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std.compress.flate.Decompress: implement more bit reading

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This commit is contained in:
Andrew Kelley
2025-07-28 13:13:59 -07:00
parent 6509fa1cf3
commit 9c8cb777d4
1 changed files with 65 additions and 30 deletions
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95
lib/std/compress/flate/Decompress.zig
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@@ -146,8 +146,8 @@ fn dynamicCodeLength(self: *Decompress, code: u16, lens: []u4, pos: usize) !usiz
// used. Shift bit reader for that much bits, those bits are used. And
// return symbol.
fn decodeSymbol(self: *Decompress, decoder: anytype) !Symbol {
const sym = try decoder.find(try self.peekBitsReverseBuffered(u15));
try self.shiftBits(sym.code_bits);
const sym = try decoder.find(@bitReverse(try self.peekBits(u15)));
try self.tossBits(sym.code_bits);
return sym;
}
@@ -245,8 +245,8 @@ fn readInner(d: *Decompress, w: *Writer, limit: std.Io.Limit) (Error || Reader.S
var dec_lens: [286 + 30]u4 = @splat(0);
var pos: usize = 0;
while (pos < hlit + hdist) {
const sym = try cl_dec.find(try d.peekBitsReverse(u7));
try d.shiftBits(sym.code_bits);
const sym = try cl_dec.find(@bitReverse(try d.peekBits(u7)));
try d.tossBits(sym.code_bits);
pos += try d.dynamicCodeLength(sym.symbol, &dec_lens, pos);
}
if (pos > hlit + hdist) {
@@ -291,7 +291,7 @@ fn readInner(d: *Decompress, w: *Writer, limit: std.Io.Limit) (Error || Reader.S
// Handles fixed block non literal (length) code.
// Length code is followed by 5 bits of distance code.
const length = try d.decodeLength(@intCast(code - 257));
const distance = try d.decodeDistance(try d.takeBitsReverseBuffered(u5));
const distance = try d.decodeDistance(@bitReverse(try d.takeBits(u5)));
remaining = try writeMatch(w, length, distance, remaining);
},
else => return error.InvalidCode,
@@ -384,9 +384,42 @@ fn takeBits(d: *Decompress, comptime T: type) !T {
};
}
fn takeBitsReverseBuffered(d: *Decompress, comptime T: type) !T {
_ = d;
@panic("TODO");
fn peekBits(d: *Decompress, comptime T: type) !T {
const U = @Type(.{ .int = .{ .signedness = .unsigned, .bits = @bitSizeOf(T) } });
const remaining_bits = d.remaining_bits;
const next_bits = d.next_bits;
if (remaining_bits >= @bitSizeOf(T)) {
const u: U = @truncate(next_bits);
return switch (@typeInfo(T)) {
.int => u,
.@"enum" => @enumFromInt(u),
else => @bitCast(u),
};
}
const in = d.input;
const next_int = try in.peekInt(usize, .little);
const needed_bits = @bitSizeOf(T) - remaining_bits;
const u: U = @intCast((next_bits << needed_bits) | (next_int & ((@as(usize, 1) << needed_bits) - 1)));
return switch (@typeInfo(T)) {
.int => u,
.@"enum" => @enumFromInt(u),
else => @bitCast(u),
};
}
fn tossBits(d: *Decompress, n: u6) !void {
const remaining_bits = d.remaining_bits;
const next_bits = d.next_bits;
if (remaining_bits >= n) {
d.next_bits = next_bits >> n;
d.remaining_bits = remaining_bits - n;
} else {
const in = d.input;
const next_int = try in.takeInt(usize, .little);
const needed_bits = n - remaining_bits;
d.next_bits = next_int >> needed_bits;
d.remaining_bits = @intCast(@bitSizeOf(usize) - @as(usize, needed_bits));
}
}
fn takeNBitsBuffered(d: *Decompress, n: u4) !u16 {
@@ -395,16 +428,6 @@ fn takeNBitsBuffered(d: *Decompress, n: u4) !u16 {
@panic("TODO");
}
fn peekBitsReverse(d: *Decompress, comptime T: type) !T {
_ = d;
@panic("TODO");
}
fn peekBitsReverseBuffered(d: *Decompress, comptime T: type) !T {
_ = d;
@panic("TODO");
}
fn alignBitsToByte(d: *Decompress) void {
const remaining_bits = d.remaining_bits;
const next_bits = d.next_bits;
@@ -422,15 +445,26 @@ fn alignBitsToByte(d: *Decompress) void {
d.next_bits = 0;
}
fn shiftBits(d: *Decompress, n: u6) !void {
_ = d;
_ = n;
@panic("TODO");
}
/// Reads first 7 bits, and then maybe 1 or 2 more to get full 7,8 or 9 bit code.
/// ref: https://datatracker.ietf.org/doc/html/rfc1951#page-12
/// Lit Value Bits Codes
/// --------- ---- -----
/// 0 - 143 8 00110000 through
/// 10111111
/// 144 - 255 9 110010000 through
/// 111111111
/// 256 - 279 7 0000000 through
/// 0010111
/// 280 - 287 8 11000000 through
/// 11000111
fn readFixedCode(d: *Decompress) !u16 {
_ = d;
@panic("TODO");
const code7 = @bitReverse(try d.takeBits(u7));
return switch (code7) {
0...0b0010_111 => @as(u16, code7) + 256,
0b0010_111 + 1...0b1011_111 => (@as(u16, code7) << 1) + @as(u16, try d.takeBits(u1)) - 0b0011_0000,
0b1011_111 + 1...0b1100_011 => (@as(u16, code7 - 0b1100000) << 1) + try d.takeBits(u1) + 280,
else => (@as(u16, code7 - 0b1100_100) << 2) + @as(u16, @bitReverse(try d.takeBits(u2))) + 144,
};
}
pub const Symbol = packed struct {
@@ -731,20 +765,21 @@ test "encode/decode literals" {
}
}
test "basic" {
// non compressed block (type 0)
test "non compressed block (type 0)" {
try testBasicCase(&[_]u8{
0b0000_0001, 0b0000_1100, 0x00, 0b1111_0011, 0xff, // deflate fixed buffer header len, nlen
'H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', 0x0a, // non compressed data
}, "Hello world\n");
}
// fixed code block (type 1)
test "fixed code block (type 1)" {
try testBasicCase(&[_]u8{
0xf3, 0x48, 0xcd, 0xc9, 0xc9, 0x57, 0x28, 0xcf, // deflate data block type 1
0x2f, 0xca, 0x49, 0xe1, 0x02, 0x00,
}, "Hello world\n");
}
// dynamic block (type 2)
test "dynamic block (type 2)" {
try testBasicCase(&[_]u8{
0x3d, 0xc6, 0x39, 0x11, 0x00, 0x00, 0x0c, 0x02, // deflate data block type 2
0x30, 0x2b, 0xb5, 0x52, 0x1e, 0xff, 0x96, 0x38,