/**
 * Variable reactivity engine — tracks variable declarations and tag
 * modifiers, then cascades changes when variables are set.
 *
 * Runs client-side (in the sender's tab, via command-dispatcher).
 * When a variable changes, it:
 *   1. Sends the base var message
 *   2. Detects tag activation/deactivation diff
 *   3. Re-evaluates declared variables whose dependencies changed
 *   4. Applies/removes tag modifiers on affected targets
 *   5. Sends var messages for every derived change
 *
 * Circular dependency detection happens at registration time
 * (topological sort). At runtime, we guard against re-entrant
 * evaluation with an in-flight set.
 */

import type { VarDeclaration, TagModifier } from "./declare-parser";
import { evaluateExpression } from "./variable-expression";

// ---------------------------------------------------------------------------
// Types
// ---------------------------------------------------------------------------

export interface VarReactivityState {
  /** All variable declarations (from role=declare blocks) */
  declarations: VarDeclaration[];
  /** All tag modifiers (from role=declare blocks) */
  tagModifiers: TagModifier[];
}

/** The runtime variable store (read from stream state). */
export type VariableStore = Record<string, string>;

// ---------------------------------------------------------------------------
// Internal state
// ---------------------------------------------------------------------------

/** Dependency graph: $dep → Set<$declaredVar> */
let depGraph: Map<string, Set<string>> | null = null;

/** Reverse: $declaredVar → its expression */
let declExprs: Map<string, string> | null = null;

/** Tag modifiers: #tag → [{target, expression}] */
let tagModMap: Map<string, Array<{ target: string; expression: string }>> | null = null;

/** Set of $vars currently being re-evaluated (cycle guard) */
const inFlight = new Set<string>();

// ---------------------------------------------------------------------------
// Public API
// ---------------------------------------------------------------------------

/**
 * Initialize (or re-initialize) the reactivity engine from declarations
 * and tag modifiers parsed from role=declare blocks.
 *
 * Throws if a circular dependency is detected.
 */
export function initReactivity(state: VarReactivityState): void {
  depGraph = new Map();
  declExprs = new Map();
  tagModMap = new Map();

  // Index tag modifiers
  for (const tm of state.tagModifiers) {
    let list = tagModMap.get(tm.tag);
    if (!list) {
      list = [];
      tagModMap.set(tm.tag, list);
    }
    list.push({ target: tm.target, expression: tm.expression });
  }

  // Index declarations and build dependency graph
  for (const decl of state.declarations) {
    declExprs.set(decl.key, decl.expression);
    const deps = extractDependencies(decl.expression);
    for (const dep of deps) {
      let dependents = depGraph.get(dep);
      if (!dependents) {
        dependents = new Set();
        depGraph.set(dep, dependents);
      }
      dependents.add(decl.key);
    }
  }

  // Circular dependency check
  checkCircular();
}

/** Extract $var names from an expression string. */
export function extractDependencies(expr: string): string[] {
  const vars: string[] = [];
  // Match $identifier patterns (not inside function names, just bare $var)
  const re = /\$([a-zA-Z_][a-zA-Z0-9_]*)/g;
  let m: RegExpExecArray | null;
  while ((m = re.exec(expr)) !== null) {
    const name = "$" + m[1];
    if (!vars.includes(name)) vars.push(name);
  }
  return vars;
}

/**
 * Compute which declared variables need re-evaluation after
 * a set of base variables changed, and what tag modifier effects
 * to apply.
 *
 * Returns the list of { key, value } pairs to publish as var messages.
 * The caller is responsible for sending these via sendMessage.
 *
 * `changedVar` is the variable that was just set (e.g. "$con").
 * `oldValue` is its previous value (for detecting tag transitions).
 * `currentVars` is the full variable store.
 */
export function computeCascade(
  changedVar: string,
  oldValue: string | undefined,
  currentVars: VariableStore,
): Array<{ key: string; value: string }> {
  if (!depGraph || !declExprs || !tagModMap) {
    return [];
  }

  const results: Array<{ key: string; value: string }> = [];

  // ---- Tag activation/deactivation ----
  const newTag = isTagValue(currentVars[changedVar]);
  const oldTag = isTagValue(oldValue);

  if (newTag !== oldTag) {
    // Deactivate old tag
    if (oldTag) {
      const removed = computeTagDeactivation(
        oldTag,
        currentVars,
      );
      for (const r of removed) {
        results.push(r);
        // Update currentVars in-place so subsequent steps see new values
        currentVars = { ...currentVars, [r.key]: r.value };
      }
    }

    // Activate new tag
    if (newTag) {
      const added = computeTagActivation(
        newTag,
        currentVars,
      );
      for (const r of added) {
        results.push(r);
        currentVars = { ...currentVars, [r.key]: r.value };
      }
    }
  }

  // ---- Declaration re-evaluation ----
  const reevaluated = reevaluateDependents(
    changedVar,
    currentVars,
  );
  for (const r of reevaluated) {
    // Avoid re-sending the same key if it was already in tag results
    if (!results.some((x) => x.key === r.key)) {
      results.push(r);
      currentVars = { ...currentVars, [r.key]: r.value };
    }
  }

  return results;
}

/**
 * Compute initial values for all declared variables.
 * Called once after initReactivity() to seed the store.
 * Unset dependencies default to 0.
 */
export function computeInitialValues(
  currentVars: VariableStore,
): Array<{ key: string; value: string }> {
  if (!declExprs) return [];

  const allKeys = [...declExprs.keys()];
  const sorted = topoSortAffected(new Set(allKeys));

  const results: Array<{ key: string; value: string }> = [];
  const localVars = { ...currentVars };

  for (const key of sorted) {
    const expr = declExprs.get(key);
    if (!expr) continue;

    try {
      const result = evaluateExpression(expr, {
        lookup: (name: string) => {
          const k = "$" + name;
          return localVars[k] ?? undefined;
        },
      });

      const newValue = String(result.value);
      const finalValue = applyTagModifiersTo(key, newValue, localVars);

      if (finalValue !== (localVars[key] ?? "")) {
        localVars[key] = finalValue;
        results.push({ key, value: finalValue });
      }
    } catch {
      // skip failed evaluations at init time
    }
  }

  return results;
}

/**
 * Compute which tags are active given the current variable store.
 * A tag is active if any variable has that tag as its value.
 */
export function computeActiveTags(vars: VariableStore): string[] {
  const active: string[] = [];
  for (const value of Object.values(vars)) {
    if (isTagValue(value) && !active.includes(value)) {
      active.push(value);
    }
  }
  return active;
}

// ---------------------------------------------------------------------------
// Internal helpers
// ---------------------------------------------------------------------------

function isTagValue(value: string | undefined): string | null {
  if (!value) return null;
  const trimmed = value.trim();
  return trimmed.startsWith("#") ? trimmed : null;
}

/**
 * Walk the dependency graph from `changedVar` and re-evaluate all
 * affected declarations. Uses topological order.
 */
function reevaluateDependents(
  changedVar: string,
  vars: VariableStore,
): Array<{ key: string; value: string }> {
  if (!depGraph || !declExprs) return [];

  // Collect all dependents reachable from changedVar (BFS)
  const affected = new Set<string>();
  const queue = [changedVar];
  while (queue.length > 0) {
    const dep = queue.shift()!;
    const dependents = depGraph.get(dep);
    if (!dependents) continue;
    for (const d of dependents) {
      if (!affected.has(d)) {
        affected.add(d);
        queue.push(d); // transitive: if C depends on B which depends on A...
      }
    }
  }

  // Topological sort: declarations that depend only on base vars go first
  const sorted = topoSortAffected(affected);

  const results: Array<{ key: string; value: string }> = [];
  const localVars = { ...vars }; // working copy

  for (const key of sorted) {
    if (inFlight.has(key)) {
      // Shouldn't happen (caught at init), but guard anyway
      throw new Error(`Circular dependency detected during evaluation of ${key}`);
    }
    inFlight.add(key);

    try {
      const expr = declExprs.get(key);
      if (!expr) continue;

      // Evaluate with current localVars (which includes previously
      // re-evaluated dependents)
      const result = evaluateExpression(expr, {
        lookup: (name: string) => {
          const k = "$" + name;
          return localVars[k] ?? undefined;
        },
      });

      const newValue = String(result.value);

      // Apply any active tag modifiers to this key
      const finalValue = applyTagModifiersTo(key, newValue, localVars);

      if (finalValue !== localVars[key]) {
        localVars[key] = finalValue;
        results.push({ key, value: finalValue });
      }
    } finally {
      inFlight.delete(key);
    }
  }

  return results;
}

/**
 * Apply all active tag modifiers that target `key`.
 * Returns the modified value (base + sum of active modifiers).
 */
function applyTagModifiersTo(
  key: string,
  baseValue: string,
  vars: VariableStore,
): string {
  if (!tagModMap) return baseValue;

  const activeTags = computeActiveTags(vars);
  const baseNum = parseFloat(baseValue);
  if (isNaN(baseNum)) return baseValue; // non-numeric, can't apply modifiers

  let modifierSum = 0;
  for (const tag of activeTags) {
    const mods = tagModMap.get(tag);
    if (!mods) continue;
    for (const mod of mods) {
      if (mod.target === key) {
        try {
          const result = evaluateExpression(mod.expression, {
            lookup: (name: string) => {
              const k = "$" + name;
              return vars[k] ?? undefined;
            },
          });
          modifierSum += result.value;
        } catch {
          // If modifier expression fails, skip it
        }
      }
    }
  }

  return String(baseNum + modifierSum);
}

/**
 * Compute the effects of activating a tag: for each modifier,
 * evaluate and add to the target variable's current value.
 */
function computeTagActivation(
  tag: string,
  vars: VariableStore,
): Array<{ key: string; value: string }> {
  if (!tagModMap) return [];

  const mods = tagModMap.get(tag);
  if (!mods) return [];

  const results: Array<{ key: string; value: string }> = [];
  const localVars = { ...vars };

  for (const mod of mods) {
    try {
      const result = evaluateExpression(mod.expression, {
        lookup: (name: string) => {
          const k = "$" + name;
          return localVars[k] ?? undefined;
        },
      });

      const currentBase = parseFloat(localVars[mod.target] ?? "0");
      if (!isNaN(currentBase)) {
        const newValue = String(currentBase + result.value);
        localVars[mod.target] = newValue;
        results.push({ key: mod.target, value: newValue });
      }
    } catch {
      // skip failed modifier
    }
  }

  return results;
}

/**
 * Compute the effects of deactivating a tag: for each modifier,
 * subtract from the target variable's current value.
 */
function computeTagDeactivation(
  tag: string,
  vars: VariableStore,
): Array<{ key: string; value: string }> {
  if (!tagModMap) return [];

  const mods = tagModMap.get(tag);
  if (!mods) return [];

  const results: Array<{ key: string; value: string }> = [];
  const localVars = { ...vars };

  for (const mod of mods) {
    try {
      const result = evaluateExpression(mod.expression, {
        lookup: (name: string) => {
          const k = "$" + name;
          return localVars[k] ?? undefined;
        },
      });

      const currentValue = parseFloat(localVars[mod.target] ?? "0");
      if (!isNaN(currentValue)) {
        const newValue = String(currentValue - result.value);
        localVars[mod.target] = newValue;
        results.push({ key: mod.target, value: newValue });
      }
    } catch {
      // skip failed modifier
    }
  }

  return results;
}

// ---------------------------------------------------------------------------
// Topological sort & circular check
// ---------------------------------------------------------------------------

/**
 * Topologically sort a set of affected declaration keys so that
 * dependents are evaluated after their dependencies.
 */
function topoSortAffected(affected: Set<string>): string[] {
  if (!depGraph || !declExprs) return [...affected];

  const result: string[] = [];
  const visited = new Set<string>();
  const temp = new Set<string>();

  function visit(key: string): void {
    if (visited.has(key)) return;
    if (temp.has(key)) {
      // This shouldn't happen if checkCircular passed, but guard
      throw new Error(`Circular dependency involving ${key}`);
    }
    temp.add(key);

    // Visit dependencies first
    const expr = declExprs!.get(key);
    if (expr) {
      const deps = extractDependencies(expr);
      for (const dep of deps) {
        if (affected.has(dep) || declExprs!.has(dep)) {
          visit(dep);
        }
      }
    }

    temp.delete(key);
    visited.add(key);
    result.push(key);
  }

  for (const key of affected) {
    visit(key);
  }

  return result;
}

/**
 * Check for circular dependencies in the full declaration graph.
 * Throws with a descriptive message if a cycle is found.
 */
function checkCircular(): void {
  if (!declExprs) return;

  const allKeys = [...declExprs.keys()];
  const state = new Map<string, "unvisited" | "visiting" | "visited">();
  for (const k of allKeys) state.set(k, "unvisited");

  const path: string[] = [];

  function dfs(key: string): void {
    const s = state.get(key);
    if (s === "visited") return;
    if (s === "visiting") {
      const cycleStart = path.indexOf(key);
      const cycle = path.slice(cycleStart).concat(key);
      throw new Error(
        `Circular dependency detected: ${cycle.join(" → ")}`,
      );
    }

    state.set(key, "visiting");
    path.push(key);

    const expr = declExprs!.get(key);
    if (expr) {
      const deps = extractDependencies(expr);
      for (const dep of deps) {
        if (declExprs!.has(dep)) {
          dfs(dep);
        }
      }
    }

    path.pop();
    state.set(key, "visited");
  }

  for (const key of allKeys) {
    dfs(key);
  }
}