#!/bin/bash

# Prevent rewriting history

refname="$1"
oldrev="$2"
newrev="$3"

check_master_is_ancestor () {
    rev="$1"
    if ! git merge-base --is-ancestor "$oldrev" "$rev"; then
        echo "Rewriting history is not allowed. The latest commit in the repository ($oldrev) is not an ancestor of the commit ($rev) you are trying to push."
        exit 1
    fi
}

count_parents() {
    rev="$1"
    git rev-parse ${rev}^@ | wc -l
}

check_master_is_ancestor_all() {
    #Check that master is the ancestor of all the commits being pushed
    rev="$1"
    if test "$rev" = "$oldrev"; then
        return
    fi

    check_master_is_ancestor "$rev"

    if test $(count_parents $rev) -gt 1; then
        #Comment out this if block to allow multiple parents.
        echo "Commit ($rev) has multiple parents."
        exit 1
    fi

    for par in $(git rev-parse ${rev}^@); do
        check_master_is_ancestor_all "$par"
    done
}

# Only operate on master branch
if test "$refname" != "refs/heads/master"; then
    exit 0
fi

check_master_is_ancestor_all "$newrev"