/**
 * 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).
 * Uses a base/mod separation:
 *   - baseValues: what /set writes (or declaration evaluation produces)
 *   - activeMods: per-target list of {tag, value, source} from tag activations
 *   - sourceActivations: per-source list of {tag, target, value} for deactivation
 *
 * Combined value = base + sum(activeMods). Tag values (starting with #)
 * are not numeric and don't receive mods.
 *
 * All functions that resolve variable values accept an explicit `fallback`
 * (the stream VariableStore) rather than relying on mutable module state.
 * This ensures correctness regardless of call order.
 */

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

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

export interface VarReactivityState {
  declarations: VarDeclaration[];
  tagModifiers: TagModifier[];
}

export type VariableStore = Record<string, string>;

/** A single modifier applied to a target variable. */
export interface ActiveMod {
  tag: string;    // "#warrior"
  value: number;  // evaluated modifier expression result
  source: string; // "$class" — which variable activated this tag
}

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

/** Base values set by /set or declaration evaluation */
const baseValues = new Map<string, string>();

/** Active mods per target: $target → [{tag, value, source}] */
const activeMods = new Map<string, ActiveMod[]>();

/** Activations per source: $source → [{tag, target, value}] */
const sourceActivations = new Map<string, Array<{ tag: string; target: string; value: number }>>();

/** 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();
  baseValues.clear();
  activeMods.clear();
  sourceActivations.clear();

  // 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);
    }
  }

  checkCircular();
}

/** Extract $var names from an expression string. */
export function extractDependencies(expr: string): string[] {
  const vars: string[] = [];
  const re = /\$([a-zA-Z_][a-zA-Z0-9_]*)/g;
  let m: RegExpExecArray | null;
  while ((m = re.exec(expr)) !== null) {
    const name = "$" + m[1];
    if (!vars.includes(name)) vars.push(name);
  }
  return vars;
}

/**
 * Get the combined value of a variable (base + sum of active mods).
 * Falls back to the provided stream store for variables not tracked locally.
 */
export function getCombined(key: string, fallback?: VariableStore): string {
  const base = baseValues.get(key);
  if (base !== undefined && isTagValue(base)) {
    return base; // tag values don't receive numeric mods
  }

  const baseNum = base !== undefined ? parseFloat(base) : NaN;
  const mods = activeMods.get(key) ?? [];
  const modSum = mods.reduce((sum, m) => sum + m.value, 0);

  if (!isNaN(baseNum)) {
    return String(baseNum + modSum);
  }

  // Fall back to stream store. The stream value is authoritative for
  // variables not tracked locally — it already includes any mods from
  // the sender's engine, so we return it as-is without adding local mods.
  const fb = fallback?.[key];
  if (fb !== undefined) return fb;

  // No base, but has mods
  if (mods.length > 0) return String(modSum);

  return "0";
}

/** Get the active mods for a variable (for UI hover display). */
export function getMods(key: string): ActiveMod[] {
  return activeMods.get(key) ?? [];
}

/** Get the declaration expression for a variable, if any. */
export function getDeclExpr(key: string): string | undefined {
  return declExprs?.get(key);
}

/**
 * Set the base value for a variable. Called by dispatchSet before
 * computing the cascade, so getCombined returns the correct new value.
 */
export function setBase(key: string, value: string): void {
  baseValues.set(key, value);
}

/**
 * Compute cascade effects after a variable change.
 *
 * @param changedVar - the variable that was just set (e.g. "$con")
 * @param oldValue - its previous combined value (for tag transition detection)
 * @param currentVars - the full stream variable store (as fallback)
 * @returns list of {key, value} pairs (combined values) to publish as var messages
 */
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 newValue = getCombined(changedVar, currentVars);
  const newTag = isTagValue(newValue);
  const oldTag = isTagValue(oldValue);

  if (newTag !== oldTag) {
    // Deactivate old tag
    if (oldTag) {
      const removed = deactivateTagFromSource(changedVar, oldTag, currentVars);
      for (const r of removed) {
        results.push(r);
      }
    }

    // Activate new tag
    if (newTag) {
      const added = activateTagFromSource(changedVar, newTag, currentVars);
      for (const r of added) {
        results.push(r);
      }
    }
  }

  // ---- Declaration re-evaluation ----
  const reevaluated = reevaluateDependents(changedVar, currentVars);
  for (const r of reevaluated) {
    if (!results.some((x) => x.key === r.key)) {
      results.push(r);
    }
  }

  return results;
}

/**
 * Compute initial values for all declared variables.
 * Called once after initReactivity() to seed the store.
 */
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 }> = [];

  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 getCombined(k, currentVars);
        },
      });

      const rawValue = String(result.value);
      baseValues.set(key, rawValue);

      // Check if this is a tag value — if so, activate it
      const tag = isTagValue(rawValue);
      if (tag) {
        const added = activateTagFromSource(key, tag, currentVars);
        for (const r of added) {
          if (!results.some((x) => x.key === r.key)) {
            results.push(r);
          }
        }
      }

      // Always emit the combined value for this key
      const combined = getCombined(key, currentVars);
      if (!results.some((x) => x.key === key)) {
        results.push({ key, value: combined });
      }
    } catch {
      // skip failed evaluations at init time
    }
  }

  return results;
}

// ---------------------------------------------------------------------------
// Tag activation / deactivation (source-based)
// ---------------------------------------------------------------------------

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

/** Activate a tag from a source variable. Evaluates modifiers once. */
function activateTagFromSource(
  source: string,
  tag: string,
  fallback: 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 sourceEntries: Array<{ tag: string; target: string; value: number }> = [];

  for (const mod of mods) {
    try {
      // Snapshot the target's current combined value as its base before
      // adding the mod, so deactivation can restore it correctly.
      if (!baseValues.has(mod.target)) {
        baseValues.set(mod.target, getCombined(mod.target, fallback));
      }

      const result = evaluateExpression(mod.expression, {
        lookup: (name: string) => {
          const k = "$" + name;
          return getCombined(k, fallback);
        },
      });

      const entry: ActiveMod = { tag, value: result.value, source };

      let targetMods = activeMods.get(mod.target);
      if (!targetMods) {
        targetMods = [];
        activeMods.set(mod.target, targetMods);
      }
      targetMods.push(entry);

      sourceEntries.push({ tag, target: mod.target, value: result.value });

      // Emit new combined value for the target
      const combined = getCombined(mod.target, fallback);
      const existing = results.findIndex((r) => r.key === mod.target);
      if (existing >= 0) {
        results[existing] = { key: mod.target, value: combined };
      } else {
        results.push({ key: mod.target, value: combined });
      }
    } catch {
      // skip failed modifier
    }
  }

  sourceActivations.set(source, sourceEntries);
  return results;
}

/** Deactivate a tag from a source variable. Removes exactly the mods it created. */
function deactivateTagFromSource(
  source: string,
  _tag: string,
  fallback: VariableStore,
): Array<{ key: string; value: string }> {
  const entries = sourceActivations.get(source);
  if (!entries) return [];

  const results: Array<{ key: string; value: string }> = [];
  const affectedTargets = new Set<string>();

  // Remove mods from activeMods
  for (const entry of entries) {
    const targetMods = activeMods.get(entry.target);
    if (!targetMods) continue;

    const idx = targetMods.findIndex(
      (m) => m.tag === entry.tag && m.source === source,
    );
    if (idx >= 0) {
      targetMods.splice(idx, 1);
      affectedTargets.add(entry.target);
    }
  }

  // Clean up empty arrays
  for (const target of affectedTargets) {
    const mods = activeMods.get(target);
    if (mods && mods.length === 0) {
      activeMods.delete(target);
    }
  }

  // Remove source activations
  sourceActivations.delete(source);

  // Emit new combined values for affected targets
  for (const target of affectedTargets) {
    results.push({ key: target, value: getCombined(target, fallback) });
  }

  return results;
}

// ---------------------------------------------------------------------------
// Declaration re-evaluation
// ---------------------------------------------------------------------------

function reevaluateDependents(
  changedVar: string,
  fallback: 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);
      }
    }
  }

  const sorted = topoSortAffected(affected);
  const results: Array<{ key: string; value: string }> = [];

  for (const key of sorted) {
    if (inFlight.has(key)) {
      throw new Error(
        `Circular dependency detected during evaluation of ${key}`,
      );
    }
    inFlight.add(key);

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

      const result = evaluateExpression(expr, {
        lookup: (name: string) => {
          const k = "$" + name;
          return getCombined(k, fallback);
        },
      });

      const rawValue = String(result.value);

      // Check for tag transition on this declared variable
      const oldCombined = getCombined(key, fallback);
      const oldTag = isTagValue(oldCombined);
      const newTag = isTagValue(rawValue);

      if (oldTag !== newTag) {
        // Deactivate old tag
        if (oldTag) {
          const removed = deactivateTagFromSource(key, oldTag, fallback);
          for (const r of removed) {
            if (!results.some((x) => x.key === r.key)) {
              results.push(r);
            }
          }
        }
        // Activate new tag
        if (newTag) {
          baseValues.set(key, rawValue);
          const added = activateTagFromSource(key, newTag, fallback);
          for (const r of added) {
            if (!results.some((x) => x.key === r.key)) {
              results.push(r);
            }
          }
        }
      }

      // Update base value
      baseValues.set(key, rawValue);

      // Emit combined value
      const combined = getCombined(key, fallback);
      if (!results.some((x) => x.key === key)) {
        results.push({ key, value: combined });
      }
    } finally {
      inFlight.delete(key);
    }
  }

  return results;
}

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

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)) {
      throw new Error(`Circular dependency involving ${key}`);
    }
    temp.add(key);

    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;
}

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);
  }
}