std/bmp/decode_bmp.wuffs

// Copyright 2020 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

pub status "#bad header"
pub status "#bad RLE compression"
pub status "#unsupported BMP file"

pri status "@internal note: short read"

pub const DECODER_WORKBUF_LEN_MAX_INCL_WORST_CASE : base.u64 = 0

pri const COMPRESSION_NONE           : base.u32 = 0
pri const COMPRESSION_RLE8           : base.u32 = 1
pri const COMPRESSION_RLE4           : base.u32 = 2
pri const COMPRESSION_BITFIELDS      : base.u32 = 3
pri const COMPRESSION_JPEG           : base.u32 = 4
pri const COMPRESSION_PNG            : base.u32 = 5
pri const COMPRESSION_ALPHABITFIELDS : base.u32 = 6
pri const COMPRESSION_LOW_BIT_DEPTH  : base.u32 = 0x100

pub struct decoder? implements base.image_decoder(
	width  : base.u32[..= 0x7FFF_FFFF],
	height : base.u32[..= 0x7FFF_FFFF],

	// Call sequence states:
	//  - 0x00: initial state.
	//  - 0x03: image config decoded.
	//  - 0x04: frame config decoded.
	//  - 0xFF: end-of-data, usually after (the non-animated) frame decoded.
	//
	// State transitions:
	//
	//  - 0x00 -> 0x03: via DIC
	//  - 0x00 -> 0x04: via DFC with implicit DIC
	//  - 0x00 -> 0xFF: via DF  with implicit DIC and DFC
	//
	//  - 0x03 -> 0x04: via DFC
	//  - 0x03 -> 0xFF: via DF  with implicit DFC
	//
	//  - 0x04 -> 0xFF: via DFC
	//  - 0x04 -> 0xFF: via DF
	//
	//  - ???? -> 0x03: via RF  for ???? > 0x00
	//
	// Where:
	//  - DF  is decode_frame
	//  - DFC is decode_frame_config, implicit means nullptr args.dst
	//  - DIC is decode_image_config, implicit means nullptr args.dst
	//  - RF  is restart_frame
	call_sequence : base.u8,

	top_down    : base.bool,
	pad_per_row : base.u32[..= 3],
	src_pixfmt  : base.u32,

	io_redirect_fourcc : base.u32,
	io_redirect_pos    : base.u64,

	frame_config_io_position : base.u64,

	bitmap_info_len : base.u32,
	padding         : base.u32,

	bits_per_pixel : base.u32,
	compression    : base.u32,

	// channel_etc's indexes are: B, G, R, A. This is in Wuffs' default order,
	// which isn't the RGBA order they're listed in the wire format.
	channel_masks    : array[4] base.u32,
	channel_shifts   : array[4] base.u8[..= 31],
	channel_num_bits : array[4] base.u8[..= 32],

	dst_x     : base.u32,
	dst_y     : base.u32,
	dst_y_inc : base.u32,

	pending_pad : base.u32[..= 3],

	rle_state   : base.u32,
	rle_length  : base.u32[..= 0xFF],
	rle_delta_x : base.u8,
	rle_padded  : base.bool,

	swizzler : base.pixel_swizzler,
	util     : base.utility,
)(
	// scratch is one of:
	//  - 2048 bytes = 256  * (8 bytes per BGRA_NONPREMUL_4X16LE).
	//  - 1024 bytes = 1024 * (1 byte  per INDEXED__BGRA_BINARY), plus
	//    1024 bytes = 256  * (4 bytes per dst_palette entry).
	scratch : array[2048] base.u8,

	src_palette : array[4 * 256] base.u8,
)

pub func decoder.set_quirk_enabled!(quirk: base.u32, enabled: base.bool) {
}

pub func decoder.decode_image_config?(dst: nptr base.image_config, src: base.io_reader) {
	var magic      : base.u32
	var width      : base.u32
	var height     : base.u32
	var planes     : base.u32
	var dst_pixfmt : base.u32
	var byte_width : base.u32

	if (this.call_sequence <> 0) or (this.io_redirect_fourcc == 1) {
		return base."#bad call sequence"
	} else if this.io_redirect_fourcc <> 0 {
		return base."@I/O redirect"
	}

	// Read the BITMAPFILEHEADER (14 bytes).

	magic = args.src.read_u16le_as_u32?()
	if magic <> 'BM'le {
		return "#bad header"
	}

	args.src.skip_u32?(n: 8)

	this.padding = args.src.read_u32le?()
	if this.padding < 14 {
		return "#bad header"
	}
	this.padding -= 14
	this.io_redirect_pos = (this.padding as base.u64) ~sat+ args.src.position()

	// Read the BITMAPINFOHEADER:
	//  - OS/2 is 12, 16 or 64 bytes.
	//  - Windows is 40, 108 or 124 bytes.

	this.bitmap_info_len = args.src.read_u32le?()
	if this.padding < this.bitmap_info_len {
		return "#bad header"
	}
	this.padding -= this.bitmap_info_len

	if this.bitmap_info_len == 12 {
		this.width = args.src.read_u16le_as_u32?()
		this.height = args.src.read_u16le_as_u32?()
		planes = args.src.read_u16le_as_u32?()
		if planes <> 1 {
			return "#unsupported BMP file"
		}
		this.bits_per_pixel = args.src.read_u16le_as_u32?()

	} else if this.bitmap_info_len == 16 {
		width = args.src.read_u32le?()
		if width >= 0x8000_0000 {
			return "#bad header"
		}
		this.width = width
		height = args.src.read_u32le?()
		if height >= 0x8000_0000 {
			return "#bad header"
		}
		this.height = height
		planes = args.src.read_u16le_as_u32?()
		if planes <> 1 {
			return "#unsupported BMP file"
		}
		this.bits_per_pixel = args.src.read_u16le_as_u32?()

	} else {
		width = args.src.read_u32le?()
		if width >= 0x8000_0000 {
			return "#bad header"
		}
		this.width = width

		height = args.src.read_u32le?()
		if height == 0x8000_0000 {
			return "#bad header"
		} else if height >= 0x8000_0000 {
			// The &0x7FFF_FFFF is redundant, but proves to the compiler that
			// the result is within this.height's refined bounds.
			this.height = (0 ~mod- height) & 0x7FFF_FFFF
			this.top_down = true
		} else {
			this.height = height
		}

		planes = args.src.read_u16le_as_u32?()
		if planes <> 1 {
			return "#unsupported BMP file"
		}

		this.bits_per_pixel = args.src.read_u16le_as_u32?()
		this.compression = args.src.read_u32le?()
		if this.bits_per_pixel == 0 {
			if this.compression == COMPRESSION_JPEG {
				this.io_redirect_fourcc = 'JPEG'be
				return base."@I/O redirect"
			} else if this.compression == COMPRESSION_PNG {
				this.io_redirect_fourcc = 'PNG 'be
				return base."@I/O redirect"
			}
			return "#unsupported BMP file"
		}

		// We've already read 20 bytes from the BITMAPINFOHEADER: size (4),
		// width (4), height (4), planes (2), bpp (2), compression (4). Skip
		// the rest of the version 3 BITMAPINFOHEADER (whose total size is 40).
		args.src.skip_u32?(n: 40 - 20)

		if this.bitmap_info_len == 40 {
			if this.bits_per_pixel >= 16 {
				// It's poorly documented, but "length 40" data can be silently
				// augmented with RGB or RGBA channel_masks (12 or 16 bytes).
				if this.padding >= 16 {
					this.bitmap_info_len = 56
					this.padding -= 16
				} else if this.padding >= 12 {
					this.bitmap_info_len = 52
					this.padding -= 12
				}
			}
		} else if (this.bitmap_info_len <> 52) and
			(this.bitmap_info_len <> 56) and
			(this.bitmap_info_len <> 64) and
			(this.bitmap_info_len <> 108) and
			(this.bitmap_info_len <> 124) {
			return "#unsupported BMP file"
		}

		// Treat COMPRESSION_ALPHABITFIELDS the same as COMPRESSION_BITFIELDS.
		if this.compression == COMPRESSION_ALPHABITFIELDS {
			this.compression = COMPRESSION_BITFIELDS
		}

		// Read the channel_masks when this.compression is
		// COMPRESSION_BITFIELDS.
		if this.compression == COMPRESSION_BITFIELDS {
			if this.bitmap_info_len >= 52 {
				this.channel_masks[2] = args.src.read_u32le?()
				this.channel_masks[1] = args.src.read_u32le?()
				this.channel_masks[0] = args.src.read_u32le?()
				if this.bitmap_info_len >= 56 {
					this.channel_masks[3] = args.src.read_u32le?()
					// Skip the rest of the BITMAPINFOHEADER.
					args.src.skip_u32?(n: this.bitmap_info_len ~mod- 56)
				}
				// If the explicit channel_masks are what the implicit ones
				// would be for no compression, treat it as no compression.
				if (this.channel_masks[0] == 0x0000_00FF) and
					(this.channel_masks[1] == 0x0000_FF00) and
					(this.channel_masks[2] == 0x00FF_0000) {
					if this.bits_per_pixel == 24 {
						this.compression = COMPRESSION_NONE
					} else if this.bits_per_pixel == 32 {
						if (this.channel_masks[3] == 0) or
							(this.channel_masks[3] == 0xFF00_0000) {
							this.compression = COMPRESSION_NONE
						}
					}
				}
				this.process_masks?()
			}
		} else if this.bitmap_info_len >= 40 {
			// Skip the rest of the BITMAPINFOHEADER.
			args.src.skip_u32?(n: this.bitmap_info_len - 40)
		} else {
			return "#unsupported BMP file"
		}
	}

	if this.compression <> COMPRESSION_BITFIELDS {
		// The palette follows the BITMAPINFOHEADER.
		if this.bits_per_pixel < 16 {
			this.read_palette?(src: args.src)
		}
	}

	if this.compression == COMPRESSION_NONE {
		if (this.bits_per_pixel == 1) or
			(this.bits_per_pixel == 2) or
			(this.bits_per_pixel == 4) {
			this.src_pixfmt = base.PIXEL_FORMAT__INDEXED__BGRA_BINARY
			this.compression = COMPRESSION_LOW_BIT_DEPTH
		} else if this.bits_per_pixel == 8 {
			this.src_pixfmt = base.PIXEL_FORMAT__INDEXED__BGRA_BINARY
		} else if this.bits_per_pixel == 16 {
			// BMP's 16-bit default is BGRX_5551.
			this.compression = COMPRESSION_BITFIELDS
			this.channel_masks[0] = 0x001F
			this.channel_masks[1] = 0x03E0
			this.channel_masks[2] = 0x7C00
			this.channel_masks[3] = 0x0000
			this.process_masks?()
			this.src_pixfmt = base.PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE
		} else if this.bits_per_pixel == 24 {
			this.src_pixfmt = base.PIXEL_FORMAT__BGR
		} else if this.bits_per_pixel == 32 {
			if this.channel_masks[3] == 0 {
				this.src_pixfmt = base.PIXEL_FORMAT__BGRX
			} else {
				this.src_pixfmt = base.PIXEL_FORMAT__BGRA_NONPREMUL
			}
		} else {
			return "#unsupported BMP file"
		}

	} else if this.compression == COMPRESSION_RLE8 {
		if this.bits_per_pixel == 8 {
			this.src_pixfmt = base.PIXEL_FORMAT__INDEXED__BGRA_BINARY
		} else {
			return "#unsupported BMP file"
		}

	} else if this.compression == COMPRESSION_RLE4 {
		if this.bits_per_pixel == 4 {
			this.src_pixfmt = base.PIXEL_FORMAT__INDEXED__BGRA_BINARY
		} else {
			return "#unsupported BMP file"
		}

	} else if this.compression == COMPRESSION_BITFIELDS {
		if (this.bits_per_pixel == 16) or (this.bits_per_pixel == 32) {
			this.src_pixfmt = base.PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE
		} else {
			return "#unsupported BMP file"
		}

	} else {
		return "#unsupported BMP file"
	}

	// OS/2 BMP files have fewer valid bits_per_pixel values.
	if ((this.bitmap_info_len < 40) or (this.bitmap_info_len == 64)) and
		(this.bits_per_pixel <> 1) and
		(this.bits_per_pixel <> 4) and
		(this.bits_per_pixel <> 8) and
		(this.bits_per_pixel <> 24) {
		return "#bad header"
	}

	// The "((x + 3) >> 2) << 2" dance rounds x up to a multiple of 4.
	if this.bits_per_pixel == 1 {
		// byte_width is this.width divided by 8, rounding up.
		byte_width = (this.width >> 3) + (((this.width & 7) + 7) >> 3)
		this.pad_per_row = (4 - (byte_width & 3)) & 3
	} else if this.bits_per_pixel == 2 {
		// byte_width is this.width divided by 4, rounding up.
		byte_width = (this.width >> 2) + (((this.width & 3) + 3) >> 2)
		this.pad_per_row = (4 - (byte_width & 3)) & 3
	} else if this.bits_per_pixel == 4 {
		// byte_width is this.width divided by 2, rounding up.
		byte_width = (this.width >> 1) + (this.width & 1)
		this.pad_per_row = (4 - (byte_width & 3)) & 3
	} else if this.bits_per_pixel == 8 {
		this.pad_per_row = (4 - (this.width & 3)) & 3
	} else if this.bits_per_pixel == 16 {
		this.pad_per_row = (this.width & 1) * 2
	} else if this.bits_per_pixel == 24 {
		this.pad_per_row = this.width & 3
	} else if this.bits_per_pixel == 32 {
		this.pad_per_row = 0
	}

	this.frame_config_io_position = args.src.position()

	if args.dst <> nullptr {
		dst_pixfmt = base.PIXEL_FORMAT__BGRA_NONPREMUL
		if (this.channel_num_bits[0] > 8) or
			(this.channel_num_bits[1] > 8) or
			(this.channel_num_bits[2] > 8) or
			(this.channel_num_bits[3] > 8) {
			dst_pixfmt = base.PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE
		}

		args.dst.set!(
			pixfmt: dst_pixfmt,
			pixsub: 0,
			width: this.width,
			height: this.height,
			first_frame_io_position: this.frame_config_io_position,
			first_frame_is_opaque: this.channel_masks[3] == 0)
	}

	this.call_sequence = 3
}

pub func decoder.decode_frame_config?(dst: nptr base.frame_config, src: base.io_reader) {
	if this.call_sequence < 3 {
		this.decode_image_config?(dst: nullptr, src: args.src)
	} else if this.call_sequence == 3 {
		if this.frame_config_io_position <> args.src.position() {
			return base."#bad restart"
		}
	} else if this.call_sequence == 4 {
		this.call_sequence = 0xFF
		return base."@end of data"
	} else {
		return base."@end of data"
	}

	if args.dst <> nullptr {
		args.dst.set!(bounds: this.util.make_rect_ie_u32(
			min_incl_x: 0,
			min_incl_y: 0,
			max_excl_x: this.width,
			max_excl_y: this.height),
			duration: 0,
			index: 0,
			io_position: this.frame_config_io_position,
			disposal: 0,
			opaque_within_bounds: true,
			overwrite_instead_of_blend: false,
			background_color: 0xFF00_0000)
	}

	this.call_sequence = 4
}

pub func decoder.decode_frame?(dst: ptr base.pixel_buffer, src: base.io_reader, blend: base.pixel_blend, workbuf: slice base.u8, opts: nptr base.decode_frame_options) {
	var status : base.status

	if this.call_sequence < 4 {
		this.decode_frame_config?(dst: nullptr, src: args.src)
	} else if this.call_sequence == 4 {
		// No-op.
	} else {
		return base."@end of data"
	}

	args.src.skip_u32?(n: this.padding)

	if (this.width > 0) and (this.height > 0) {
		this.dst_x = 0
		if this.top_down {
			this.dst_y = 0
			this.dst_y_inc = 1
		} else {
			this.dst_y = this.height ~mod- 1
			this.dst_y_inc = 0xFFFF_FFFF  // -1 as a base.u32.
		}

		status = this.swizzler.prepare!(
			dst_pixfmt: args.dst.pixel_format(),
			dst_palette: args.dst.palette_or_else(fallback: this.scratch[1024 ..]),
			src_pixfmt: this.util.make_pixel_format(repr: this.src_pixfmt),
			src_palette: this.src_palette[..],
			blend: args.blend)
		if not status.is_ok() {
			return status
		}

		while true {
			if this.compression == COMPRESSION_NONE {
				status = this.swizzle_none!(dst: args.dst, src: args.src)
			} else if this.compression < COMPRESSION_BITFIELDS {
				status = this.swizzle_rle!(dst: args.dst, src: args.src)
			} else if this.compression == COMPRESSION_BITFIELDS {
				status = this.swizzle_bitfields!(dst: args.dst, src: args.src)
			} else {
				status = this.swizzle_low_bit_depth!(dst: args.dst, src: args.src)
			}

			if status.is_ok() {
				break
			} else if status <> "@internal note: short read" {
				return status
			}
			yield? base."$short read"
		} endwhile

		args.src.skip_u32?(n: this.pending_pad)
		this.pending_pad = 0
	}

	this.call_sequence = 0xFF
}

pri func decoder.swizzle_none!(dst: ptr base.pixel_buffer, src: base.io_reader) base.status {
	var dst_pixfmt          : base.pixel_format
	var dst_bits_per_pixel  : base.u32[..= 256]
	var dst_bytes_per_pixel : base.u64[..= 32]
	var dst_bytes_per_row   : base.u64
	var dst_palette         : slice base.u8
	var tab                 : table base.u8
	var dst                 : slice base.u8
	var i                   : base.u64
	var n                   : base.u64

	// TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
	// to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
	dst_pixfmt = args.dst.pixel_format()
	dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()
	if (dst_bits_per_pixel & 7) <> 0 {
		return base."#unsupported option"
	}
	dst_bytes_per_pixel = (dst_bits_per_pixel / 8) as base.u64
	dst_bytes_per_row = (this.width as base.u64) * dst_bytes_per_pixel
	dst_palette = args.dst.palette_or_else(fallback: this.scratch[1024 ..])
	tab = args.dst.plane(p: 0)

	while.outer true {
		while this.pending_pad > 0 {
			if args.src.length() <= 0 {
				return "@internal note: short read"
			}
			this.pending_pad -= 1
			args.src.skip_u32_fast!(actual: 1, worst_case: 1)
		} endwhile

		while.inner true {
			if this.dst_x == this.width {
				this.dst_x = 0
				this.dst_y ~mod+= this.dst_y_inc
				if this.dst_y >= this.height {
					if this.height > 0 {
						this.pending_pad = this.pad_per_row
					}
					break.outer
				} else if this.pad_per_row <> 0 {
					this.pending_pad = this.pad_per_row
					continue.outer
				}
			}

			dst = tab.row(y: this.dst_y)
			if dst_bytes_per_row < dst.length() {
				dst = dst[.. dst_bytes_per_row]
			}
			i = (this.dst_x as base.u64) * dst_bytes_per_pixel
			if i >= dst.length() {
				// TODO: advance args.src if the dst pixel_buffer bounds is
				// smaller than this BMP's image bounds?
				continue.inner
			}
			n = this.swizzler.swizzle_interleaved_from_reader!(
				dst: dst[i ..],
				dst_palette: dst_palette,
				src: args.src)
			if n == 0 {
				return "@internal note: short read"
			}
			this.dst_x ~sat+= (n & 0xFFFF_FFFF) as base.u32
		} endwhile.inner
	} endwhile.outer

	return ok
}

pri const RLE_STATE_NEUTRAL : base.u32 = 0
pri const RLE_STATE_RUN     : base.u32 = 1
pri const RLE_STATE_ESCAPE  : base.u32 = 2
pri const RLE_STATE_LITERAL : base.u32 = 3
pri const RLE_STATE_DELTA_X : base.u32 = 4
pri const RLE_STATE_DELTA_Y : base.u32 = 5

pri func decoder.swizzle_rle!(dst: ptr base.pixel_buffer, src: base.io_reader) base.status {
	var dst_pixfmt          : base.pixel_format
	var dst_bits_per_pixel  : base.u32[..= 256]
	var dst_bytes_per_pixel : base.u64[..= 32]
	var dst_bytes_per_row   : base.u64
	var dst_palette         : slice base.u8
	var tab                 : table base.u8
	var row                 : slice base.u8
	var dst                 : slice base.u8
	var i                   : base.u64
	var n                   : base.u64

	var p0      : base.u32[..= 259]
	var code    : base.u8
	var indexes : array[2] base.u8

	var rle_state : base.u32

	var chunk_bits  : base.u32
	var chunk_count : base.u32[..= 64]

	// TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
	// to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
	dst_pixfmt = args.dst.pixel_format()
	dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()
	if (dst_bits_per_pixel & 7) <> 0 {
		return base."#unsupported option"
	}
	dst_bytes_per_pixel = (dst_bits_per_pixel / 8) as base.u64
	dst_bytes_per_row = (this.width as base.u64) * dst_bytes_per_pixel
	dst_palette = args.dst.palette_or_else(fallback: this.scratch[1024 ..])
	tab = args.dst.plane(p: 0)

	rle_state = this.rle_state

	while.outer true {
		row = tab.row(y: this.dst_y)
		if dst_bytes_per_row < row.length() {
			row = row[.. dst_bytes_per_row]
		}

		while.middle true {
			i = (this.dst_x as base.u64) * dst_bytes_per_pixel
			if i <= row.length() {
				dst = row[i ..]
			} else {
				dst = this.util.empty_slice_u8()
			}

			while.goto_suspend true {{
			while.inner true {
				if rle_state == RLE_STATE_NEUTRAL {
					if args.src.length() < 1 {
						break.goto_suspend
					}
					code = args.src.peek_u8()
					args.src.skip_u32_fast!(actual: 1, worst_case: 1)
					if code == 0 {
						rle_state = RLE_STATE_ESCAPE
						continue.inner
					}
					this.rle_length = code as base.u32
					rle_state = RLE_STATE_RUN
					continue.inner

				} else if rle_state == RLE_STATE_RUN {
					if args.src.length() < 1 {
						break.goto_suspend
					}
					code = args.src.peek_u8()
					args.src.skip_u32_fast!(actual: 1, worst_case: 1)
					if this.bits_per_pixel == 8 {
						p0 = 0
						while p0 < this.rle_length {
							assert p0 < 255 via "a < b: a < c; c <= b"(c: this.rle_length)
							this.scratch[p0] = code
							p0 += 1
						} endwhile
					} else {
						indexes[0] = (code >> 4) as base.u8
						indexes[1] = code & 0x0F
						p0 = 0
						while p0 < this.rle_length {
							assert p0 < 255 via "a < b: a < c; c <= b"(c: this.rle_length)
							this.scratch[p0 + 0] = indexes[0]
							this.scratch[p0 + 1] = indexes[1]
							p0 += 2
						} endwhile
					}
					this.swizzler.swizzle_interleaved_from_slice!(
						dst: dst,
						dst_palette: dst_palette,
						src: this.scratch[.. this.rle_length])
					this.dst_x ~sat+= this.rle_length
					rle_state = RLE_STATE_NEUTRAL
					continue.middle

				} else if rle_state == RLE_STATE_ESCAPE {
					if args.src.length() < 1 {
						break.goto_suspend
					}
					code = args.src.peek_u8()
					args.src.skip_u32_fast!(actual: 1, worst_case: 1)
					if code < 2 {  // 0=EOL, 1=EOF.
						if (this.dst_y >= this.height) and (code == 0) {
							return "#bad RLE compression"
						}
						this.swizzler.swizzle_interleaved_transparent_black!(
							dst: dst,
							dst_palette: dst_palette,
							num_pixels: 0xFFFF_FFFF_FFFF_FFFF)
						this.dst_x = 0
						this.dst_y ~mod+= this.dst_y_inc
						if code > 0 {
							break.outer
						}
						rle_state = RLE_STATE_NEUTRAL
						continue.outer
					} else if code == 2 {  // 2=DELTA.
						rle_state = RLE_STATE_DELTA_X
						continue.inner
					}
					this.rle_length = code as base.u32
					this.rle_padded = (this.bits_per_pixel == 8) and ((code & 1) <> 0)
					rle_state = RLE_STATE_LITERAL
					continue.inner

				} else if rle_state == RLE_STATE_LITERAL {
					if this.bits_per_pixel == 8 {
						n = this.swizzler.limited_swizzle_u32_interleaved_from_reader!(
							up_to_num_pixels: this.rle_length,
							dst: dst,
							dst_palette: dst_palette,
							src: args.src)
						this.dst_x ~sat+= (n & 0xFFFF_FFFF) as base.u32
						this.rle_length ~sat-= (n & 0xFFFF_FFFF) as base.u32
					} else {
						// Calculate the remaining number of 16-bit chunks. At
						// 4 bits per pixel there are 4 pixels per chunk.
						// Division rounds up.
						chunk_count = (this.rle_length + 3) / 4
						p0 = 0
						while (chunk_count > 0) and (args.src.length() >= 2) {
							chunk_bits = args.src.peek_u16be_as_u32()
							args.src.skip_u32_fast!(actual: 2, worst_case: 2)
							this.scratch[p0 + 0x00] = (0x0F & (chunk_bits >> 0x0C)) as base.u8
							this.scratch[p0 + 0x01] = (0x0F & (chunk_bits >> 0x08)) as base.u8
							this.scratch[p0 + 0x02] = (0x0F & (chunk_bits >> 0x04)) as base.u8
							this.scratch[p0 + 0x03] = (0x0F & (chunk_bits >> 0x00)) as base.u8
							p0 = (p0 & 255) + 0x04
							chunk_count -= 1
						} endwhile
						p0 = p0.min(a: this.rle_length)
						this.swizzler.swizzle_interleaved_from_slice!(
							dst: dst,
							dst_palette: dst_palette,
							src: this.scratch[.. p0])
						this.dst_x ~sat+= p0
						this.rle_length ~sat-= p0
					}

					if this.rle_length > 0 {
						break.goto_suspend
					}
					if this.rle_padded {
						if args.src.length() < 1 {
							break.goto_suspend
						}
						args.src.skip_u32_fast!(actual: 1, worst_case: 1)
						this.rle_padded = false
					}
					rle_state = RLE_STATE_NEUTRAL
					continue.middle

				} else if rle_state == RLE_STATE_DELTA_X {
					if args.src.length() < 1 {
						break.goto_suspend
					}
					this.rle_delta_x = args.src.peek_u8()
					args.src.skip_u32_fast!(actual: 1, worst_case: 1)
					rle_state = RLE_STATE_DELTA_Y
					continue.inner

				}  // else (rle_state == RLE_STATE_DELTA_Y).
				if args.src.length() < 1 {
					break.goto_suspend
				}
				code = args.src.peek_u8()
				args.src.skip_u32_fast!(actual: 1, worst_case: 1)
				if this.rle_delta_x > 0 {
					this.swizzler.swizzle_interleaved_transparent_black!(
						dst: dst,
						dst_palette: dst_palette,
						num_pixels: this.rle_delta_x as base.u64)
					this.dst_x ~sat+= this.rle_delta_x as base.u32
					this.rle_delta_x = 0
					if this.dst_x > this.width {
						return "#bad RLE compression"
					}
				}

				// Paint the skipped rows (and the partial final row) with
				// transparent black.
				if code > 0 {
					code -= 1
					while true {
						this.dst_y ~mod+= this.dst_y_inc
						if this.dst_y >= this.height {
							return "#bad RLE compression"
						}
						row = tab.row(y: this.dst_y)
						if dst_bytes_per_row < row.length() {
							row = row[.. dst_bytes_per_row]
						}
						if code <= 0 {
							this.swizzler.swizzle_interleaved_transparent_black!(
								dst: row,
								dst_palette: dst_palette,
								num_pixels: this.dst_x as base.u64)
							break
						}
						this.swizzler.swizzle_interleaved_transparent_black!(
							dst: row,
							dst_palette: dst_palette,
							num_pixels: 0xFFFF_FFFF_FFFF_FFFF)
						code -= 1
					} endwhile
				}

				rle_state = RLE_STATE_NEUTRAL
				continue.middle

			} endwhile.inner
			}} endwhile.goto_suspend
			this.rle_state = rle_state
			return "@internal note: short read"
		} endwhile.middle
	} endwhile.outer

	while this.dst_y < this.height {
		row = tab.row(y: this.dst_y)
		if dst_bytes_per_row < row.length() {
			row = row[.. dst_bytes_per_row]
		}
		this.swizzler.swizzle_interleaved_transparent_black!(
			dst: row,
			dst_palette: dst_palette,
			num_pixels: 0xFFFF_FFFF_FFFF_FFFF)
		this.dst_y ~mod+= this.dst_y_inc
	} endwhile

	return ok
}

pri func decoder.swizzle_bitfields!(dst: ptr base.pixel_buffer, src: base.io_reader) base.status {
	var dst_pixfmt          : base.pixel_format
	var dst_bits_per_pixel  : base.u32[..= 256]
	var dst_bytes_per_pixel : base.u64[..= 32]
	var dst_bytes_per_row   : base.u64
	var dst_palette         : slice base.u8
	var tab                 : table base.u8
	var dst                 : slice base.u8
	var i                   : base.u64
	var n                   : base.u64

	var p0      : base.u32[..= 256]
	var p1      : base.u32[..= 256]
	var p1_temp : base.u32

	var num_bits : base.u32[..= 32]
	var c        : base.u32
	var c32      : base.u32
	var channel  : base.u32[..= 4]

	// TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
	// to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
	dst_pixfmt = args.dst.pixel_format()
	dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()
	if (dst_bits_per_pixel & 7) <> 0 {
		return base."#unsupported option"
	}
	dst_bytes_per_pixel = (dst_bits_per_pixel / 8) as base.u64
	dst_bytes_per_row = (this.width as base.u64) * dst_bytes_per_pixel
	dst_palette = args.dst.palette_or_else(fallback: this.scratch[1024 ..])
	tab = args.dst.plane(p: 0)

	while.outer true {
		while this.pending_pad > 0 {
			if args.src.length() <= 0 {
				return "@internal note: short read"
			}
			this.pending_pad -= 1
			args.src.skip_u32_fast!(actual: 1, worst_case: 1)
		} endwhile

		while.inner true {
			if this.dst_x == this.width {
				this.dst_x = 0
				this.dst_y ~mod+= this.dst_y_inc
				if this.dst_y >= this.height {
					if this.height > 0 {
						this.pending_pad = this.pad_per_row
					}
					break.outer
				} else if this.pad_per_row <> 0 {
					this.pending_pad = this.pad_per_row
					continue.outer
				}
			}

			// -------- BEGIN convert to PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE.
			p1_temp = this.width ~mod- this.dst_x
			p1 = p1_temp.min(a: 256)
			p0 = 0
			while p0 < p1 {
				assert p0 < 256 via "a < b: a < c; c <= b"(c: p1)
				if this.bits_per_pixel == 16 {
					if args.src.length() < 2 {
						break
					}
					c32 = args.src.peek_u16le_as_u32()
					args.src.skip_u32_fast!(actual: 2, worst_case: 2)
				} else {
					if args.src.length() < 4 {
						break
					}
					c32 = args.src.peek_u32le()
					args.src.skip_u32_fast!(actual: 4, worst_case: 4)
				}

				channel = 0
				while channel < 4,
					inv p0 < 256,
				{
					if this.channel_num_bits[channel] == 0 {
						this.scratch[(8 * p0) + (2 * channel) + 0] = 0xFF
						this.scratch[(8 * p0) + (2 * channel) + 1] = 0xFF
					} else {
						c = (c32 & this.channel_masks[channel]) >> this.channel_shifts[channel]
						num_bits = this.channel_num_bits[channel] as base.u32
						while num_bits < 16,
							inv p0 < 256,
							inv channel < 4,
							post num_bits >= 16,
						{
							c |= c ~mod<< num_bits
							num_bits *= 2
						} endwhile
						c >>= num_bits - 16
						this.scratch[(8 * p0) + (2 * channel) + 0] = (0xFF & (c >> 0)) as base.u8
						this.scratch[(8 * p0) + (2 * channel) + 1] = (0xFF & (c >> 8)) as base.u8
					}

					channel += 1
				} endwhile

				p0 += 1
			} endwhile
			// -------- END   convert to PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE.

			dst = tab.row(y: this.dst_y)
			if dst_bytes_per_row < dst.length() {
				dst = dst[.. dst_bytes_per_row]
			}
			i = (this.dst_x as base.u64) * dst_bytes_per_pixel
			if i >= dst.length() {
				// TODO: advance args.src if the dst pixel_buffer bounds is
				// smaller than this BMP's image bounds?
				continue.inner
			}

			n = this.swizzler.swizzle_interleaved_from_slice!(
				dst: dst[i ..],
				dst_palette: dst_palette,
				src: this.scratch[.. 8 * p0])
			if n == 0 {
				return "@internal note: short read"
			}
			this.dst_x ~sat+= (n & 0xFFFF_FFFF) as base.u32
		} endwhile.inner
	} endwhile.outer

	return ok
}

pri func decoder.swizzle_low_bit_depth!(dst: ptr base.pixel_buffer, src: base.io_reader) base.status {
	var dst_pixfmt          : base.pixel_format
	var dst_bits_per_pixel  : base.u32[..= 256]
	var dst_bytes_per_pixel : base.u64[..= 32]
	var dst_bytes_per_row   : base.u64
	var dst_palette         : slice base.u8
	var tab                 : table base.u8
	var dst                 : slice base.u8
	var i                   : base.u64
	var n                   : base.u64

	var p0 : base.u32[..= 543]

	var chunk_bits  : base.u32
	var chunk_count : base.u32

	// TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
	// to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
	dst_pixfmt = args.dst.pixel_format()
	dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()
	if (dst_bits_per_pixel & 7) <> 0 {
		return base."#unsupported option"
	}
	dst_bytes_per_pixel = (dst_bits_per_pixel / 8) as base.u64
	dst_bytes_per_row = (this.width as base.u64) * dst_bytes_per_pixel
	dst_palette = args.dst.palette_or_else(fallback: this.scratch[1024 ..])
	tab = args.dst.plane(p: 0)

	while.loop true {
		if this.dst_x == this.width {
			this.dst_x = 0
			this.dst_y ~mod+= this.dst_y_inc
			if this.dst_y >= this.height {
				break.loop
			}
		}

		dst = tab.row(y: this.dst_y)
		if dst_bytes_per_row < dst.length() {
			dst = dst[.. dst_bytes_per_row]
		}
		i = (this.dst_x as base.u64) * dst_bytes_per_pixel
		if i >= dst.length() {
			// TODO: advance args.src if the dst pixel_buffer bounds is
			// smaller than this BMP's image bounds?
			continue.loop
		}
		dst = dst[i ..]
		p0 = 0

		if this.bits_per_pixel == 1 {
			// Calculate the remaining number of 32-bit chunks. At 1 bit per
			// pixel there are 32 pixels per chunk. Division rounds up.
			chunk_count = ((this.width ~sat- this.dst_x) + 31) / 32
			chunk_count = chunk_count.min(a: 16)  // Keep p0 <= 512.
			while (chunk_count > 0) and (args.src.length() >= 4) {
				chunk_bits = args.src.peek_u32be()
				args.src.skip_u32_fast!(actual: 4, worst_case: 4)
				this.scratch[p0 + 0x00] = (0x01 & (chunk_bits >> 0x1F)) as base.u8
				this.scratch[p0 + 0x01] = (0x01 & (chunk_bits >> 0x1E)) as base.u8
				this.scratch[p0 + 0x02] = (0x01 & (chunk_bits >> 0x1D)) as base.u8
				this.scratch[p0 + 0x03] = (0x01 & (chunk_bits >> 0x1C)) as base.u8
				this.scratch[p0 + 0x04] = (0x01 & (chunk_bits >> 0x1B)) as base.u8
				this.scratch[p0 + 0x05] = (0x01 & (chunk_bits >> 0x1A)) as base.u8
				this.scratch[p0 + 0x06] = (0x01 & (chunk_bits >> 0x19)) as base.u8
				this.scratch[p0 + 0x07] = (0x01 & (chunk_bits >> 0x18)) as base.u8
				this.scratch[p0 + 0x08] = (0x01 & (chunk_bits >> 0x17)) as base.u8
				this.scratch[p0 + 0x09] = (0x01 & (chunk_bits >> 0x16)) as base.u8
				this.scratch[p0 + 0x0A] = (0x01 & (chunk_bits >> 0x15)) as base.u8
				this.scratch[p0 + 0x0B] = (0x01 & (chunk_bits >> 0x14)) as base.u8
				this.scratch[p0 + 0x0C] = (0x01 & (chunk_bits >> 0x13)) as base.u8
				this.scratch[p0 + 0x0D] = (0x01 & (chunk_bits >> 0x12)) as base.u8
				this.scratch[p0 + 0x0E] = (0x01 & (chunk_bits >> 0x11)) as base.u8
				this.scratch[p0 + 0x0F] = (0x01 & (chunk_bits >> 0x10)) as base.u8
				this.scratch[p0 + 0x10] = (0x01 & (chunk_bits >> 0x0F)) as base.u8
				this.scratch[p0 + 0x11] = (0x01 & (chunk_bits >> 0x0E)) as base.u8
				this.scratch[p0 + 0x12] = (0x01 & (chunk_bits >> 0x0D)) as base.u8
				this.scratch[p0 + 0x13] = (0x01 & (chunk_bits >> 0x0C)) as base.u8
				this.scratch[p0 + 0x14] = (0x01 & (chunk_bits >> 0x0B)) as base.u8
				this.scratch[p0 + 0x15] = (0x01 & (chunk_bits >> 0x0A)) as base.u8
				this.scratch[p0 + 0x16] = (0x01 & (chunk_bits >> 0x09)) as base.u8
				this.scratch[p0 + 0x17] = (0x01 & (chunk_bits >> 0x08)) as base.u8
				this.scratch[p0 + 0x18] = (0x01 & (chunk_bits >> 0x07)) as base.u8
				this.scratch[p0 + 0x19] = (0x01 & (chunk_bits >> 0x06)) as base.u8
				this.scratch[p0 + 0x1A] = (0x01 & (chunk_bits >> 0x05)) as base.u8
				this.scratch[p0 + 0x1B] = (0x01 & (chunk_bits >> 0x04)) as base.u8
				this.scratch[p0 + 0x1C] = (0x01 & (chunk_bits >> 0x03)) as base.u8
				this.scratch[p0 + 0x1D] = (0x01 & (chunk_bits >> 0x02)) as base.u8
				this.scratch[p0 + 0x1E] = (0x01 & (chunk_bits >> 0x01)) as base.u8
				this.scratch[p0 + 0x1F] = (0x01 & (chunk_bits >> 0x00)) as base.u8
				p0 = (p0 & 511) + 0x20
				chunk_count -= 1
			} endwhile

		} else if this.bits_per_pixel == 2 {
			// Calculate the remaining number of 32-bit chunks. At 2 bits per
			// pixel there are 16 pixels per chunk. Division rounds up.
			chunk_count = ((this.width ~sat- this.dst_x) + 15) / 16
			chunk_count = chunk_count.min(a: 32)  // Keep p0 <= 512.
			while (chunk_count > 0) and (args.src.length() >= 4) {
				chunk_bits = args.src.peek_u32be()
				args.src.skip_u32_fast!(actual: 4, worst_case: 4)
				this.scratch[p0 + 0x00] = (0x03 & (chunk_bits >> 0x1E)) as base.u8
				this.scratch[p0 + 0x01] = (0x03 & (chunk_bits >> 0x1C)) as base.u8
				this.scratch[p0 + 0x02] = (0x03 & (chunk_bits >> 0x1A)) as base.u8
				this.scratch[p0 + 0x03] = (0x03 & (chunk_bits >> 0x18)) as base.u8
				this.scratch[p0 + 0x04] = (0x03 & (chunk_bits >> 0x16)) as base.u8
				this.scratch[p0 + 0x05] = (0x03 & (chunk_bits >> 0x14)) as base.u8
				this.scratch[p0 + 0x06] = (0x03 & (chunk_bits >> 0x12)) as base.u8
				this.scratch[p0 + 0x07] = (0x03 & (chunk_bits >> 0x10)) as base.u8
				this.scratch[p0 + 0x08] = (0x03 & (chunk_bits >> 0x0E)) as base.u8
				this.scratch[p0 + 0x09] = (0x03 & (chunk_bits >> 0x0C)) as base.u8
				this.scratch[p0 + 0x0A] = (0x03 & (chunk_bits >> 0x0A)) as base.u8
				this.scratch[p0 + 0x0B] = (0x03 & (chunk_bits >> 0x08)) as base.u8
				this.scratch[p0 + 0x0C] = (0x03 & (chunk_bits >> 0x06)) as base.u8
				this.scratch[p0 + 0x0D] = (0x03 & (chunk_bits >> 0x04)) as base.u8
				this.scratch[p0 + 0x0E] = (0x03 & (chunk_bits >> 0x02)) as base.u8
				this.scratch[p0 + 0x0F] = (0x03 & (chunk_bits >> 0x00)) as base.u8
				p0 = (p0 & 511) + 0x10
				chunk_count -= 1
			} endwhile

		} else if this.bits_per_pixel == 4 {
			// Calculate the remaining number of 32-bit chunks. At 4 bits per
			// pixel there are 8 pixels per chunk. Division rounds up.
			chunk_count = ((this.width ~sat- this.dst_x) + 7) / 8
			chunk_count = chunk_count.min(a: 64)  // Keep p0 <= 512.
			while (chunk_count > 0) and (args.src.length() >= 4) {
				chunk_bits = args.src.peek_u32be()
				args.src.skip_u32_fast!(actual: 4, worst_case: 4)
				this.scratch[p0 + 0x00] = (0x0F & (chunk_bits >> 0x1C)) as base.u8
				this.scratch[p0 + 0x01] = (0x0F & (chunk_bits >> 0x18)) as base.u8
				this.scratch[p0 + 0x02] = (0x0F & (chunk_bits >> 0x14)) as base.u8
				this.scratch[p0 + 0x03] = (0x0F & (chunk_bits >> 0x10)) as base.u8
				this.scratch[p0 + 0x04] = (0x0F & (chunk_bits >> 0x0C)) as base.u8
				this.scratch[p0 + 0x05] = (0x0F & (chunk_bits >> 0x08)) as base.u8
				this.scratch[p0 + 0x06] = (0x0F & (chunk_bits >> 0x04)) as base.u8
				this.scratch[p0 + 0x07] = (0x0F & (chunk_bits >> 0x00)) as base.u8
				p0 = (p0 & 511) + 0x08
				chunk_count -= 1
			} endwhile
		}

		p0 = p0.min(a: this.width ~sat- this.dst_x)
		n = this.swizzler.swizzle_interleaved_from_slice!(
			dst: dst,
			dst_palette: dst_palette,
			src: this.scratch[.. p0])
		if n == 0 {
			return "@internal note: short read"
		}
		this.dst_x ~sat+= (n & 0xFFFF_FFFF) as base.u32
	} endwhile.loop

	return ok
}

pub func decoder.frame_dirty_rect() base.rect_ie_u32 {
	return this.util.make_rect_ie_u32(
		min_incl_x: 0,
		min_incl_y: 0,
		max_excl_x: this.width,
		max_excl_y: this.height)
}

pub func decoder.num_animation_loops() base.u32 {
	return 0
}

pub func decoder.num_decoded_frame_configs() base.u64 {
	if this.call_sequence > 3 {
		return 1
	}
	return 0
}

pub func decoder.num_decoded_frames() base.u64 {
	if this.call_sequence > 4 {
		return 1
	}
	return 0
}

pub func decoder.restart_frame!(index: base.u64, io_position: base.u64) base.status {
	if this.call_sequence < 3 {
		return base."#bad call sequence"
	}
	if args.index <> 0 {
		return base."#bad argument"
	}
	this.call_sequence = 3
	this.frame_config_io_position = args.io_position
	return ok
}

pub func decoder.set_report_metadata!(fourcc: base.u32, report: base.bool) {
	// No-op. BMP doesn't support metadata.
}

pub func decoder.tell_me_more?(dst: base.io_writer, minfo: nptr base.more_information, src: base.io_reader) {
	if this.io_redirect_fourcc <= 1 {
		return base."#no more information"
	}
	if args.minfo <> nullptr {
		args.minfo.set!(
			flavor: 1,  // WUFFS_BASE__MORE_INFORMATION__FLAVOR__IO_REDIRECT
			w: this.io_redirect_fourcc,
			x: 0,
			y: this.io_redirect_pos,
			z: 0xFFFF_FFFF_FFFF_FFFF)
	}
	// Setting io_redirect_fourcc to a placeholder value of 1 will cause future
	// calls to return an error.
	this.io_redirect_fourcc = 1
}

pub func decoder.workbuf_len() base.range_ii_u64 {
	return this.util.make_range_ii_u64(min_incl: 0, max_incl: 0)
}

pri func decoder.read_palette?(src: base.io_reader) {
	var i    : base.u32
	var argb : base.u32

	if this.bitmap_info_len == 12 {
		while (i < 256) and (this.padding >= 3) {
			this.padding -= 3
			argb = args.src.read_u24le_as_u32?()
			argb |= 0xFF00_0000
			this.src_palette[(4 * i) + 0] = ((argb >> 0) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 1] = ((argb >> 8) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 2] = ((argb >> 16) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 3] = ((argb >> 24) & 0xFF) as base.u8
			i += 1
		} endwhile
	} else {
		while (i < 256) and (this.padding >= 4) {
			this.padding -= 4
			argb = args.src.read_u32le?()
			argb |= 0xFF00_0000
			this.src_palette[(4 * i) + 0] = ((argb >> 0) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 1] = ((argb >> 8) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 2] = ((argb >> 16) & 0xFF) as base.u8
			this.src_palette[(4 * i) + 3] = ((argb >> 24) & 0xFF) as base.u8
			i += 1
		} endwhile
	}

	while i < 256 {
		this.src_palette[(4 * i) + 0] = 0x00
		this.src_palette[(4 * i) + 1] = 0x00
		this.src_palette[(4 * i) + 2] = 0x00
		this.src_palette[(4 * i) + 3] = 0xFF
		i += 1
	} endwhile
}

pri func decoder.process_masks?() {
	var i    : base.u32
	var mask : base.u32
	var n    : base.u32

	while i < 4 {
		mask = this.channel_masks[i]
		if mask <> 0 {
			n = 0
			while (mask & 1) == 0,
				inv i < 4,
			{
				n ~mod+= 1
				mask >>= 1
			} endwhile
			this.channel_shifts[i] = (n & 31) as base.u8

			n = 0
			while (mask & 1) == 1,
				inv i < 4,
			{
				n ~mod+= 1
				mask >>= 1
			} endwhile
			if (mask <> 0) or (n > 32) {
				return "#bad header"
			}
			this.channel_num_bits[i] = n as base.u8
		} else if i <> 3 {
			return "#bad header"
		}

		i += 1
	} endwhile
}