main.go

/*
embd is a tool to help embed various files into go source code.

It has signature
	embd [options] file

The options are

	Example: 'embd a.sql' creates a.sql.go which which has 'const a = "...contents of a.sql..."'
	Example: 'embd a.sql -n foo' creates a.sql.go with 'const foo = ...'
	Options:
	  -n string
	    	name of the variable to generate (default <filename>)

For example suppose `schema.sql` contains the schema for some
database and we want to use. We can't read the file at runtime like this:

	ioutil.ReadAll("schema.sql")

unless the server is running from the source directory, which is usually
not the case. An easier workaround is to copy and paste the contents of
`schema.sql` into the source code via declaring some variable

	const schema = "...sql code..."

That is what `embd` does for you. It also works great for embedding
any small text~ish files like `JSON`, `YAML`, `SQL`, `CSS`, `HTML`, etc.

I wrote it because I missed the ability to `require("really.anything")` from `webpack`
in the javascript ecosystem.

Note: this tool always saves the generated file in the directory where it was called.
So `embd foo/bar.txt` generates `./bar.txt.go`.
*/
package main

import (
	"flag"
	"fmt"
	"go/format"
	"go/parser"
	"go/token"
	"io/ioutil"
	"path/filepath"
	"strings"
)

func main() {
	fileName, variableName := parseArgs()
	packageName := getPackageName(".")
	writeEmbedding(fileName, variableName, packageName)
}

func parseArgs() (fileName, variableName string) {
	// step 1: read input
	flag.CommandLine.Usage = func() {
		fmt.Println(`embed a file into go source code
Example: 'embd a.sql' creates a.sql.go which which has 'const a = "...contents of a.sql..."'
Example: 'embd a.sql -n foo' creates a.sql.go with 'const foo = ...'
Options:`)
		flag.CommandLine.PrintDefaults()
	}
	flag.StringVar(&variableName, "n", "", "name of the variable to generate (default <filename>)")
	flag.Parse()
	fileName = flag.Arg(0)
	if variableName == "" {
		// TODO replace bad file name characters
		// also maybe use full file name and replace . with _
		_, s := filepath.Split(fileName)
		variableName = strings.Split(s, ".")[0]
	}
	return
}

func getPackageName(directory string) (packageName string) {
	packageMap, err := parser.ParseDir(token.NewFileSet(), directory, nil, parser.PackageClauseOnly)
	if err != nil {
		panic(err)
	}
	for packageName = range packageMap {
		break
	}
	if packageName == "" {
		panic("could not find package name")
	}
	return packageName
}

func writeEmbedding(fileName, variableName, packageName string) {
	s, err := ioutil.ReadFile(fileName)
	if err != nil {
		panic(err)
	}
	_, outputFile := filepath.Split(fileName)
	outputFile += ".go"
	out := fmt.Sprintln("package " + packageName)
	out += fmt.Sprintln("//generated by embd")
	out += fmt.Sprintf("const %s = %q", variableName, s)
	fmtout, err := format.Source([]byte(out))
	if err != nil {
		panic(err)
	}
	ioutil.WriteFile(outputFile, fmtout, 0666)
}