git-rm-merged-branches

#!/bin/bash
set -euo pipefail

run() {
    echo >&2 "+ $*"
    "$@"
}

# usage: prompt_yn [PROMPT]
# Prompt the user for a yes/no response.
#
# Exit codes:
#   0: user entered yes
#   2: STDIN is not a TTY
#   10: user entered no
#
prompt_yn() {
    local prompt ans
    if [ $# -ge 1 ]; then
        prompt="$1"
    else
        prompt="Continue?"
    fi

    if [ ! -t 0 ]; then
        echo >&2 "$prompt [y/n]"
        echo >&2 "prompt_yn: error: stdin is not a TTY!"
        return 2
    fi

    while true; do
        read -r -p "$prompt [y/n] " ans
        case "$ans" in
            Y|y|yes|YES|Yes)
                return
                ;;
            N|n|no|NO|No)
                return 10
                ;;
        esac
    done
}


# Determine the main/master branch and echo its name.
git-main-branch() {
    local ret
    git rev-parse --verify --quiet main >/dev/null && ret=$? || ret=$?

    case "$ret" in
        0)
            echo main
            return
            ;;
        1)
            # pass
            ;;
        *)
            # probably not a git repo / some other error
            return "$ret"
            ;;
    esac

    if git rev-parse --verify --quiet master >/dev/null; then
        echo master
    else
        echo >&2 "Neither master nor main exists"
        return 1
    fi
}


git-is-branch-cherry-equivalent() {
    local main branch
    main="$1"
    branch="$2"

    local merge_base
    merge_base="$(git merge-base "$main" "$branch")"

    # get the tree content as of the branch
    local tree
    tree="$(git rev-parse "$branch^{tree}")"

    # make a commit that's a single squashed commit of the tree, with
    # the main/master merge base as the parent
    local tmp_commit
    tmp_commit="$(git commit-tree "$tree" -p "$merge_base" -m "tmp from $0")"

    # TODO: is there any way to gc this tmp commit when we're done?

    # determine whether cherry-pick would do anything
    if [[ $(git cherry "$main" "$tmp_commit") == "-"* ]]; then
        # - prefix from git cherry means cherry-pick would be a no-op
        return 0
    fi

    return 1
}

# usage: is-branch-excluded BRANCH CURRENT_BRANCH
is-branch-excluded() {
    local branch current_branch

    branch="$1"
    current_branch="$2"

    # branches excluded from pruning
    case "$branch" in
        master|main)       return 0 ;;
        stages/*)          return 0 ;;
        env/*)             return 0 ;;
        "$current_branch") return 0 ;;
    esac

    return 1
}


main() {
    local branch current_branch
    local nothing_to_do

    current_branch="$(git branch --show-current)"
    nothing_to_do=1

    # prune merged branches
    for branch in $(git branch --format='%(refname:short)' --merged); do

        if is-branch-excluded "$branch" "$current_branch"; then
            continue
        fi

        nothing_to_do=
        if prompt_yn "Delete '$branch'?"; then
            run git branch -d "$branch"
        fi
    done

    local main
    main="$(git-main-branch)"

    # prune squashed branches
    for branch in $(git for-each-ref refs/heads/ "--format=%(refname:short)")
    do

        if is-branch-excluded "$branch" "$current_branch"; then
            continue
        fi

        if git-is-branch-cherry-equivalent "$main" "$branch"; then
            nothing_to_do=
            if prompt_yn "Delete squashed '$branch'?"; then
                run git branch -D "$branch"
            fi
        fi
    done


    if [ -n "$nothing_to_do" ]; then
        echo "Nothing to do"
    else
        echo "Done"
    fi
}

main