// Package auth handles password hashing (argon2id), session
// management, CSRF tokens, and bearer-token verification for agents.
package auth

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"strings"
	"testing"

	"golang.org/x/crypto/argon2"
)

// argon2id parameters following RFC 9106 §4 "second
// recommended option" (memory-constrained):
//   - 64 MiB memory, 3 iterations, 4 lanes, 32-byte tag.
//
// These are tunable per-deployment if a beefy controller wants to
// crank them; we ship a defensible default.
const (
	defaultMemoryKiB  = 64 * 1024
	defaultIterations = 3
	defaultParallel   = 4
	defaultSaltLen    = 16
	defaultKeyLen     = 32
)

// Cheap params used only when the binary is a `go test` binary
// (testing.Testing() == true). Argon2id at production params costs
// 300–500 ms per hash and dominates wall time on CI runners under
// `-race`. Tests don't need real KDF strength — VerifyPassword reads
// params from the encoded hash, so verifying a cheap-params hash
// works the same way.
const (
	testMemoryKiB  = 8
	testIterations = 1
	testParallel   = 1
)

// HashPassword returns an argon2id-encoded string of the form
//
//	$argon2id$v=19$m=...,t=...,p=...$<salt>$<hash>
//
// safe to store in a TEXT column. The salt is freshly random per call.
func HashPassword(password string) (string, error) {
	mem, iter, par := uint32(defaultMemoryKiB), uint32(defaultIterations), uint8(defaultParallel)
	if testing.Testing() {
		mem, iter, par = testMemoryKiB, testIterations, testParallel
	}
	salt := make([]byte, defaultSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("auth: read salt: %w", err)
	}
	hash := argon2.IDKey([]byte(password), salt,
		iter, mem, par, defaultKeyLen)

	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		mem, iter, par,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(hash),
	), nil
}

// VerifyPassword returns nil if password matches the encoded hash.
// On any decode error or mismatch the error is non-nil — callers
// should treat all non-nil returns as "invalid credentials" and not
// leak which case it was.
func VerifyPassword(encoded, password string) error {
	parts := strings.Split(encoded, "$")
	// "$argon2id$v=...$m=...,t=...,p=...$<salt>$<hash>" → 6 parts (leading empty)
	if len(parts) != 6 || parts[1] != "argon2id" {
		return errors.New("auth: unrecognised hash format")
	}
	var version int
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
		return fmt.Errorf("auth: parse version: %w", err)
	}
	if version != argon2.Version {
		return fmt.Errorf("auth: unsupported argon2 version %d", version)
	}
	var memory, iterations uint32
	var parallel uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d",
		&memory, &iterations, &parallel); err != nil {
		return fmt.Errorf("auth: parse params: %w", err)
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return fmt.Errorf("auth: decode salt: %w", err)
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return fmt.Errorf("auth: decode hash: %w", err)
	}

	got := argon2.IDKey([]byte(password), salt,
		iterations, memory, parallel, uint32(len(want)))

	if subtle.ConstantTimeCompare(got, want) != 1 {
		return errors.New("auth: invalid password")
	}
	return nil
}
