module('hige', package.seeall)

local tags = { open = '{{', close = '}}' }
local r = {}

local function merge_environment(...)
    local numargs, out = select('#', ...), {}
    for i = 1, numargs do
        local t = select(i, ...)
        if type(t) == 'table' then
            for k, v in pairs(t) do
                if (type(v) == 'function') then
                    out[k] = setfenv(v, setmetatable(out, {
                        __index = getmetatable(getfenv()).__index
                    }))
                else
                    out[k] = v
                end
            end
        end
    end
    return out
end

local function escape(str)
    return str:gsub('[&"<>]', function(c)
        if c == '&' then return '&amp;'
        elseif c == '"' then return '\"'
        elseif c == '\\' then return '\\\\'
        elseif c == '<' then return '&lt;'
        elseif c == '>' then return '&gt;'
        else return c end
    end)
end

local function find(name, context)
    local value = context[name]
    if value == nil then
        return ''
    elseif type(value) == 'function' then
        return merge_environment(context, value)[name]()
    else
        return value
    end
end

local operators = {
    -- comments
    ['!'] = function(state, outer, name, context)
        return state.tag_open .. '!' .. outer .. state.tag_close
    end,
    -- the triple hige is unescaped
    ['{'] = function(state, outer, name, context)
        return find(name, context)
    end,
    -- render partial
    ['<'] = function(state, outer, name, context)
        return r.partial(state, name, context)
    end,
    -- set new delimiters
    ['='] = function(state, outer, name, context)
        -- FIXME!
        error('setting new delimiters in the template is currently broken')
        --[[
        return name:gsub('^(.-)%s+(.-)$', function(open, close)
            state.tag_open, state.tag_close = open, close
            return ''
        end)
        ]]
    end,
}

function r.partial(state, name, context)
    local target_mt   = setmetatable(context, { __index = state.lookup_env })
    local target_name = setfenv(loadstring('return ' .. name), target_mt)()
    local target_type = type(target_name)

    if target_type == 'string' then
        return r.render(state, target_name, context)
    elseif target_type == 'table' then
        local target_template = setfenv(loadstring('return '..name..'_template'), target_mt)()
        return r.render(state, target_template, merge_environment(target_name, context))
    else
        error('unknown partial type "' .. tostring(name) .. '"')
    end
end

function r.tags(state, template, context)
    local tag_path = state.tag_open..'([=!<{]?)(%s*([^#/]-)%s*)[=}]?%s*'..state.tag_close

    return template:gsub(tag_path, function(op, outer, name)
        if operators[op] ~= nil then
            return tostring(operators[op](state, outer, name, context))
        else
            return escape(tostring((function()
                if name ~= '.' then
                    return find(name, context)
                else
                    return context
                end
            end)()))
        end
    end)
end

function r.section(state, template, context)
    for section_name in template:gmatch(state.tag_open..'#%s*([^#/]-)%s*'..state.tag_close) do
        local found, value = context[section_name] ~= nil, find(section_name, context)
        local section_path = state.tag_open..'#'..section_name..state.tag_close..'%s*(.*)'..state.tag_open..'/'..section_name..state.tag_close..'%s*'

        template = template:gsub(section_path, function(inner)
            if found == false then return '' end

            if value == true then
                return r.render(state, inner, context)
            elseif type(value) == 'table' then
                local output = {}
                for _, row in pairs(value) do
                    local new_context
                    if type(row) == 'table' then
                        new_context = merge_environment(context, row)
                    else
                        new_context = row
                    end
                    table.insert(output, (r.render(state, inner, new_context)))
                end
                return table.concat(output)
            else
                return ''
            end
        end)
    end

    return template
end

function r.render(state, template, context)
    return r.tags(state, r.section(state, template, context), context)
end

function render(template, context, env)
    if template:find(tags.open) == nil then return template end

    local state = {
        lookup_env = env or _G,
        tag_open   = tags.open,
        tag_close  = tags.close,
    }

    return r.render(state, template, context or {})
end