computePrefix

fn computePrefix(
    buf: []u8,
    start_i: usize,
    nl_n: usize,
    serialized: Serialized,
    children: []const Children,
    node_index: Node.Index,
) usize {
    var i = start_i;
    const parent_index = serialized.parents[@backingInt(node_index)].unwrap() orelse return i;
    if (serialized.parents[@backingInt(parent_index)] == .none) return i;
    if (@backingInt(serialized.parents[@backingInt(parent_index)]) == 0 and
        serialized.storage[0].name[0] == 0)
    {
        return i;
    }
    i = computePrefix(buf, i, nl_n, serialized, children, parent_index);
    if (children[@backingInt(parent_index)].sibling == .none) {
        const prefix = "   ";
        const upper_bound_len = prefix.len + lineUpperBoundLen(nl_n);
        if (i + upper_bound_len > buf.len) return buf.len;
        buf[i..][0..prefix.len].* = prefix.*;
        i += prefix.len;
    } else {
        const upper_bound_len = TreeSymbol.line.maxByteLen() + lineUpperBoundLen(nl_n);
        if (i + upper_bound_len > buf.len) return buf.len;
        i = appendTreeSymbol(.line, buf, i);
    }
    return i;
}