--[[
    a prompt consists of a parent card, an index into the note,
    the review timestamp, the review delay,
    counts of successful and failed reviews, and an ease measurement

    timestamps are seconds from the Unix epoch
    review delay is in seconds
]]

local Prompt = {}
Prompt.__index = Prompt

function Prompt:new(prtable)
    local o = { card = {}, ind = 0, time = os.time(), delay = 30, succ = 0, fail = 0, ease = 2.5 }
    setmetatable(o, self)
    for k, v in pairs(prtable) do
        if o[k] then o[k] = v end
    end
    return o
end

function Prompt:__tostring()
    return string.format(
        "\"%s\" (\"%s\") %d -> %d+%d*%f %d/%d",
        self:text(), self:ans(), self.card.created or 0,
        self.time, math.floor(self.delay), self.ease,
        self.succ, self.fail
    )
end

--[[
    the answer-insertion point is indicated with a "%A" in the prompt text
]]
function Prompt:text()
    if self.ind == 0 then
        return "error: unpopulated ind"
    end
    if self.card.ct == "bmc" then
        local _, eoa = self.card.note:find("{[^}]+}", self.ind)
        return self.card.note:sub(1, self.ind - 1) .. "%A" .. self.card.note:sub(eoa + 1)
    else
        return "error: unrecognised card type " .. self.card.ct
    end
end

function Prompt:ans()
    if self.ind == 0 then
        return "error: unpopulated ind"
    end
    if self.card.ct == "bmc" then
        return self.card.note:match("{([^}]+)}", self.ind)
    else
        return "error: unrecognised card type " .. self.card.ct
    end
end

function Prompt:retention()
    return self.succ / (self.succ + self.fail)
end

return Prompt