hacking on trying to write git objects (https://github.com/ImJasonH/ideas/issues/77)

git.go

package main

import (
	"bytes"
	"compress/zlib"
	"crypto/sha1"
	"fmt"
	"io"
	"log"
	"os"
)

func writeCommit(files map[string]string, message, person string) (string, error) {
	//TODO: parents

	treeSHA, err := writeTree(files)
	if err != nil {
		return "", err
	}
	commit := fmt.Sprintf(
		`tree %s
author %s
committer %s

%s
`, treeSHA, person, person, message)

	size := len(commit)
	obj := fmt.Sprintf("commit %d\000%s", size, commit)
	log.Println("writing commit...")
	return writeObject(obj)
}

// TODO: subdirectries/subtrees
func writeTree(files map[string]string) (string, error) {
	var buf bytes.Buffer
	for fn, c := range files {
		log.Println("writing blob...")
		size := len(c)
		obj := fmt.Sprintf("blob %d\000%s", size, c)
		sha, err := writeObject(obj)
		if err != nil {
			return "", err
		}
		fmt.Fprintf(&buf, "100644 blob %s      %s\n", sha, fn)
	}
	c := buf.String()
	obj := fmt.Sprintf("tree %d\000%s", len(c), c)
	log.Println("writing tree...")
	return writeObject(obj)
}

func writeObject(obj string) (string, error) {
	h := sha1.New()
	io.WriteString(h, obj)
	sha := fmt.Sprintf("%x", h.Sum(nil))
	log.Println("writing object", sha, obj)

	prefix := sha[0:2]
	rest := sha[2:]
	os.MkdirAll(fmt.Sprintf(".git/objects/%s", prefix), 0777)
	// TODO:O skip if exists
	f, err := os.Create(fmt.Sprintf(".git/objects/%s/%s", prefix, rest))
	if err != nil {
		return "", err
	}
	defer f.Close()

	zw := zlib.NewWriter(f)
	defer zw.Close()
	fmt.Fprint(zw, obj)
	return sha, nil
}

func main() {
	files := map[string]string{
		"date": `Sun Jul 28 08:47:45 EDT 2019
`,
	}
	message := "the date is"
	person := "Jason Hall <jasonhall@google.com> 1564318077 -0400"
	_, err := writeCommit(files, message, person)
	if err != nil {
		log.Fatal(err)
	}
}