dotfiles/nvim/.config/nvim/plugin/split_line.lua

local SPLIT_DELIMETERS = { -- single characters only
    [','] = true,
    [';'] = true,
    ['|'] = true,
}
local SPLIT_BETWEEN = { -- single characters only
    ['('] = ')',
    ['['] = ']',
    ['{'] = '}',
    ['<'] = '>',
}
local SPLIT_IGNORE_BETWEEN = { --single characters only
    ['"'] = '"',
    ["'"] = "'",
}

local split_line = function()
    local SPLIT_WHITESPACE = '	'
    if vim.o.expandtab then
        SPLIT_WHITESPACE = ''
        for _ = 1, vim.o.tabstop do
            SPLIT_WHITESPACE = SPLIT_WHITESPACE .. ' '
        end
    end

    local line = vim.api.nvim_get_current_line()
    local _, col =  unpack(vim.api.nvim_win_get_cursor(0))
    col = col + 1 -- Doing this to make it 1-indexed

    ---@type integer?
    local first_bracket_i = nil
    for i = col, #line do
        local char = line:sub(i, i)
        if SPLIT_BETWEEN[char] ~= nil then
            first_bracket_i = i
            break
        end
    end

    if not first_bracket_i then
        print('No opening brackets found after cursor on this line.')
        return
    end

    ---@type integer[]
    local split_indexes = {} -- Populate this array
    ---@type integer?
    local last_bracket_i = nil -- And find this index

    do
        ---@type string[]
        local closing_bracket_stack = {}
        local icon_to_close_ignore = ''
        local in_ignore = false

        for i = first_bracket_i, #line do
            local char = line:sub(i,i)

            if in_ignore then
                in_ignore = not (char == icon_to_close_ignore)
                goto continue
            end

            if SPLIT_IGNORE_BETWEEN[char] ~= nil then
                icon_to_close_ignore = SPLIT_IGNORE_BETWEEN[char]
                in_ignore = true
                goto continue
            end
            -- string handling complete

            if SPLIT_BETWEEN[char] ~= nil then
                table.insert(
                    closing_bracket_stack,
                    SPLIT_BETWEEN[char]
                )
            end

            if char == closing_bracket_stack[#closing_bracket_stack] then
                table.remove(closing_bracket_stack)
            end

            if #closing_bracket_stack == 1 and SPLIT_DELIMETERS[char] then
                table.insert(split_indexes, i)
            end

            if #closing_bracket_stack == 0 then
                last_bracket_i = i
                break
            end
            ::continue::
        end
    end

    if not last_bracket_i then
        print('The first opening bracket found after the cursor was not closed on this line.')
        return
    end

    ---@type string
    local leading_whitespace = string.match(line, '^%s*')
    local first_line = line:sub(1, first_bracket_i)
    local last_line = leading_whitespace .. line:sub(last_bracket_i, #line)
    if #split_indexes == 0 then
        if (last_bracket_i - first_bracket_i == 1) then return end
        local row, _ = unpack(vim.api.nvim_win_get_cursor(0))
        line = line:sub(first_bracket_i+1, last_bracket_i-1)
        local leading_pattern = '^[%s'
        for k, _ in pairs(SPLIT_DELIMETERS) do
            leading_pattern = leading_pattern .. k
        end
        leading_pattern = leading_pattern .. ']*'
        line = line:gsub(leading_pattern, leading_whitespace .. SPLIT_WHITESPACE, 1)
        line = line:gsub('[%s]*$', '', 1)
        vim.api.nvim_buf_set_lines(0, row-1, row, false, {first_line})
        vim.api.nvim_buf_set_lines(0, row, row, false, {last_line})
        vim.api.nvim_buf_set_lines(0, row, row, false, {line})
        return
    end

    local middle_lines = {}
    table.insert(
        middle_lines,
        line:sub(first_bracket_i+1, split_indexes[1])
    )
    for i = 1, #split_indexes-1 do
        table.insert(
            middle_lines,
            line:sub(split_indexes[i], split_indexes[i+1])
        )
    end
    table.insert(
        middle_lines,
        line:sub(split_indexes[#split_indexes], last_bracket_i-1)
    )

    local leading_pattern = '^[%s'
    for k, _ in pairs(SPLIT_DELIMETERS) do
        leading_pattern = leading_pattern .. k
    end
    leading_pattern = leading_pattern .. ']*'

    -- Cleanup step
    for i, middle_line in ipairs(middle_lines) do
        middle_line = middle_line:gsub(leading_pattern, leading_whitespace .. SPLIT_WHITESPACE, 1)
        middle_line = middle_line:gsub('[%s]*$', '', 1)
        middle_lines[i] = middle_line
    end

    if middle_lines[#middle_lines]:match('^%s*$') ~= nil then
        table.remove(middle_lines)
    end

    local row, _ = unpack(vim.api.nvim_win_get_cursor(0))
    vim.api.nvim_buf_set_lines(0, row-1, row, false, {first_line})
    vim.api.nvim_buf_set_lines(0, row, row, false, {last_line})
    vim.api.nvim_buf_set_lines(0, row, row, false, middle_lines)
end

vim.keymap.set(
    'n',
    ',s',
    split_line,
    { silent = true }
)