Implement a more consistent CLI interface

By default, read from stdin, unless `-i infile` is given.
By default, write to stdout, unless `-o outfile` is given
or a binary format with `-f` is selected.
By default, it writes in SVG format, unless another format is
specified with `-f`.

Author: dmacvicar

README.md

@@ -20,18 +20,25 @@ instead I recommend you look at these forks:
 
 * [@bep] is the fork currently used by Hugo, which I expect to be more active
   over time.
-* [@dmacvicar] has improved SVG/PDF rendering, font handling, and CLI
-  experience.
+* [@dmacvicar] has improved SVG/PNG/PDF rendering.
 * [@sw46] has implemented a really wonderful hand-drawn style worth checking
   out.
 
 ## Usage
 
 ```bash
 $ go get github.com/blampe/goat
-$ goat my-cool-diagram.txt > my-cool-diagram.svg
+$ cat my-cool-diagram.txt | goat > my-cool-diagram.svg
 ```
 
+By default, the program reads from stdin, unless `-i infile` is given.
+
+By default, the program writes to stdout, unless `-o outfile` is given or a
+binary format with `-f` is selected.
+
+By default, it writes in [SVG] format, unless another format is specified with
+`-f`.
+
 ## TODO
 
 - Dashed lines signaled by `:` or `=`.
@@ -195,5 +202,6 @@ More examples are available [here](examples).
 [@bep]: https://github.com/bep/goat/
 [@dmacvicar]: https://github.com/dmacvicar/goat
 [@sw46]: https://github.com/sw46/goat/
+[SVG]: https://en.wikipedia.org/wiki/Scalable_Vector_Graphics
 [markdeep.mini.js]: http://casual-effects.com/markdeep/
 [v0.93.0]: https://github.com/gohugoio/hugo/releases/tag/v0.93.0

cmd/goat/main.go

@@ -0,0 +1,71 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/blampe/goat"
+)
+
+func main() {
+	log.SetFlags(0)
+
+	var inputFilename string
+	var outputFilename string
+	var format string
+
+	flag.StringVar(&inputFilename, "i", "", "Input filename (default stdin)")
+	flag.StringVar(&outputFilename, "o", "", "Output filename (default stdout for SVG)")
+	flag.StringVar(&format, "f", "svg", "Output format: svg (default: svg)")
+	flag.Parse()
+
+	format = strings.ToLower(format)
+	if format != "svg" {
+		log.Fatalf("unrecognized format: %s", format)
+	}
+
+	input := os.Stdin
+	if inputFilename != "" {
+		if _, err := os.Stat(inputFilename); os.IsNotExist(err) {
+			log.Fatalf("input file not found: %s", inputFilename)
+		}
+		var err error
+		input, err = os.Open(inputFilename)
+		defer input.Close()
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	output := os.Stdout
+	if outputFilename != "" {
+		var err error
+		output, err = os.Create(outputFilename)
+		defer output.Close()
+		if err != nil {
+			log.Fatal(err)
+		}
+		// warn the user if he is writing to an extension different to the
+		// file format
+		ext := filepath.Ext(outputFilename)
+		if fmt.Sprintf(".%s", format) != ext {
+			log.Printf("Warning: writing to '%s' with extension '%s' and format %s", outputFilename, ext, strings.ToUpper(format))
+		}
+	} else {
+		// check that we are not writing binary data to terminal
+		fileInfo, _ := os.Stdout.Stat()
+		isTerminal := (fileInfo.Mode() & os.ModeCharDevice) != 0
+		if isTerminal && format != "svg" {
+			log.Fatalf("refuse to write binary data to terminal: %s", format)
+		}
+	}
+
+	switch format {
+	case "svg":
+		goat.BuildAndWriteSVG(input, output)
+	}
+}