How does git compare two files while merging?

Question

How does git compare two files? Which algorithms are used to compare two files? Does it compare line by line while merging?

I can't be sure whether the comparison of the two files produce conflict or not while merging.

Answer 1

There are several merge strategies.

3-way merge algorithm recurse is used in Git by default.

For example:

master: A -> B -> C

Create new branch

master: A -> B -> C

                   \

branch:             D

Some new commits

master: A -> B -> C -> E

                   \

branch:             D -> F

Here all changes made in 1.txt 

 commit C         commit E         commit F 

----------       ----------       ----------

  line a                            line a

  line b         new line d

  line c                          new line e

                   line a           line b

                   line b         new line f

                   line c           

                 new line g         line c

What happens if we merge two branches (commit E, commit F). Does it produce a merge conflict? No, Because git does not compare a file line by line rather It compares the context of the lines.

Align the 1.txt file

commit C         commit E         commit F 

----------       ----------       ----------

                 new line d

line a-----------line a-----------line a

                                 new line e

  line b-----------line b-----------line b

                                  new line f

  line c-----------line c-----------line c

                 new line g

In the above table, changes are aligned. lines in the commit C are our references. Git compares the neighbor of the reference lines. In the example, we have 4 slots:

above the line a : commit e adds new line d

below the line a : commit f adds new line e

below the line b : commit e adds new line f

below the line c : commit g adds new line g

Here, only one of the branches (commit E, commit F) may add something new or both of them may add the same thing. Otherwise, A merge conflict will occur.

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