decode_huffman_fast64.wuffs

// Copyright 2017 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.

// TODO: describe how the xxx_fastxx version differs from the xxx_slow one, the
// assumptions that xxx_fastxx makes, and how that makes it fast.
pri func decoder.decode_huffman_fast64!(dst: base.io_writer, src: base.io_reader) base.status,
	choosy,
{
	// When editing this function, consider making the equivalent change to the
	// decode_huffman_slow function. Keep the diff between the two
	// decode_huffman_*.wuffs files as small as possible, while retaining both
	// correctness and performance.

	var bits               : base.u64
	var n_bits             : base.u32
	var table_entry        : base.u32
	var table_entry_n_bits : base.u32[..= 15]
	var lmask              : base.u64[..= 511]
	var dmask              : base.u64[..= 511]
	var redir_top          : base.u32[..= 0xFFFF]
	var redir_mask         : base.u32[..= 0x7FFF]
	var length             : base.u32[..= 258]
	var dist_minus_1       : base.u32[..= 0x7FFF]
	var hlen               : base.u32[..= 0x7FFF]
	var hdist              : base.u32
	var hdist_adjustment   : base.u32

	if (this.n_bits >= 8) or ((this.bits >> (this.n_bits & 7)) <> 0) {
		return "#internal error: inconsistent n_bits"
	}

	bits = this.bits as base.u64
	n_bits = this.n_bits

	lmask = ((1 as base.u64) << this.n_huffs_bits[0]) - 1
	dmask = ((1 as base.u64) << this.n_huffs_bits[1]) - 1

	if this.transformed_history_count < args.dst.history_position() {
		return base."#bad I/O position"
	}
	hdist_adjustment = ((this.transformed_history_count - args.dst.history_position()) & 0xFFFF_FFFF) as base.u32

	// Check up front, on each iteration, that we have enough buffer space to
	// both read (8 bytes) and write (266 bytes) as much as we need to. Doing
	// this check once (per iteration), up front, removes the need to check
	// multiple times inside the loop body, so it's faster overall.
	//
	// For writing, a literal code obviously corresponds to writing 1 byte, and
	// 258 is the maximum length in a length-distance pair, as specified in the
	// RFC section 3.2.5. Compressed blocks (length and distance codes).
	//
	// We adjust 258 up to (258 + 8) = 266 since it can be faster to overshoot
	// a little and make multiple-of-8-byte copies even when the length in the
	// length-distance pair isn't an exact multiple-of-8. Strictly speaking,
	// 264 (an exact multiple-of-8) is the tight bound but (258 + 8) is easier
	// for the Wuffs proof system, as length's type is refined to [..= 258],
	//
	// For reading, strictly speaking, we only need 6 bytes (48 bits) of
	// available input, because the H-L Literal/Length code is up to 15 bits
	// plus up to 5 extra bits, the H-D Distance code is up to 15 bits plus up
	// to 13 extra bits and 15 + 5 + 15 + 13 == 48. However, it's faster to
	// read 64 bits than 48 bits or (48 - n_bits) bits.
	while.loop(args.dst.length() >= 266) and (args.src.length() >= 8) {
		// Ensure that we have at least 56 bits of input.
		//
		// This is "Variant 4" of
		// https://fgiesen.wordpress.com/2018/02/20/reading-bits-in-far-too-many-ways-part-2/
		//
		// 56, the number of bits in 7 bytes, is a property of that "Variant 4"
		// bit-reading technique, and not related to the DEFLATE format per se.
		//
		// The "& 63" is a no-op, and not part of the original "Variant 4"
		// technique, but satisfies Wuffs' overflow/underflow checks.
		bits |= args.src.peek_u64le() ~mod<< (n_bits & 63)
		args.src.skip_u32_fast!(actual: (63 - (n_bits & 63)) >> 3, worst_case: 8)
		n_bits |= 56

		// Decode an lcode symbol from H-L.
		table_entry = this.huffs[0][bits & lmask]
		table_entry_n_bits = table_entry & 0x0F
		bits >>= table_entry_n_bits
		n_bits ~mod-= table_entry_n_bits

		if (table_entry >> 31) <> 0 {
			// Literal.
			args.dst.write_u8_fast!(a: ((table_entry >> 8) & 0xFF) as base.u8)
			continue.loop
		} else if (table_entry >> 30) <> 0 {
			// No-op; code continues past the if-else chain.
		} else if (table_entry >> 29) <> 0 {
			// End of block.
			this.end_of_block = true
			break.loop
		} else if (table_entry >> 28) <> 0 {
			// Redirect.
			redir_top = (table_entry >> 8) & 0xFFFF
			redir_mask = ((1 as base.u32) << ((table_entry >> 4) & 0x0F)) - 1
			table_entry = this.huffs[0][
				(redir_top + (((bits & 0xFFFF_FFFF) as base.u32) & redir_mask)) & HUFFS_TABLE_MASK]
			table_entry_n_bits = table_entry & 0x0F
			bits >>= table_entry_n_bits
			n_bits ~mod-= table_entry_n_bits

			if (table_entry >> 31) <> 0 {
				// Literal.
				args.dst.write_u8_fast!(a: ((table_entry >> 8) & 0xFF) as base.u8)
				continue.loop
			} else if (table_entry >> 30) <> 0 {
				// No-op; code continues past the if-else chain.
			} else if (table_entry >> 29) <> 0 {
				// End of block.
				this.end_of_block = true
				break.loop
			} else if (table_entry >> 28) <> 0 {
				return "#internal error: inconsistent Huffman decoder state"
			} else if (table_entry >> 27) <> 0 {
				return "#bad Huffman code"
			} else {
				return "#internal error: inconsistent Huffman decoder state"
			}

		} else if (table_entry >> 27) <> 0 {
			return "#bad Huffman code"
		} else {
			return "#internal error: inconsistent Huffman decoder state"
		}

		// length = base_number_minus_3 + 3 + extra_bits.
		//
		// The -3 is from the bias in script/print-deflate-magic-numbers.go.
		// That bias makes the "& 0xFF" 1 and 15-ish lines below correct.
		length = ((table_entry >> 8) & 0xFF) + 3
		table_entry_n_bits = (table_entry >> 4) & 0x0F
		if table_entry_n_bits > 0 {
			// The "+ 253" is the same as "- 3", after the "& 0xFF", but the
			// plus form won't require an underflow check.
			length = ((length + 253 + (bits.low_bits(n: table_entry_n_bits) as base.u32)) & 0xFF) + 3
			bits >>= table_entry_n_bits
			n_bits ~mod-= table_entry_n_bits
		}

		// Decode a dcode symbol from H-D.
		table_entry = this.huffs[1][bits & dmask]
		table_entry_n_bits = table_entry & 15
		bits >>= table_entry_n_bits
		n_bits ~mod-= table_entry_n_bits

		// Check for a redirect.
		if (table_entry >> 28) == 1 {
			redir_top = (table_entry >> 8) & 0xFFFF
			redir_mask = ((1 as base.u32) << ((table_entry >> 4) & 0x0F)) - 1
			table_entry = this.huffs[1][
				(redir_top + (((bits & 0xFFFF_FFFF) as base.u32) & redir_mask)) & HUFFS_TABLE_MASK]
			table_entry_n_bits = table_entry & 0x0F
			bits >>= table_entry_n_bits
			n_bits ~mod-= table_entry_n_bits
		}

		// For H-D, all symbols should be base_number + extra_bits.
		if (table_entry >> 24) <> 0x40 {
			if (table_entry >> 24) == 0x08 {
				return "#bad Huffman code"
			}
			return "#internal error: inconsistent Huffman decoder state"
		}

		// dist_minus_1 = base_number_minus_1 + extra_bits.
		// distance     = dist_minus_1 + 1.
		//
		// The -1 is from the bias in script/print-deflate-magic-numbers.go.
		// That bias makes the "& 0x7FFF" 2 and 15-ish lines below correct and
		// undoing that bias makes proving (dist_minus_1 + 1) > 0 trivial.
		dist_minus_1 = (table_entry >> 8) & 0x7FFF
		table_entry_n_bits = (table_entry >> 4) & 0x0F

		dist_minus_1 = (dist_minus_1 + (bits.low_bits(n: table_entry_n_bits) as base.u32)) & 0x7FFF
		bits >>= table_entry_n_bits
		n_bits ~mod-= table_entry_n_bits

		// The "while true { etc; break }" is a redundant version of "etc", but
		// its presence minimizes the diff between decode_huffman_fastxx and
		// decode_huffman_slow.
		while true,
			pre args.dst.length() >= 266,
		{
			// We can therefore prove:
			assert (length as base.u64) <= args.dst.length() via "a <= b: a <= c; c <= b"(c: 266)
			assert ((length + 8) as base.u64) <= args.dst.length() via "a <= b: a <= c; c <= b"(c: 266)

			// Copy from this.history.
			if ((dist_minus_1 + 1) as base.u64) > args.dst.history_length() {
				// Set (hlen, hdist) to be the length-distance pair to copy
				// from this.history, and (length, distance) to be the
				// remaining length-distance pair to copy from args.dst.
				hlen = 0
				hdist = (((dist_minus_1 + 1) as base.u64) - args.dst.history_length()) as base.u32
				if length > hdist {
					assert hdist < length via "a < b: b > a"()
					assert hdist < 0x8000 via "a < b: a < c; c <= b"(c: length)
					length -= hdist
					hlen = hdist
				} else {
					hlen = length
					length = 0
				}
				hdist ~mod+= hdist_adjustment
				if this.history_index < hdist {
					return "#bad distance"
				}

				// Copy from this.history[(this.history_index - hdist) ..].
				//
				// This copying is simpler than the decode_huffman_slow version
				// because it cannot yield. We have already checked that
				// args.dst.length() is large enough.
				args.dst.limited_copy_u32_from_slice!(
					up_to: hlen, s: this.history[(this.history_index - hdist) & 0x7FFF ..])
				if length == 0 {
					// No need to copy from args.dst.
					continue.loop
				}

				if (((dist_minus_1 + 1) as base.u64) > args.dst.history_length()) or
					((length as base.u64) > args.dst.length()) or
					(((length + 8) as base.u64) > args.dst.length()) {
					return "#internal error: inconsistent distance"
				}
			}
			// Once again, redundant but explicit assertions.
			assert (dist_minus_1 + 1) >= 1
			assert ((dist_minus_1 + 1) as base.u64) <= args.dst.history_length()
			assert (length as base.u64) <= args.dst.length()
			assert ((length + 8) as base.u64) <= args.dst.length()

			// Copy from args.dst.
			//
			// For short distances, less than 8 bytes, copying atomic 8-byte
			// chunks can result in incorrect output, so we fall back to a
			// slower 1-byte-at-a-time copy. Deflate's copy-from-history can
			// pick up freshly written bytes. A length = 5, distance = 2 copy
			// starting with "abc" should give "abcbcbcb" but the 8-byte chunk
			// technique would give "abcbc???", the exact output depending on
			// what was previously in the writer buffer.
			if (dist_minus_1 + 1) >= 8 {
				args.dst.limited_copy_u32_from_history_8_byte_chunks_fast!(
					up_to: length, distance: (dist_minus_1 + 1))
			} else if (dist_minus_1 + 1) == 1 {
				// (distance == 1) is essentially RLE (Run Length Encoding). It
				// happens often enough that it's worth special-casing.
				args.dst.limited_copy_u32_from_history_8_byte_chunks_distance_1_fast!(
					up_to: length, distance: (dist_minus_1 + 1))
			} else {
				args.dst.limited_copy_u32_from_history_fast!(
					up_to: length, distance: (dist_minus_1 + 1))
			}
			break
		} endwhile
	} endwhile.loop

	// Ensure n_bits < 8 by rewindng args.src, if we loaded too many of its
	// bytes into the bits variable.
	//
	// Note that we can unconditionally call undo_read (without resulting in an
	// "invalid I/O operation" error code) only because this whole function can
	// never suspend, as all of its I/O operations were checked beforehand for
	// sufficient buffer space. Otherwise, resuming from the suspension could
	// mean that the (possibly different) args.src is no longer rewindable,
	// even if conceptually, this function was responsible for reading the
	// bytes we want to rewind.
	if n_bits > 63 {
		return "#internal error: inconsistent n_bits"
	}
	while n_bits >= 8,
		post n_bits < 8,
	{
		n_bits -= 8
		if args.src.can_undo_byte() {
			args.src.undo_byte!()
		} else {
			return "#internal error: inconsistent I/O"
		}
	} endwhile

	this.bits = (bits & (((1 as base.u64) << n_bits) - 1)) as base.u32
	this.n_bits = n_bits

	if (this.n_bits >= 8) or ((this.bits >> this.n_bits) <> 0) {
		return "#internal error: inconsistent n_bits"
	}
}