--
-- formspec_anim SSCSM helper
--
-- Copyright © 2019 by luk3yx
-- License: https://git.minetest.land/luk3yx/formspec_anim/src/branch/master/LICENSE.md
--

local open_formname, open_formspec, elapsed, next_frame

local function cleanup()
    open_formname, open_formspec, elapsed, next_frame = nil, nil, nil, nil
end

local function cmd(data)
    minetest.run_server_chatcommand('formspec_anim', '\001' .. data)
end

local function send_formspec_input(formname, fields)
    assert(not formname:find('\001', nil, true))
    cmd(formname .. '\001' .. minetest.write_json(fields))
end

minetest.register_on_formspec_input(function(formname, fields)
    if formname:sub(1, 14) ~= 'formspec_anim:' then return end
    if fields.quit then
        cleanup()
    end
    send_formspec_input(formname:sub(15), fields)
    return true
end)

-- (Re)draw the formspec. This uses a primitive pattern that works with edge
-- cases because the formspec has been interpreted by formspec_ast on the
-- server.
-- Note that duration is in seconds here, it gets converted server-side.
local p = '(image%[[^;]+;[^;]+;)\\%[\001formspec_anim\001:([^,]+)([%+%.])' ..
    '\\,([^,]+)\\,([^\001]*)\001'
local function redraw()
    next_frame = math.huge
    local formspec = open_formspec:gsub(p, function(prefix, count, one_shot,
            duration, texture)
        count, duration = tonumber(count), tonumber(duration)
        local step = math.floor(elapsed / duration)
        if one_shot == '.' then
            -- One-shot
            step = math.min(step + 1, count)
            if step < count then
                next_frame = math.min(next_frame, step * duration)
            end
        else
            -- Loop
            next_frame = math.min(next_frame, (step + 1) * duration)
            step = (step % count) + 1
        end

        -- Numbers are casted to strings automatically.
        return prefix .. texture .. step
    end)
    minetest.show_formspec('formspec_anim:' .. open_formname, formspec)
end

-- Draw the next frame when ready
minetest.register_globalstep(function(dtime)
    if not open_formname then return end
    elapsed = elapsed + dtime
    if elapsed >= next_frame then
        redraw()
    end
end)

-- Handle formspec opens
local sscsm_msg_prefix = '\001FORMSPEC_ANIM\001'
minetest.register_on_receiving_chat_message(function(message)
    if message:sub(1, #sscsm_msg_prefix) ~= sscsm_msg_prefix then
        return
    end

    message = message:sub(#sscsm_msg_prefix + 1)
    if message == '' then
        cleanup()
        return true
    end

    local s, e = message:find('\001', nil, true)
    if not s or not e then return end

    local formname = message:sub(1, s - 1)
    if formname ~= open_formname then
        elapsed = 0
        open_formname = formname
    end
    open_formspec = message:sub(e + 1)
    redraw()
    return true
end)

cmd('OK')