In this blog post, we'll create a powerful command-line tool using TypeScript. This tool will showcase the difference between the two files and allow us to apply patches.

Prerequisites

Before diving into the implementation details, let's ensure you are comfortable with typescript and have a basic understanding of dynamic programming(DP). Familiarity with these concepts will make our journey more enjoyable and insightful.

The purpose of using dynamic programming

Dynamic programming is our chosen approach for a reason - efficiency. When dealing with file differences, especially in the scenarios of large files we want our tool to perform optimally. Dynamic programming enables us to break complex problems into smaller subproblems and store the results, to reduce duplicative computations.

Understanding the Edit Distance Algorithm

At the heart of our tool lies the edit distance algorithm. This algorithm calculates the minimal number of operations required to transfer one sequence into another. In our project's context, these sequences represent lines of text in two files. By using dynamic programming, we efficiently compute the edit distance, allowing us to identify insertions, deletions, and replacements.

Components of our tool

1. File operations

   Our tool relies on the Node.js fs module for reading and writing files.

2. Subcommands Architecture

   To provide a clean and organized structure, we'll be implementing subcommand architecture. Each subcommand represents a distinct functionality of our tool. Currently, we support three subcommands: diff, patch, and help.

3. Diff Subcommand

   The diff subcommand compares two files and outputs the differences. Leveraging the edit distance algorithm, we generate a patch that succinctly represents the changes between the files.

4. Patch Subcommand

   The patch subcommand takes a file and a corresponding patch file as input, efficiently applying the specified changes. This functionality ensures that our tool not only identifies differences but can also bring files in sync.

5. Help Subcommand

   The help subcommand provides users with essential usage information, helping them navigate through the tool's capabilities effortlessly.

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Part 2: Tackling the Diff Subcommand

In this segment, we'll understand the inner workings of the diff subcommand — the powerhouse behind our file comparison tool. Buckle up, as we dissect each component of the codebase to understand how it efficiently calculates the differences between two files.

The core of our file diffing capability lies in the editDistance function. This dynamic programming-based algorithm computes the edit distance between two sequences, representing lines of text from our input files. Let's break down its key components:

function editDistance<T>(s1: T[], s2: T[]): [Action, number, T][] {
 // Initialization and setup
 // ...

  function calculateDistances(strip: number[]): number[] {
    // Calculating distances using dynamic programming
    return distances;
  }

  // Backtracking to generate patch
  // ...

  // Sorting and reversing for clarity
  // ...

  return patch;
}

You can see the complete code here

Initialization and setup

The function initializes variables and arrays required for the dynamic programming approach. The strip1 and strip2 arrays play a crucial role in calculating the edit distance efficiently.

Dynamic Programming Calculation

The heart of the algorithm. The nested loops iterate through the sequences, updating the strip2 array based on the edit distance calculations. The goal is to find the minimum number of operations needed to transform one sequence into the other.

Backtracking to Generate Patch

Once the edit distance is calculated, the function performs backtracking to generate a patch representing the differences between the two sequences. This patch is an array of tuples, each containing an action ("A" for addition, "R" for removal), the line number, and the line content.

Sorting and Reversing

To present the patch in a logical order, we sort it based on line numbers and then reverse the order. This ensures that when we apply the patch later, it follows the correct sequence of operations.

TheDiffSubcommand Class

Let's take a closer look at the DiffSubcommand class, which is in charge of managing the process of finding the differences.

class DiffSubcommand extends Subcommand {
  // ...

  run(program: string, args: string[]): number {
    // Argument validation
    // ...

    let [file_path1, file_path2, ...rest] = args;

    try {
      // Reading file contents
      const lines1 = readEntireFile(file_path1).split("\n");
      const lines2 = readEntireFile(file_path2).split("\n");

      // Calculating the patch using editDistance
      const patch = editDistance(lines1, lines2);

      // Displaying the patch
      for (const [action, n, line] of patch) {
        console.log(`${action} ${n} ${line}`);
      }

      return 0;
    } catch (error) {
      // Error handling
      console.error(`Error during execution: ${error}`);
      return 1;
    }
  }
}

Argument Validation

The run method of the DiffSubcommand class starts by validating the command-line arguments. It ensures that two files are provided for comparison.

Calculating the Patch

Using the readEntireFile function, we read the contents of the two input files (file_path1 and file_path2) and split them into arrays of lines. The real magic happens here. We call the editDistance function with the lines from both files, obtaining a patch that represents the differences between the files. Finally, we iterate through the patch and display each action along with the line number and content.

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Part 3: Grasping the Patching Process

Now, let's dig into the patch subcommand, the last part of our TypeScript tool. This subcommand does something cool—it uses a file and a patch file to make specific changes to the file. It's like magic! We'll break down the code and see how our tool makes files match the changes we want.

ThePatchSubcommand Class

The PatchSubcommand class is at the forefront of the patching process. It's responsible for reading the patch file, interpreting each line, and effectively applying changes to the target file.

class PatchSubcommand extends Subcommand {
  // ...

  run(program: string, args: string[]): number {
    console.log(`Executing ${this.name} subcommand...`);
    // Argument validation
    // ...

    let [file_path, patch_path, ...rest] = args;
    const lines = readEntireFile(file_path).split("\n");
    const patch: [Action, number, string][] = [];
    let ok = true;

    // Reading and processing the patch file
    for (const [row, line] of readEntireFile(patch_path).split("\n").entries()) {
      if (line.length === 0) {
        continue;
      }
      const m = line.match(PATCH_LINE_REGEXP);
      if (m === null) {
        console.log(`${patch_path}:${row + 1}: Invalid patch action: ${line}`);
        ok = false;
        continue;
      }
      patch.push([m[1] as Action, parseInt(m[2]), m[3]]);
    }
    if (!ok) {
      return 1;
    }

    // Applying the patch in reverse order
    for (const [action, row, line] of patch.reverse()) {
      if (action === "A") {
        lines.splice(row, 0, line);
      } else if (action === "R") {
        lines.splice(row, 1);
      } else {
        throw new Error("unreachable");
      }
    }

    // Writing the updated content back to the file
    fs.writeFileSync(file_path, lines.join("\n"));
    return 0;
  }
}

Argument Validation

As usual, the run method starts with argument validation, ensuring that the correct number of arguments is provided.

Reading File Contents

Similar to the DiffSubcommand, we read the contents of the target file using the readEntireFile function.

Reading and Processing the Patch File

The code then reads and processes the patch file, interpreting each line and populating the patch array. Any inconsistencies or errors in the patch file are flagged and handled gracefully.

Applying the Patch in Reverse Order

To ensure that the changes are applied in the correct order, the patch array is reversed. The actual patching process involves either adding lines (A action) or removing lines (R action) based on the patch content and action.

Writing the Updated Content Back

Finally, the tool writes the updated content back to the target file, completing the patching process.

Similarly, we can understand how HelpSubcommand class works.

And there you have it — a comprehensive TypeScript-based file diff and patch tool ready to streamline your file management tasks.

読んでくれてありがとう

(Yonde kurete arigatou) - Thanks for reading