/**
 * AST evaluator.
 *
 * `evaluate(ast, host)` walks the tree and returns a {@link CellValue}. It never
 * recurses into other cells directly -- it asks the {@link EngineHost} for their
 * *already computed* values, which the recalculation driver guarantees are ready
 * (topological order). Circular cells are detected up front by the graph module
 * and never reach the evaluator.
 *
 * Evaluation model
 * ----------------
 *   - Arithmetic is numeric. `null` (empty cell) counts as 0. A numeric-looking
 *     string is coerced; any other string yields `#VALUE!`.
 *   - Comparisons (`= <> > < >= <=`) result `1`/`0`. Two numbers (or an empty
 *     cell, which takes on `0`) compare numerically; two texts (or an empty
 *     cell, which takes on `''`) compare case-insensitively and trimmed --
 *     same convention as XLOOKUP's own text matching, see evalXLookup(). A
 *     number against a real (non-empty) text never crossmatches: `=`/`<>`
 *     answer definitively false/true, `> < >= <=` have no sane answer and
 *     yield `#VALUE!` (see compareValues()).
 *   - `&` concatenates as text: a number renders plain (`String(n)`, no
 *     locale formatting), an empty cell renders as `''` (see toText()).
 *   - Any error among operands / arguments propagates unchanged (first wins).
 *   - A range may only appear as a function argument. Used in scalar position it
 *     yields `#VALUE!`.
 *   - A `NamedRefNode` (a bare, unqualified name that isn't a cell) resolves
 *     live against `host.resolveNamedRange()` into the cell/range it stands
 *     for, then evaluates exactly like that reference would have (see
 *     `resolveNamedRef()`). Unknown name -> `#NAME?`.
 *   - Unknown sheet name -> `#REF`. Unknown function -> `#NAME?`.
 *   - Division by zero -> `#DIV/0!`.
 *
 * `kind: 'block'` custom functions (FOR/IF/RETURN "code" bodies -- see
 * userFunctions.ts and src/engine/README.md's "Schleifen" section) are the
 * one thing here that isn't a pure, static AST walk: a `BlockCallNode`
 * embedded in a cell's AST is *interpreted* right here, at evaluation time
 * (`evaluateBlockCall`, below `evalScalar`) -- real cell values are needed
 * to decide an `IF` branch or when an early `RETURN` fires, so unlike an
 * ordinary function call this can't be resolved any earlier.
 */

import type { BlockCallNode, BlockNode, CellNode, FuncNode, NamedRefNode, Node, RangeNode, Statement } from './parser';
import { getFunction } from './functions';
import { expandRange } from './refs';
import { substitute } from './userFunctions';
import { FormulaError, isError, type CellValue, type EngineHost } from './types';

export function evaluate(ast: Node, host: EngineHost): CellValue {
  return evalScalar(ast, host);
}

function evalScalar(node: Node, host: EngineHost): CellValue {
  switch (node.kind) {
    case 'number':
      return node.value;
    case 'string':
      return node.value;

    case 'unary': {
      const v = toNumber(evalScalar(node.operand, host));
      if (isError(v)) return v;
      return node.op === '-' ? -v : v;
    }

    case 'binary': {
      if (node.op === '^') {
        const l = toNumber(evalScalar(node.left, host));
        if (isError(l)) return l;
        const r = toNumber(evalScalar(node.right, host));
        if (isError(r)) return r;
        return Math.pow(l, r);
      }
      if (isComparisonOp(node.op)) {
        const lv = evalScalar(node.left, host);
        if (isError(lv)) return lv;
        const rv = evalScalar(node.right, host);
        if (isError(rv)) return rv;
        return compareValues(node.op, lv, rv);
      }
      if (node.op === '&') {
        const lv = toText(evalScalar(node.left, host));
        if (isError(lv)) return lv;
        const rv = toText(evalScalar(node.right, host));
        if (isError(rv)) return rv;
        return lv + rv;
      }
      const l = toNumber(evalScalar(node.left, host));
      if (isError(l)) return l;
      const r = toNumber(evalScalar(node.right, host));
      if (isError(r)) return r;
      switch (node.op) {
        case '+':
          return l + r;
        case '-':
          return l - r;
        case '*':
          return l * r;
        case '/':
          return r === 0 ? new FormulaError('#DIV/0!') : l / r;
      }
      return new FormulaError('#ERR', `unknown operator ${node.op}`);
    }

    case 'cell': {
      const tableId =
        node.qualifier == null
          ? host.homeTableId
          : host.resolveTableName(node.qualifier, node.sheetQualifier ?? null);
      if (!tableId) return new FormulaError('#REF', `unknown table '${node.qualifier}'`);
      return host.getComputed(tableId, node.col, node.row);
    }

    case 'range':
      // A bare range can't collapse to a scalar.
      return new FormulaError('#VALUE!', 'range used where a value is expected');

    case 'namedref': {
      const resolved = resolveNamedRef(node, host);
      if (isError(resolved)) return resolved;
      return evalScalar(resolved, host); // a resolved 1x1 range is a 'cell', so this recursion terminates
    }

    case 'func': {
      // IF is the one built-in that must NOT evaluate its arguments eagerly
      // (see functions.ts's FUNCTIONS.IF comment): only the taken branch may
      // be computed, so e.g. IF(B1=0; 0; A1/B1) never divides by zero when
      // B1=0. Special-cased here, before the generic eager-args dispatch.
      const upperName = node.name.toUpperCase();
      if (upperName === 'IF') return evalIf(node, host);
      // XLOOKUP needs its two range arguments kept separate (not flattened
      // into one combined list like every other function gets, see
      // functions.ts's FUNCTIONS.XLOOKUP comment) and its if_not_found
      // argument evaluated lazily -- both require handling here, before the
      // generic eager/flattening dispatch below.
      if (upperName === 'XLOOKUP') return evalXLookup(node, host);

      const fn = getFunction(node.name);
      if (!fn) return new FormulaError('#NAME?', `unknown function ${node.name}`);
      const values: CellValue[] = [];
      for (const arg of node.args) {
        const part = evalArg(arg, host);
        if (isError(part)) return part;
        for (const v of part) {
          if (isError(v)) return v;
          values.push(v);
        }
      }
      return fn(values);
    }

    case 'param':
      // userFunctions.ts's expandUserFunctions() substitutes every ParamNode
      // away before a cell's AST is ever stored -- this should be unreachable
      // in practice. Fails as a cell error rather than however an unhandled
      // case would otherwise behave, matching the engine's "no crash" rule.
      return new FormulaError('#ERR', `unresolved parameter ${node.name}`);

    case 'blockcall':
      return evaluateBlockCall(node, host);

    case 'omitted':
      // an empty argument slot (`XLOOKUP(x;a;b;;-1)`'s skipped 4th
      // argument) evaluates like any other missing value -- same as an
      // empty cell. A function that needs to distinguish "omitted" from
      // "evaluates to null" checks the raw arg Node's kind directly before
      // ever calling evalScalar on it (see evalXLookup below).
      return null;
  }
}

/** IF(cond; then; else?) -- the real, short-circuiting implementation (see
 *  the `case 'func'` special-case above and functions.ts's FUNCTIONS.IF
 *  comment). Only `node.args[0]` is always evaluated; whichever of the
 *  other two branches is taken is evaluated next, and the other is never
 *  touched at all -- so an error (or, in a block-mode call, any other side
 *  effect) in the untaken branch never surfaces. Same truthy convention as
 *  everywhere else in this engine (`execStatement`'s `ifstmt` case, above):
 *  a number is truthy unless it's exactly 0; text/refs are truthy unless
 *  empty. */
function evalIf(node: FuncNode, host: EngineHost): CellValue {
  if (node.args.length !== 2 && node.args.length !== 3) {
    return new FormulaError('#ERR', 'IF expects 2 or 3 arguments');
  }
  const cond = evalScalar(node.args[0], host);
  if (isError(cond)) return cond;
  const truthy = typeof cond === 'number' ? cond !== 0 : cond !== null && cond !== '';
  if (truthy) return evalScalar(node.args[1], host);
  return node.args.length === 3 ? evalScalar(node.args[2], host) : 0;
}

/** Exact-match equality for XLOOKUP -- deliberately its own thing, not the
 *  `=` operator (which coerces both sides through `toNumber()` and so can
 *  never match on text, XLOOKUP's single most common real-world use: "find
 *  the row where this name/ID matches"). Numbers compare numerically, text
 *  compares case-insensitively with surrounding whitespace ignored (matches
 *  this engine's existing case-insensitive conventions -- function/table
 *  names, `!`/`::` qualifiers). A number is never treated as equal to text
 *  (no cross-type coercion -- predictable, no "5" == 5 surprises); two
 *  empty cells are equal to each other. */
function xlookupEquals(a: CellValue, b: CellValue): boolean {
  if (isError(a) || isError(b)) return false;
  if (a === null || b === null) return a === b;
  if (typeof a === 'number' && typeof b === 'number') return a === b;
  if (typeof a === 'string' && typeof b === 'string') {
    return a.trim().toLowerCase() === b.trim().toLowerCase();
  }
  return false;
}

/**
 * XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; [WennNichtGefunden];
 * [Vergleichsmodus]; [Suchmodus]) -- see functions.ts's FUNCTIONS.XLOOKUP
 * comment for why this has to live here instead. `Suchbereich` and
 * `Rückgabebereich` are evaluated separately via `evalArg` (which already
 * returns one argument's values as their own array -- it's only `case
 * 'func'`'s generic dispatch above that flattens every argument together)
 * and must come out the same length.
 *
 * `Vergleichsmodus`: `0` (default) exact match; `-1` exact match or next
 * *smaller*; `1` exact match or next *larger* (both numeric-only -- a
 * nearest match against text isn't supported, documented scope limit,
 * see README.md). `2` (Excel's wildcard mode) is not supported.
 * `Suchmodus`: `1` (default) first-to-last; `-1` last-to-first. Excel's
 * binary-search modes (`2`/`-2`) are not supported -- pointless at this
 * app's table sizes and would require an assumed-sorted precondition this
 * engine can't verify.
 *
 * No match and no `WennNichtGefunden` given -> `#N/A` (a documented
 * extension to this engine's original error set, exactly the same
 * reasoning `#VALUE!` was added under). `WennNichtGefunden` is evaluated
 * lazily -- only when actually needed, same short-circuit principle as
 * `IF`'s branches -- so an error inside it never surfaces on a successful
 * match.
 */
function evalXLookup(node: FuncNode, host: EngineHost): CellValue {
  const args = node.args;
  if (args.length < 3 || args.length > 6) {
    return new FormulaError('#ERR', 'XLOOKUP expects 3 to 6 arguments');
  }
  // An optional argument counts as "given" only if the slot both exists AND
  // wasn't left empty (`XLOOKUP(x;a;b;;-1)`'s skipped 4th argument, an
  // OmittedNode -- see parser.ts). `args.length` alone can't tell "not
  // provided at all" apart from "explicitly skipped to reach a later one".
  const given = (i: number) => i < args.length && args[i].kind !== 'omitted';

  const lookupValue = evalScalar(args[0], host);
  if (isError(lookupValue)) return lookupValue;

  const lookupArr = evalArg(args[1], host);
  if (isError(lookupArr)) return lookupArr;
  const returnArr = evalArg(args[2], host);
  if (isError(returnArr)) return returnArr;
  if (lookupArr.length !== returnArr.length) {
    return new FormulaError(
      '#VALUE!',
      'XLOOKUP: Suchbereich und Rückgabebereich müssen gleich viele Zellen haben',
    );
  }
  for (const v of lookupArr) if (isError(v)) return v;
  for (const v of returnArr) if (isError(v)) return v;

  let matchMode = 0;
  if (given(4)) {
    const mm = toNumber(evalScalar(args[4], host));
    if (isError(mm)) return mm;
    if (mm !== -1 && mm !== 0 && mm !== 1) {
      return new FormulaError(
        '#ERR',
        'XLOOKUP: Vergleichsmodus muss -1, 0 oder 1 sein (2 wird nicht unterstützt)',
      );
    }
    matchMode = mm;
  }

  let searchMode = 1;
  if (given(5)) {
    const sm = toNumber(evalScalar(args[5], host));
    if (isError(sm)) return sm;
    if (sm !== 1 && sm !== -1) {
      return new FormulaError(
        '#ERR',
        'XLOOKUP: Suchmodus muss 1 oder -1 sein (2/-2 werden nicht unterstützt)',
      );
    }
    searchMode = sm;
  }

  const n = lookupArr.length;
  const order = searchMode === -1 ? [...Array(n).keys()].reverse() : [...Array(n).keys()];

  if (matchMode === 0) {
    for (const i of order) {
      if (xlookupEquals(lookupValue, lookupArr[i])) return returnArr[i];
    }
  } else if (typeof lookupValue === 'number') {
    // Nearest-match modes: numeric only. An exact match still wins outright
    // wherever it's found; otherwise track the closest qualifying candidate.
    let bestIdx = -1;
    let bestVal = 0;
    for (const i of order) {
      const cand = lookupArr[i];
      if (xlookupEquals(lookupValue, cand)) return returnArr[i];
      if (typeof cand !== 'number') continue;
      const qualifies = matchMode === -1 ? cand < lookupValue : cand > lookupValue;
      if (!qualifies) continue;
      const better =
        bestIdx === -1 || (matchMode === -1 ? cand > bestVal : cand < bestVal);
      if (better) {
        bestIdx = i;
        bestVal = cand;
      }
    }
    if (bestIdx !== -1) return returnArr[bestIdx];
  }

  if (given(3)) return evalScalar(args[3], host);
  return new FormulaError('#N/A', 'XLOOKUP: kein Treffer');
}

// ---------------------------------------------------------------------------
// `kind: 'block'` custom functions: a small tree-walking interpreter for a
// BlockCallNode's `block` (see parser.ts's grammar comment and
// src/engine/README.md's "Schleifen" section). Everything else in this file
// is a pure, static AST walk; this is the one place real control flow
// (branching, early exit) happens, so it needs real cell values as it goes
// -- unlike a `kind: 'expression'` call, which is fully resolved to a plain
// expression at parse time and never reaches any special-case code here.
// ---------------------------------------------------------------------------

/** Thrown by a `return` statement to unwind out of `execBlock`/`execStatement`
 *  immediately, carrying the function's result. Caught only by
 *  `evaluateBlockCall`, right where it's thrown from -- a plain, standard
 *  "unwind to a known boundary" pattern for early-exit control flow. */
class ReturnSignal {
  constructor(public readonly value: CellValue) {}
}

/** Total loop iterations one `evaluateBlockCall` may run, summed across
 *  every FOR loop (including nested ones) it executes -- a safety net
 *  against a pathologically large range (e.g. `FOR x IN A1:Z10000 DO ...`),
 *  matching the "no crash, no hang" guarantee. A chain of nested
 *  block-kind calls is separately bounded by expandUserFunctions()'s own
 *  MAX_EXPANSION_DEPTH, so total work across one formula stays finite
 *  either way. Generous for anything this app's tables realistically hold. */
const MAX_LOOP_STEPS = 10_000;

function evaluateBlockCall(node: BlockCallNode, host: EngineHost): CellValue {
  const env = new Map<string, Node>(node.paramNames.map((p, i) => [p.toUpperCase(), node.args[i]]));
  const budget = { steps: 0 };
  try {
    execBlock(node.block, env, host, budget);
  } catch (e) {
    if (e instanceof ReturnSignal) return e.value;
    throw e; // a real problem (bad FOR source, step budget, ...) -- propagates
    // to evaluateCell()'s existing catch-all in workbook.ts, becoming #ERR,
    // exactly like a plain evaluation exception already does.
  }
  throw new Error(`'${node.label}' ended without RETURN`);
}

function execBlock(block: BlockNode, env: Map<string, Node>, host: EngineHost, budget: { steps: number }): void {
  for (const stmt of block.statements) {
    execStatement(stmt, env, host, budget);
  }
}

/** Resolve `expr` against the current environment (substitute() replaces
 *  any name reference with its bound Node -- the caller's argument, or an
 *  earlier statement's/iteration's literal result) and evaluate it. */
function evalInEnv(expr: Node, env: Map<string, Node>, host: EngineHost): CellValue {
  return evalScalar(substitute(expr, env), host);
}

/** Wrap an already-computed scalar back into a literal `Node` so it can be
 *  stored in `env` and later be resolved/evaluated exactly like any other
 *  bound name (a parameter, or the caller's argument expression). `null`
 *  (an empty cell's value) becomes the number `0`, matching how every other
 *  arithmetic context in this engine already treats an empty cell. Callers
 *  check `isError` before ever calling this -- an error value is never
 *  passed in (it aborts the whole function via `ReturnSignal` instead). */
function literalNodeOf(v: number | string | null): Node {
  if (v === null) return { kind: 'number', value: 0 };
  return typeof v === 'number' ? { kind: 'number', value: v } : { kind: 'string', value: v };
}

function execStatement(
  stmt: Statement,
  env: Map<string, Node>,
  host: EngineHost,
  budget: { steps: number },
): void {
  switch (stmt.kind) {
    case 'assign': {
      const v = evalInEnv(stmt.value, env, host);
      if (isError(v)) throw new ReturnSignal(v); // an error in a plain assignment aborts the function with that error
      env.set(stmt.name.toUpperCase(), literalNodeOf(v));
      return;
    }
    case 'return': {
      const v = evalInEnv(stmt.value, env, host);
      throw new ReturnSignal(v);
    }
    case 'ifstmt': {
      const c = evalInEnv(stmt.cond, env, host);
      if (isError(c)) throw new ReturnSignal(c);
      const truthy = typeof c === 'number' ? c !== 0 : c !== null && c !== '';
      if (truthy) execBlock(stmt.then, env, host, budget);
      else if (stmt.else) execBlock(stmt.else, env, host, budget);
      return;
    }
    case 'for': {
      const source = substitute(stmt.source, env);
      const cells = rangeOrCellToCellNodes(source, stmt.varName, host);
      for (const cell of cells) {
        budget.steps++;
        if (budget.steps > MAX_LOOP_STEPS) {
          throw new Error(`FOR-Schleife(n) über zu viele Zellen (> ${MAX_LOOP_STEPS} insgesamt)`);
        }
        env.set(stmt.varName.toUpperCase(), cell);
        execBlock(stmt.body, env, host, budget);
      }
      return;
    }
  }
}

/** A FOR loop's source, once names are resolved against the current
 *  environment, must be a single cell, a range, or a named range -- never a
 *  computed value (mirrors "a range may only appear as a function argument"
 *  for ordinary formulas). Expands a range to its individual cells via
 *  `expandRange` (the same helper `evalArg` uses for a range function
 *  argument), carrying the same table/sheet qualifier through to each. A
 *  named range resolves first (via `host`), then falls into the same
 *  cell/range handling as any other reference. */
function rangeOrCellToCellNodes(node: Node, loopVarName: string, host: EngineHost): CellNode[] {
  if (node.kind === 'namedref') {
    const resolved = resolveNamedRef(node, host);
    if (isError(resolved)) {
      throw new Error(`FOR ${loopVarName} IN ...: unbekannter Name '${node.name}'`);
    }
    node = resolved;
  }
  if (node.kind === 'cell') return [node];
  if (node.kind === 'range') {
    return expandRange(node.start, node.end).map(
      (c): CellNode => ({
        kind: 'cell',
        qualifier: node.qualifier,
        ...(node.sheetQualifier === undefined ? {} : { sheetQualifier: node.sheetQualifier }),
        col: c.col,
        row: c.row,
      }),
    );
  }
  throw new Error(`FOR ${loopVarName} IN ...: die Quelle muss ein Zell- oder Bereichsbezug sein`);
}

/** Evaluate one function argument into a flat list of values (ranges expand). */
function evalArg(node: Node, host: EngineHost): FormulaError | CellValue[] {
  if (node.kind === 'namedref') {
    const resolved = resolveNamedRef(node, host);
    if (isError(resolved)) return resolved;
    return evalArg(resolved, host); // now a plain 'cell'/'range', handled below/by evalScalar
  }
  if (node.kind === 'range') {
    const tableId =
      node.qualifier == null
        ? host.homeTableId
        : host.resolveTableName(node.qualifier, node.sheetQualifier ?? null);
    if (!tableId) return new FormulaError('#REF', `unknown table '${node.qualifier}'`);
    return expandRange(node.start, node.end).map((c) => host.getComputed(tableId, c.col, c.row));
  }
  const v = evalScalar(node, host);
  return [v];
}

/** Resolve a `NamedRefNode` to the concrete `CellNode`/`RangeNode` it stands
 *  for right now (via `host.resolveNamedRange()`), so every other place that
 *  already knows how to evaluate a cell/range (`evalScalar`, `evalArg`,
 *  `rangeOrCellToCellNodes`) can just fall through to that existing logic
 *  instead of duplicating it. `#NAME?` when the name is unknown -- the same
 *  error class an unknown function name gets. */
function resolveNamedRef(node: NamedRefNode, host: EngineHost): CellNode | RangeNode | FormulaError {
  const def = host.resolveNamedRange(node.name);
  if (!def) return new FormulaError('#NAME?', `unknown name '${node.name}'`);
  const sheetField = def.sheetQualifier === undefined ? {} : { sheetQualifier: def.sheetQualifier };
  if (def.start.col === def.end.col && def.start.row === def.end.row) {
    return { kind: 'cell', qualifier: def.qualifier, ...sheetField, col: def.start.col, row: def.start.row };
  }
  return { kind: 'range', qualifier: def.qualifier, ...sheetField, start: def.start, end: def.end };
}

/** Coerce a cell value to a number for arithmetic, or return an error value. */
function toNumber(v: CellValue): number | FormulaError {
  if (isError(v)) return v;
  if (v === null) return 0;
  if (typeof v === 'number') return v;
  const trimmed = v.trim();
  if (trimmed === '') return 0;
  const n = Number(trimmed);
  return Number.isFinite(n) ? n : new FormulaError('#VALUE!', `'${v}' is not a number`);
}

/** Coerce a cell value to text for `&` concatenation, or return an error
 *  value. An empty cell becomes `''` (not `'0'`), matching how it's ignored
 *  everywhere else in this engine rather than treated as a literal number.
 *  A number renders plain (`String(n)`, no thousands separator/locale --
 *  same "locale-neutral" convention as CSV/XLSX export), never `Intl`. */
function toText(v: CellValue): string | FormulaError {
  if (isError(v)) return v;
  if (v === null) return '';
  return typeof v === 'string' ? v : String(v);
}

type ComparisonOp = '=' | '<>' | '>' | '<' | '>=' | '<=';

function isComparisonOp(op: string): op is ComparisonOp {
  return op === '=' || op === '<>' || op === '>' || op === '<' || op === '>=' || op === '<=';
}

/** `= <> > < >= <=` on two already-evaluated scalars -- see the module
 *  comment's "Comparisons" bullet for the full rule. An empty cell (`null`)
 *  takes on whichever type the other side is (`0` for a number, `''` for
 *  text), so both `=A1=0` and `=A1=""` work on a genuinely empty `A1`. Text
 *  is matched the same way XLOOKUP matches text (case-insensitive, trimmed)
 *  -- kept as one convention across the engine rather than two. */
function compareValues(op: ComparisonOp, l: CellValue, r: CellValue): CellValue {
  const lIsText = typeof l === 'string';
  const rIsText = typeof r === 'string';

  // A real number against real (non-empty) text is a genuine type mismatch
  // -- an empty cell (null) is handled below, taking on whichever type the
  // OTHER side is, so it never reaches this branch.
  if ((typeof l === 'number' && rIsText) || (lIsText && typeof r === 'number')) {
    if (op === '=') return 0;
    if (op === '<>') return 1;
    return new FormulaError('#VALUE!', 'text and number are not comparable with > < >= <=');
  }

  if (lIsText || rIsText) {
    const lt = (l ?? '').toString().trim().toLowerCase();
    const rt = (r ?? '').toString().trim().toLowerCase();
    return applyCompare(op, lt, rt);
  }
  const ln = l === null ? 0 : (l as number);
  const rn = r === null ? 0 : (r as number);
  return applyCompare(op, ln, rn);
}

function applyCompare(op: ComparisonOp, l: number, r: number): CellValue;
function applyCompare(op: ComparisonOp, l: string, r: string): CellValue;
function applyCompare(op: ComparisonOp, l: number | string, r: number | string): CellValue {
  switch (op) {
    case '=':
      return l === r ? 1 : 0;
    case '<>':
      return l !== r ? 1 : 0;
    case '>':
      return l > r ? 1 : 0;
    case '<':
      return l < r ? 1 : 0;
    case '>=':
      return l >= r ? 1 : 0;
    case '<=':
      return l <= r ? 1 : 0;
  }
}