import {
	extractFrontmatterBlock,
	findFieldLineRange,
	findFieldRange,
	type TextRange,
} from "./frontmatterBlock";

export { findFieldLineRange, findFieldRange };
export type { TextRange };

export type ReplaceFrontmatterFieldResult =
	| { ok: true; content: string; range: TextRange }
	| { ok: false; reason: "no-frontmatter" | "no-field" };

/**
 * Replace a scalar frontmatter field value on its line.
 * Does not re-serialize YAML; only rewrites the value after `:`.
 */
export function replaceFrontmatterField(
	content: string,
	key: string,
	value: string,
): ReplaceFrontmatterFieldResult {
	const block = extractFrontmatterBlock(content);
	if (!block || block.closeLine < 0) {
		return { ok: false, reason: "no-frontmatter" };
	}

	const lineRange = findFieldLineRange(content, key);
	if (!lineRange) {
		return { ok: false, reason: "no-field" };
	}

	const lines = content.split(/\r?\n/);
	const line = lines[lineRange.startLine] ?? "";
	const colonIndex = line.indexOf(":");
	if (colonIndex < 0) {
		return { ok: false, reason: "no-field" };
	}

	const prefix = line.slice(0, colonIndex + 1);
	const spacing = line.slice(colonIndex + 1).match(/^\s*/)?.[0] ?? " ";
	const newLine = `${prefix}${spacing || " "}${value}`;
	lines[lineRange.startLine] = newLine;

	const eol = content.includes("\r\n") ? "\r\n" : "\n";
	const next = lines.join(eol);

	const valueRange = findFieldRange(next, key);
	return {
		ok: true,
		content: next,
		range: valueRange ?? {
			startLine: lineRange.startLine,
			startCol: 0,
			endLine: lineRange.startLine,
			endCol: newLine.length,
		},
	};
}