import { createHash } from "node:crypto";
import type { ValueType } from "../capability-model/types.js";
import { canonicalJson } from "../capability-model/evolution.js";
import { QueryCompileError, type CheckedQuery } from "./types.js";

/** Bindings refer to checked values, never executable callbacks. A path cannot
 * silently cross a nullable value or expand an unbounded collection. */
export type QueryBinding =
  | { flatMap: { source: QueryBinding; path: string[] } }
  | { literal: string | boolean }
  | { equal: [QueryBinding, QueryBinding] }
  | { from: "input"; path: string[] }
  | { from: "item"; path: string[] }
  | { from: "result"; name: string; path: string[] }
  | { record: Record<string, QueryBinding> };

export interface QueryCompositionStep {
  name: string;
  query: string;
  root: QueryBinding | null;
  arguments: QueryBinding;
  /** Null roots skip this step and produce null, rather than querying an
   * arbitrary object or treating unavailable query inputs as absence. */
  optional?: boolean;
  when?: QueryBinding;
  /** An optional object is unwrapped as item; absence skips the query. */
  with?: QueryBinding;
  each?: { source: QueryBinding; limit: number };
}

export interface QueryComposition {
  id: string;
  input: ValueType;
  steps: QueryCompositionStep[];
  maxQueries: number;
}

export interface CheckedQueryComposition extends QueryComposition {
  output: ValueType;
  definitionDigest: string;
  dependencies: { query: string; digest: string }[];
}

/** Checks a finite dependency graph. Query identities are resolved by the
 * caller's package/workspace scope; no presentation concepts participate. */
export function compileQueryComposition(
  source: QueryComposition,
  queries: ReadonlyMap<string, CheckedQuery>,
): CheckedQueryComposition {
  const fail = (message: string): never => {
    throw new QueryCompileError("QUERY_COMPOSITION", `${source.id}: ${message}`);
  };
  if (!Number.isSafeInteger(source.maxQueries) || source.maxQueries < 1) fail("maxQueries must be a positive integer");
  if (source.input.kind !== "record") fail("input must be a record");
  const outputs = new Map<string, ValueType>();
  const dependencies = new Map<string, string>();
  let count = 0;
  const binding = (expression: QueryBinding, item?: ValueType) =>
    queryBindingType(expression, source.input, outputs, item);
  const equal = (a: ValueType, b: ValueType) => canonicalJson(a) === canonicalJson(b);
  for (const step of source.steps) {
    if (!step.name || outputs.has(step.name)) fail(`duplicate or empty step name ${step.name}`);
    const query = queries.get(step.query);
    if (!query) return fail(`unknown query ${step.query}`);
    let item: ValueType | undefined;
    if (step.with) {
      if (step.each) fail(`${step.name} cannot combine with and each`);
      const value = binding(step.with);
      item = value.kind === "optional" ? value.value : value;
    }
    if (step.each) {
      const collection = binding(step.each.source);
      if (collection.kind !== "list") return fail(`${step.name}.each requires a list`);
      if (!Number.isSafeInteger(step.each.limit) || step.each.limit < 1)
        fail(`${step.name}.each.limit must be a positive integer`);
      item = collection.value;
    }
    count += step.each?.limit ?? 1;
    if (!Number.isSafeInteger(count) || count > source.maxQueries) fail("declared steps exceed maxQueries");
    if (step.when && !equal(binding(step.when, item), { kind: "scalar", name: "bool" }))
      fail(`${step.name}.when requires a boolean`);
    if (query.declaration.root.kind === "workspace-interface") {
      if (step.root !== null) fail(`${step.name} workspace query takes no object root`);
    } else {
      if (step.root === null) return fail(`${step.name} requires an object root`);
      let root = binding(step.root, item);
      if (root.kind === "optional" && step.optional) root = root.value;
      const expected: ValueType = {
        kind: "object-ref",
        expectation: {
          kind: "interface",
          interfaceRevisionId: query.declaration.root.interface,
        },
      };
      if (!equal(root, expected)) fail(`${step.name} root must be a reference to ${query.declaration.root.interface}`);
    }
    if (!equal(binding(step.arguments, item), query.variables))
      fail(`${step.name} arguments do not match ${step.query}`);
    let output = query.output;
    if (step.optional || step.with || step.when) output = { kind: "optional", value: output };
    if (step.each) output = { kind: "list", value: output };
    outputs.set(step.name, output);
    dependencies.set(step.query, query.definitionDigest);
  }
  const result = {
    ...structuredClone(source),
    output: { kind: "record" as const, fields: Object.fromEntries(outputs) },
    dependencies: [...dependencies].map(([query, digest]) => ({ query, digest })),
  };
  return { ...result, definitionDigest: createHash("sha256").update(canonicalJson(result)).digest("hex") };
}

export function queryBindingType(
  expression: QueryBinding,
  input: ValueType,
  outputs: ReadonlyMap<string, ValueType>,
  item?: ValueType,
): ValueType {
  const fail = (message: string): never => {
    throw new QueryCompileError("QUERY_COMPOSITION", message);
  };
  const binding = (expression: QueryBinding, item?: ValueType) => queryBindingType(expression, input, outputs, item);
  const equal = (a: ValueType, b: ValueType) => canonicalJson(a) === canonicalJson(b);
  if ("flatMap" in expression) {
    const rows = binding(expression.flatMap.source, item);
    if (rows.kind !== "list") return fail("flatMap requires a list");
    const children = binding({ from: "item", path: expression.flatMap.path }, rows.value);
    if (children.kind !== "list") return fail("flatMap path requires a list");
    return children;
  }
  if ("literal" in expression)
    return { kind: "scalar", name: typeof expression.literal === "string" ? "string" : "bool" };
  if ("equal" in expression) {
    const left = binding(expression.equal[0], item);
    const right = binding(expression.equal[1], item);
    if (left.kind !== "scalar" || !equal(left, right)) fail("equal requires matching scalar operands");
    return { kind: "scalar", name: "bool" };
  }
  if ("record" in expression)
    return {
      kind: "record",
      fields: Object.fromEntries(Object.entries(expression.record).map(([key, value]) => [key, binding(value, item)])),
    };
  let type = expression.from === "input" ? input : expression.from === "item" ? item : outputs.get(expression.name);
  if (!type)
    return fail(
      expression.from === "result"
        ? `result ${expression.name} must name an earlier step (forward references and cycles are unsupported)`
        : "item is only available within a bounded each step",
    );
  for (const key of expression.path) {
    if (type.kind !== "record" || !Object.hasOwn(type.fields, key))
      return fail(`cannot select ${key} from ${type.kind}; nulls and collections need an explicit boundary`);
    type = type.fields[key]!;
  }
  return type;
}