#!/bin/sh
#
# Filter the history of a git repository to only include the named
# files.

set -e

if [ $# -lt 1 ]; then
  echo "Usage $0 <files to keep>"
  exit 1
fi

set -x

files_to_keep="$@"

# Build the grep pattern for ignoring files that we want to keep
keep_pattern="\($(echo $files_to_keep | sed -e 's/ /\\|/g')\)"
# Prune all other files in every commit
pruner="git ls-files | grep -v \"$keep_pattern\" | git update-index --force-remove --stdin; git ls-files > /dev/stderr"

# Find all first commits with listed files and find a subset of them that
# predates all others

roots=""
for file in $files_to_keep; do
    file_root=$(git rev-list --reverse HEAD -- $file | head -n1)
    fail=0
    for root in $roots; do
        if git merge-base --is-ancestor $root $file_root; then
            fail=1
            break
        elif ! git merge-base --is-ancestor $file_root $root; then
            new_roots="$new_roots $root"
        fi
    done
    if [ $fail -ne 1 ]; then
        roots="$new_roots $file_root"
    fi
done

# Purge all parents for those commits

set_roots="
if [ 1 -eq 0 $(for root in $roots; do echo " -o \"\$GIT_COMMIT\" = '$root' "; done) ]; then
    echo '';
else
    cat;
fi"

# Enhance git_commit_non_empty_tree to skip merges with:
# a) either two equal parents (commit that was about to land got purged as well
# as all commits on mainline);
# b) or with second parent being an ancestor to the first one (just as with a)
# but when there are some commits on mainline).
# In both cases drop second parent and let git_commit_non_empty_tree to decide
# if commit worth doing (most likely not).

skip_empty=$(cat << \EOF
if [ $# = 5 ] && git merge-base --is-ancestor $5 $3; then
    git_commit_non_empty_tree $1 -p $3
else
    git_commit_non_empty_tree "$@"
fi
EOF
)

# Filter out commits for unrelated files
echo "Pruning commits for unrelated files..."
git filter-branch --index-filter "$pruner" --parent-filter "$set_roots" --commit-filter "$skip_empty" HEAD