import { readFileSync } from 'node:fs';
import { join } from 'node:path';
import { buildSchema, type GraphQLSchema } from 'graphql';
import type { SchemaGroupConfig } from './types';

const EXTEND_RE = /\bextend\s+(type|interface)\b/g;

export interface LoadedSchemaGroup {
  name: string;
  schema: GraphQLSchema;
}

/**
 * Build one `GraphQLSchema` from a group's SDL file(s). Files are concatenated
 * in the order given. When `mergeExtends` is set, `extend type X` / `extend
 * interface X` in every file is rewritten to `type X` / `interface X` first —
 * this lets a file that declares `extend type Query` against root fields only
 * present in the composed supergraph (not in the group's own base SDL) build
 * as a single self-contained schema instead of requiring the extend target to
 * pre-exist.
 */
export function loadSchemaGroup(group: SchemaGroupConfig, repoRoot: string): LoadedSchemaGroup {
  const parts = group.files.map((file) => {
    const text = readFileSync(join(repoRoot, file), 'utf8');
    return group.mergeExtends ? text.replace(EXTEND_RE, '$1') : text;
  });
  const sdl = parts.join('\n\n');
  return {
    name: group.name,
    schema: buildSchema(sdl, { assumeValidSDL: true, assumeValid: true }),
  };
}

export function loadSchemaGroups(
  groups: readonly SchemaGroupConfig[],
  repoRoot: string
): LoadedSchemaGroup[] {
  if (groups.length === 0) {
    throw new Error('schema-drift.config.json must declare at least one entry in schemaGroups.');
  }
  return groups.map((g) => loadSchemaGroup(g, repoRoot));
}