// Package auth turns a password into a session and a session back into a
// principal. It holds no database code: everything here is pure, so the rules
// that decide who gets in are testable without Postgres.
package auth

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/base32"
	"encoding/base64"
	"errors"
	"fmt"
	"net/http"
	"strings"
	"time"

	"golang.org/x/crypto/bcrypt"
)

var (
	ErrBadCredentials = errors.New("email or password is incorrect")
	ErrNoSession      = errors.New("not signed in")
	ErrExpired        = errors.New("session expired")
	ErrForbidden      = errors.New("not allowed")
)

// Lifetimes. The access token is short because it is copied onto shop-floor
// PCs and into a tray app's config file; the refresh token is long because a
// store that reboots overnight must come back working rather than waiting for
// someone with a password to arrive in the morning.
const (
	AccessTTL  = 12 * time.Hour
	RefreshTTL = 60 * 24 * time.Hour
)

// bcryptCost 12 is roughly 250 ms on the 2 vCPU box this runs on. Deliberately
// slow: login happens once a shift, and the cost is the entire defence if the
// hashes ever leak - and with the password floor now at 8 characters it is
// most of what stands between a leaked hash and a working credential.
//
// ProductionBcryptCost is the real one, kept as a const so a test can assert
// on it without depending on whatever the suite has temporarily set.
//
// A var, not a const, solely so the test suite can lower it. At cost 12 nearly
// every handler test pays ~500 ms for a hash and a verify, which under the race
// detector pushed the package past `go test`'s ten-minute default and turned a
// passing suite into a CI failure with no failing assertion in it. Nothing in
// production writes to this; UseTestCost exists to make that obvious at the
// call site.
const ProductionBcryptCost = 12

var bcryptCost = ProductionBcryptCost

// UseTestCost drops the hashing cost to bcrypt's minimum and returns a function
// restoring it.
//
// For tests only. Named so that a production caller reads as obviously wrong,
// rather than a bare exported knob somebody could set from config and quietly
// destroy the only defence a leaked hash has.
func UseTestCost() func() {
	previous := bcryptCost
	bcryptCost = bcrypt.MinCost
	// DummyHash is generated once at startup at the old cost. Regenerate it, or
	// the unknown-address path keeps burning 250 ms per attempt and the timing
	// equivalence the login handler depends on is measured against the wrong
	// number.
	previousDummy := DummyHash
	if h, err := bcrypt.GenerateFromPassword([]byte("no user"), bcrypt.MinCost); err == nil {
		DummyHash = string(h)
	}
	return func() { bcryptCost = previous; DummyHash = previousDummy }
}

// RandomPassword mints a credential for somebody else - a merchant owner
// created by the platform admin, a salesperson created by their manager, a
// reset. 80 bits as 16 lowercase base32 characters: long enough that guessing
// it is not a plan, and a shape a person can read down a phone line without
// spelling out case. One generator rather than one per caller, so nobody
// later writes a shorter one for the "less important" account.
func RandomPassword() (string, error) {
	b := make([]byte, 10)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return strings.ToLower(base32.StdEncoding.
		WithPadding(base32.NoPadding).EncodeToString(b)), nil
}

func HashPassword(plain string) (string, error) {
	if err := CheckPasswordPolicy(plain); err != nil {
		return "", err
	}
	b, err := bcrypt.GenerateFromPassword([]byte(plain), bcryptCost)
	return string(b), err
}

// VerifyPassword reports whether the password matches.
//
// It takes the same time whether the user exists or not — the caller passes
// DummyHash for an unknown address. Without that, response time alone tells an
// attacker which addresses are registered, which for a B2B product is a list
// of your customer's staff.
func VerifyPassword(hash, plain string) bool {
	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(plain)) == nil
}

// DummyHash is a bcrypt hash, at the real cost, of a value nothing can match.
// Used to burn the same CPU on an unknown email as on a known one.
//
// Generated at startup rather than pasted in as a constant: a hardcoded string
// with a typo in it fails to parse, CompareHashAndPassword returns immediately,
// and the timing leak this exists to close is silently back — the one failure
// mode no test would notice.
var DummyHash = func() string {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		panic("auth: no entropy: " + err.Error())
	}
	h, err := bcrypt.GenerateFromPassword(b[:], bcryptCost)
	if err != nil {
		panic("auth: cannot build dummy hash: " + err.Error())
	}
	return string(h)
}()

// MinPasswordLength is the whole policy, alongside the 200-character ceiling.
//
// Set to 8 by the product owner. Recording the trade rather than the number:
// eight characters of anything is inside the reach of an offline attack on a
// leaked hash, and these accounts read customer face data. What stands between
// the two is bcrypt at cost 12 (~250 ms a guess, so an online list is
// hopeless) and the per-account throttle of 10 failures in 15 minutes. Those
// make ONLINE guessing impractical at any length; they do nothing if the
// hashes themselves ever leak.
const MinPasswordLength = 8

// CheckPasswordPolicy is length-only on purpose. Composition rules ("one
// capital, one symbol") push people towards Passw0rd! and are worse than
// length for the same annoyance.
func CheckPasswordPolicy(plain string) error {
	if len(plain) < MinPasswordLength {
		return fmt.Errorf("password must be at least %d characters", MinPasswordLength)
	}
	if len(plain) > 200 {
		// bcrypt silently truncates at 72 bytes; a 4 KB password is either a
		// mistake or an attempt to make us hash something enormous.
		return errors.New("password must be at most 200 characters")
	}
	return nil
}

// Token is a freshly minted secret and the hash to store for it. The plaintext
// exists only in the response to the client; only Hash is ever persisted.
type Token struct {
	Plain string
	Hash  []byte
}

// NewToken mints 256 bits from crypto/rand.
//
// Not a UUID: v4 gives 122 bits and, more importantly, uuid is the type used
// for row ids all over this schema, so a token that looks like one invites
// somebody to eventually store it in a uuid column where it would be logged,
// joined and pasted around like an identifier rather than a secret.
func NewToken() (Token, error) {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		return Token{}, fmt.Errorf("cannot generate token: %w", err)
	}
	plain := base64.RawURLEncoding.EncodeToString(b[:])
	return Token{Plain: plain, Hash: HashToken(plain)}, nil
}

// HashToken is SHA-256, not bcrypt. The input is 256 bits of entropy, so there
// is no dictionary for a slow hash to protect against — only a per-request
// cost, paid on every authenticated call.
func HashToken(plain string) []byte {
	sum := sha256.Sum256([]byte(plain))
	return sum[:]
}

// Principal is who the request is. ClientID empty means a platform admin, who
// is the only kind of user not scoped to one tenant.
type Principal struct {
	UserID     string
	SessionID  string
	ClientID   string
	ClientName string
	Email      string
	FullName   string
	Role       string
}

func (p Principal) IsAdmin() bool { return p.Role == "admin" }

// CanWriteProfiles gates the in-store customer form. Staff can fill it in —
// that is the job — but not everyone who can read a report should be able to
// attach a name and a phone number to a face.
func (p Principal) CanWriteProfiles() bool {
	switch p.Role {
	case "admin", "owner", "manager", "staff":
		return true
	}
	return false
}

// CanManageSites gates enrolment tokens and site configuration.
func (p Principal) CanManageSites() bool {
	switch p.Role {
	case "admin", "owner", "manager":
		return true
	}
	return false
}

// BearerToken pulls the credential out of an Authorization header.
//
// Header only, never a query parameter: URLs end up in access logs, proxy logs
// and browser history, and a session token in any of those is a session token
// leaked.
func BearerToken(r *http.Request) string {
	h := r.Header.Get("Authorization")
	const p = "Bearer "
	if len(h) > len(p) && strings.EqualFold(h[:len(p)], p) {
		return strings.TrimSpace(h[len(p):])
	}
	return ""
}

// NormalizeEmail lowercases and trims. The unique index is on lower(email), so
// anything reaching the database must already agree with it or the constraint
// silently stops meaning what it says.
func NormalizeEmail(s string) string {
	return strings.ToLower(strings.TrimSpace(s))
}

// NormalizeCode strips an operator's formatting from an enrolment code.
//
// Codes are read off a screen, dictated down a phone and typed in, so spaces,
// dashes and the shift key are presentation, not part of the secret. Both the
// side that issues a code and the side that redeems one must agree exactly on
// what gets hashed, which is why this is one function and not two.
func NormalizeCode(s string) string {
	return strings.ToUpper(strings.NewReplacer(" ", "", "-", "").Replace(s))
}

// NewEnrolmentCode mints the code an installer types once: 120 bits of
// randomness, base32 so it survives being read down a phone line, grouped in
// sixes so it can be read aloud at all.
//
// Here rather than beside either caller because two of them mint codes now -
// the provisioning command and the API an owner uses to replace a shop PC - and
// a second implementation that formatted or cased a code differently would hash
// to something the redeemer never produces. Same reason NormalizeCode above is
// one function.
func NewEnrolmentCode() (string, error) {
	b := make([]byte, 15)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	raw := strings.ToUpper(base32.StdEncoding.WithPadding(base32.NoPadding).
		EncodeToString(b))
	var parts []string
	for i := 0; i < len(raw); i += 6 {
		parts = append(parts, raw[i:i+6])
	}
	return strings.Join(parts, "-"), nil
}