/**
 * Apply a parsed list of {@link Edit}s to a text body and return the
 * post-edit lines plus any diagnostic warnings. Pure function: no FS, no
 * mutation of the input.
 *
 * Replacement groups are first normalized by {@link repairReplacementBoundaries},
 * which absorbs common model mistakes where a payload restates unchanged range
 * boundaries or duplicates/drops structural closers.
 */

import { resolveClipboardEdits } from "./clipboard";
import {
	afterInsertLandingShiftWarning,
	ambiguousBoundaryEchoMessage,
	ambiguousCloserSpareMessage,
	blockInsertLandingShiftWarning,
	REPLACEMENT_INDENT_AUTO_SHIFT_WARNING,
	UNRESOLVED_BLOCK_INTERNAL,
} from "./messages";
import { cloneCursor } from "./tokenizer";
import type { Anchor, ApplyResult, Clipboard, Cursor, Edit } from "./types";

type LineOrigin = "original" | "insert" | "replacement";

type InsertEdit = Extract<Edit, { kind: "insert" }>;
type DeleteEdit = Extract<Edit, { kind: "delete" }>;
type AppliedEdit = InsertEdit | DeleteEdit;

interface IndexedEdit {
	edit: AppliedEdit;
	idx: number;
}

function isReplacementInsert(edit: Edit): edit is InsertEdit & { mode: "replacement" } {
	return edit.kind === "insert" && edit.mode === "replacement";
}

function getCursorAnchors(cursor: Cursor): Anchor[] {
	return cursor.kind === "before_anchor" || cursor.kind === "after_anchor" ? [cursor.anchor] : [];
}

function getEditAnchors(edit: AppliedEdit): Anchor[] {
	if (edit.kind === "delete") return [edit.anchor];
	return getCursorAnchors(edit.cursor);
}

function trailingPhantomLine(fileLines: readonly string[]): number {
	// `split("\n")` on a newline-terminated file yields a trailing "" sentinel.
	// It is addressable for inserts (append-past-end), but it is not real
	// content. Deleting it only strips the file's final newline, so ignore delete
	// edits that land there; inclusive ranges ending at EOF then do the intended
	// thing and delete through the last concrete line.
	return fileLines.length > 1 && fileLines[fileLines.length - 1] === "" ? fileLines.length : 0;
}

function dropTrailingPhantomDeletes(edits: AppliedEdit[], fileLines: readonly string[]): AppliedEdit[] {
	const phantomLine = trailingPhantomLine(fileLines);
	if (phantomLine === 0) return edits;
	return edits.filter(edit => edit.kind !== "delete" || edit.anchor.line !== phantomLine);
}

/**
 * Verify every anchored edit points at an existing line. File-version binding is
 * checked once per section via the header hash before this function runs.
 */
function validateLineBounds(edits: readonly AppliedEdit[], fileLines: readonly string[]): void {
	for (const edit of edits) {
		for (const anchor of getEditAnchors(edit)) {
			if (anchor.line < 1 || anchor.line > fileLines.length) {
				throw new Error(`Line ${anchor.line} does not exist (file has ${fileLines.length} lines)`);
			}
		}
	}
}

function cloneAppliedEdit(edit: AppliedEdit, index: number): AppliedEdit {
	if (edit.kind === "delete") return { ...edit, anchor: { ...edit.anchor }, index };
	return { ...edit, cursor: cloneCursor(edit.cursor), index };
}

function insertAtStart(fileLines: string[], lineOrigins: LineOrigin[], lines: string[]): void {
	if (lines.length === 0) return;
	const origins = lines.map((): LineOrigin => "insert");
	if (fileLines.length === 1 && fileLines[0] === "") {
		fileLines.splice(0, 1, ...lines);
		lineOrigins.splice(0, 1, ...origins);
		return;
	}
	fileLines.splice(0, 0, ...lines);
	lineOrigins.splice(0, 0, ...origins);
}

function insertAtEnd(fileLines: string[], lineOrigins: LineOrigin[], lines: string[]): number | undefined {
	if (lines.length === 0) return undefined;
	const origins = lines.map((): LineOrigin => "insert");
	if (fileLines.length === 1 && fileLines[0] === "") {
		fileLines.splice(0, 1, ...lines);
		lineOrigins.splice(0, 1, ...origins);
		return 1;
	}
	const hasTrailingNewline = fileLines.length > 0 && fileLines[fileLines.length - 1] === "";
	const insertIndex = hasTrailingNewline ? fileLines.length - 1 : fileLines.length;
	fileLines.splice(insertIndex, 0, ...lines);
	lineOrigins.splice(insertIndex, 0, ...origins);
	return insertIndex + 1;
}

function bucketAnchorEditsByLine(edits: IndexedEdit[]): Map<number, IndexedEdit[]> {
	const byLine = new Map<number, IndexedEdit[]>();
	for (const entry of edits) {
		const line =
			entry.edit.kind === "delete"
				? entry.edit.anchor.line
				: entry.edit.cursor.kind === "before_anchor" || entry.edit.cursor.kind === "after_anchor"
					? entry.edit.cursor.anchor.line
					: 0;
		const bucket = byLine.get(line);
		if (bucket) bucket.push(entry);
		else byLine.set(line, [entry]);
	}
	return byLine;
}

// ═══════════════════════════════════════════════════════════════════════════
// Replacement-boundary repair
//
// Models routinely miscount a replacement range's edges. Sometimes the payload
// re-states unchanged lines that still live on both sides of the range
// (duplicating a function header and final statement); sometimes it only
// re-states or omits a structural closer, which leaves delimiter balance broken.
//
// A balance-neutral boundary-echo repair fires only when both the leading and
// trailing payload edges are exact copies of the surviving lines outside the
// range. One-sided content echoes are left alone unless delimiter-balance repair
// proves they are duplicated structural boundaries. This preserves intended
// duplicate statements while absorbing the common "body includes the unchanged
// wrapper" mistake.

/** A line that is nothing but closing delimiters: `}`, `)`, `];`, `})`, `},`. */
export const STRUCTURAL_CLOSER_RE = /^\s*[)\]}]+[;,]?\s*$/;

/** A JSX/XML closing boundary that carries structure but no bracket tokens. */
const JSX_CLOSER_RE = /^\s*(?:<\/>|<\/[A-Za-z][\w.:-]*>|\/>)\s*[;,]?\s*$/;
const JSX_NAMED_CLOSER_RE = /^\s*<\/([A-Za-z][\w.:-]*)>\s*[;,]?\s*$/;
const JSX_FRAGMENT_CLOSER_RE = /^\s*<\/>\s*[;,]?\s*$/;

function isStructuralCloserLine(text: string): boolean {
	return STRUCTURAL_CLOSER_RE.test(text) || JSX_CLOSER_RE.test(text);
}

function jsxCloserName(text: string): string | undefined {
	if (JSX_FRAGMENT_CLOSER_RE.test(text)) return "";
	const match = JSX_NAMED_CLOSER_RE.exec(text);
	return match?.[1];
}

interface JsxPayloadTag {
	readonly name: string;
	readonly closing: boolean;
	readonly selfClosing: boolean;
}

function isJsxTagStart(text: string, index: number): boolean {
	const next = text[index + 1];
	return next === ">" || next === "/" || (next >= "A" && next <= "Z") || (next >= "a" && next <= "z");
}

function findJsxTagEnd(text: string, start: number): number {
	let quote: string | undefined;
	let braces = 0;
	for (let i = start + 1; i < text.length; i++) {
		const ch = text[i];
		if (quote) {
			if (ch === "\\" && i + 1 < text.length) {
				i++;
			} else if (ch === quote) {
				quote = undefined;
			}
			continue;
		}
		if (ch === '"' || ch === "'" || ch === "`") {
			quote = ch;
		} else if (ch === "{") {
			braces++;
		} else if (ch === "}" && braces > 0) {
			braces--;
		} else if (ch === ">" && braces === 0) {
			return i;
		}
	}
	return -1;
}

function parseJsxPayloadTag(raw: string): JsxPayloadTag | undefined {
	if (raw === "<>") return { name: "", closing: false, selfClosing: false };
	if (raw === "</>") return { name: "", closing: true, selfClosing: false };
	const closing = raw.startsWith("</");
	const nameStart = closing ? 2 : 1;
	let nameEnd = nameStart;
	while (nameEnd < raw.length && /[\w.:-]/.test(raw[nameEnd])) nameEnd++;
	if (nameEnd === nameStart) return undefined;
	return {
		name: raw.slice(nameStart, nameEnd),
		closing,
		selfClosing: !closing && /\/>\s*$/.test(raw),
	};
}

function readJsxPayloadTags(text: string): JsxPayloadTag[] {
	const tags: JsxPayloadTag[] = [];
	for (let start = text.indexOf("<"); start >= 0; start = text.indexOf("<", start + 1)) {
		if (!isJsxTagStart(text, start)) continue;
		const end = findJsxTagEnd(text, start);
		if (end < 0) break;
		const tag = parseJsxPayloadTag(text.slice(start, end + 1));
		if (tag) tags.push(tag);
		start = end;
	}
	return tags;
}

function payloadHasJsxOpenerForEcho(payloadPrefix: readonly string[], echoLines: readonly string[]): boolean {
	const openTags: string[] = [];
	for (const tag of readJsxPayloadTags(payloadPrefix.join("\n"))) {
		if (tag.closing) {
			if (openTags[openTags.length - 1] === tag.name) openTags.pop();
		} else if (!tag.selfClosing) {
			openTags.push(tag.name);
		}
	}
	for (const line of echoLines) {
		const name = jsxCloserName(line);
		if (name !== undefined && openTags.includes(name)) return true;
	}
	return false;
}

interface DelimiterBalance {
	paren: number;
	bracket: number;
	brace: number;
}

/**
 * Net `()` / `[]` / `{}` delta across `lines`, skipping delimiters inside line
 * comments (`//`), block comments, and string/template literals. Block-comment
 * and backtick-template state carry across lines; `"` / `'` reset at EOL since
 * they cannot span lines. Deliberately language-light: constructs it cannot
 * classify (e.g. regex literals) are counted naively, which can only suppress a
 * repair (the safe direction), never force one.
 */
function computeDelimiterBalance(lines: readonly string[]): DelimiterBalance {
	const balance: DelimiterBalance = { paren: 0, bracket: 0, brace: 0 };
	let inBlockComment = false;
	let quote = "";
	for (const line of lines) {
		for (let i = 0; i < line.length; i++) {
			const ch = line[i];
			if (inBlockComment) {
				if (ch === "*" && line[i + 1] === "/") {
					inBlockComment = false;
					i++;
				}
				continue;
			}
			if (quote) {
				if (ch === "\\") i++;
				else if (ch === quote) quote = "";
				continue;
			}
			if (ch === '"' || ch === "'" || ch === "`") {
				quote = ch;
				continue;
			}
			if (ch === "/" && line[i + 1] === "/") break;
			if (ch === "/" && line[i + 1] === "*") {
				inBlockComment = true;
				i++;
				continue;
			}
			switch (ch) {
				case "(":
					balance.paren++;
					break;
				case ")":
					balance.paren--;
					break;
				case "[":
					balance.bracket++;
					break;
				case "]":
					balance.bracket--;
					break;
				case "{":
					balance.brace++;
					break;
				case "}":
					balance.brace--;
					break;
			}
		}
		// `"` / `'` cannot span lines; only backtick templates and block comments do.
		if (quote === '"' || quote === "'") quote = "";
	}
	return balance;
}

function balanceDelta(a: DelimiterBalance, b: DelimiterBalance): DelimiterBalance {
	return { paren: a.paren - b.paren, bracket: a.bracket - b.bracket, brace: a.brace - b.brace };
}

function balanceNegate(a: DelimiterBalance): DelimiterBalance {
	return { paren: -a.paren, bracket: -a.bracket, brace: -a.brace };
}

function balanceEqual(a: DelimiterBalance, b: DelimiterBalance): boolean {
	return a.paren === b.paren && a.bracket === b.bracket && a.brace === b.brace;
}

function balanceIsZero(a: DelimiterBalance): boolean {
	return a.paren === 0 && a.bracket === 0 && a.brace === 0;
}

function balanceSum(a: DelimiterBalance, b: DelimiterBalance): DelimiterBalance {
	return { paren: a.paren + b.paren, bracket: a.bracket + b.bracket, brace: a.brace + b.brace };
}

function balanceComponentCovers(candidate: number, target: number): boolean {
	if (target === 0) return true;
	return candidate > 0 === target > 0 && Math.abs(candidate) >= Math.abs(target);
}

function balanceCovers(candidate: DelimiterBalance, target: DelimiterBalance): boolean {
	return (
		balanceComponentCovers(candidate.paren, target.paren) &&
		balanceComponentCovers(candidate.bracket, target.bracket) &&
		balanceComponentCovers(candidate.brace, target.brace)
	);
}

interface ReplacementGroup {
	/** Positions in the edit array of the payload inserts, in payload order. */
	insertIndices: number[];
	/** Positions in the edit array of the range deletes, ascending by line. */
	deleteIndices: number[];
	payload: string[];
	/** First deleted line (1-indexed). */
	startLine: number;
	/** Last deleted line (1-indexed). */
	endLine: number;
}

/**
 * Detect a replacement group starting at `start`: a run of `before_anchor`
 * replacement inserts sharing one source op line, immediately followed by the
 * contiguous range deletes for that same op. Mirrors how the parser lowers an
 * `replace N.=M:` hunk with a body.
 */
function findReplacementGroup(edits: readonly AppliedEdit[], start: number): ReplacementGroup | undefined {
	const first = edits[start];
	if (first?.kind !== "insert" || first.mode !== "replacement" || first.cursor.kind !== "before_anchor") {
		return undefined;
	}
	const { lineNum } = first;
	const anchorLine = first.cursor.anchor.line;
	const insertIndices: number[] = [];
	const payload: string[] = [];
	let i = start;
	for (; i < edits.length; i++) {
		const edit = edits[i];
		if (edit.kind !== "insert" || edit.mode !== "replacement" || edit.lineNum !== lineNum) break;
		if (edit.cursor.kind !== "before_anchor" || edit.cursor.anchor.line !== anchorLine) break;
		insertIndices.push(i);
		payload.push(edit.text);
	}
	const deleteIndices: number[] = [];
	let expectedLine = anchorLine;
	for (; i < edits.length; i++) {
		const edit = edits[i];
		if (edit.kind !== "delete" || edit.lineNum !== lineNum || edit.anchor.line !== expectedLine) break;
		deleteIndices.push(i);
		expectedLine++;
	}
	if (deleteIndices.length === 0) return undefined;
	return {
		insertIndices,
		deleteIndices,
		payload,
		startLine: anchorLine,
		endLine: anchorLine + deleteIndices.length - 1,
	};
}

/**
 * Restore a uniformly omitted base indent only when the payload would escape
 * a surviving `{` opener immediately above the replacement. Matching unchanged
 * rows then prove the uniform shift; ordinary indentation-only edits stay exact.
 */
function repairReplacementIndentation(edits: AppliedEdit[], fileLines: readonly string[]): string[] {
	let repaired = false;
	for (let start = 0; start < edits.length; ) {
		const group = findReplacementGroup(edits, start);
		if (group === undefined) {
			start++;
			continue;
		}
		const lastDeleteIndex = group.deleteIndices.at(-1);
		if (lastDeleteIndex === undefined) continue;
		start = lastDeleteIndex + 1;
		if (group.payload.length !== group.deleteIndices.length) continue;
		const preceding = fileLines[group.startLine - 2] ?? "";
		const sourceFirst = fileLines[group.startLine - 1] ?? "";
		const payloadFirst = group.payload[0] ?? "";
		if (
			!preceding.trimEnd().endsWith("{") ||
			!isIndentDeeper(leadingIndent(sourceFirst), leadingIndent(preceding)) ||
			isIndentDeeper(leadingIndent(payloadFirst), leadingIndent(preceding))
		) {
			continue;
		}

		let shift: string | undefined;
		let matches = 0;
		let consistent = true;
		for (let offset = 0; offset < group.payload.length; offset++) {
			const source = fileLines[group.startLine - 1 + offset] ?? "";
			const payload = group.payload[offset];
			if (source.trim().length === 0 || source.trimStart() !== payload.trimStart()) continue;
			const sourceIndent = leadingIndent(source);
			const payloadIndent = leadingIndent(payload);
			if (!sourceIndent.endsWith(payloadIndent)) {
				consistent = false;
				break;
			}
			const candidate = sourceIndent.slice(0, sourceIndent.length - payloadIndent.length);
			if (shift === undefined) shift = candidate;
			else if (shift !== candidate) {
				consistent = false;
				break;
			}
			matches++;
		}
		if (!consistent || !shift || matches < 2 || matches * 2 <= group.payload.length) continue;
		for (const index of group.insertIndices) {
			const edit = edits[index];
			if (edit.kind !== "insert" || edit.text.trim().length === 0) continue;
			edits[index] = { ...edit, text: `${shift}${edit.text}` };
		}
		repaired = true;
	}
	return repaired ? [REPLACEMENT_INDENT_AUTO_SHIFT_WARNING] : [];
}

/**
 * Largest `k` such that the payload's last `k` lines exactly equal the `k`
 * surviving file lines just below the range AND dropping them zeroes `delta`.
 * Requires a non-zero `delta`: a zero-balance candidate can never account for
 * the imbalance, so intentional duplicates of ordinary statements stay intact,
 * while duplicated structural lines (closers like `});`, openers like `foo(`)
 * are dropped when they exactly explain the imbalance.
 */
function findDuplicateSuffix(group: ReplacementGroup, fileLines: readonly string[], delta: DelimiterBalance): number {
	if (balanceIsZero(delta)) return 0;
	const { payload, endLine } = group;
	const maxK = Math.min(payload.length, fileLines.length - endLine);
	for (let k = maxK; k >= 1; k--) {
		let matches = true;
		for (let t = 0; t < k; t++) {
			if (payload[payload.length - k + t] !== fileLines[endLine + t]) {
				matches = false;
				break;
			}
		}
		if (!matches) continue;
		if (balanceEqual(computeDelimiterBalance(payload.slice(payload.length - k)), delta)) return k;
	}
	return 0;
}

/**
 * Largest `j` such that the payload's first `j` lines exactly equal the `j`
 * surviving file lines just above the range AND dropping them zeroes `delta`.
 * Requires a non-zero `delta`; see {@link findDuplicateSuffix}.
 */
function findDuplicatePrefix(group: ReplacementGroup, fileLines: readonly string[], delta: DelimiterBalance): number {
	if (balanceIsZero(delta)) return 0;
	const { payload, startLine } = group;
	const maxJ = Math.min(payload.length, startLine - 1);
	for (let j = maxJ; j >= 1; j--) {
		let matches = true;
		for (let t = 0; t < j; t++) {
			if (payload[t] !== fileLines[startLine - 1 - j + t]) {
				matches = false;
				break;
			}
		}
		if (!matches) continue;
		if (balanceEqual(computeDelimiterBalance(payload.slice(0, j)), delta)) return j;
	}
	return 0;
}
interface DroppedSuffixClosers {
	readonly startLine: number;
	readonly count: number;
	readonly balance: DelimiterBalance;
}

function countPayloadRestatedSuffixHead(payload: readonly string[], suffixLines: readonly string[]): number {
	const maxCount = Math.min(payload.length, suffixLines.length);
	for (let count = maxCount; count >= 1; count--) {
		let matches = true;
		for (let offset = 0; offset < count; offset++) {
			if (payload[payload.length - count + offset] !== suffixLines[offset]) {
				matches = false;
				break;
			}
		}
		if (matches) return count;
	}
	return 0;
}

function countProjectedBelowSuffixTail(
	group: ReplacementGroup,
	fileLines: readonly string[],
	deletedLines: ReadonlySet<number>,
	insertedLineMaps: InsertedLineMaps,
	suffixLines: readonly string[],
): number {
	const below: string[] = [];
	const appendCloserLines = (lines: readonly string[] | undefined): boolean => {
		if (!lines) return true;
		for (const text of lines) {
			if (!STRUCTURAL_CLOSER_RE.test(text)) return false;
			below.push(text);
		}
		return true;
	};
	if (!appendCloserLines(insertedLineMaps.after.get(group.endLine))) return 0;
	for (let line = group.endLine + 1; line <= fileLines.length; line++) {
		if (!appendCloserLines(insertedLineMaps.before.get(line))) break;
		if (!deletedLines.has(line)) {
			const text = fileLines[line - 1] ?? "";
			if (!STRUCTURAL_CLOSER_RE.test(text)) break;
			below.push(text);
		}
		if (!appendCloserLines(insertedLineMaps.after.get(line))) break;
	}
	const maxCount = Math.min(below.length, suffixLines.length);
	for (let count = maxCount; count >= 1; count--) {
		let matches = true;
		for (let offset = 0; offset < count; offset++) {
			if (below[offset] !== suffixLines[suffixLines.length - count + offset]) {
				matches = false;
				break;
			}
		}
		if (matches) return count;
	}
	return 0;
}

interface InsertedLineMaps {
	readonly before: ReadonlyMap<number, readonly string[]>;
	readonly after: ReadonlyMap<number, readonly string[]>;
}

function computeProjectedPrefixBalance(
	group: ReplacementGroup,
	fileLines: readonly string[],
	deletedLines: ReadonlySet<number>,
	insertedByLine: ReadonlyMap<number, readonly string[]>,
	insertedLineMaps: InsertedLineMaps,
): DelimiterBalance {
	const prefix: string[] = [];
	for (let line = 1; line < group.startLine; line++) {
		const inserted = insertedByLine.get(line);
		if (inserted) prefix.push(...inserted);
		if (!deletedLines.has(line)) prefix.push(fileLines[line - 1] ?? "");
	}
	const insertedAtStart = insertedLineMaps.before.get(group.startLine);
	if (insertedAtStart) prefix.push(...insertedAtStart);
	prefix.push(...group.payload);
	return computeDelimiterBalance(prefix);
}

function prefixCanCoverSuffixClosers(
	group: ReplacementGroup,
	fileLines: readonly string[],
	suffixBalance: DelimiterBalance,
	coveredBelowBalance: DelimiterBalance,
	deletedLines: ReadonlySet<number>,
	insertedByLine: ReadonlyMap<number, readonly string[]>,
	insertedLineMaps: InsertedLineMaps,
): boolean {
	const neededOpeners = balanceNegate(suffixBalance);
	const prefixBalance = computeProjectedPrefixBalance(
		group,
		fileLines,
		deletedLines,
		insertedByLine,
		insertedLineMaps,
	);
	const uncoveredPrefixBalance = balanceSum(prefixBalance, coveredBelowBalance);
	return balanceCovers(uncoveredPrefixBalance, neededOpeners);
}

/**
 * Missing segment of the range's deleted structural-closer suffix that should
 * be spared. Payload lines that already restate the suffix head are not kept
 * again, and projected closers immediately below the range satisfy the suffix
 * tail. The remaining middle segment is kept only when backed by unmatched
 * openers plus the whole-patch residual.
 */
function findDroppedSuffixClosers(
	group: ReplacementGroup,
	fileLines: readonly string[],
	delta: DelimiterBalance,
	remainingDelta: DelimiterBalance,
	deletedPrefixBalance: DelimiterBalance,
	deletedLines: ReadonlySet<number>,
	insertedByLine: ReadonlyMap<number, readonly string[]>,
	insertedLineMaps: InsertedLineMaps,
): DroppedSuffixClosers | undefined {
	let suffixLength = 0;
	while (
		suffixLength < group.deleteIndices.length &&
		STRUCTURAL_CLOSER_RE.test(fileLines[group.endLine - suffixLength - 1] ?? "")
	) {
		suffixLength++;
	}
	if (suffixLength === 0) return undefined;

	const suffixStartLine = group.endLine - suffixLength + 1;
	const suffixLines = fileLines.slice(group.endLine - suffixLength, group.endLine);
	const restatedHead = countPayloadRestatedSuffixHead(group.payload, suffixLines);
	const coveredTail = countProjectedBelowSuffixTail(group, fileLines, deletedLines, insertedLineMaps, suffixLines);
	const keepStart = restatedHead;
	const keepEnd = suffixLength - coveredTail;
	if (keepStart >= keepEnd) return undefined;

	const keptLines = suffixLines.slice(keepStart, keepEnd);
	const keptBalance = computeDelimiterBalance(keptLines);
	const neededOpeners = balanceNegate(keptBalance);
	const coveredBelowBalance = computeDelimiterBalance(suffixLines.slice(keepEnd));
	if (!balanceCovers(delta, neededOpeners)) return undefined;
	if (balanceCovers(deletedPrefixBalance, neededOpeners)) return undefined;
	if (!balanceCovers(remainingDelta, neededOpeners)) return undefined;
	if (
		!prefixCanCoverSuffixClosers(
			group,
			fileLines,
			keptBalance,
			coveredBelowBalance,
			deletedLines,
			insertedByLine,
			insertedLineMaps,
		)
	) {
		return undefined;
	}
	return { startLine: suffixStartLine + keepStart, count: keepEnd - keepStart, balance: keptBalance };
}

interface BoundaryEcho {
	leading: number;
	trailing: number;
}

function hasNonWhitespace(text: string): boolean {
	for (let i = 0; i < text.length; i++) {
		const code = text.charCodeAt(i);
		if (code !== 9 && code !== 10 && code !== 11 && code !== 12 && code !== 13 && code !== 32) return true;
	}
	return false;
}

function countDuplicateLeadingBoundaryLines(group: ReplacementGroup, fileLines: readonly string[]): number {
	const { payload, startLine } = group;
	const max = Math.min(payload.length, startLine - 1);
	for (let count = max; count >= 1; count--) {
		let matches = true;
		let hasContent = false;
		for (let offset = 0; offset < count; offset++) {
			const line = payload[offset];
			if (line !== fileLines[startLine - 1 - count + offset]) {
				matches = false;
				break;
			}
			hasContent ||= hasNonWhitespace(line);
		}
		if (matches && hasContent) return count;
	}
	return 0;
}

function countDuplicateTrailingBoundaryLines(group: ReplacementGroup, fileLines: readonly string[]): number {
	const { payload, endLine } = group;
	const max = Math.min(payload.length, fileLines.length - endLine);
	for (let count = max; count >= 1; count--) {
		let matches = true;
		let hasContent = false;
		for (let offset = 0; offset < count; offset++) {
			const line = payload[payload.length - count + offset];
			if (line !== fileLines[endLine + offset]) {
				matches = false;
				break;
			}
			hasContent ||= hasNonWhitespace(line);
		}
		if (matches && hasContent) return count;
	}
	return 0;
}

function findBoundaryEcho(group: ReplacementGroup, fileLines: readonly string[]): BoundaryEcho | undefined {
	const leadingMax = countDuplicateLeadingBoundaryLines(group, fileLines);
	if (leadingMax === 0) return undefined;
	const trailingMax = countDuplicateTrailingBoundaryLines(group, fileLines);
	if (trailingMax === 0) return undefined;
	// Bail when every payload line could be claimed by a boundary echo: any
	// repair would strip explicit replacement content with no signal that the
	// payload was a mistake rather than an intentional duplication.
	if (leadingMax + trailingMax >= group.payload.length) return undefined;
	// Balance-neutrality guard (see header comment): the dropped echo lines must
	// either be delimiter-neutral on their own or exactly cancel the payload/range
	// balance delta. In brace-heavy code where bare closer lines repeat, an
	// "echo" that shifts delimiter balance is structural content the payload
	// placed intentionally — stripping it would corrupt the result.
	const leadingBalance = computeDelimiterBalance(group.payload.slice(0, leadingMax));
	const trailingBalance = computeDelimiterBalance(group.payload.slice(group.payload.length - trailingMax));
	const droppedBalance = balanceDelta(leadingBalance, balanceNegate(trailingBalance));
	if (!balanceIsZero(droppedBalance)) {
		const delta = balanceDelta(
			computeDelimiterBalance(group.payload),
			computeDelimiterBalance(fileLines.slice(group.startLine - 1, group.endLine)),
		);
		if (!balanceEqual(droppedBalance, delta)) return undefined;
	}
	return { leading: leadingMax, trailing: trailingMax };
}

function describeBoundaryEchoRepair(group: ReplacementGroup, echo: BoundaryEcho): string {
	return (
		`Auto-repaired a replacement boundary echo at line ${group.startLine}: ` +
		`dropped ${echo.leading} leading and ${echo.trailing} trailing payload line(s) already present outside the range. ` +
		`Issue the payload as the final desired content for the selected range only — never restate unchanged lines bordering the range.`
	);
}

function describeBoundaryRepair(group: ReplacementGroup, action: string): string {
	return (
		`Auto-repaired a delimiter-balance mismatch in the replacement at line ${group.startLine}: ${action}. ` +
		`Issue the payload as the final desired content only — never restate or omit a closing bracket bordering the range.`
	);
}

/**
 * A single-sided boundary echo in an otherwise delimiter-balanced *multi-line*
 * replacement: the payload's leading XOR trailing edge exactly restates the
 * surviving line(s) just outside the range — the off-by-one "range one line
 * short of the keeper I retyped" mistake (e.g. att: payload ends with
 * `const x = [];` and line B+1 is the same `const x = [];`). Two-sided echoes
 * are handled by {@link findBoundaryEcho}; delimiter-imbalanced one-sided echoes
 * by {@link findDuplicateSuffix}/{@link findDuplicatePrefix}.
 *
 * Scoped broadly for multi-line ranges (a construct rewrite) because retouched
 * neutral keepers are usually boundary mistakes there. Single-line expansions
 * are riskier — ordinary duplicated statements may be intentional — so they are
 * only repaired when the duplicated edge is a structural closer line that
 * carries no delimiter-balance signal itself, such as a JSX `</section>` close.
 * The dropped lines must keep the already-balanced result balanced, and must
 * not consume the whole payload.
 *
 * A detected echo is only *repairable* when the payload is long enough to be
 * the widened range's full content (`payload ≥ range + echo`). Shorter
 * payloads are ambiguous — the echo may instead mean the range itself was
 * shifted by the echo, which keeps the far boundary line(s) the repair would
 * delete — and the caller rejects the edit instead of guessing.
 */
function findOneSidedBoundaryEcho(
	group: ReplacementGroup,
	fileLines: readonly string[],
): { side: "leading" | "trailing"; count: number } | undefined {
	const leading = countDuplicateLeadingBoundaryLines(group, fileLines);
	const trailing = countDuplicateTrailingBoundaryLines(group, fileLines);
	if (leading > 0 === trailing > 0) return undefined;
	const side = leading > 0 ? "leading" : "trailing";
	const count = leading > 0 ? leading : trailing;
	if (count >= group.payload.length) return undefined;
	const echoLines =
		side === "leading" ? group.payload.slice(0, count) : group.payload.slice(group.payload.length - count);
	if (!balanceIsZero(computeDelimiterBalance(echoLines))) return undefined;
	if (group.deleteIndices.length <= 1) {
		if (side !== "trailing" || !echoLines.every(isStructuralCloserLine)) return undefined;
		const payloadPrefix = group.payload.slice(0, group.payload.length - count);
		if (payloadHasJsxOpenerForEcho(payloadPrefix, echoLines)) return undefined;
	}
	return { side, count };
}

function describeOneSidedEchoRepair(group: ReplacementGroup, side: "leading" | "trailing", count: number): string {
	const where = side === "leading" ? "above" : "below";
	return (
		`Auto-repaired a replacement boundary echo at line ${group.startLine}: ` +
		`dropped ${count} ${side} payload line(s) identical to the surviving line(s) just ${where} the range. ` +
		`The range was one line short of the content you retyped — issue the payload as the final content for the ` +
		`selected range only, and widen the range to consume any keeper you restate.`
	);
}

/**
 * One pass-1 outcome per source position: resolved edits (with an optional
 * warning) or a deferred missing-closer candidate, resolved against the
 * whole-patch residual in pass 2.
 */
type RepairSlot =
	| { kind: "edits"; edits: AppliedEdit[]; warning?: string }
	| {
			kind: "candidate";
			group: ReplacementGroup;
			inserts: AppliedEdit[];
			deletes: AppliedEdit[];
			delta: DelimiterBalance;
	  };

/**
 * Delimiter balance of the lines immediately above a group's range that are
 * themselves deleted by other hunks, netted against any payload inserted at
 * those lines. When this covers the group's own delta the matching opener was
 * deleted (or replaced by an opener of the same shape) just above — a deliberate
 * wrapper removal — so the range's deleted closer must stay deleted, not be
 * "kept". Scanned over its own contiguous lines so quote/comment state never
 * bleeds in from elsewhere in the patch.
 */
function netDeletedPrefixBalance(
	group: ReplacementGroup,
	deletedLines: ReadonlySet<number>,
	insertedByLine: ReadonlyMap<number, readonly string[]>,
	fileLines: readonly string[],
): DelimiterBalance {
	const deleted: string[] = [];
	const inserted: string[] = [];
	for (let line = group.startLine - 1; line >= 1 && deletedLines.has(line); line--) {
		deleted.unshift(fileLines[line - 1] ?? "");
		const insertedAtLine = insertedByLine.get(line);
		if (insertedAtLine) inserted.unshift(...insertedAtLine);
	}
	return balanceDelta(computeDelimiterBalance(deleted), computeDelimiterBalance(inserted));
}

/**
 * Net delimiter balance a slot contributes, computed over the slot's own
 * contiguous insert/delete lines only. Summing these per-slot deltas — never one
 * concatenated scan across non-adjacent hunks — keeps backtick/block-comment
 * state local, so an unterminated quote in one hunk cannot mask a real delimiter
 * in another.
 */
function slotPatchDelta(slot: RepairSlot, fileLines: readonly string[]): DelimiterBalance {
	if (slot.kind === "candidate") return slot.delta;
	const inserted: string[] = [];
	const deleted: string[] = [];
	for (const edit of slot.edits) {
		if (edit.kind === "insert") inserted.push(edit.text);
		else deleted.push(fileLines[edit.anchor.line - 1] ?? "");
	}
	return balanceDelta(computeDelimiterBalance(inserted), computeDelimiterBalance(deleted));
}

/**
 * Normalize replacement groups so common off-by-one boundaries do not duplicate
 * unchanged surrounding lines or wrongly drop/keep structural closers. Local
 * repairs run in pass 1; the missing-closer repair is deferred to pass 2 and
 * weighed against the whole-patch delimiter residual, so a closer the range
 * deleted is only kept when the patch as a whole is missing it — never when
 * another hunk already removed the matching opener. Returns the repaired edits
 * plus one warning per repaired group.
 *
 * Repairs fire only when exactly one reading explains the mistake. When the
 * evidence is ambiguous — a one-sided echo whose payload is too short for the
 * widened range, or a spared closer the payload neither opens nor indents
 * into — the function throws instead of guessing, so the author re-issues the
 * edit rather than shipping silently corrupted content.
 */
function repairReplacementBoundaries(
	edits: readonly AppliedEdit[],
	fileLines: readonly string[],
): {
	edits: AppliedEdit[];
	warnings: string[];
} {
	// Pass 1: apply every repair whose correctness is local to one group
	// (boundary echo, duplicate prefix/suffix). Defer the missing-closer repair:
	// it must weigh a group's imbalance against the whole patch, which is only
	// known once the local repairs above have settled.
	const slots: RepairSlot[] = [];
	let i = 0;
	while (i < edits.length) {
		const group = findReplacementGroup(edits, i);
		if (!group) {
			slots.push({ kind: "edits", edits: [edits[i]] });
			i++;
			continue;
		}
		const inserts = group.insertIndices.map(idx => edits[idx]);
		const deletes = group.deleteIndices.map(idx => edits[idx]);
		i = group.deleteIndices[group.deleteIndices.length - 1] + 1;

		const boundaryEcho = findBoundaryEcho(group, fileLines);
		if (boundaryEcho) {
			slots.push({
				kind: "edits",
				edits: [...inserts.slice(boundaryEcho.leading, inserts.length - boundaryEcho.trailing), ...deletes],
				warning: describeBoundaryEchoRepair(group, boundaryEcho),
			});
			continue;
		}

		const delta = balanceDelta(
			computeDelimiterBalance(group.payload),
			computeDelimiterBalance(fileLines.slice(group.startLine - 1, group.endLine)),
		);
		if (balanceIsZero(delta)) {
			const oneSided = findOneSidedBoundaryEcho(group, fileLines);
			if (oneSided) {
				// A payload shorter than range+echo cannot be the widened
				// range's full content: the repair would delete range line(s)
				// the payload never restates, while the "shifted range"
				// reading keeps them. Reject rather than guess.
				if (group.payload.length < group.deleteIndices.length + oneSided.count) {
					throw new Error(
						ambiguousBoundaryEchoMessage(group.startLine, group.endLine, oneSided.side, oneSided.count),
					);
				}
				const trimmed =
					oneSided.side === "leading"
						? inserts.slice(oneSided.count)
						: inserts.slice(0, inserts.length - oneSided.count);
				slots.push({
					kind: "edits",
					edits: [...trimmed, ...deletes],
					warning: describeOneSidedEchoRepair(group, oneSided.side, oneSided.count),
				});
				continue;
			}
			slots.push({ kind: "edits", edits: [...inserts, ...deletes] });
			continue;
		}

		const dupSuffix = findDuplicateSuffix(group, fileLines, delta);
		if (dupSuffix > 0) {
			slots.push({
				kind: "edits",
				edits: [...inserts.slice(0, inserts.length - dupSuffix), ...deletes],
				warning: describeBoundaryRepair(
					group,
					`dropped ${dupSuffix} duplicated trailing payload line(s) already present below the range`,
				),
			});
			continue;
		}
		const dupPrefix = findDuplicatePrefix(group, fileLines, delta);
		if (dupPrefix > 0) {
			slots.push({
				kind: "edits",
				edits: [...inserts.slice(dupPrefix), ...deletes],
				warning: describeBoundaryRepair(
					group,
					`dropped ${dupPrefix} duplicated leading payload line(s) already present above the range`,
				),
			});
			continue;
		}
		slots.push({ kind: "candidate", group, inserts, deletes, delta });
	}

	const projected: AppliedEdit[] = [];
	for (const slot of slots) {
		projected.push(...(slot.kind === "candidate" ? [...slot.inserts, ...slot.deletes] : slot.edits));
	}
	const deletedLines = new Set<number>();
	for (const edit of projected) {
		if (edit.kind === "delete") deletedLines.add(edit.anchor.line);
	}
	const insertedByLine = new Map<number, string[]>();
	const insertedLineMaps: { before: Map<number, string[]>; after: Map<number, string[]> } = {
		before: new Map(),
		after: new Map(),
	};
	for (const edit of projected) {
		if (edit.kind === "insert" && edit.cursor.kind === "bof") {
			const inserted = insertedByLine.get(1);
			if (inserted) inserted.push(edit.text);
			else insertedByLine.set(1, [edit.text]);
			const before = insertedLineMaps.before.get(1);
			if (before) before.push(edit.text);
			else insertedLineMaps.before.set(1, [edit.text]);
			continue;
		}
		if (edit.kind !== "insert") continue;
		for (const anchor of getCursorAnchors(edit.cursor)) {
			const lines = insertedByLine.get(anchor.line);
			if (lines) lines.push(edit.text);
			else insertedByLine.set(anchor.line, [edit.text]);
		}
		if (edit.cursor.kind === "before_anchor" || edit.cursor.kind === "after_anchor") {
			const bySide = edit.cursor.kind === "before_anchor" ? insertedLineMaps.before : insertedLineMaps.after;
			const lines = bySide.get(edit.cursor.anchor.line);
			if (lines) lines.push(edit.text);
			else bySide.set(edit.cursor.anchor.line, [edit.text]);
		}
	}
	let remainingDelta: DelimiterBalance = { paren: 0, bracket: 0, brace: 0 };
	for (const slot of slots) remainingDelta = balanceSum(remainingDelta, slotPatchDelta(slot, fileLines));

	const out: AppliedEdit[] = [];
	const warnings: string[] = [];
	for (const slot of slots) {
		if (slot.kind !== "candidate") {
			if (slot.warning !== undefined) warnings.push(slot.warning);
			out.push(...slot.edits);
			continue;
		}
		const deletedPrefixBalance = netDeletedPrefixBalance(slot.group, deletedLines, insertedByLine, fileLines);
		const droppedClosers = findDroppedSuffixClosers(
			slot.group,
			fileLines,
			slot.delta,
			remainingDelta,
			deletedPrefixBalance,
			deletedLines,
			insertedByLine,
			insertedLineMaps,
		);
		if (droppedClosers) {
			// Sparing a closer re-inserts it *after* the payload, which claims
			// the payload lives inside the block the closer terminates. That
			// claim needs evidence: the payload carries the closer's unmatched
			// opener itself, or its indentation sits deeper than the closer.
			// Without either, "before or after the closer" is a coin flip —
			// reject rather than guess (e.g. a statement swapped onto a lone
			// `}` at the closer's own depth belongs after the block).
			const keptIndent = leadingIndent(fileLines[droppedClosers.startLine - 1] ?? "");
			const payloadIndent = bodyTargetIndent(slot.group.payload);
			const payloadOpens = balanceCovers(
				computeDelimiterBalance(slot.group.payload),
				balanceNegate(droppedClosers.balance),
			);
			if (!payloadOpens && !(payloadIndent !== undefined && isIndentDeeper(payloadIndent, keptIndent))) {
				throw new Error(
					ambiguousCloserSpareMessage(
						slot.group.startLine,
						slot.group.endLine,
						droppedClosers.startLine,
						droppedClosers.count,
					),
				);
			}
			warnings.push(
				describeBoundaryRepair(
					slot.group,
					`kept ${droppedClosers.count} structural closing line(s) the range deleted without restating`,
				),
			);
			out.push(
				...slot.inserts,
				...slot.deletes.filter(
					edit =>
						edit.kind !== "delete" ||
						edit.anchor.line < droppedClosers.startLine ||
						edit.anchor.line >= droppedClosers.startLine + droppedClosers.count,
				),
			);
			for (let line = droppedClosers.startLine; line < droppedClosers.startLine + droppedClosers.count; line++) {
				deletedLines.delete(line);
			}
			remainingDelta = balanceSum(remainingDelta, droppedClosers.balance);
			continue;
		}
		out.push(...slot.inserts, ...slot.deletes);
	}
	return { edits: out, warnings };
}

// ═══════════════════════════════════════════════════════════════════════════
// After-insert landing correction
//
// The body rows of an `insert after N:` hunk carry an implicit depth claim:
// their leading indentation says how deep the author expects the new lines
// to sit. Two corrections share that claim, in opposite directions:
//
// Outward (any after-insert): when the depth is shallower than line N itself,
// the hunk is inserting a sibling of some enclosing construct while anchored
// inside it — the common shape is anchoring on the last statement of a block
// and writing the body at the parent's depth. Sliding the landing point
// forward across the structural closer lines that follow (and nothing else —
// content lines are never crossed) places the body at the depth its
// indentation names.
//
// Inward (block-lowered inserts only): `insert_after_block N:` anchors on the
// resolved block's closing line, but a body indented deeper than that closer
// claims a depth inside the block — the common misreading of the op as
// "append at the end of block N's body". Sliding the landing point backward
// across the block's trailing closer lines places the body inside, at its
// claimed depth. Scoped to block-lowered inserts because there the author
// named the opener and never saw the closer; a plain `insert after M:` on a
// closer line stays literal (the escape hatch for genuinely-after content
// such as method-chain continuations).
//
// Both shifts are deliberately conservative: they fire only when the body
// and anchor indentation are comparable (one is a prefix of the other),
// cross only pure closing-delimiter lines, stop as soon as depth matches the
// body's claim, and are abandoned when any other edit in the patch targets a
// crossed line. Every shift is reported as a warning so the author can
// re-issue when the original landing was intended.

/** Leading run of tabs and spaces. */
function leadingIndent(line: string): string {
	let end = 0;
	while (end < line.length) {
		const code = line.charCodeAt(end);
		if (code !== 9 && code !== 32) break;
		end++;
	}
	return line.slice(0, end);
}

/** `deeper` strictly extends `shallower` (same indent style, more depth). */
function isIndentDeeper(deeper: string, shallower: string): boolean {
	return deeper.length > shallower.length && deeper.startsWith(shallower);
}

interface AfterInsertGroup {
	/** Anchor line shared by every insert row of the hunk. */
	anchor: number;
	/** Indices into the edit list, in patch order. */
	members: number[];
	/** First line of the resolved block when lowered from `insert_after_block N:`. */
	blockStart?: number;
}

/**
 * Depth of an after-insert hunk's body: the shallowest indentation across its
 * non-blank rows. Returns `undefined` when no depth claim can be made — an
 * all-blank or all-closer body, or rows whose indentation styles are not
 * mutually comparable (tabs vs spaces).
 */
function bodyTargetIndent(rows: readonly string[]): string | undefined {
	const nonBlank = rows.filter(hasNonWhitespace);
	if (nonBlank.length === 0) return undefined;
	// A body of pure closers re-balances delimiters; it claims no depth.
	if (nonBlank.every(row => STRUCTURAL_CLOSER_RE.test(row))) return undefined;
	let target = leadingIndent(nonBlank[0] ?? "");
	for (const row of nonBlank) {
		const indent = leadingIndent(row);
		if (indent.startsWith(target)) continue;
		if (target.startsWith(indent)) target = indent;
		else return undefined;
	}
	return target;
}

/**
 * Resolve where an after-insert hunk anchored on `group.anchor` should land
 * given its body depth `target`: the last structural closer line in the run
 * directly below the anchor whose indentation still covers `target`. Returns
 * `undefined` when the landing stays put.
 */
function resolveShiftedLanding(
	group: AfterInsertGroup,
	target: string,
	fileLines: readonly string[],
	targetedLines: ReadonlySet<number>,
): { line: number; crossed: number } | undefined {
	const anchorText = fileLines[group.anchor - 1];
	if (anchorText === undefined || !hasNonWhitespace(anchorText)) return undefined;
	if (!isIndentDeeper(leadingIndent(anchorText), target)) return undefined;

	let landing = group.anchor;
	let crossed = 0;
	for (let line = group.anchor + 1; line <= fileLines.length; line++) {
		const text = fileLines[line - 1] ?? "";
		if (!hasNonWhitespace(text)) continue; // look past blanks, never land on them
		if (!STRUCTURAL_CLOSER_RE.test(text)) break; // content is never crossed
		const indent = leadingIndent(text);
		if (!indent.startsWith(target)) break; // shallower than the body — crossing would over-escape
		if (targetedLines.has(line)) return undefined; // another hunk owns this closer
		landing = line;
		crossed++;
		if (indent.length === target.length) break; // depth returned to the body's level
	}
	return landing === group.anchor ? undefined : { line: landing, crossed };
}

/**
 * Resolve where a block-lowered after-insert anchored on the block's closing
 * line should land given a body depth `target` deeper than that closer: just
 * above the block's trailing run of closer lines, bounded below by
 * `blockStart` (an empty block lands the body right after its opener).
 * Returns `undefined` when the landing stays put.
 */
function resolveInwardLanding(
	group: AfterInsertGroup,
	target: string,
	blockStart: number,
	fileLines: readonly string[],
	targetedLines: ReadonlySet<number>,
): number | undefined {
	const anchorText = fileLines[group.anchor - 1];
	if (anchorText === undefined || !hasNonWhitespace(anchorText)) return undefined;
	// Fires only when the block ends in a pure closer the body out-indents.
	// Blocks ending in content (indentation-only languages) already land the
	// body inside the block — nothing to correct.
	if (!STRUCTURAL_CLOSER_RE.test(anchorText)) return undefined;
	if (!isIndentDeeper(target, leadingIndent(anchorText))) return undefined;

	let landing = group.anchor;
	for (let line = group.anchor; line > blockStart; line--) {
		const text = fileLines[line - 1] ?? "";
		if (!hasNonWhitespace(text)) {
			landing = line - 1; // look past trailing blanks, never land after one
			continue;
		}
		if (!STRUCTURAL_CLOSER_RE.test(text)) break; // content reached — land right after it
		const indent = leadingIndent(text);
		if (!isIndentDeeper(target, indent)) break; // closer at the body's depth — land after it
		// Another hunk owns this closer (the group's own rows put the anchor
		// itself in `targetedLines`; that one is ours to cross).
		if (line !== group.anchor && targetedLines.has(line)) return undefined;
		landing = line - 1;
	}
	return landing === group.anchor ? undefined : landing;
}

/**
 * Slide mis-anchored after-insert hunks to the depth their body indentation
 * claims: outward past the structural closer lines that follow the anchor
 * when the body is shallower, or — for `insert_after_block N:` lowerings —
 * inward across the block's trailing closers when the body is deeper than
 * the block's closing line. Returns the corrected edit list plus one warning
 * per shifted hunk.
 */
function repairAfterInsertLandings(
	edits: readonly AppliedEdit[],
	fileLines: readonly string[],
): { edits: readonly AppliedEdit[]; warnings: string[] } {
	// Group plain (non-replacement) after-anchor inserts per authored hunk:
	// rows of one hunk share the anchor line and the patch header line.
	const groups = new Map<string, AfterInsertGroup>();
	edits.forEach((edit, idx) => {
		if (edit.kind !== "insert" || edit.mode === "replacement") return;
		if (edit.cursor.kind !== "after_anchor") return;
		const key = `${edit.cursor.anchor.line}:${edit.lineNum}`;
		const group = groups.get(key);
		if (group === undefined)
			groups.set(key, { anchor: edit.cursor.anchor.line, members: [idx], blockStart: edit.blockStart });
		else group.members.push(idx);
	});
	if (groups.size === 0) return { edits, warnings: [] };

	// Lines explicitly targeted by any edit; a shift never crosses them.
	const targetedLines = new Set<number>();
	for (const edit of edits) {
		if (edit.kind === "delete") targetedLines.add(edit.anchor.line);
		else if (edit.cursor.kind === "before_anchor" || edit.cursor.kind === "after_anchor")
			targetedLines.add(edit.cursor.anchor.line);
	}

	let out: AppliedEdit[] | undefined;
	const warnings: string[] = [];
	const retarget = (group: AfterInsertGroup, line: number): void => {
		out ??= [...edits];
		for (const idx of group.members) {
			const edit = out[idx] as InsertEdit;
			out[idx] = { ...edit, cursor: { kind: "after_anchor", anchor: { line } } };
		}
	};
	for (const group of groups.values()) {
		const target = bodyTargetIndent(group.members.map(idx => (edits[idx] as InsertEdit).text));
		if (target === undefined) continue;
		const outward = resolveShiftedLanding(group, target, fileLines, targetedLines);
		if (outward !== undefined) {
			retarget(group, outward.line);
			warnings.push(afterInsertLandingShiftWarning(group.anchor, outward.line, outward.crossed));
			continue;
		}
		if (group.blockStart === undefined) continue;
		const inward = resolveInwardLanding(group, target, group.blockStart, fileLines, targetedLines);
		if (inward === undefined) continue;
		retarget(group, inward);
		warnings.push(blockInsertLandingShiftWarning(group.blockStart, group.anchor, inward));
	}
	return { edits: out ?? edits, warnings };
}

/** Optional knobs for {@link applyEdits}. */
export interface ApplyEditsOptions {
	/**
	 * Clipboard register filled by `cut` edits and read by `paste` edits.
	 * Thread one register through every section of a batch to move content
	 * across files; omitted, the call gets a private register.
	 */
	clipboard?: Clipboard;
	/** `PASTE` with an empty register: `throw` (default) or `drop` (streaming previews). */
	onEmptyPaste?: "throw" | "drop";
}

/**
 * Apply a parsed list of edits to a text body. Pure function — no I/O.
 *
 * Returns the post-edit text and the first changed line number (1-indexed).
 * Throws if an anchor is out of bounds.
 */
export function applyEdits(text: string, edits: readonly Edit[], options: ApplyEditsOptions = {}): ApplyResult {
	if (edits.length === 0) return { text, firstChangedLine: undefined };

	const fileLines = text.split("\n");

	// Clipboard pre-pass: capture `cut` ranges from the original lines and
	// expand `paste` edits into plain inserts in authored order.
	const concrete = resolveClipboardEdits(edits, fileLines, options.clipboard ?? {}, {
		...(options.onEmptyPaste === undefined ? {} : { onEmptyPaste: options.onEmptyPaste }),
	});

	// Block edits are deferred until `resolveBlockEdits` expands them into
	// concrete inserts + deletes. Reaching the applier with one still present
	// is an internal wiring bug, not authored-input error.
	for (const edit of concrete) {
		if (edit.kind === "block") throw new Error(UNRESOLVED_BLOCK_INTERNAL);
	}
	const appliedEdits = concrete as readonly AppliedEdit[];

	const lineOrigins: LineOrigin[] = fileLines.map(() => "original");

	let firstChangedLine: number | undefined;
	const trackFirstChanged = (line: number) => {
		if (firstChangedLine === undefined || line < firstChangedLine) firstChangedLine = line;
	};

	const targetEdits = dropTrailingPhantomDeletes(
		appliedEdits.map((edit, index) => cloneAppliedEdit(edit, index)),
		fileLines,
	);
	validateLineBounds(targetEdits, fileLines);
	const indentationWarnings = repairReplacementIndentation(targetEdits, fileLines);
	const { edits: repaired, warnings: boundaryWarnings } = repairReplacementBoundaries(targetEdits, fileLines);
	const { edits: landed, warnings: landingWarnings } = repairAfterInsertLandings(repaired, fileLines);
	const warnings = [...indentationWarnings, ...boundaryWarnings, ...landingWarnings];

	// Partition edits into bof, eof, and anchor-targeted buckets.
	const bofLines: string[] = [];
	const eofLines: string[] = [];
	const anchorEdits: IndexedEdit[] = [];
	landed.forEach((edit, idx) => {
		if (edit.kind === "insert" && edit.cursor.kind === "bof") {
			bofLines.push(edit.text);
		} else if (edit.kind === "insert" && edit.cursor.kind === "eof") {
			eofLines.push(edit.text);
		} else {
			anchorEdits.push({ edit, idx });
		}
	});

	// Apply per-line buckets bottom-up so earlier indices stay valid.
	const byLine = bucketAnchorEditsByLine(anchorEdits);
	for (const line of [...byLine.keys()].sort((a, b) => b - a)) {
		const bucket = byLine.get(line);
		if (!bucket) continue;
		bucket.sort((a, b) => a.idx - b.idx);

		const idx = line - 1;
		const currentLine = fileLines[idx] ?? "";
		const beforeInsertLines: string[] = [];
		const afterInsertLines: string[] = [];
		const replacementLines: string[] = [];
		let deleteLine = false;

		for (const { edit } of bucket) {
			if (isReplacementInsert(edit)) {
				replacementLines.push(edit.text);
			} else if (edit.kind === "insert" && edit.cursor.kind === "after_anchor") {
				afterInsertLines.push(edit.text);
			} else if (edit.kind === "insert") {
				beforeInsertLines.push(edit.text);
			} else if (edit.kind === "delete") {
				deleteLine = true;
			}
		}
		if (
			beforeInsertLines.length === 0 &&
			replacementLines.length === 0 &&
			afterInsertLines.length === 0 &&
			!deleteLine
		)
			continue;

		const replacement = deleteLine
			? [...beforeInsertLines, ...replacementLines, ...afterInsertLines]
			: [...beforeInsertLines, ...replacementLines, currentLine, ...afterInsertLines];
		const origins: LineOrigin[] = [];
		for (let i = 0; i < beforeInsertLines.length; i++) origins.push("insert");
		for (let i = 0; i < replacementLines.length; i++) origins.push(deleteLine ? "replacement" : "insert");
		if (!deleteLine) origins.push(lineOrigins[idx] ?? "original");
		for (let i = 0; i < afterInsertLines.length; i++) origins.push("insert");

		fileLines.splice(idx, 1, ...replacement);
		lineOrigins.splice(idx, 1, ...origins);
		trackFirstChanged(line);
	}

	if (bofLines.length > 0) {
		insertAtStart(fileLines, lineOrigins, bofLines);
		trackFirstChanged(1);
	}
	const eofChangedLine = insertAtEnd(fileLines, lineOrigins, eofLines);
	if (eofChangedLine !== undefined) trackFirstChanged(eofChangedLine);

	return {
		text: fileLines.join("\n"),
		firstChangedLine,
		...(warnings.length > 0 ? { warnings } : {}),
	};
}