/**
 * Parse the sectioned form of an `api` block body.
 *
 * Returns null when no section keyword is present, which is how the legacy
 * bare-body form stays valid: the caller keeps treating the body as the
 * response expression.
 *
 * Detection and slicing both drive the tokenizer's own string/comment-aware
 * scanning (`skipLiteralOrComment`, `Lexer.readBalancedBraces`) instead of a
 * second hand-rolled brace counter, so a `}` inside a string or a `request {`
 * mentioned in a comment can't be mistaken for a real section.
 */
import { Lexer, LexError, isIdentPart, isIdentStart, skipLiteralOrComment } from "./tokenizer.ts";

export interface ApiFieldDecl {
  name: string;
  optional: boolean;
  type: string;
}

export interface ApiSections {
  parameters: ApiFieldDecl[];
  body: ApiFieldDecl[];
  response: string;
  error: string;
}

const SECTION_NAMES = ["request", "response", "error"] as const;
const REQUEST_SUBSECTION_NAMES = ["parameters", "body"] as const;

interface Span {
  text: string;
  /** Offset of `text[0]` within the source that was scanned. */
  start: number;
}

/**
 * Walk `source` at brace-depth 0, looking for `name { ... }` where `name` is
 * one of `names`. Strings, template literals, and comments are skipped via
 * `skipLiteralOrComment` — the same rules `readBalancedBraces` uses — so a
 * keyword mentioned inside a string or comment, or nested inside an unrelated
 * `{ }` (e.g. an object literal in a legacy body), is never mistaken for a
 * section. Matched blocks are sliced via `Lexer.readBalancedBraces()` itself,
 * not a reimplementation of it.
 */
function scanTopLevelBlocks(source: string, names: readonly string[]): Map<string, Span> {
  const found = new Map<string, Span>();
  const lx = new Lexer(source);
  let depth = 0;
  let i = 0;
  let atLineStart = true;

  while (i < source.length) {
    const c = source[i]!;

    if (c === "\n") {
      atLineStart = true;
      i++;
      continue;
    }

    const skipped = skipLiteralOrComment(source, i, atLineStart);
    if (skipped !== null) {
      i = skipped;
      atLineStart = false;
      continue;
    }

    if (c !== " " && c !== "\t" && c !== "\r") atLineStart = false;

    if (depth === 0 && isIdentStart(c)) {
      let j = i + 1;
      while (j < source.length && isIdentPart(source[j]!)) j++;
      const word = source.slice(i, j);

      // Skip trivia between the identifier and a possible '{' without
      // treating anything in between as significant yet.
      let k = j;
      let lineStartAtK = false;
      while (k < source.length) {
        const kc = source[k]!;
        if (kc === " " || kc === "\t" || kc === "\r") {
          k++;
          continue;
        }
        if (kc === "\n") {
          lineStartAtK = true;
          k++;
          continue;
        }
        const kSkipped = skipLiteralOrComment(source, k, lineStartAtK);
        if (kSkipped !== null) {
          k = kSkipped;
          lineStartAtK = false;
          continue;
        }
        break;
      }

      if (names.includes(word) && source[k] === "{") {
        lx.pos = k;
        const start = k + 1;
        const text = lx.readBalancedBraces();
        if (!found.has(word)) found.set(word, { text, start });
        i = lx.pos;
        continue;
      }

      i = j;
      continue;
    }

    if (c === "{") depth++;
    else if (c === "}") depth = Math.max(0, depth - 1);
    i++;
  }

  return found;
}

/** Rebase a span captured from `outer.text` back onto the original source. */
function absolutize(span: Span | undefined, outer: Span | undefined): Span | undefined {
  if (!span) return undefined;
  return outer ? { text: span.text, start: outer.start + span.start } : span;
}

/** `name?: string` -> { name, optional, type }. Blank lines and comments are skipped. */
function parseFields(span: Span | undefined): ApiFieldDecl[] {
  if (!span) return [];
  const fields: ApiFieldDecl[] = [];
  let cursor = 0;

  for (const rawLine of span.text.split("\n")) {
    const lineOffset = span.start + cursor;
    cursor += rawLine.length + 1;

    const line = rawLine.trim().replace(/,$/, "");
    if (!line || line.startsWith("//")) continue;

    const match = /^([A-Za-z_$][A-Za-z0-9_$]*)(\?)?\s*:\s*(.+)$/.exec(line);
    if (!match) {
      throw new LexError(
        `Expected "name: type" in an api request section, got "${line}" at offset ${lineOffset}`,
      );
    }

    fields.push({ name: match[1]!, optional: match[2] === "?", type: match[3]!.trim() });
  }

  return fields;
}

export function parseApiSections(source: string): ApiSections | null {
  const top = scanTopLevelBlocks(source, SECTION_NAMES);
  if (top.size === 0) return null;

  const request = top.get("request");
  const sub = request
    ? scanTopLevelBlocks(request.text, REQUEST_SUBSECTION_NAMES)
    : new Map<string, Span>();

  return {
    parameters: parseFields(absolutize(sub.get("parameters"), request)),
    body: parseFields(absolutize(sub.get("body"), request)),
    response: top.get("response")?.text ?? "",
    error: top.get("error")?.text ?? "",
  };
}

/** True when the block declares a `request` section. */
export function hasRequestSection(source: string): boolean {
  return scanTopLevelBlocks(source, SECTION_NAMES).has("request");
}