05. Diffs

🔍 Diffs

GitObject = Commit | Branch | Index | WorkDir
def diff(a: GitObject, b: GitObject, files: list[Path]):

• A diff shows changes between two versions of one or multiple files.
• You can compare any file pair at any point in the graph:

    main      dev ◄ HEAD + 𝚒𝚠
     ▼         ▼
A ◄─ B ◄─ C ◄─ D
          ▲
          E ◄─ F
               ▲
             bugfix

• For example, we could compare:
  • 🏗️ The workdir (𝚠) with the index (𝚒).
  • ⏳ A file in the workdir (𝚠) with that same file in main.
  • 🔄 D with F, or equivalently, dev with bugfix
• 🧙 A diff has all the information needed to turn one version into the other.

🧩 Hunks

• A hunk is a section of text containing changes within a file.
  • 📜 Part of the hunk serves only as context.
• Git uses a line-by-line comparison to identify what has changed:

--- a/kiwi.py
+++ b/kiwi.py
@@ -20,5 +22,5 @@
     total = 0
     for kiwi in kiwis:
-        total += kiwi["price"]
+        total += kiwi["price"] * (1 - kiwi["discount"])
     return total
@@ -43,2 +44,6 @@
 
+
+# Todo list:
+# - Retrieve prices/discounts from supermarket websites
+# - Setup alerts for big kiwi discounts
 

• Diff header shows we are comparing two versions of kiwi.py.
• There are two hunks in the diff, with hunk headers @@ -x,n +y,m @@:
  • -/+ refer to the a/b versions of the file.
  • x/y refer to the line number of the first line of the hunk in the a/b versions.
    • In this case, total = 0 is at line 20 in a and line 22 in b.
  • n/m refer to number of lines in the hunk in each version.

🌲 Compare Versions

• Hold Ctrl and select both commits/branches.

• Right-click and select diff against each other (or use Diff panel at the bottom).

• Workdir Changes: Compares the state at the index (𝚒) with the workdir (𝚠).

• Staged Changes: Compares the HEAD with the index (𝚒).

• History View: Shows diffs between a commit and its parent.

🌲 Diff Tools

• SourceTree integrates with popular diff tools like:
  • Meld
  • P4Merge
  • Visual Studio Code

🖥️ Set VS Code as SourceTree diff tool

1. Go to Tools → Options → Diff.
2. Under External Diff / Merge, set:
   • Diff Tool: Custom
   • Merge Tool: Custom
3. In the Diff Command field, enter the path to VS Code executable:

"C:\Program Files\Microsoft VS Code\Code.exe" # or similar

4. In Arguments, enter:
   • Diff: --diff "$LOCAL" "$REMOTE"
   • Merge: -n "$MERGED"

📏 Commit Distance

• 🛤️ To more quickly understand the state of a repository, it is common to report the "distance" of a branch to another reference branch.
• ⬆️ If a branch is a successor of the reference, then the distance is how many commits ahead the branch is:
  • (⬆️1) means the branch is 1 commit ahead (is a child).
  • Example: bugfix is 3 commits ahead of main: (⬆️3).
  • Another interpretation is that we must advance 3 commits from main to reach bugfix.

    main      dev ◄ HEAD + 𝚒𝚠
     ▼         ▼
A ◄─ B ◄─ C ◄─ D
          ▲
          E ◄─ F
               ▲
             bugfix

• ⬇️ Similarly if a branch is an ancestor of the reference: main is ⬇️3 behind bugfix.
• ↕️ If the branches being compared do not share a direct history, we count steps both ways:
  • Example: bugfix is 1 commit behind, 2 commits ahead of dev: (⬇️1⬆️2).
  • Or, from dev to bugfix, we go back one commit and go forward 2 commits.