qr-bmp.ts

const startBufferLength = 0x20
const startBuffer = Buffer.from([
	0x42, 0x4d, // magic ('BM')
	0x00, 0x00, 0x00, 0x00, // length -- @2
	0x00, 0x00, 0x00, 0x00, // should be [0u16, 0u16].
	startBufferLength, 0x00, 0x00, 0x00, // offset

	// dib
	0x0c, 0x00, 0x00, 0x00, // dib length
	0x00, 0x00, // width -- @18
	0x00, 0x00, // height -- @20
	0x01, 0x00, // should be 1u16
	0x01, 0x00, // color bits

	// palette
	0xff, 0xff, 0xff, // white - 0
	0x00, 0x00, 0x00, // black - 1
])

export const render = (data: (0|1)[][]): Buffer => {
	const width = data[0].length
	const height = data.length

	// data with given color bits (1 bit),
	// and add some paddings to make the row length into multiple of 4 bytes
	// ceil(bytes / 4) * 4 where bytes = width * color_bits / 8
	const bitmapRowLength = Math.ceil(width / 32) * 4
	const bitmapLength = bitmapRowLength * height

	const newBuffer = Buffer.from(startBuffer)

	newBuffer.writeUInt32LE(bitmapLength + startBufferLength, 2)
	newBuffer.writeUInt16LE(width, 18)
	newBuffer.writeUInt16LE(height, 20)

	const dataBytes = data.map(v => {
		const buf = Buffer.alloc(bitmapRowLength, 0)
		const len = v.length

		for (let i = 0; i*8 < len; ++i) {
			// bits to octets
			const data = v.slice(i * 8, (i+1) * 8).reduce(
					(st, v, i) => st | v << 8-1-i,
				0 as number
			)

			buf.writeUInt8(data, i)
		}

		return buf
	
	// https://stackoverflow.com/questions/8346115/why-are-bmps-stored-upside-down
	}).reverse()

	dataBytes.unshift(newBuffer)

	return Buffer.concat(dataBytes)
}