--[[
    wan-watcher.lua - Monitor WAN link state and DHCP changes
    Copyright (c) 2026 Luis Rosales - MIT License
    
    Use case: Detect when router is moved from one modem to another
    - Physical link flap (cable unplugged/plugged)
    - New DHCP lease (connected to new ISP)
    
    Runs as daemon, reports events to central server
]]

local socket = require("socket")
local json = require("json")

-- ============================================================================
-- CONFIG
-- ============================================================================

local cfg = {
    wan_interface = "wan",        -- UCI interface name
    physical_device = "eth0",    -- Physical interface (e.g., eth0, wan)
    check_interval = 5,          -- Seconds between checks
    
    -- Server (optional - could just buffer like client2server)
    server_url = os.getenv("SERVER_URL") or "",
    server_token = os.getenv("SERVER_TOKEN") or "",
    
    -- Paths
    status_file = "/var/run/wan-watcher.status",
    pid_file = "/var/run/wan-watcher.pid",
}

-- Load config from UCI
pcall(function()
    local uci = require("luci.model.uci").cursor()
    cfg.wan_interface = uci:get("wan-watcher", "general", "interface") or cfg.wan_interface
    cfg.physical_device = uci:get("wan-watcher", "general", "device") or cfg.physical_device
    cfg.server_url = uci:get("wan-watcher", "server", "url") or cfg.server_url
    cfg.server_token = uci:get("wan-watcher", "server", "token") or cfg.server_token
end)

-- ============================================================================
-- UTILITIES
-- ============================================================================

local function log(msg)
    os.execute(string.format('logger -t "wan-watcher" -p user.info "%s"', msg:gsub('"', '\\"')))
end

local function log_err(msg)
    os.execute(string.format('logger -t "wan-watcher" -p user.err "%s"', msg:gsub('"', '\\"')))
end

local function get_hostname()
    local f = io.popen("hostname")
    local h = f and f:read("*a"):gsub("%s+$", "") or "unknown"
    if f then f:close() end
    return h
end

-- ============================================================================
-- WAN STATUS CHECKS
-- ============================================================================

-- Check physical link state
function check_link(device)
    local f = io.popen("cat /sys/class/net/" .. device .. "/operstate 2>/dev/null")
    if not f then return nil end
    
    local state = f:read("*a"):gsub("%s+$", "")
    f:close()
    
    return state  -- "up", "down", "unknown", "dormant", etc.
end

-- Check if interface has carrier
function has_carrier(device)
    local f = io.popen("cat /sys/class/net/" .. device .. "/carrier 2>/dev/null")
    if not f then return false end
    
    local carrier = f:read("*a"):gsub("%s+$", "")
    f:close()
    
    return carrier == "1"
end

-- Get current DHCP lease info
function get_dhcp_info(interface)
    local uci = require("luci.model.uci").cursor()
    local cursor = uci.cursor()
    
    -- Get interface data from network config
    local proto = cursor:get("network", interface, "proto")
    
    -- Get IP from ubus
    local f = io.popen("ubus call network.interface." .. interface .. " status 2>/dev/null")
    if not f then return nil end
    
    local status = f:read("*a")
    f:close()
    
    if not status then return nil end
    
    -- Parse JSON manually (without json library)
    local ip = status:match('"address"%s*:%s*"([^"]+)"')
    local prefix = status:match('"prefix"%s*:%s*(%d+)')
    
    return {
        ip = ip,
        prefix = tonumber(prefix),
        proto = proto,
    }
end

-- ============================================================================
-- REPORTING
-- ============================================================================

function report_event(event_type, data)
    local event = {
        router_id = get_hostname(),
        event_type = event_type,
        timestamp = os.date("!%Y-%m-%dT%H:%M:%SZ"),
        payload = data,
    }
    
    local json_str = json.encode(event)
    
    log(event_type .. ": " .. (data.message or ""))
    
    -- Send to server if configured
    if cfg.server_url ~= "" and cfg.server_token ~= "" then
        local cmd = string.format(
            'curl -s -X POST "%s" -H "Authorization: Bearer %s" -H "Content-Type: application/json" -d "%s" 2>/dev/null',
            cfg.server_url,
            cfg.server_token,
            json_str:gsub('"', '\\"')
        )
        os.execute(cmd .. " &")
    end
    
    -- Also could buffer for offline (reuse client2server logic)
    return event
end

-- ============================================================================
-- SAVE STATE
-- ============================================================================

local function save_state(state)
    local f = io.open(cfg.status_file, "w")
    if f then
        f:write(state .. "\n")
        f:close()
    end
end

local function load_state()
    local f = io.open(cfg.status_file, "r")
    if not f then return nil end
    
    local state = f:read("*a"):gsub("%s+$", "")
    f:close()
    return state
end

-- ============================================================================
-- MAIN LOOP
-- ============================================================================

local function main()
    log("Starting WAN watcher...")
    log("Interface: " .. cfg.wan_interface .. " (" .. cfg.physical_device .. ")")
    
    -- Save PID
    local pf = io.open(cfg.pid_file, "w")
    if pf then
        pf:write(tostring(os.getpid()))
        pf:close()
    end
    
    -- Initial states
    local last_link_state = nil
    local last_ip = nil
    
    -- Check initial link state
    last_link_state = has_carrier(cfg.physical_device)
    local info = get_dhcp_info(cfg.wan_interface)
    if info then last_ip = info.ip end
    
    save_state(last_link_state and "up" or "down")
    
    log("Initial state: link=" .. tostring(last_link_state) .. ", ip=" .. tostring(last_ip))
    
    -- Main monitoring loop
    while true do
        socket.sleep(cfg.check_interval)
        
        -- Check physical link
        local current_link = has_carrier(cfg.physical_device)
        local current_ip_info = get_dhcp_info(cfg.wan_interface)
        local current_ip = current_ip_info and current_ip_info.ip or nil
        
        -- Link state change
        if current_link ~= last_link_state then
            if current_link then
                -- Link came UP
                report_event("wan_link_up", {
                    device = cfg.physical_device,
                    message = "Physical link detected on " .. cfg.physical_device,
                })
            else
                -- Link went DOWN
                report_event("wan_link_down", {
                    device = cfg.physical_device,
                    message = "Physical link lost on " .. cfg.physical_device,
                })
            end
            
            last_link_state = current_link
            save_state(current_link and "up" or "down")
        end
        
        -- DHCP lease change (new IP)
        if current_ip and current_ip ~= last_ip then
            if last_ip then
                -- IP changed!
                report_event("wan_dhcp_changed", {
                    old_ip = last_ip,
                    new_ip = current_ip,
                    message = "New DHCP lease: " .. (last_ip or "none") .. " -> " .. current_ip,
                })
            else
                -- Got first IP
                report_event("wan_dhcp_renew", {
                    new_ip = current_ip,
                    message = "DHCP lease obtained: " .. current_ip,
                })
            end
            
            last_ip = current_ip
        end
        
        -- No link but had IP before - connection dropped
        if not current_link and last_ip and current_ip ~= nil then
            report_event("wan_dropped", {
                ip = last_ip,
                message = "WAN connection dropped (still held lease)",
            })
        end
    end
end

-- Run
main()