package cepis

import (
	"bytes"
	"context"
	"encoding/csv"
	"encoding/json"
	"errors"
	"io"
	"math/rand"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"
)

// Version is the version of this library, as reported in the User-Agent.
const Version = "0.1.0"

// DefaultBaseURL is where the public API lives.
const DefaultBaseURL = "https://cep.is"

// MaxBatch is the API's ceiling on CEPs per batch request. [Client.LookupMany]
// splits longer input into chunks of this size on its own.
const MaxBatch = 500

const acceptHeader = "application/json, text/csv;q=0.9, */*;q=0.1"

func isRetriable(status int) bool {
	switch status {
	case http.StatusTooManyRequests,
		http.StatusInternalServerError,
		http.StatusBadGateway,
		http.StatusServiceUnavailable,
		http.StatusGatewayTimeout:
		return true
	}
	return false
}

// Client talks to the cep.is API.
//
// Build one with [New] and reuse it: the configuration is fixed after
// construction and no method keeps state between calls, so a single Client is
// safe to share across goroutines. Reuse also lets the underlying
// [net/http.Client] pool connections, which a fresh one per call cannot do.
//
// For thousands of CEPs do not write a loop over [Client.Lookup] — use
// [Client.LookupMany], which resolves up to [MaxBatch] of them per request.
type Client struct {
	baseURL      string
	httpClient   *http.Client
	timeout      time.Duration
	maxAttempts  int
	backoffBase  time.Duration
	backoffMax   time.Duration
	totalTimeout time.Duration
	userAgent    string
}

// Option configures a [Client]. Options are applied in order by [New], and an
// invalid value is reported as an error rather than silently ignored.
type Option func(*Client) error

// WithBaseURL points the client at another instance of the API — a local one
// for development (the API repo brings one up on http://localhost:8080 with
// `just up`), or a test server. It is the only setting that usually changes
// between environments. A trailing slash is stripped, so it cannot produce the
// doubled separator that some proxies normalise and others answer 404 to.
func WithBaseURL(baseURL string) Option {
	return func(c *Client) error {
		trimmed := strings.TrimRight(strings.TrimSpace(baseURL), "/")
		if trimmed == "" {
			return newError(ErrInvalidConfig, "base URL must not be empty")
		}
		c.baseURL = trimmed
		return nil
	}
}

// WithTimeout caps a single attempt: connection, response and body read
// together. It is not a cap on the whole call, because a call may make several
// attempts with waits between them — that cap is [WithTotalTimeout].
func WithTimeout(timeout time.Duration) Option {
	return func(c *Client) error {
		if timeout <= 0 {
			return newError(ErrInvalidConfig, "timeout must be > 0")
		}
		c.timeout = timeout
		return nil
	}
}

// WithMaxAttempts sets how many attempts a call makes in total, counting the
// first. 1 disables retrying. The default of 3 covers the common case (one
// transient failure) without turning a real outage into a long wait.
func WithMaxAttempts(attempts int) Option {
	return func(c *Client) error {
		if attempts < 1 {
			return newError(ErrInvalidConfig, "max attempts must be >= 1")
		}
		c.maxAttempts = attempts
		return nil
	}
}

// WithBackoff sets the exponential wait between attempts.
//
// base is the first ceiling: attempt n waits a random duration drawn from
// [0, min(base*2^(n-1), max)] — full jitter. The randomness is not decoration.
// Without it, N clients that fail together come back together and reproduce
// the spike that took the service down.
//
// max caps each individual wait. It applies to a server-sent Retry-After too,
// or a `Retry-After: 3600` from a misconfigured proxy would park the caller
// for an hour.
func WithBackoff(base, max time.Duration) Option {
	return func(c *Client) error {
		if base < 0 || max < 0 {
			return newError(ErrInvalidConfig, "backoff must not be negative")
		}
		c.backoffBase = base
		c.backoffMax = max
		return nil
	}
}

// WithTotalTimeout caps the whole call, attempts and waits included. Zero
// leaves the attempt count as the only bound. Set it in code serving a web
// request: without it the worst case is max attempts times the per-attempt
// timeout, plus the sum of the backoffs.
//
// A deadline already carried by the context passed to a call is honoured
// regardless, and the earlier of the two wins.
func WithTotalTimeout(timeout time.Duration) Option {
	return func(c *Client) error {
		if timeout < 0 {
			return newError(ErrInvalidConfig, "total timeout must not be negative")
		}
		c.totalTimeout = timeout
		return nil
	}
}

// WithUserAgent overrides the User-Agent header. The default identifies the
// library and its version, which is what lets the server diagnose an old
// client misbehaving.
func WithUserAgent(userAgent string) Option {
	return func(c *Client) error {
		if strings.TrimSpace(userAgent) == "" {
			return newError(ErrInvalidConfig, "user agent must not be empty")
		}
		c.userAgent = userAgent
		return nil
	}
}

// WithHTTPClient supplies the [net/http.Client] used for every request — for a
// custom transport, a proxy, or instrumentation. Its own Timeout field is left
// untouched but is redundant here: this library bounds each attempt through
// the request context.
func WithHTTPClient(httpClient *http.Client) Option {
	return func(c *Client) error {
		if httpClient == nil {
			return newError(ErrInvalidConfig, "http client must not be nil")
		}
		c.httpClient = httpClient
		return nil
	}
}

// New builds a client. With no options it targets the public API with the
// defaults: 10s per attempt, 3 attempts, 0.5s to 8s of jittered backoff and no
// deadline on the call as a whole.
func New(opts ...Option) (*Client, error) {
	c := &Client{
		baseURL:     DefaultBaseURL,
		httpClient:  &http.Client{},
		timeout:     10 * time.Second,
		maxAttempts: 3,
		backoffBase: 500 * time.Millisecond,
		backoffMax:  8 * time.Second,
		userAgent:   "cepis-go/" + Version + " (+https://cep.is)",
	}
	for _, opt := range opts {
		if opt == nil {
			continue
		}
		if err := opt(c); err != nil {
			return nil, err
		}
	}
	return c, nil
}

// BaseURL reports where this client sends its requests, without a trailing
// slash.
func (c *Client) BaseURL() string { return c.baseURL }

// UserAgent reports the User-Agent this client sends.
func (c *Client) UserAgent() string { return c.userAgent }

// Lookup resolves a single CEP, masked or unmasked.
//
//	addr, err := client.Lookup(ctx, "01001-000")
//	addr.Localidade // "São Paulo"
//
// A CEP that is not 8 digits fails with [ErrInvalidCEP] before any request is
// made. One that is well-formed but absent from the database fails with
// [ErrCEPNotFound]. Neither is retried: both are definitive. A 400 from the
// API also becomes [ErrInvalidCEP], carrying the API's own message — the
// format already passed local validation, so the server knows something this
// client does not.
func (c *Client) Lookup(ctx context.Context, cep string) (*Address, error) {
	number, err := NormalizeCEP(cep)
	if err != nil {
		return nil, err
	}

	resp, err := c.do(ctx, "/"+number)
	if err != nil {
		return nil, err
	}

	switch resp.status {
	case http.StatusNotFound:
		// The body may name the municipality the CEP would fall in; when it
		// does, say so in the message too, because most callers only ever
		// print the error.
		faixa := decodeFaixa(resp.body)
		message := "CEP " + number + " does not exist in the DNE database"
		if faixa != nil {
			message += " (range of " + faixa.Localidade + "/" + faixa.UF +
				", which goes from " + faixa.CEPIni + " to " + faixa.CEPFim + ")"
		}
		return nil, &Error{
			Kind:    ErrCEPNotFound,
			CEP:     number,
			Status:  resp.status,
			Faixa:   faixa,
			Message: message,
		}
	case http.StatusBadRequest:
		return nil, &Error{
			Kind:    ErrInvalidCEP,
			CEP:     number,
			Status:  resp.status,
			Message: apiMessage(resp.body, "CEP "+number+" rejected by the API"),
		}
	}

	var address Address
	if err := json.Unmarshal(resp.body, &address); err != nil {
		return nil, &Error{
			Kind:   ErrInvalidResponse,
			CEP:    number,
			Status: resp.status,
			Message: "response is not valid JSON, something between you and the API is " +
				"answering instead of it: " + preview(resp.body),
			Cause: err,
		}
	}
	return &address, nil
}

// LookupMany resolves many CEPs in as few requests as possible.
//
// The result has the same length and the same order as the input. A CEP that
// does not exist is nil at its position — it does not vanish from the slice
// and it is not an error. That is the deliberate difference from
// [Client.Lookup]: in a batch of a thousand, one bad CEP must not cost the
// other nine hundred and ninety-nine. A malformed CEP is nil for the same
// reason; validate with [NormalizeCEP] first if you need to know which one it
// was.
//
// Input longer than [MaxBatch] is chunked automatically.
//
// Batches carry no coordinate. The endpoint behind them is CSV, and the CSV
// has no geo column, so every Address returned here has a nil Geo — including
// the CEPs that do have one. Need Geo? Use [Client.Lookup], one at a time.
func (c *Client) LookupMany(ctx context.Context, ceps []string) ([]*Address, error) {
	if len(ceps) == 0 {
		return []*Address{}, nil
	}

	result := make([]*Address, 0, len(ceps))
	for start := 0; start < len(ceps); start += MaxBatch {
		end := start + MaxBatch
		if end > len(ceps) {
			end = len(ceps)
		}
		chunk, err := c.lookupChunk(ctx, ceps[start:end])
		if err != nil {
			return nil, err
		}
		result = append(result, chunk...)
	}
	return result, nil
}

// Health queries GET /health: does the API answer, and with which database.
//
// An API reporting itself degraded is not an error — that arrives in
// Health.Status, and checking health is exactly when you want the diagnosis
// rather than a failure. Network failures still fail.
func (c *Client) Health(ctx context.Context) (*Health, error) {
	resp, err := c.do(ctx, "/health", http.StatusOK, http.StatusServiceUnavailable)
	if err != nil {
		return nil, err
	}

	var health Health
	if err := json.Unmarshal(resp.body, &health); err != nil {
		return nil, &Error{
			Kind:    ErrInvalidResponse,
			Status:  resp.status,
			Message: "health response is not valid JSON: " + preview(resp.body),
			Cause:   err,
		}
	}
	return &health, nil
}

func (c *Client) lookupChunk(ctx context.Context, ceps []string) ([]*Address, error) {
	query := url.Values{}
	query.Set("ceps", strings.Join(ceps, ","))

	resp, err := c.do(ctx, "/csv?"+query.Encode())
	if err != nil {
		return nil, err
	}

	rows, err := parseCSV(resp.body)
	if err != nil {
		return nil, &Error{
			Kind:    ErrInvalidResponse,
			Status:  resp.status,
			Message: "batch response is not valid CSV: " + preview(resp.body),
			Cause:   err,
		}
	}

	if len(rows) > len(ceps) {
		rows = rows[:len(ceps)]
	}
	for len(rows) < len(ceps) {
		rows = append(rows, nil)
	}
	return rows, nil
}

func parseCSV(body []byte) ([]*Address, error) {
	reader := csv.NewReader(bytes.NewReader(bytes.TrimPrefix(body, []byte("\xef\xbb\xbf"))))
	reader.FieldsPerRecord = -1

	records, err := reader.ReadAll()
	if err != nil {
		return nil, err
	}
	if len(records) == 0 {
		return nil, errors.New("empty CSV: not even a header row")
	}

	header := records[0]
	rows := make([]*Address, 0, len(records)-1)
	for _, record := range records[1:] {
		fields := make(map[string]string, len(header))
		for i, name := range header {
			if i < len(record) {
				fields[name] = record[i]
			} else {
				fields[name] = ""
			}
		}
		if strings.TrimSpace(fields["localidade"]) == "" {
			rows = append(rows, nil)
			continue
		}
		address, err := addressFromFields(fields)
		if err != nil {
			return nil, err
		}
		rows = append(rows, address)
	}
	return rows, nil
}

func addressFromFields(fields map[string]string) (*Address, error) {
	encoded, err := json.Marshal(fields)
	if err != nil {
		return nil, err
	}
	var address Address
	if err := json.Unmarshal(encoded, &address); err != nil {
		return nil, err
	}
	return &address, nil
}

type response struct {
	status int
	body   []byte
	header http.Header
}

func (c *Client) do(ctx context.Context, path string, accepted ...int) (*response, error) {
	if ctx == nil {
		ctx = context.Background()
	}
	if c.totalTimeout > 0 {
		var cancel context.CancelFunc
		ctx, cancel = context.WithTimeout(ctx, c.totalTimeout)
		defer cancel()
	}

	target := c.baseURL + path
	if _, err := url.Parse(target); err != nil {
		return nil, &Error{
			Kind:    ErrNetwork,
			Message: "cannot build a request for " + target,
			Cause:   err,
		}
	}

	var (
		lastStatus     int
		lastRetryAfter time.Duration
		hasRetryAfter  bool
		lastCause      error
	)

	for attempt := 1; attempt <= c.maxAttempts; attempt++ {
		if err := ctx.Err(); err != nil {
			if lastStatus == 0 && lastCause == nil {
				lastCause = err
			}
			break
		}

		resp, err := c.attempt(ctx, target)
		switch {
		case err != nil:
			lastCause, lastStatus, lastRetryAfter, hasRetryAfter = err, 0, 0, false
		case isAccepted(resp.status, accepted) || !isRetriable(resp.status):
			return resp, nil
		default:
			lastCause, lastStatus = nil, resp.status
			lastRetryAfter, hasRetryAfter = retryAfter(resp.header)
		}

		if attempt == c.maxAttempts {
			break
		}
		if !sleepContext(ctx, c.backoff(attempt, lastRetryAfter, hasRetryAfter)) {
			break
		}
	}

	attempts := " (after " + strconv.Itoa(c.maxAttempts) + " attempt(s))"
	switch {
	case lastStatus == http.StatusTooManyRequests:
		return nil, &Error{
			Kind:       ErrRateLimited,
			Status:     lastStatus,
			RetryAfter: lastRetryAfter,
			Message:    "rate limit reached at " + target + attempts,
		}
	case lastStatus != 0:
		return nil, &Error{
			Kind:    ErrServer,
			Status:  lastStatus,
			Message: "the API answered " + strconv.Itoa(lastStatus) + " at " + target + attempts,
		}
	default:
		return nil, &Error{
			Kind:    ErrNetwork,
			Message: "could not reach " + target + attempts,
			Cause:   lastCause,
		}
	}
}

func (c *Client) attempt(ctx context.Context, target string) (*response, error) {
	ctx, cancel := context.WithTimeout(ctx, c.timeout)
	defer cancel()

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("User-Agent", c.userAgent)
	req.Header.Set("Accept", acceptHeader)

	resp, err := c.httpClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	return &response{status: resp.StatusCode, body: body, header: resp.Header}, nil
}

func (c *Client) backoff(attempt int, serverAsked time.Duration, hasRetryAfter bool) time.Duration {
	if hasRetryAfter {
		if serverAsked > c.backoffMax {
			return c.backoffMax
		}
		return serverAsked
	}
	ceiling := c.backoffBase << (attempt - 1)
	if ceiling > c.backoffMax || ceiling <= 0 {
		ceiling = c.backoffMax
	}
	if ceiling <= 0 {
		return 0
	}
	return time.Duration(rand.Int63n(int64(ceiling) + 1))
}

func isAccepted(status int, accepted []int) bool {
	if len(accepted) == 0 {
		return status == http.StatusOK
	}
	for _, candidate := range accepted {
		if status == candidate {
			return true
		}
	}
	return false
}

func retryAfter(header http.Header) (time.Duration, bool) {
	if header == nil {
		return 0, false
	}
	value := strings.TrimSpace(header.Get("Retry-After"))
	if value == "" {
		return 0, false
	}
	if seconds, err := strconv.ParseFloat(value, 64); err == nil {
		if seconds <= 0 {
			return 0, true
		}
		return time.Duration(seconds * float64(time.Second)), true
	}
	target, err := http.ParseTime(value)
	if err != nil {
		return 0, false
	}
	wait := time.Until(target)
	if wait < 0 {
		return 0, true
	}
	return wait, true
}

func sleepContext(ctx context.Context, wait time.Duration) bool {
	if wait <= 0 {
		return ctx.Err() == nil
	}
	timer := time.NewTimer(wait)
	defer timer.Stop()
	select {
	case <-timer.C:
		return true
	case <-ctx.Done():
		return false
	}
}

func preview(body []byte) string {
	const limit = 120
	if len(body) > limit {
		return strconv.Quote(string(body[:limit])) + "..."
	}
	return strconv.Quote(string(body))
}