#!/bin/bash
# 把当前分支 merge 进目标分支；目标已在其它 worktree 时直接在那边 merge
set -euo pipefail

usage() {
    cat <<'EOF'
用法: git merge2 <branch> [git merge 参数...]

把当前分支合进 <branch>，然后留在当前分支。

若 <branch> 已在其它 worktree 里 checkout，则在那个 worktree 里 merge，
不会在当前目录 checkout（避免 "already used by worktree"）。
EOF
}

if [[ "${1:-}" == "-h" || "${1:-}" == "--help" ]]; then
    usage
    exit 0
fi

if [[ $# -lt 1 ]]; then
    usage >&2
    exit 1
fi

current="$(git symbolic-ref --short HEAD)"
target="$1"
shift

if [[ "$current" == "$target" ]]; then
    echo "已经在 $target 上" >&2
    exit 1
fi

# porcelain 含主 worktree；%(worktreepath) 只填 linked worktree，不能用
worktree_for_branch() {
    local branch="$1"
    local wt=""
    local line
    while IFS= read -r line || [[ -n "$line" ]]; do
        case "$line" in
            worktree\ *)
                wt="${line#worktree }"
                ;;
            "branch refs/heads/${branch}")
                printf '%s\n' "$wt"
                return 0
                ;;
            "")
                wt=""
                ;;
        esac
    done < <(git worktree list --porcelain)
    return 1
}

# alias / 子进程会继承当前 worktree 的 GIT_DIR，必须清掉才能 git -C 到另一个 worktree
merge_in_worktree() {
    local wt="$1"
    shift
    env -u GIT_DIR -u GIT_WORK_TREE git -C "$wt" merge "$@"
}

wt="$(worktree_for_branch "$target" || true)"
if [[ -n "$wt" ]]; then
    echo "在 worktree 中 merge $current -> $target: $wt"
    merge_in_worktree "$wt" "$current" "$@"
else
    git checkout "$target"
    git merge "$current" "$@"
    git checkout "$current"
fi