import { useEffect, useState } from "react";
import { useNavigate, useParams, useLocation } from "react-router-dom";
import { useForm, useWatch } from "react-hook-form";
import { z } from "zod";
import { zodResolver } from "@hookform/resolvers/zod";
import { ChevronLeft, ChevronRight, Check, FileText, ListChecks, Link2, Plus } from "lucide-react";

import { useCourses } from "@/contexts/AdminCoursesContext";
import { useAuth } from "@/contexts/AuthContext";
import { PageMeta } from "@/contexts/MetadataContext";
import { cn } from "@/lib/utils";
import api from "@/utils/api.util";
import { Button } from "@/components/ui/button";
import { Input } from "@/components/ui/input";
import { Label } from "@/components/ui/label";
import { Textarea } from "@/components/ui/textarea";
import { Spinner } from "@/components/ui/spinner";
import {
  Select, SelectContent, SelectItem, SelectTrigger, SelectValue,
} from "@/components/ui/select";
import { useUnsavedChangesGuard } from "@/hooks/useUnsavedChangesGuard";
import DraftRequirementsEditor from "@/modules/admin/components/courses/DraftRequirementsEditor";
import AttachUnitsDialog from "@/modules/admin/components/library/AttachUnitsDialog";

const schema = z.object({
  title: z.string().min(1, "Title is required."),
  description: z.string().optional(),
  order: z.coerce.number().min(0).default(0),
  subscription: z.string().optional(),
});

const STEPS = [
  { id: 0, label: "Details", icon: FileText },
  { id: 1, label: "Completion Requirements", icon: ListChecks },
];

function FieldError({ message }) {
  if (!message) return null;
  return <p className="text-xs text-destructive mt-1">{message}</p>;
}

export default function AddUnit() {
  const navigate = useNavigate();
  const location = useLocation();
  const { courseId } = useParams();
  const { createUnit, fetchCourse, course, loading, syncUnitRequirements } = useCourses();
  const { user } = useAuth();

  const backTarget = location.state?.returnTo ?? `/admin/courses/${courseId}/units`;

  const [mode, setMode] = useState(null); // null | "create" — the Details/Requirements form only shows once "Create" is chosen
  const [step, setStep] = useState(0);
  const [requirements, setRequirements] = useState([]);
  const [attachOpen, setAttachOpen] = useState(false);

  const [tierCategories, setTierCategories] = useState([]);
  useEffect(() => {
    api.get("/admin/tiers/categories")
      .then(({ data }) => setTierCategories((data.data ?? []).filter((c) => c.is_active)))
      .catch(() => {});
  }, []);

  const { register, trigger, getValues, control, setValue, formState: { errors, isDirty } } = useForm({
    resolver: zodResolver(schema),
    defaultValues: { title: "", description: "", order: 0, subscription: "free" },
  });

  const watchedSubscr = useWatch({ control, name: "subscription" });
  const defaultTierSlug = tierCategories.find((c) => c.is_default)?.slug || "free";

  const { bypassOnce, dialog: unsavedChangesDialog } = useUnsavedChangesGuard(isDirty || requirements.length > 0);

  useEffect(() => {
    fetchCourse(courseId);
  }, [courseId]);

  const handleNext = async () => {
    const valid = await trigger();
    if (valid) setStep(1);
  };

  // The one and only persistence point — nothing is created until this final
  // click at Requirements (the last step), which is purely a draft form
  // until now.
  const handleCreate = async () => {
    const values = getValues();
    const result = await createUnit(courseId, { ...values, subscription: values.subscription || defaultTierSlug, createdBy: user?.user_id });
    const newUnitId = result?.data?.data?.unit_id;
    if (!newUnitId) return;

    if (requirements.length > 0) {
      const clean = requirements.map(({ _key, ...r }) => r);
      await syncUnitRequirements(courseId, newUnitId, clean);
    }

    bypassOnce();
    navigate(backTarget);
  };

  return (
    <section className="bg-muted h-full">
      <PageMeta title={course ? `Add Unit – ${course.title} - STARR` : undefined} />
      <div className="lg:container lg:mx-auto lg:px-0 flex flex-col items-center px-4 py-10">

        <div className="w-full max-w-2xl mx-auto space-y-6">
          <div className="flex items-center gap-3">
            <Button
              type="button"
              variant="ghost"
              size="icon"
              onClick={() => (mode === "create" && step > 0 ? setStep(0) : navigate(backTarget))}
            >
              <ChevronLeft className="h-4 w-4" />
            </Button>
            <div>
              <h1 className="text-xl font-semibold">Create Unit</h1>
              <p className="text-sm text-muted-foreground">
                Creates a new unit in the shared Units Library and attaches it to this course.
              </p>
            </div>
          </div>

          {/* Select existing vs. create new */}
          <div className="flex items-start justify-between gap-3 pb-3 border-b border-border">
            <p className="text-xs text-muted-foreground">
              Attach an existing library unit to reuse its content, or create a new one from scratch.
            </p>
            <div className="flex items-center gap-2 shrink-0">
              <Button type="button" variant="outline" size="sm" onClick={() => setAttachOpen(true)}>
                <Link2 className="h-3.5 w-3.5 mr-1.5" /> Select
              </Button>
              <Button type="button" variant="outline" size="sm" onClick={() => setMode("create")}>
                <Plus className="h-3.5 w-3.5 mr-1.5" /> Create
              </Button>
            </div>
          </div>

          {mode === "create" && (
            <>
              {/* Stepper */}
              <div className="flex items-center gap-0">
                {STEPS.map((s, i) => {
                  const Icon = s.icon;
                  const isActive = step === i;
                  const isDone = step > i;

                  return (
                    <div key={s.id} className="flex items-center flex-1 last:flex-none">
                      <div className="flex flex-col items-center gap-1">
                        <div className={cn(
                          "h-8 w-8 rounded-full flex items-center justify-center border transition-colors",
                          isDone   && "bg-emerald-600 border-emerald-600 text-white",
                          isActive && "border-primary bg-primary text-primary-foreground",
                          !isActive && !isDone && "border-border bg-background text-muted-foreground"
                        )}>
                          {isDone ? <Check className="h-4 w-4" /> : <Icon className="h-4 w-4" />}
                        </div>
                        <span className={cn(
                          "text-[11px] font-medium whitespace-nowrap hidden sm:block",
                          isActive ? "text-foreground"  : "text-muted-foreground",
                          isDone   ? "text-emerald-600" : ""
                        )}>
                          {s.label}
                        </span>
                      </div>
                      {i < STEPS.length - 1 && (
                        <div className={cn(
                          "flex-1 h-px mx-2 mb-4 transition-colors",
                          step > i ? "bg-emerald-600" : "bg-border"
                        )} />
                      )}
                    </div>
                  );
                })}
              </div>

              {/* Step content */}
              <div className="rounded-lg border bg-card p-6 space-y-5">
                {step === 0 && (
                  <div className="space-y-5">
                    <div className="space-y-1.5">
                      <Label htmlFor="title">Title <span className="text-destructive">*</span></Label>
                      <Input id="title" placeholder="Unit title" {...register("title")} />
                      <FieldError message={errors.title?.message} />
                    </div>

                    <div className="space-y-1.5">
                      <Label htmlFor="description">Description</Label>
                      <Textarea id="description" placeholder="Optional description" rows={3} {...register("description")} />
                    </div>

                    <div className="space-y-1.5 max-w-[120px]">
                      <Label htmlFor="order">Order</Label>
                      <Input id="order" type="number" min={0} {...register("order")} />
                    </div>

                    <div className="space-y-1.5">
                      <Label>Subscription</Label>
                      <Select
                        value={watchedSubscr || defaultTierSlug}
                        onValueChange={(val) => setValue("subscription", val, { shouldDirty: true })}
                      >
                        <SelectTrigger>
                          <SelectValue placeholder="No tier gate" />
                        </SelectTrigger>
                        <SelectContent>
                          {tierCategories.map((c) => (
                            <SelectItem key={c.slug} value={c.slug}>
                              {c.name}
                            </SelectItem>
                          ))}
                        </SelectContent>
                      </Select>
                      <p className="text-xs text-muted-foreground">
                        Optional. Gates this unit directly, independent of the course it's attached to.
                      </p>
                    </div>
                  </div>
                )}

                {step === 1 && (
                  <div className="space-y-4">
                    <p className="text-sm text-muted-foreground">
                      Configure how learners complete this unit — optional, sensible defaults apply automatically. This is created together with the rest of the unit when you finish.
                    </p>
                    <DraftRequirementsEditor entityType="unit" items={requirements} onChange={setRequirements} />
                  </div>
                )}
              </div>

              <div className="flex justify-between gap-3">
                <Button
                  type="button"
                  variant="outline"
                  onClick={() => (step === 0 ? navigate(backTarget) : setStep(0))}
                  disabled={loading}
                >
                  <ChevronLeft className="h-4 w-4 mr-1" />
                  {step === 0 ? "Cancel" : "Back"}
                </Button>

                {step === 0 ? (
                  <Button type="button" onClick={handleNext}>
                    Next
                    <ChevronRight className="h-4 w-4 ml-1" />
                  </Button>
                ) : (
                  // Only the true final step (Requirements) actually persists
                  // anything — the unit and any draft requirements are created
                  // together in one shot here.
                  <Button type="button" onClick={handleCreate} disabled={loading}>
                    {loading && <Spinner className="h-4 w-4 mr-2" />}
                    Create Unit
                  </Button>
                )}
              </div>
            </>
          )}
        </div>
      </div>

      {unsavedChangesDialog}

      <AttachUnitsDialog
        open={attachOpen}
        onOpenChange={setAttachOpen}
        attachedUnitIds={course?.units?.map((u) => u.unit_id) ?? []}
        onAttach={async (unitIds) => {
          await api.post(`/admin/courses/${courseId}/units/attach`, { unit_ids: unitIds });
          bypassOnce();
          navigate(backTarget);
        }}
        loading={loading}
      />
    </section>
  );
}