reverse-engineering-anything/src/domain/analysisView/analysisViewText.ts

import { analysisViewInputError } from "./analysisViewErrors.js";
import type { AnalysisError } from "../analysisErrorBase.js";
import { err, ok, type Result } from "../result.js";
import type { AnalysisViewCoverage } from "./analysisView.js";
import { pageViewCoverage } from "./analysisViewCoverage.js";

const viewError = (message: string): AnalysisError =>
  analysisViewInputError([
    { path: ["view"], reason: "invalid_value", message },
  ]);

/** Slice text by UTF-16 code units without losing a partial Unicode character. */
export const analysisTextPage = (
  body: string,
  offset: number,
  limit: number,
  label = "Pseudocode",
): Result<
  {
    readonly item: { readonly text: string; readonly unit: "utf16-code-units" };
    readonly coverage: AnalysisViewCoverage;
  },
  AnalysisError
> => {
  const start = Math.min(offset, body.length);
  const high = (n: number) => n >= 0xd800 && n <= 0xdbff;
  const low = (n: number) => n >= 0xdc00 && n <= 0xdfff;
  if (
    start > 0 &&
    start < body.length &&
    high(body.charCodeAt(start - 1)) &&
    low(body.charCodeAt(start))
  )
    return err(viewError(`${label} offset splits a UTF-16 surrogate pair.`));
  let end = Math.min(start + limit, body.length);
  if (
    end > start &&
    end < body.length &&
    high(body.charCodeAt(end - 1)) &&
    low(body.charCodeAt(end))
  )
    end--;
  if (end === start && start < body.length)
    return err(
      viewError(
        `Increase ${label.toLowerCase()} limit to include the next complete Unicode character.`,
      ),
    );
  return ok({
    item: { text: body.slice(start, end), unit: "utf16-code-units" },
    coverage: pageViewCoverage(start, end - start, body.length),
  });
};