Text Diff

About Text Diff

This text diff tool compares two text blocks line-by-line using a set-based comparison algorithm to identify lines that exist only in the original (A), only in the modified (B), or in both versions. Output uses standard unified diff notation with - for deletions, + for additions, and space prefix for unchanged lines.

It is useful for code review before commits, comparing configuration files, reviewing document revisions, validating API response changes, debugging environment differences, checking log file variations, reviewing legal contract changes, and any scenario requiring visual comparison of text versions.

Diff Output Format

The diff output uses unified diff notation:

Legend:
  (space) = Unchanged line (exists in both A and B)
  -       = Line removed (exists only in A)
  +       = Line added (exists only in B)

Example Output:
  line 1 unchanged
  line 2 unchanged
- removed line from original
  line 3 unchanged
+ added line in modified version
  line 4 unchanged

Diff Algorithm Explanation

This tool uses a set-based comparison approach:

Algorithm Steps:
1. Split Text A into lines → array A
2. Split Text B into lines → array B
3. Create Set from B for O(1) lookup
4. Create Set from A for O(1) lookup
5. For each line in A:
   - If in Set B → unchanged (prefix with space)
   - If not in Set B → deletion (prefix with -)
6. For each line in B:
   - If not in Set A → addition (prefix with +)

Note: This is a simplified diff.
Advanced tools (Git, diff utility) use
Myers' algorithm for optimal LCS matching.

Common Diff Formats

Diff Example: Code Revision

Original (A):
function calculateTotal(items) {
  let total = 0;
  for (let i = 0; i < items.length; i++) {
    total += items[i].price;
  }
  return total;
}

Modified (B):
function calculateTotal(items) {
  let total = 0;
  for (let i = 0; i < items.length; i++) {
    total += items[i].price * items[i].quantity;
  }
  return total;
}

Diff Output:
  function calculateTotal(items) {
    let total = 0;
    for (let i = 0; i < items.length; i++) {
-     total += items[i].price;
+     total += items[i].price * items[i].quantity;
    }
    return total;
  }

Diff Example: Configuration Change

Original config.env:
DEBUG=false
PORT=3000
DATABASE_URL=postgres://localhost/mydb
LOG_LEVEL=info

Modified config.env:
DEBUG=true
PORT=3000
DATABASE_URL=postgres://localhost/mydb
LOG_LEVEL=debug
CACHE_TTL=3600

Diff Output:
- DEBUG=false
+ DEBUG=true
  PORT=3000
  DATABASE_URL=postgres://localhost/mydb
- LOG_LEVEL=info
+ LOG_LEVEL=debug
+ CACHE_TTL=3600

Common Use Cases

Git Diff Commands Reference

Related Git commands for diff operations:

# Working directory vs staged
git diff

# Staged vs last commit
git diff --staged

# Between two commits
git diff HEAD~1 HEAD

# Between two branches
git diff main feature-branch

# Specific file diff
git diff -- path/to/file.txt

# Ignore whitespace
git diff -w

# Word-by-word diff (better for docs)
git diff --word-diff

Limitations of Simple Diff

• No move detection: Moved lines appear as delete + add, not as moves
• No context awareness: Doesn't understand code structure or semantics
• Exact match only: "hello " vs "hello" (trailing space) counts as different
• No character-level diff: Shows entire line changed, not which characters
• Order insensitive: Same lines in different order may show as unchanged

Tips for Better Diff Results

• Format code consistently before comparing (use Prettier, eslint --fix)
• Remove trailing whitespace to avoid false positives
• Use meaningful line breaks (one statement per line)
• For config files, keep keys in consistent order
• Consider using specialized diff tools for JSON, XML, or YAML
• For large files, focus on specific sections or hunks