git-swap-br

#!/usr/bin/env perl

# purpose/use-case:
#
# It frequently happens that after pushing changes to a shared remote branch
# (e.g. main/master) I forget to switch back to my $dev_branch before resuming
# dev work, resulting in commits to local shared branch which should have been
# committed to my $dev_branch *atop* local shared branch.
#
# This program will automatically fix this situation.
# If the following preconditions
# 1. $current_branch has a remote-tracking peer from which it has diverged.
# 2. A non-remote-tracked $dev_branch is at the same commit as $current_branch's remote-tracking peer.
# are present, take action:
# 1. move $dev_branch to the current commit.
# 2. switch to $dev_branch
# 3. move/reset $current_branch to match its remote-tracking peer.
#
# Optional behavior: supply on the cmdline a $specified_branch to be used as $dev_branch.

use strict;
use warnings;

die "Usage: $0 [branchname]\n" if @ARGV > 1;
my $specified_branch = $ARGV[0];  # undef if no args

# Get the current branch name
my $current_branch = `git rev-parse --abbrev-ref HEAD`;
chomp $current_branch;

# Get the remote tracking branch for current branch
my $remote_tracking = `git rev-parse --abbrev-ref $current_branch\@{upstream} 2>/dev/null`;
chomp $remote_tracking;

if ($? != 0) {
    die "Current branch '$current_branch' has no upstream branch\n";
}

# Get the commit hash of remote tracking branch
my $remote_commit = `git rev-parse $remote_tracking`;
chomp $remote_commit;

# Get the current commit hash
my $current_commit = `git rev-parse HEAD`;
chomp $current_commit;

# Verify current branch is ahead of remote
if ($current_commit eq $remote_commit) {
    die "Current branch is not ahead of remote tracking branch\n";
}

my $dev_branch;

if ($specified_branch) {
    $dev_branch = $specified_branch;

    # Check if specified branch exists
    my $branch_exists = system("git show-ref --verify --quiet refs/heads/$dev_branch") == 0;

    if ($branch_exists) {
        # If branch exists, verify it's not the current branch
        if ($dev_branch eq $current_branch) {
            die "Specified branch '$dev_branch' is the current branch\n";
        }

        # Get the commit hash of specified branch
        my $specified_commit = `git rev-parse $dev_branch`;
        chomp $specified_commit;

        # Warn if specified branch isn't at remote commit
        if ($specified_commit ne $remote_commit) {
            print "\nWarning: Specified branch '$dev_branch' is not at the remote tracking commit.\n";
            print "Remote tracking commit: $remote_commit\n";
            print "Specified branch commit: $specified_commit\n";
            print "Continue anyway? (y/n): ";
            my $confirm = <STDIN>;
            chomp $confirm;
            die "Aborted\n" unless $confirm eq 'y';
        }
    } else {
        print "\nNote: Branch '$dev_branch' doesn't exist and will be created.\n";
    }
} else {
    # Get all local branches that point to the remote tracking commit
    my @matching_branches = split /\n/, `git branch --points-at $remote_commit`;
    @matching_branches = map { s/^\s*\*?\s*//r } @matching_branches;  # Clean branch names
    @matching_branches = grep { $_ ne $current_branch } @matching_branches;  # Remove current branch

    if (!@matching_branches) {
        die "No local branches found at remote tracking commit $remote_commit\n" .
            "Please specify a branch name as an argument\n";
    }

    # If multiple branches match, let user choose
    if (@matching_branches > 1) {
        print "Multiple branches found at remote commit:\n";
        for (my $i = 0; $i < @matching_branches; $i++) {
            print "$i: $matching_branches[$i]\n";
        }
        print "Enter number of branch to use (Ctrl-C to abort): ";
        my $choice = <STDIN>;
        chomp $choice;
        if ($choice !~ /^\d+$/ || $choice >= @matching_branches) {
            die "Invalid choice\n";
        }
        $dev_branch = $matching_branches[$choice];
    } else {
        $dev_branch = $matching_branches[0];
    }
}

# Show the current state
print "\nBEFORE: HEAD -> $remote_tracking (inclusive):\n";
system("git log --oneline --decorate --graph HEAD ^$remote_tracking^");

# Confirm with user
print "\nWill perform the following branch pointer movements:\n";
print "1. Point '$dev_branch' at current commit ($current_commit)\n";
print "2. Switch to '$dev_branch'\n";
print "3. Point '$current_branch' at $remote_tracking ($remote_commit)\n";
print "\nProceed? (y/n): ";
my $confirm = <STDIN>;
chomp $confirm;
die "Aborted\n" unless $confirm eq 'y';

# Perform the operations
system("git branch -f $dev_branch $current_commit") == 0
    or die "Failed to move $dev_branch pointer\n";
system("git checkout $dev_branch") == 0
    or die "Failed to switch to $dev_branch\n";
system("git branch -f $current_branch $remote_tracking") == 0
    or die "Failed to move $current_branch pointer\n";

print "\nSuccess!\n";
# print "Current branch: $dev_branch\n";
# print "$dev_branch now points to: $current_commit\n";
# print "$current_branch now points to: $remote_commit\n";

# Show the final state
print "\nAFTER: HEAD -> $remote_tracking (inclusive):\n";
system("git log --oneline --decorate --graph HEAD ^$remote_tracking^");