local function safeRequire(moduleName)
    local ok, result = pcall(require, moduleName)
    if ok and type(result) == "table" then
        return result
    end
    return {}
end

local function appendArray(target, source)
    if type(source) ~= "table" then
        return
    end
    for _, value in ipairs(source) do
        target[#target + 1] = value
    end
end

local function mergeSpec(baseSpec, extraSpec)
    local merged = {
        items = {},
        prefixes = {},
    }

    if type(baseSpec) == "table" then
        appendArray(merged.items, baseSpec.items)
        appendArray(merged.prefixes, baseSpec.prefixes)
    end

    if type(extraSpec) == "table" then
        appendArray(merged.items, extraSpec.items)
        appendArray(merged.prefixes, extraSpec.prefixes)
    end

    return merged
end

local function mergeAliasTables(baseAliases, extraAliases)
    local merged = {}
    if type(baseAliases) == "table" then
        for key, list in pairs(baseAliases) do
            merged[key] = merged[key] or {}
            appendArray(merged[key], list)
        end
    end
    if type(extraAliases) == "table" then
        for key, list in pairs(extraAliases) do
            merged[key] = merged[key] or {}
            appendArray(merged[key], list)
        end
    end
    return merged
end

local function mergeRuleTables(baseRules, extraRules)
    local merged = {}
    appendArray(merged, baseRules)
    appendArray(merged, extraRules)
    return merged
end

local function mergeByList(baseByList, extraByList)
    local merged = {}

    if type(baseByList) == "table" then
        for listName, spec in pairs(baseByList) do
            merged[listName] = mergeSpec(nil, spec)
        end
    end

    if type(extraByList) == "table" then
        for listName, spec in pairs(extraByList) do
            merged[listName] = mergeSpec(merged[listName], spec)
        end
    end

    return merged
end

local defaults = safeRequire("OFBlockRules_Default")
local user = safeRequire("OFBlockRules_User")
local sourceCatalog = safeRequire("OFSourceCatalog")

local aliasCatalog = {
    firearms = sourceCatalog.firearms or {},
    attachments = sourceCatalog.attachments or {},
    ggs_all = sourceCatalog.ggs_all or {},
}

local merged = {
    global = mergeSpec(defaults.global, user.global),
    byList = mergeByList(defaults.byList, user.byList),
    rules = mergeRuleTables(defaults.rules, user.rules),
    aliases = mergeAliasTables(defaults.aliases, user.aliases),
}

merged.aliases = mergeAliasTables(aliasCatalog, merged.aliases)

return merged