chore: split functionality internally (#117)

## PR Checklist

- [x] Addresses an existing open issue: fixes #116
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/eslint-plugin-expect-type/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/eslint-plugin-expect-type/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Splits out the roughly-not-specific-to-ESLint logic from the `expect`
rule into:

* `src/assertions`: parsing assertions out from source code
* `src/failures`: determined unmatched/unmet assertions
* `src/utils`: miscellaneous helpers

src/assertions/parseAssertions.ts

@@ -0,0 +1,136 @@
+import ts from "typescript";
+
+import { getTypeSnapshot } from "../utils/snapshot.js";
+import { parseTwoslashAssertion } from "./parseTwoslashAssertion.js";
+import { Assertion, TwoSlashAssertion } from "./types.js";
+import { Assertions, SyntaxError } from "./types.js";
+
+export function parseAssertions(sourceFile: ts.SourceFile): Assertions {
+	const errorLines = new Set<number>();
+	const typeAssertions = new Map<number, Assertion>();
+	const duplicates: number[] = [];
+	const syntaxErrors: SyntaxError[] = [];
+	const twoSlashAssertions: TwoSlashAssertion[] = [];
+
+	const { text } = sourceFile;
+	const commentRegexp = /\/\/(.*)/g;
+	const lineStarts = sourceFile.getLineStarts();
+	let curLine = 0;
+
+	while (true) {
+		const commentMatch = commentRegexp.exec(text);
+		if (commentMatch === null) {
+			break;
+		}
+
+		// Match on the contents of that comment so we do nothing in a commented-out assertion,
+		// i.e. `// foo; // $ExpectType number`
+		const comment = commentMatch[1];
+		// eslint-disable-next-line regexp/no-unused-capturing-group
+		const matchExpect = /^ ?\$Expect(TypeSnapshot|Type|Error)( (.*))?$/.exec(
+			comment,
+		) as [never, "Error" | "Type" | "TypeSnapshot", never, string?] | null;
+		const commentIndex = commentMatch.index;
+		const line = getLine(commentIndex);
+		if (matchExpect) {
+			const directive = matchExpect[1];
+			const payload = matchExpect[3];
+			switch (directive) {
+				case "TypeSnapshot":
+					const snapshotName = payload;
+					if (snapshotName) {
+						if (typeAssertions.delete(line)) {
+							duplicates.push(line);
+						} else {
+							typeAssertions.set(line, {
+								assertionType: "snapshot",
+								expected: getTypeSnapshot(sourceFile.fileName, snapshotName),
+								snapshotName,
+							});
+						}
+					} else {
+						syntaxErrors.push({
+							line,
+							type: "MissingSnapshotName",
+						});
+					}
+
+					break;
+
+				case "Error":
+					if (errorLines.has(line)) {
+						duplicates.push(line);
+					}
+
+					errorLines.add(line);
+					break;
+
+				case "Type": {
+					const expected = payload;
+					if (expected) {
+						// Don't bother with the assertion if there are 2 assertions on 1 line. Just fail for the duplicate.
+						if (typeAssertions.delete(line)) {
+							duplicates.push(line);
+						} else {
+							typeAssertions.set(line, { assertionType: "manual", expected });
+						}
+					} else {
+						syntaxErrors.push({
+							line,
+							type: "MissingExpectType",
+						});
+					}
+
+					break;
+				}
+			}
+		} else {
+			// Maybe it's a twoslash assertion
+			const assertion = parseTwoslashAssertion(
+				comment,
+				commentIndex,
+				line,
+				text,
+				lineStarts,
+			);
+			if (assertion) {
+				if ("type" in assertion) {
+					syntaxErrors.push(assertion);
+				} else {
+					twoSlashAssertions.push(assertion);
+				}
+			}
+		}
+	}
+
+	return {
+		duplicates,
+		errorLines,
+		syntaxErrors,
+		twoSlashAssertions,
+		typeAssertions,
+	};
+
+	function getLine(pos: number): number {
+		// advance curLine to be the line preceding 'pos'
+		while (lineStarts[curLine + 1] <= pos) {
+			curLine++;
+		}
+
+		// If this is the first token on the line, it applies to the next line.
+		// Otherwise, it applies to the text to the left of it.
+		return isFirstOnLine(text, lineStarts[curLine], pos)
+			? curLine + 1
+			: curLine;
+	}
+}
+
+function isFirstOnLine(text: string, lineStart: number, pos: number): boolean {
+	for (let i = lineStart; i < pos; i++) {
+		if (/\S/.test(text[i])) {
+			return false;
+		}
+	}
+
+	return true;
+}

src/assertions/parseTwoslashAssertion.ts

@@ -0,0 +1,90 @@
+import { SyntaxError, TwoSlashAssertion } from "./types.js";
+
+export function parseTwoslashAssertion(
+	comment: string,
+	commentIndex: number,
+	commentLine: number,
+	sourceText: string,
+	lineStarts: readonly number[],
+): SyntaxError | TwoSlashAssertion | null {
+	const matchTwoslash = /^( *)\^\?(.*)$/.exec(comment) as
+		| [never, string, string]
+		| null;
+	if (!matchTwoslash) {
+		return null;
+	}
+
+	const whitespace = matchTwoslash[1];
+	const rawPayload = matchTwoslash[2];
+	if (rawPayload.length && !rawPayload.startsWith(" ")) {
+		// This is an error: there must be a space after the ^?
+		return {
+			line: commentLine - 1,
+			type: "InvalidTwoslash",
+		};
+	}
+
+	let expected = rawPayload.slice(1); // strip leading space, or leave it as "".
+	if (commentLine === 1) {
+		// This will become an attachment error later.
+		return {
+			assertionType: "twoslash",
+			expected,
+			expectedPrefix: "",
+			expectedRange: [-1, -1],
+			insertSpace: false,
+			position: -1,
+		};
+	}
+
+	// The position of interest is wherever the "^" (caret) is, but on the previous line.
+	const caretIndex = commentIndex + whitespace.length + 2; // 2 = length of "//"
+	const position =
+		caretIndex - (lineStarts[commentLine - 1] - lineStarts[commentLine - 2]);
+
+	const expectedRange: [number, number] = [
+		commentIndex + whitespace.length + 5,
+		commentLine < lineStarts.length
+			? lineStarts[commentLine] - 1
+			: sourceText.length,
+	];
+	// Peak ahead to the next lines to see if the expected type continues
+	const expectedPrefix =
+		sourceText.slice(
+			lineStarts[commentLine - 1],
+			commentIndex + 2 + whitespace.length,
+		) + "   ";
+	for (let nextLine = commentLine; nextLine < lineStarts.length; nextLine++) {
+		const thisLineEnd =
+			nextLine + 1 < lineStarts.length
+				? lineStarts[nextLine + 1] - 1
+				: sourceText.length;
+		const lineText = sourceText.slice(lineStarts[nextLine], thisLineEnd + 1);
+		if (lineText.startsWith(expectedPrefix)) {
+			if (nextLine === commentLine) {
+				expected += "\n";
+			}
+
+			expected += lineText.slice(expectedPrefix.length);
+			expectedRange[1] = thisLineEnd;
+		} else {
+			break;
+		}
+	}
+
+	let insertSpace = false;
+	if (expectedRange[0] > expectedRange[1]) {
+		// this happens if the line ends with "^?" and nothing else
+		expectedRange[0] = expectedRange[1];
+		insertSpace = true;
+	}
+
+	return {
+		assertionType: "twoslash",
+		expected,
+		expectedPrefix,
+		expectedRange,
+		insertSpace,
+		position,
+	};
+}

src/assertions/types.ts

@@ -0,0 +1,76 @@
+export interface SyntaxError {
+	readonly line: number;
+	readonly type:
+		| "InvalidTwoslash"
+		| "MissingExpectType"
+		| "MissingSnapshotName";
+}
+
+export interface ManualAssertion {
+	readonly assertionType: "manual";
+	readonly expected: string;
+}
+
+export interface SnapshotAssertion {
+	readonly assertionType: "snapshot";
+	readonly expected: string | undefined;
+	readonly snapshotName: string;
+}
+
+export interface TwoSlashAssertion {
+	readonly assertionType: "twoslash";
+
+	/**
+	 * The expected type in the twoslash comment
+	 */
+	readonly expected: string;
+
+	/**
+	 * Text before the "^?" (used to produce continuation lines for fixer)
+	 */
+	readonly expectedPrefix: string;
+
+	/**
+	 * Range of positions corresponding to the "expected" string (for fixer)
+	 */
+	readonly expectedRange: [number, number];
+
+	/**
+	 * Does a space need to be added after "^?" when fixing? (If "^?" ends the line.)
+	 */
+	readonly insertSpace: boolean;
+
+	/**
+	 * Position in the source file that the twoslash assertion points at
+	 */
+	readonly position: number;
+}
+
+export type Assertion = ManualAssertion | SnapshotAssertion | TwoSlashAssertion;
+
+export interface Assertions {
+	/**
+	 * Lines with more than one assertion (these are errors).
+	 */
+	readonly duplicates: readonly number[];
+
+	/**
+	 * Lines with an $ExpectError.
+	 */
+	readonly errorLines: ReadonlySet<number>;
+
+	/**
+	 * Syntax Errors
+	 */
+	readonly syntaxErrors: readonly SyntaxError[];
+
+	/**
+	 * Twoslash-style type assertions in the file
+	 */
+	readonly twoSlashAssertions: readonly TwoSlashAssertion[];
+
+	/**
+	 * Map from a line number to the expected type at that line.
+	 */
+	readonly typeAssertions: Map<number, Assertion>;
+}