export function jsonParser<OutputT extends object = object, KeysT extends keyof OutputT = keyof OutputT, InputT extends { readonly [key in KeysT]: string } = { readonly [key in KeysT]: string }>(keys: readonly KeysT[]): (value: InputT) => Pick<OutputT, KeysT> {
	const keysCopy = keys.slice()
	return (value) => {
		const result: Partial<Pick<OutputT, KeysT>> = {}
		for (const key of keysCopy) {
			result[key] = JSON.parse(value[key])
		}
		return result as Pick<OutputT, KeysT>
	}
}

export function rows<OutputT extends object = object, InputT extends object = object>(transformer?: (value: InputT) => OutputT): (result: D1Result<object>) => OutputT[] {
	if (transformer) {
		return (result) => (result.results as InputT[]).map(transformer)
	} else {
		return (result) => result.results as OutputT[];
	}
}

export function guaranteedSingleton<OutputT extends object = object, InputT extends object = object>(transformer?: (value: InputT) => OutputT): (result: D1Result<object>) => OutputT {
	const inner = singleton<OutputT, InputT>(transformer)

	return (result) => {
		const out = inner(result)
		if (out === null) {
			throw Error('expected exactly one result but got none')
		}
		return out
	}
}

export function singleton<OutputT extends object = object, InputT extends object = object>(transformer?: (value: InputT) => OutputT): (result: D1Result<object>) => OutputT | null {
	if (transformer) {
		return (result) => {
			if (result.results.length > 1) {
				throw Error(`expected single result but got ${result.results.length}`);
			}
			const resultRow = result.results[0] as InputT|undefined
			return resultRow ? transformer(resultRow) : null;
		};
	} else {
		return (result) => {
			if (result.results.length > 1) {
				throw Error(`expected single result but got ${result.results.length}`);
			}
			const resultRow = result.results[0] as OutputT|undefined
			return resultRow ?? null;
		};
	}
}

export function nothing(): (result: D1Result) => void {
	return (result) => {
		if (result.results.length > 0) {
			throw Error(`expected no results but got ${result.results.length}`);
		}
	};
}

export function writeCount(): (result: D1Result) => number {
	return (result) => result.meta.rows_written;
}