std/gif/decode_lzw.puffs

// Copyright 2017 The Puffs 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.

// TODO: some mechanism for fixing the error code's value, such as a trailing
// "= 100", to maintain API stability.
pub error "LZW code is out of range"
pub error "LZW prefix chain is cyclical"

pub struct lzw_decoder?(
	literal_width u32[2..8] = 8,
	stack[4096] u8,
	suffixes[4096] u8,
	prefixes[4096] u16[..4095],
)

// TODO: add a ! as this function is impure.
pub func lzw_decoder.set_literal_width(lw u32[2..8] = 8)() {
	this.literal_width = in.lw
}

pub func lzw_decoder.decode?(dst writer1, src reader1)() {
	// These variables don't change over the lifetime of this func.
	var clear_code u32[4..256] = (1 as u32) << this.literal_width
	var end_code u32[5..257] = clear_code + 1

	// These variables do change.
	//
	// save_code is the code for which, after decoding a code, we save what the
	// next back-reference expands to. The README.md file also calls this value
	// `max`. 4096 means do not save.
	var save_code u32[..4096] = end_code
	var prev_code u32[..4095]
	var width u32[..12] = this.literal_width + 1

	// These variables yield src's bits in Least Significant Bits order.
	var bits u32
	var n_bits u32

	while true,
		pre n_bits < 8,
	{
		assert n_bits < (width + 8) via "a < (b + c): a < c; 0 <= b"()
		while n_bits < width,
			inv n_bits < (width + 8),
			post n_bits >= width,
		{
			assert n_bits < 12 via "a < b: a < c; c <= b"(c:width)
			bits |= (in.src.read_u8?() as u32) << n_bits
			n_bits += 8
		}
		var code u32[..4095] = bits.low_bits(n:width)
		bits >>= width
		n_bits -= width

		if code < clear_code {
			assert code < 256 via "a < b: a < c; c <= b"(c:clear_code)
			in.dst.write_u8?(x:code as u8)
			if save_code <= 4095 {
				this.suffixes[save_code] = code as u8
				this.prefixes[save_code] = prev_code as u16
			}

		} else if code == clear_code {
			save_code = end_code
			prev_code = 0
			width = this.literal_width + 1
			continue

		} else if code == end_code {
			return

		} else if code <= save_code {
			var s u32[..4095] = 4095
			var c u32[..4095] = code

			if code == save_code {
				s -= 1
				c = prev_code
			}

			while c >= clear_code,
				inv n_bits < 8,
				post c < 256 via "a < b: a < c; c <= b"(c:clear_code),
			{
				this.stack[s] = this.suffixes[c]
				if s == 0 {
					return error "LZW prefix chain is cyclical"
				}
				s -= 1
				c = this.prefixes[c] as u32
			}
			this.stack[s] = c as u8

			if code == save_code {
				this.stack[4095] = c as u8
			}

			while true,
				inv n_bits < 8,
				inv c < 256,
			{
				var expansion[] u8 = this.stack[s:]
				// TODO: "x:" instead of "s:" should be a compiler error.
				var n_copied u64 = in.dst.copy_from_slice(x:expansion)
				if n_copied == expansion.length() {
					break
				}
				s = (s + ((n_copied & 4095) as u32)) & 4095
				// TODO: "closed for writes" instead?
				return suspension "short write"
			}

			if save_code <= 4095 {
				this.suffixes[save_code] = c as u8
				this.prefixes[save_code] = prev_code as u16
			}

		} else {
			return error "LZW code is out of range"
		}

		if save_code <= 4095 {
			save_code += 1
			if (save_code == ((1 as u32) << width)) and (width < 12) {
				width += 1
			}
		}
		prev_code = code
	}
}