Rewrite bit_reader and bit_writer to take advantage of current zig se…

…mantics and enhance readability (#21689)

Co-authored-by: Tanner Schultz <tgschultz@tgschultz-dl.tail7ba92.ts.net>

lib/std/compress/zstandard/decode/block.zig

@@ -405,7 +405,7 @@ pub const DecodeState = struct {
     };
     fn readLiteralsBits(
         self: *DecodeState,
-        bit_count_to_read: usize,
+        bit_count_to_read: u16,
     ) LiteralBitsError!u16 {
         return self.literal_stream_reader.readBitsNoEof(u16, bit_count_to_read) catch bits: {
             if (self.literal_streams == .four and self.literal_stream_index < 3) {

lib/std/compress/zstandard/decode/huffman.zig

@@ -63,7 +63,7 @@ fn decodeFseHuffmanTreeSlice(src: []const u8, compressed_size: usize, weights: *
 
 fn assignWeights(
     huff_bits: *readers.ReverseBitReader,
-    accuracy_log: usize,
+    accuracy_log: u16,
     entries: *[1 << 6]Table.Fse,
     weights: *[256]u4,
 ) !usize {
@@ -73,7 +73,7 @@ fn assignWeights(
 
     while (i < 254) {
         const even_data = entries[even_state];
-        var read_bits: usize = 0;
+        var read_bits: u16 = 0;
         const even_bits = huff_bits.readBits(u32, even_data.bits, &read_bits) catch unreachable;
         weights[i] = std.math.cast(u4, even_data.symbol) orelse return error.MalformedHuffmanTree;
         i += 1;

lib/std/compress/zstandard/readers.zig

@@ -42,11 +42,11 @@ pub const ReverseBitReader = struct {
         if (i == 8) return error.BitStreamHasNoStartBit;
     }
 
-    pub fn readBitsNoEof(self: *@This(), comptime U: type, num_bits: usize) error{EndOfStream}!U {
+    pub fn readBitsNoEof(self: *@This(), comptime U: type, num_bits: u16) error{EndOfStream}!U {
         return self.bit_reader.readBitsNoEof(U, num_bits);
     }
 
-    pub fn readBits(self: *@This(), comptime U: type, num_bits: usize, out_bits: *usize) error{}!U {
+    pub fn readBits(self: *@This(), comptime U: type, num_bits: u16, out_bits: *u16) error{}!U {
         return try self.bit_reader.readBits(U, num_bits, out_bits);
     }
 
@@ -55,19 +55,19 @@ pub const ReverseBitReader = struct {
     }
 
     pub fn isEmpty(self: ReverseBitReader) bool {
-        return self.byte_reader.remaining_bytes == 0 and self.bit_reader.bit_count == 0;
+        return self.byte_reader.remaining_bytes == 0 and self.bit_reader.count == 0;
     }
 };
 
 pub fn BitReader(comptime Reader: type) type {
     return struct {
         underlying: std.io.BitReader(.little, Reader),
 
-        pub fn readBitsNoEof(self: *@This(), comptime U: type, num_bits: usize) !U {
+        pub fn readBitsNoEof(self: *@This(), comptime U: type, num_bits: u16) !U {
             return self.underlying.readBitsNoEof(U, num_bits);
         }
 
-        pub fn readBits(self: *@This(), comptime U: type, num_bits: usize, out_bits: *usize) !U {
+        pub fn readBits(self: *@This(), comptime U: type, num_bits: u16, out_bits: *u16) !U {
             return self.underlying.readBits(U, num_bits, out_bits);
         }