HTTP Caching Layer

local cache = {}

if fs.exists("/.http_cache") then
    local stat, err = pcall(fs.delete, "/.http_cache")
    if not stat then
        error("Error clearing \".http_cache\".", 2)
    end
end

local stat, err = pcall(fs.makeDir, "/.http_cache")
if not stat then
    error("Error making \".http_cache\".", 2)
end

local function makeFileSafe(url)
    local url = string.gsub(url, "http%:%/%/", "")
    url = string.gsub(url, "[%/%\\%?%%%*%:%|%\"%<%>%.% ]", "_")
    return url
end

local function basicGet(url, data, cacheTimeout)
    cacheTimeout = cacheTimeout or 300 -- cache lifetime in seconds
    local fileSafeUrl = makeFileSafe(url)
    if not data then -- Don't lookup the cache on POST requests
        -- Cache lookup
        if (cache[fileSafeUrl]) then
            if (cache[fileSafeUrl][1] == url) then
                if fs.exists(fs.combine("/.http_cache", fileSafeUrl)) and (cache[fileSafeUrl][3] >= (os.clock() - cache[fileSafeUrl][2])) then
                    local stat, ret = pcall(fs.open, fs.combine("/.http_cache", fileSafeUrl), "r")
                    if stat then
                        ret.getResponseCode = function()
                            return cache[fileSafeUrl][4]
                        end
                        return ret
                    end
                else
                    pcall(fs.delete, fs.combine("/.http_cache", fileSafeUrl))
                    cache[fileSafeUrl] = nil
                end
            end
        end
    end

    -- Network request
    local req = {}
    http.request(url, data)
    while true do
        local e, rUrl, handle = os.pullEvent()
        if (e == "http_success") and (url == rUrl) then
            req = handle
            break
        elseif (e == "http_failure") and (url == rUrl) then
            return
        end
    end

    -- Cache writeback
    local rCode = req.getResponseCode()
    local data = req.readAll()
    req.close()
    local localHandle = fs.open(fs.combine("/.http_cache", fileSafeUrl), "w")
    localHandle.write(data)
    localHandle.close()
    cache[fileSafeUrl] = {
        url,
        os.clock(),
        cacheTimeout,
        rCode,
    }
    local h = fs.open(fs.combine("/.http_cache", fileSafeUrl), "r")
    h.getResponseCode = function()
        return cache[fileSafeUrl][4]
    end
    return h
end

http.get = function(url, cacheTimeout)
    return basicGet(url, nil, cacheTimeout)
end

http.post = function(url, data, cacheTimeout)
    return basicGet(url, data, cacheTimeout)
end

http.clearCache = function(url)
    if url then
        if cache[makeFileSafe(url)] then
            cache[makeFileSafe(url)] = nil
        end
        if fs.exists(fs.combine("/.http_cache", makeFileSafe(url))) then
            local stat, err = pcall(fs.delete, fs.combine("/.http_cache", makeFileSafe(url)))
            if not stat then
                error("Error deleting cache file.", 2)
            end
        end
    else
        if fs.exists("/.http_cache") then
            local stat, err = pcall(fs.delete, "/.http_cache")
            if not stat then
                error("Error clearing \".http_cache\".", 2)
            end
        end

        local stat, err = pcall(fs.makeDir, "/.http_cache")
        if not stat then
            error("Error making \".http_cache\".", 2)
        end
        cache = {}
    end
end

http.selectiveClearCache = function() -- Only clear the entries that have timed out
    local l = {}
    for i,v in pairs(cache) do
        if not (v[3] >= (os.clock() - v[2])) then
            pcall(fs.delete, fs.combine("/.http_cache", i))
            table.insert(l, i)
        end
    end
    for i=1, #l do
        cache[l[i]] = nil
    end
end