write-tree command

The write-tree command is used to create a tree object from the current index file (staging area). As we said earlier, a tree object represents the directory structure of a repository at a specific point in time.
Git processes the current index and writes a tree object that can be referenced later by a commit. The output is a SHA-1 hash that uniquely identifies the tree object.

Command Implementation

Check if index file exists:

./index.php
if (!file_exists('.mygit/index')) {
  echo "Error: No index file found\n";
  return;
}

Check if the index file doesn't exist and output an error message.

Reading and validating the index file:

This part ensures that only valid file entries are processed before generating the tree object.

./index.php
$entries = [];
$index_content = file_get_contents('.mygit/index');

if($index_content === ''){
  echo "Error: Index is empty\n";
  return;
}

$lines = explode("\n", trim($index_content));
foreach ($lines as $line) {
  if (empty(trim($line)))
    continue;

  $parts = explode(" ", trim($line));
  if (count($parts) < 4)
    continue;

  $entries[] = $parts;
}

$tree_content = '';
foreach ($entries as $entry) {
  $tree_content .= implode(" ", $entry) . "\n";
}

We start by reading the index file contents and storing them in $index_contents.
Check if the index file is empty, output an error, and return if it is.
We split the index contents into lines.
We iterate over each line and validate that it is not empty and we skip this iteration if it is.
Then we split each line into parts to validate that each line has the correct data format which is <mode> <type> <hash> <path>.
Then we append the $path array to $entries.
At the end, we reconstruct the tree content again by concatenating each entry to $tree_content.

Add a tree object header:

./index.php
$header = "tree " . strlen($tree_content) . "\0";
$full_content = $header . $tree_content;

We make our header by concatenating tree with length of the tree content and null byte (we explained Git storage format earlier).
Then we concatenate the header with the tree content to get the full content that will be stored.

Compute the sha-1 hash:

./index.php
$hash = sha1($full_content);

A SHA-1 hash is computed for the full tree object which uniquely identifies the object.

Compress the tree object:

./index.php
$compressed = gzcompress($full_content);

Compress the tree content with Zlib compression.

Store the tree object:

$dir = ".mygit/objects/" . substr($hash, 0, 2);
if (!is_dir($dir)) {
  mkdir($dir, 0777, true);
}

$path = $dir . "/" . substr($hash, 2);
file_put_contents($path, $compressed);

We create our directory path $dir by concatenating '.mygit/objects/' with the first two characters in $hash.
If the path $dir didn't exist we make a new directory using mkdir().
We create our file path $path by concatenating $dir path with / and the rest of the file hash. Then we save the compressed content into our file.

Output and return the hash:

./index.php
echo $hash . PHP_EOL;
return $hash;

At the end, we output the calculated hash and return it.

Full code for this section:

./index.php
function command_write_tree()
{
  if (!file_exists('.mygit/index')) {
    echo "Error: No index file found\n";
    return;
  }

  $entries = [];
  $index_content = file_get_contents('.mygit/index');

  if($index_content === ''){
    echo "Error: Index is empty\n";
    return;
  }

  $lines = explode("\n", trim($index_content));
  foreach ($lines as $line) {
    if (empty(trim($line)))
      continue;

    $parts = explode(" ", trim($line));
    if (count($parts) < 4)
      continue;

    $entries[] = $parts;
  }

  $tree_content = '';
  foreach ($entries as $entry) {
    $tree_content .= implode(" ", $entry) . "\n";
  }

  $header = "tree " . strlen($tree_content) . "\0";
  $full_content = $header . $tree_content;

  $hash = sha1($full_content);
  $compressed = gzcompress($full_content);

  $dir = ".mygit/objects/" . substr($hash, 0, 2);
  if (!is_dir($dir)) {
    mkdir($dir, 0777, true);
  }

  $path = $dir . "/" . substr($hash, 2);
  file_put_contents($path, $compressed);
  echo $hash . PHP_EOL;
  return $hash;
}

Command Implementation​

Command Implementation