/info/shapes.html
    fn node_shape(&'a self, _node: &Self::Node) -> Option<LabelText<'a>> {
        None
    }

    /// Maps `n` to a label that will be used in the rendered output.
    /// The label need not be unique, and may be the empty string; the
    /// default is just the output from `node_id`.
    fn node_label(&'a self, n: &Self::Node) -> LabelText<'a> {
        LabelStr(self.node_id(n).name)
    }

    /// Maps `e` to a label that will be used in the rendered output.
    /// The label need not be unique, and may be the empty string; the
    /// default is in fact the empty string.
    fn edge_label(&'a self, _e: &Self::Edge) -> LabelText<'a> {
        LabelStr("".into())
    }

    /// Maps `n` to a style that will be used in the rendered output.
    fn node_style(&'a self, _n: &Self::Node) -> Style {
        Style::None
    }

    /// Maps `e` to a style that will be used in the rendered output.
    fn edge_style(&'a self, _e: &Self::Edge) -> Style {
        Style::None
    }
}

/// Escape tags in such a way that it is suitable for inclusion in a
/// Graphviz HTML label.
pub fn escape_html(s: &str) -> String {
    s.replace('&', "&amp;")
        .replace('\"', "&quot;")
        .replace('<', "&lt;")
        .replace('>', "&gt;")
        .replace('\n', "<br align=\"left\"/>")
}

impl<'a> LabelText<'a> {
    pub fn label<S: Into<Cow<'a, str>>>(s: S) -> LabelText<'a> {
        LabelStr(s.into())
    }

    pub fn html<S: Into<Cow<'a, str>>>(s: S) -> LabelText<'a> {
        HtmlStr(s.into())
    }

    fn escape_char<F>(c: char, mut f: F)
    where
        F: FnMut(char),
    {
        match c {
            // not escaping \\, since Graphviz escString needs to
            // interpret backslashes; see EscStr above.
            '\\' => f(c),
            _ => {
                for c in c.escape_default() {
                    f(c)
                }
            }
        }
    }
    fn escape_str(s: &str) -> String {
        let mut out = String::with_capacity(s.len());
        for c in s.chars() {
            LabelText::escape_char(c, |c| out.push(c));
        }
        out
    }

    /// Renders text as string suitable for a label in a .dot file.
    /// This includes quotes or suitable delimiters.
    pub fn to_dot_string(&self) -> String {
        match *self {
            LabelStr(ref s) => format!("\"{}\"", s.escape_default()),
            EscStr(ref s) => format!("\"{}\"", LabelText::escape_str(s)),
            HtmlStr(ref s) => format!("<{s}>"),
        }
    }

    /// Decomposes content into string suitable for making EscStr that
    /// yields same content as self. The result obeys the law
    /// render(`lt`) == render(`EscStr(lt.pre_escaped_content())`) for
    /// all `lt: LabelText`.
    fn pre_escaped_content(self) -> Cow<'a, str> {
        match self {
            EscStr(s) => s,
            LabelStr(s) => {
                if s.contains('\\') {
                    s.escape_default().to_string().into()
                } else {
                    s
                }
            }
            HtmlStr(s) => s,
        }
    }

    /// Puts `suffix` on a line below this label, with a blank line separator.
    pub fn suffix_line(self, suffix: LabelText<'_>) -> LabelText<'static> {
        let mut prefix = self.pre_escaped_content().into_owned();
        let suffix = suffix.pre_escaped_content();
        prefix.push_str(r"\n\n");
        prefix.push_str(&suffix);
        EscStr(prefix.into())
    }
}

pub type Nodes<'a, N> = Cow<'a, [N]>;
pub type Edges<'a, E> = Cow<'a, [E]>;

// (The type parameters in GraphWalk should be associated items,
// when/if Rust supports such.)

/// GraphWalk is an abstraction over a directed graph = (nodes,edges)
/// made up of node handles `N` and edge handles `E`, where each `E`
/// can be mapped to its source and target nodes.
///
/// The lifetime parameter `'a` is exposed in this trait (rather than
/// introduced as a generic parameter on each method declaration) so
/// that a client impl can choose `N` and `E` that have substructure
/// that is bound by the self lifetime `'a`.
///
/// The `nodes` and `edges` method each return instantiations of
/// `Cow<[T]>` to leave implementors the freedom to create
/// entirely new vectors or to pass back slices into internally owned
/// vectors.
pub trait GraphWalk<'a> {
    type Node: Clone;
    type Edge: Clone;

    /// Returns all the nodes in this graph.
    fn nodes(&'a self) -> Nodes<'a, Self::Node>;
    /// Returns all of the edges in this graph.
    fn edges(&'a self) -> Edges<'a, Self::Edge>;
    /// The source node for `edge`.
    fn source(&'a self, edge: &Self::Edge) -> Self::Node;
    /// The target node for `edge`.
    fn target(&'a self, edge: &Self::Edge) -> Self::Node;
}

#[derive(Clone, PartialEq, Eq, Debug)]
pub enum RenderOption {
    NoEdgeLabels,
    NoNodeLabels,
    NoEdgeStyles,
    NoNodeStyles,

    Fontname(String),
    DarkTheme,
}

/// Renders directed graph `g` into the writer `w` in DOT syntax.
/// (Simple wrapper around `render_opts` that passes a default set of options.)
pub fn render<'a, N, E, G, W>(g: &'a G, w: &mut W) -> io::Result<()>
where
    N: Clone + 'a,
    E: Clone + 'a,
    G: Labeller<'a, Node = N, Edge = E> + GraphWalk<'a, Node = N, Edge = E>,
    W: Write,
{
    render_opts(g, w, &[])
}

/// Renders directed graph `g` into the writer `w` in DOT syntax.
/// (Main entry point for the library.)
pub fn render_opts<'a, N, E, G, W>(g: &'a G, w: &mut W, options: &[RenderOption]) -> io::Result<()>
where
    N: Clone + 'a,
    E: Clone + 'a,
    G: Labeller<'a, Node = N, Edge = E> + GraphWalk<'a, Node = N, Edge = E>,
    W: Write,
{
    writeln!(w, "digraph {} {{", g.graph_id().as_slice())?;

    // Global graph properties
    let mut graph_attrs = Vec::new();
    let mut content_attrs = Vec::new();
    let font;
    if let Some(fontname) = options.iter().find_map(|option| {
        if let RenderOption::Fontname(fontname) = option { Some(fontname) } else { None }
    }) {
        font = format!(r#"fontname="{fontname}""#);
        graph_attrs.push(&font[..]);
        content_attrs.push(&font[..]);
    }
    if options.contains(&RenderOption::DarkTheme) {
        graph_attrs.push(r#"bgcolor="black""#);
        graph_attrs.push(r#"fontcolor="white""#);
        content_attrs.push(r#"color="white""#);
        content_attrs.push(r#"fontcolor="white""#);
    }
    if !(graph_attrs.is_empty() && content_attrs.is_empty()) {
        writeln!(w, r#"    graph[{}];"#, graph_attrs.join(" "))?;
        let content_attrs_str = content_attrs.join(" ");
        writeln!(w, r#"    node[{content_attrs_str}];"#)?;
        writeln!(w, r#"    edge[{content_attrs_str}];"#)?;
    }

    let mut text = Vec::new();
    for n in g.nodes().iter() {
        write!(w, "    ")?;
        let id = g.node_id(n);

        let escaped = &g.node_label(n).to_dot_string();

        write!(text, "{}", id.as_slice()).unwrap();

        if !options.contains(&RenderOption::NoNodeLabels) {
            write!(text, "[label={escaped}]").unwrap();
        }

        let style = g.node_style(n);
        if !options.contains(&RenderOption::NoNodeStyles) && style != Style::None {
            write!(text, "[style=\"{}\"]", style.as_slice()).unwrap();
        }

        if let Some(s) = g.node_shape(n) {
            write!(text, "[shape={}]", &s.to_dot_string()).unwrap();
        }

        writeln!(text, ";").unwrap();
        w.write_all(&text)?;

        text.clear();
    }

    for e in g.edges().iter() {
        let escaped_label = &g.edge_label(e).to_dot_string();
        write!(w, "    ")?;
        let source = g.source(e);
        let target = g.target(e);
        let source_id = g.node_id(&source);
        let target_id = g.node_id(&target);

        write!(text, "{} -> {}", source_id.as_slice(), target_id.as_slice()).unwrap();

        if !options.contains(&RenderOption::NoEdgeLabels) {
            write!(text, "[label={escaped_label}]").unwrap();
        }

        let style = g.edge_style(e);
        if !options.contains(&RenderOption::NoEdgeStyles) && style != Style::None {
            write!(text, "[style=\"{}\"]", style.as_slice()).unwrap();
        }

        writeln!(text, ";").unwrap();
        w.write_all(&text)?;

        text.clear();
    }

    writeln!(w, "}}")
}

#[cfg(test)]
mod tests;